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Heat treatment can be applied to many metal alloys to significantly improve key physical properties such as hardness, strength, or machinability. These changes are due to changes in the microstructure and sometimes due to changes in the chemical composition of the material. These treatments include heating the metal alloy to (usually) extreme temperatures followed by cooling under controlled conditions. The temperature to which the material is heated, the time to maintain the temperature and the cooling rate will greatly affect the final physical properties of the metal alloy. In this paper, we review the heat treatment related to the most commonly used metal alloys in CNC machining. By describing the impact of these processes on the final part properties, this article will help you choose the right material for your application. When will the heat treatment be carried out Heat treatment can be applied to metal alloys throughout the manufacturing process. For CNC machined parts, heat treatment is generally applicable to: Before CNC machining: when it is required to provide ready-made standard grade metal alloys, CNC service providers will directly process parts from inventory materials. This is usually the best choice to shorten the lead time. After CNC machining: some heat treatments significantly increase the hardness of the material, or are used as finishing steps after forming. In these cases, the heat treatment is performed after CNC machining, because high hardness reduces the machinability of the material. For example, this is the standard practice when CNC machining tool steel parts. Common heat treatment of CNC materials: annealing, stress relief and tempering Annealing, tempering and stress relief all involve heating the metal alloy to a high temperature and then slowly cooling the material, usually in air or in an oven. They differ in the temperature at which the material is heated and in the order of the manufacturing process. During annealing, the metal is heated to a very high temperature and then slowly cooled to obtain the desired microstructure. Annealing is usually applied to all metal alloys after forming and before any further processing to soften them and improve their workability. If no other heat treatment is specified, most CNC machined parts will have material properties in the annealed state. Stress relief includes heating the parts to a high temperature (but lower than annealing), which is usually used after CNC machining to eliminate the residual stress generated in the manufacturing process. This can produce parts with more consistent mechanical properties. Tempering also heats parts at a temperature lower than the annealing temperature. It is usually used after quenching of low carbon steel (1045 and A36) and alloy steel (4140 and 4240) to reduce its brittleness and improve its mechanical properties. quench Quenching involves heating the metal to a very high temperature, followed by rapid cooling, usually by immersing the material in oil or water or exposing it to a cold air stream. Rapid cooling "locks" the microstructure changes that occur when the material is heated, resulting in extremely high hardness of the parts. Parts are usually quenched after CNC machining as the last step of the manufacturing process (think of blacksmith immersing the blade in oil), because the increase in hardness makes the material more difficult to process. Tool steels are quenched after CNC machining to obtain extremely high surface hardness characteristics. The resulting hardness can then be controlled using a tempering process. For example, the hardness of tool steel A2 after quenching is 63-65 Rockwell C, but it can be tempered to a hardness between 42-62 HRC. Tempering can prolong the service life of parts because tempering can reduce brittleness (the best results can be obtained when the hardness is 56-58 HRC). Precipitation hardening (aging) Precipitation hardening or aging are two terms commonly used to describe the same process. Precipitation hardening is a three-step process: first, the material is heated to a high temperature, then quenched, and finally heated to a low temperature (aging) for a long time. This leads to the dissolution and uniform distribution of alloying elements initially in the form of discrete particles of different compositions in the metal matrix, just as sugar crystals dissolve in water when the solution is heated. After precipitation hardening, the strength and hardness of the metal alloy increase sharply. For example, 7075 is an aluminum alloy, which is usually used in the aerospace industry to manufacture parts with tensile strength equivalent to that of stainless steel, and its weight is less than 3 times. The following table illustrates the effect of precipitation hardening in aluminum 7075: Not all metals can be heat treated in this way, but compatible materials are considered as superalloys and are suitable for very high performance applications. The most common precipitation hardening alloys used in CNC are summarized as follows: Case hardening and carburizing Case hardening is a series of heat treatment, which can make the surface of parts have high hardness while the underlining material remains soft. This is generally better than increasing the hardness of the part over the entire volume (e.g., by quenching) because the harder part is also more brittle. Carburizing is the most common case hardening heat treatment. It involves heating low carbon steel in a carbon rich environment and then quenching the parts to lock the carbon in the metal matrix. This increases the surface hardness of steel, just as anodizing increases the surface hardness of aluminum alloy.

Heat treatment can be applied to many metal alloys to significantly improve key physical properties such as hardness, strength, or machinability. These changes are due to changes in the microstructure and sometimes due to changes in the chemical composition of the material. These treatments include heating the metal alloy to (usually) extreme temperatures followed by cooling under controlled conditions. The temperature to which the material is heated, the time to maintain the temperature and the cooling rate will greatly affect the final physical properties of the metal alloy. In this paper, we review the heat treatment related to the most commonly used metal alloys in CNC machining. By describing the impact of these processes on the final part properties, this article will help you choose the right material for your application. When will the heat treatment be carried out Heat treatment can be applied to metal alloys throughout the manufacturing process. For CNC machined parts, heat treatment is generally applicable to: Before CNC machining: when it is required to provide ready-made standard grade metal alloys, CNC service providers will directly process parts from inventory materials. This is usually the best choice to shorten the lead time. After CNC machining: some heat treatments significantly increase the hardness of the material, or are used as finishing steps after forming. In these cases, the heat treatment is performed after CNC machining, because high hardness reduces the machinability of the material. For example, this is the standard practice when CNC machining tool steel parts. Common heat treatment of CNC materials: annealing, stress relief and tempering Annealing, tempering and stress relief all involve heating the metal alloy to a high temperature and then slowly cooling the material, usually in air or in an oven. They differ in the temperature at which the material is heated and in the order of the manufacturing process. During annealing, the metal is heated to a very high temperature and then slowly cooled to obtain the desired microstructure. Annealing is usually applied to all metal alloys after forming and before any further processing to soften them and improve their workability. If no other heat treatment is specified, most CNC machined parts will have material properties in the annealed state. Stress relief includes heating the parts to a high temperature (but lower than annealing), which is usually used after CNC machining to eliminate the residual stress generated in the manufacturing process. This can produce parts with more consistent mechanical properties. Tempering also heats parts at a temperature lower than the annealing temperature. It is usually used after quenching of low carbon steel (1045 and A36) and alloy steel (4140 and 4240) to reduce its brittleness and improve its mechanical properties. quench Quenching involves heating the metal to a very high temperature, followed by rapid cooling, usually by immersing the material in oil or water or exposing it to a cold air stream. Rapid cooling "locks" the microstructure changes that occur when the material is heated, resulting in extremely high hardness of the parts. Parts are usually quenched after CNC machining as the last step of the manufacturing process (think of blacksmith immersing the blade in oil), because the increase in hardness makes the material more difficult to process. Tool steels are quenched after CNC machining to obtain extremely high surface hardness characteristics. The resulting hardness can then be controlled using a tempering process. For example, the hardness of tool steel A2 after quenching is 63-65 Rockwell C, but it can be tempered to a hardness between 42-62 HRC. Tempering can prolong the service life of parts because tempering can reduce brittleness (the best results can be obtained when the hardness is 56-58 HRC). Precipitation hardening (aging) Precipitation hardening or aging are two terms commonly used to describe the same process. Precipitation hardening is a three-step process: first, the material is heated to a high temperature, then quenched, and finally heated to a low temperature (aging) for a long time. This leads to the dissolution and uniform distribution of alloying elements initially in the form of discrete particles of different compositions in the metal matrix, just as sugar crystals dissolve in water when the solution is heated. After precipitation hardening, the strength and hardness of the metal alloy increase sharply. For example, 7075 is an aluminum alloy, which is usually used in the aerospace industry to manufacture parts with tensile strength equivalent to that of stainless steel, and its weight is less than 3 times. The following table illustrates the effect of precipitation hardening in aluminum 7075: Not all metals can be heat treated in this way, but compatible materials are considered as superalloys and are suitable for very high performance applications. The most common precipitation hardening alloys used in CNC are summarized as follows: Case hardening and carburizing Case hardening is a series of heat treatment, which can make the surface of parts have high hardness while the underlining material remains soft. This is generally better than increasing the hardness of the part over the entire volume (e.g., by quenching) because the harder part is also more brittle. Carburizing is the most common case hardening heat treatment. It involves heating low carbon steel in a carbon rich environment and then quenching the parts to lock the carbon in the metal matrix. This increases the surface hardness of steel, just as anodizing increases the surface hardness of aluminum alloy.

In order to make full use of the ability of CNC machining, designers must follow specific manufacturing rules. But this can be a challenge because there is no specific industry standard. In this article, we have compiled a comprehensive guide with best design practices for CNC machining. We focus on describing the feasibility of modern CNC systems, ignoring the related costs. For guidance on designing cost-effective parts for CNC, please refer to this article. CNC machining CNC machining is a subtractive machining technology. In CNC, various high-speed rotating (thousands of RPM) tools are used to remove materials from solid blocks to produce parts according to CAD models. Metal and plastic can be processed by CNC. CNC machining parts have high dimensional accuracy and strict tolerance. CNC is suitable for mass production and one-time work. In fact, CNC machining is currently the most cost-effective way to produce metal prototypes, even compared to 3D printing. Main design limitations of CNC CNC provides great design flexibility, but there are some design limitations. These limitations are related to the basic mechanics of the cutting process, mainly related to tool geometry and tool access. 1. Tool geometry The most common CNC tools (end mills and drills) are cylindrical with limited cutting length. When the material is removed from the workpiece, the geometry of the tool is transferred to the machined part. This means that, for example, no matter how small a tool is used, the internal angle of a CNC part always has a radius. 2. Tool access In order to remove the material, the tool approaches the workpiece directly from above. Functions that cannot be accessed in this way cannot be CNC processed. There is one exception to this rule: undercut. We will learn how to use undercuts in design in the next section. A good design practice is to align all features of the model (holes, cavities, vertical walls, etc.) with one of the six main directions. This rule is considered a recommendation, not a limitation, because the 5-axis CNC system provides advanced workpiece holding capability. Tool access is also an issue when machining features with large aspect ratios. For example, to reach the bottom of the deep cavity, a special tool with a long axis is required. This reduces the stiffness of the end effector, increases vibration and reduces achievable accuracy. CNC experts recommend designing parts that can be machined with tools with the maximum possible diameter and the shortest possible length. CNC design rules One of the challenges often encountered when designing parts for CNC machining is that there is no specific industry standard: CNC machine tool and tool manufacturers constantly improve their technical capabilities and expand the range of possibilities. In the following table, we summarize the recommended and feasible values of the most common features encountered in CNC machining parts. 1. Cavity and groove Recommended cavity depth: 4 times cavity width The cutting length of the end mill is limited (usually 3-4 times its diameter). When the depth width ratio is small, the tool deflection, chip discharge and vibration become more prominent. Limiting the depth of the cavity to four times its width ensures good results. If a greater depth is required, consider designing a part with a variable cavity depth (see the figure above for an example). Deep cavity milling: a cavity with a depth greater than 6 times the tool diameter is considered as a deep cavity. The ratio of tool diameter to cavity depth can be 30:1 by using special tools (using end mills with a diameter of 1 inch, the maximum depth is 30 cm). 2. Inner edge Vertical corner radius: recommended ⅓ x cavity depth (or greater) Using the recommended value of the internal corner radius ensures that the appropriate diameter tool can be used and aligned with the guidelines for the recommended cavity depth. Increasing the corner radius slightly above the recommended value (e.g. by 1 mm) allows the tool to cut along a circular path instead of a 90 ° angle. This is preferred because it can obtain a higher quality surface finish. If an internal angle of 90 ° sharpness is required, consider adding a T-shaped undercut instead of reducing the angle radius. The recommended bottom plate radius is 0.5mm, 1mm or no radius; Any radius is feasible The lower edge of the end mill is a flat edge or a slightly round edge. Other floor radii can be processed with ball head tools. It is a good design practice to use the recommended value because it is the first choice of the machinist. 3. Thin wall Recommended minimum wall thickness: 0.8mm (metal) and 1.5mm (plastic); 0.5mm (metal) and 1.0mm (plastic) are feasible Reducing the wall thickness will reduce the stiffness of the material, thereby increasing the vibration in the machining process and reducing the achievable accuracy. Plastics tend to warp (due to residual stress) and soften (due to temperature rise), so it is recommended to use a larger minimum wall thickness. 4. Hole Diameter recommended standard drill size; Any diameter greater than 1mm is acceptable Use a drill or end mill to machine holes. Standardization of drill bit size (metric and English units). Reamers and boring cutters are used to finish holes requiring strict tolerances. For sizes less than ▽ 20 mm, standard diameters are recommended. Maximum depth recommended 4 x nominal diameter; Typically 10 x nominal diameter; 40 x nominal diameter where feasible Non standard diameter holes must be processed with end mills. In this case, the maximum cavity depth limit applies and the recommended maximum depth value should be used. Use a special drill (minimum diameter 3 mm) to machine holes with a depth exceeding the typical value. The blind hole machined by the drill has a conical bottom plate (135 ° angle), while the hole machined by the end mill is flat. In CNC machining, there is no special preference between through holes and blind holes. 5. Thread The minimum thread size is m2; M6 or larger is recommended The internal thread is cut with a tap, and the external thread is cut with a die. Taps and dies can be used to cut threads to m2. CNC threading tools are common and preferred by machinists because they limit the risk of tap breakage. CNC thread tools can be used to cut threads to M6. The minimum thread length is 1.5 x nominal diameter; 3 x nominal diameter recommended Most of the load applied to the thread is borne by a few first teeth (up to 1.5 times the nominal diameter). Therefore, no more than 3 times the nominal diameter of the thread is required. For threads in blind holes cut with a tap (i.e. all threads smaller than M6), add a non threaded length equal to 1.5 x nominal diameter at the bottom of the hole. When a CNC thread tool can be used (i.e. the thread is larger than M6), the hole can run through its entire length. 6. Small features The minimum hole diameter is recommended to be 2.5 mm (0.1 inch); 0.05 mm (0.005 in) is feasible Most machine shops will be able to accurately machine cavities and holes using tools less than 2.5 mm (0.1 inch) in diameter. Anything below this limit is considered micromachining. Special tools (micro drills) and expert knowledge are required to process such features (the physical changes in the cutting process are within this range), so it is recommended to avoid using them unless absolutely necessary. 7. Tolerance Standard: ± 0.125 mm (0.005 in) Typical: ± 0.025 mm (0.001 in) Feasible: ± 0.0125 mm (0.0005 in) Tolerances define the boundaries of acceptable dimensions. The achievable tolerances depend on the basic dimensions and geometry of the part. The above values are reasonable guidelines. If no tolerance is specified, most machine shops will use a standard ± 0.125 mm (0.005 in) tolerance. 8. Words and lettering The recommended font size is 20 (or larger), 5mm lettering Engraved characters are preferably embossed characters because less material is removed. It is recommended to use sans serif fonts (such as Arial or Verdana) with a size of at least 20 points. Many CNC machines have pre programmed routines for these fonts. Machine settings and part orientation The schematic diagram of parts that need to be set several times is as follows: As mentioned earlier, tool access is one of the main design limitations of CNC machining. To reach all the surfaces of the model, the workpiece must be rotated several times. For example, the part of the above image must be rotated three times in total: two holes are machined in two main directions, and the third enters the back of the part. Whenever the workpiece rotates, the machine must be recalibrated and a new coordinate system must be defined. It is important to consider the machine settings in design for two reasons: The total number of machine settings affects costs. Rotating and realigning parts requires manual operation and increases the total processing time. If the part needs to be rotated 3-4 times, this is generally acceptable, but any exceeding this limit is redundant. In order to obtain maximum relative positional accuracy, two features must be machined in the same setup. This is because the new call step introduces a small (but not negligible) error. Five axis CNC machining When using 5-axis CNC machining, the need for multiple machine settings can be eliminated. Multi axis CNC machining can manufacture parts with complex geometry because they provide 2 additional rotational axes. Five axis CNC machining allows the tool to always be tangent to the cutting surface. More complex and efficient tool paths can be followed, resulting in better surface finish and lower machining time. Of course, 5-axis CNC also has its limitations. The basic tool geometry and tool access restrictions still apply (for example, parts with internal geometry cannot be machined). In addition, the cost of using such systems is higher. Design undercut Undercuts are features that cannot be machined with standard cutting tools because some of their surfaces cannot be directly accessed from above. There are two main types of undercuts: T-grooves and dovetails. Undercut can be single-sided or double-sided and processed with special tools. The T-groove cutting tool is basically made of a horizontal cutting insert connected to a vertical axis. The width of the undercut may vary between 3 mm and 40 mm. It is recommended to use standard dimensions for widths (i.e., full millimeter increments or standard inch fractions) as tools are more likely to be available. For dovetail tools, the angle defines the feature size. 45 ° and 60 ° dovetail tools are considered standard. When designing parts with undercuts on the inner wall, remember to add enough clearance for the tool. A good rule of thumb is to add at least four times the undercut depth between the machined wall and any other inner wall. For standard tools, the typical ratio between the cutting diameter and the shaft diameter is 2:1, which limits the cutting depth. When non-standard undercut is required, the machine shop usually makes customized undercut tools by itself. This increases lead times and costs and should be avoided as much as possible. T-shaped groove (left), dovetail groove undercut (middle) and unilateral undercut (right) on the inner wall Drafting technical drawings Note that some design criteria cannot be included in step or IGES files. If your model contains one or more of the following, 2D technical drawings must be provided: Threaded hole or shaft Tolerance dimension Specific surface finish requirements Instructions for CNC machine tool operators Rule of thumb 1. Design the parts that can be processed with the largest diameter tool. 2. Add large fillets (at least ⅓ x cavity depth) to all internal vertical angles. 3. Limit the depth of the cavity to 4 times its width. 4. Align the main functions of the design along one of the six main directions. If this is not possible, 5-axis CNC machining can be selected. 5. When your design includes thread, tolerance, surface finish specification or other comments of the machine operator, please submit technical drawings with the drawings.

Inconel: another heat-resistant superalloy (HRSA), Inconel is the best choice for extreme temperatures or corrosive environments. In addition to jet engines, Inconel 625 and its harder and stronger brother Inconel 718 are also used in nuclear power plants, oil and gas drilling platforms, chemical processing facilities, etc. Both are quite weldable, but they are expensive and even more difficult to process than CoCr. Therefore, they should be avoided unless necessary. Stainless steel: by adding the minimum 10.5% chromium, the carbon content is reduced to the maximum 1.2%, and adding alloy elements such as nickel and molybdenum, the metallurgist converts ordinary rusty steel into stainless steel, which is the killer of anti-corrosion switch in the manufacturing industry. However, because there are dozens of levels and categories to choose from, it may be difficult to determine which is best for a given application. For example, the crystal structure of austenitic stainless steels 304 and 316L makes them non-magnetic, non hardenable, ductile and quite ductile. On the other hand, martensitic stainless steel (grade 420 is grade 1) is magnetic and hardenable, making it an ideal choice for surgical instruments and various wear-resistant parts. There are also ferritic stainless steel (mostly 400 Series), duplex steel (think of oil and natural gas), and precipitation hardening stainless steel 15-5 pH and 17-4 PH, all of which are favored for their excellent mechanical properties. Machinability ranges from fairly good (416 stainless steel) to moderately poor (347 stainless steel). Steel: like stainless steel, there are too many alloys and properties. However, four important issues to be considered are: 1. The cost of steel is usually lower than that of stainless steel and high-temperature alloy 2. In the presence of air and moisture, all steel will corrode 3. Except for some tool steels, most steels have good machinability 4. The lower the carbon content, the lower the hardness of the steel (represented by the first two digits of the alloy, such as 1018, 4340 or 8620). That is, steel and its close relatives iron are by far the most commonly used of all metals, followed by aluminum. The list does not mention the red metals copper, brass and bronze, or titanium, another super important superalloy. There is also no mention of some polymers. For example, ABS is the material of Lego building blocks and drainage pipes, which can be molded and processed, and has excellent toughness and impact resistance. Engineering grade plastic acetal is a remarkable example, applicable to all products from gears to sporting goods. The combination of strength and flexibility of nylon has replaced silk as the preferred material for parachutes. There are also polycarbonate, polyvinyl chloride (PVC), high density and low density polyethylene. The key is that the selection of materials is extensive, so as a part designer, it is meaningful to explore what is available, what is good, and how to process. Quick plus offers more than 40 different grades of plastic and metal materials.

Inconel: another heat-resistant superalloy (HRSA), Inconel is the best choice for extreme temperatures or corrosive environments. In addition to jet engines, Inconel 625 and its harder and stronger brother Inconel 718 are also used in nuclear power plants, oil and gas drilling platforms, chemical processing facilities, etc. Both are quite weldable, but they are expensive and even more difficult to process than CoCr. Therefore, they should be avoided unless necessary. Stainless steel: by adding the minimum 10.5% chromium, the carbon content is reduced to the maximum 1.2%, and adding alloy elements such as nickel and molybdenum, the metallurgist converts ordinary rusty steel into stainless steel, which is the killer of anti-corrosion switch in the manufacturing industry. However, because there are dozens of levels and categories to choose from, it may be difficult to determine which is best for a given application. For example, the crystal structure of austenitic stainless steels 304 and 316L makes them non-magnetic, non hardenable, ductile and quite ductile. On the other hand, martensitic stainless steel (grade 420 is grade 1) is magnetic and hardenable, making it an ideal choice for surgical instruments and various wear-resistant parts. There are also ferritic stainless steel (mostly 400 Series), duplex steel (think of oil and natural gas), and precipitation hardening stainless steel 15-5 pH and 17-4 PH, all of which are favored for their excellent mechanical properties. Machinability ranges from fairly good (416 stainless steel) to moderately poor (347 stainless steel). Steel: like stainless steel, there are too many alloys and properties. However, four important issues to be considered are: 1. The cost of steel is usually lower than that of stainless steel and high-temperature alloy 2. In the presence of air and moisture, all steel will corrode 3. Except for some tool steels, most steels have good machinability 4. The lower the carbon content, the lower the hardness of the steel (represented by the first two digits of the alloy, such as 1018, 4340 or 8620). That is, steel and its close relatives iron are by far the most commonly used of all metals, followed by aluminum. The list does not mention the red metals copper, brass and bronze, or titanium, another super important superalloy. There is also no mention of some polymers. For example, ABS is the material of Lego building blocks and drainage pipes, which can be molded and processed, and has excellent toughness and impact resistance. Engineering grade plastic acetal is a remarkable example, applicable to all products from gears to sporting goods. The combination of strength and flexibility of nylon has replaced silk as the preferred material for parachutes. There are also polycarbonate, polyvinyl chloride (PVC), high density and low density polyethylene. The key is that the selection of materials is extensive, so as a part designer, it is meaningful to explore what is available, what is good, and how to process. Quick plus offers more than 40 different grades of plastic and metal materials.

From the 1950s to the present, injection molding has been dominating the consumer goods manufacturing industry, bringing us everything from action figures to denture containers. Despite the incredible versatility of injection molding, it does have some design limitations. The basic injection molding process is to heat and pressurize the plastic particles until they flow into the mold cavity; Cooling the mold; Open the mold; Eject parts; And then close the mold. Repeat and repeat, usually 10000 times for one plastic manufacturing run, one million times during the life of the mold. It is not easy to produce hundreds of thousands of parts, but there are some changes in the design of plastic parts, the simplest of which is to pay attention to the design wall thickness. Wall thickness limit of injection molding If you disassemble any plastic appliance around your home, you will notice that the wall thickness of most parts is about 1mm to 4mm (the best thickness for molding), and the wall thickness of the whole part is uniform. Why? There are two reasons. First of all, the cooling speed of the thinner wall is faster, which shortens the cycle time of the mold and shortens the time required for manufacturing each part. If the plastic part can be cooled faster after the mold is filled, it can be safely pushed out faster without warping, and because the time cost on the injection molding machine is high, the production cost of the part is low. The second reason is uniformity: in the cooling cycle, the outer surface of the plastic part is cooled first. Shrinkage due to cooling; If the part has a uniform thickness, the whole part will shrink uniformly from the mold during cooling, and the part will be taken out smoothly. However, if the thick section and the thin section of the part are adjacent, the melting center of the thicker area will continue to cool and shrink after the thinner area and the surface have solidified. As this thick area continues to cool, it shrinks and it can only pull material from the surface. As a result, there is a small dent on the surface of the part, which is called a shrinkage mark. The shrink marks only indicate that the engineering design of the hidden areas is poor, but on the decorative surface, they may require tens of thousands of yuan for re installation. How do you know if your parts have these "thick wall" problems during injection molding? Thick wall solutions Fortunately, thick walls have some simple solutions. The first thing to do is to pay attention to the problem area. In the following sections, you can see two common problems: the thickness around the screw hole and the thickness in the part that requires strength. For screw holes in injection molded parts, the solution is to use "screw bosses": a small cylinder of material directly surrounding the screw holes, connected to the rest of the shell with a reinforcing rib or material flange. This allows for a more uniform wall thickness and fewer shrinkage marks. When an area of the part needs to be particularly strong, but the wall is too thick, the solution is also simple: reinforcement. Instead of making the whole part thicker and difficult to cool, it is better to thin the outer surface into a shell, and then add vertical material ribs inside to improve the strength and rigidity. In addition to being easier to form, this also reduces the amount of material required and reduces the cost. After completing these changes, you can use the DFM tool again to check whether the changes have solved the problem. Of course, after everything has been resolved, the part prototype can be made in the 3D printer to test it before continuing manufacturing.

The design of injection molding has clear rules: add draft, no undercut, round edge, clear parting line, and the wall should be uniform and not too thick. Sharp edges require additional processing costs and time; Changes in wall thickness will leave unsightly shrinkage marks and undercuts. Although it can act on the side of the mold, it will increase the cost and cycle time. Injection mould Basic injection molding consists of two mold halves joined together, the plastic is heated and pressed into the cavity between the two mold halves, and the mold halves are separated to release the parts from the mold. The last step is the reason why the undercut in the part is difficult to form. Undercuts are essentially part surfaces that are not visible from the top or bottom. If you look at the cross-section of the part below, you can see that most of the surface is easily formed by the upper or lower half of the mold, but the small shelf on the right will cause the part to get stuck with the lower half of the mold. In other types of casting, such as dewaxing or sand casting, the mold is disposable. However, in injection molding, mold parts are designed to produce hundreds of thousands of pieces. Therefore, each mold part needs to be easily separated from the mold when it is opened, and these undercuts provide a special design for manufacturing challenges. If your design needs undercut, is this the rule that can be bent? Yes, this is where you enter the picture from the side. Side effect in undercut tool Undercut is not a new problem and a solution has been developed. Instead of just joining the two half parts of the tool together to form a part, create another part (or multiple parts, as required) to move in from the side, allowing the formation of a surface that could not have been formed, while still allowing the part to be easily demoulded from the mold. It makes more sense if you look at the molding method of the above parts. To create this shelf, the lower half of the mold will have a side action that will move vertically with the bottom mold part and horizontally as part of the molding cycle. When the mold is closed, this side action forms part of the mold cavity, but when the mold is opened, it will slide away from the part, so that the part can be easily removed from the mold. Although it is ingenious and can produce truly amazing parts, otherwise it cannot be formed, the side action does have shortcomings. Designing molds with lateral action requires additional mold engineering to deal with the high forces, heating and cooling cycles, and additional moving parts present in all molds. These parts also require additional processing time to produce and assemble mold tools. All these greatly increase the cost of the molds, which require auxiliary operations. How do you judge whether your part needs to take auxiliary measures? With experience, engineers who often deal with injection molding can quickly analyze and design. Alternative to side action: avoid undercut The most common solution for undercut, and the resulting increased mold cost and lead time for side actions, is to cut the material below the undercut. In the following figure, you can see how the groove on the side of the molded part allows the buckle to be formed without any undercut, and how the hinge barrel can be formed without side action. Another possible solution is to split the part. The part is molded into a single unit with multiple side effects, and the design is molded into several smaller parts and ultrasonically welded together after molding. Although this also increases unit cost and tool cost, it is usually worth exploring and referring to as a manufacturing option, especially when your geometry is very complex (such as the golf training tool below), or when your part needs to contain a volume. Undercut in design With the continuous improvement of injection molding technology for more than a century, the mold design rules are rarely absolute. However, deviating from the standard DFM rules does increase the cost of tools and each unit, and side actions that produce undercuts on parts are no exception.

When to choose CNC machining instead of casting If you start with die casting, why do you choose to redesign your parts and use CNC machining instead? Although casting is more cost-effective for high volume parts, CNC machining is the best choice for low to medium volume parts. CNC processing can better meet the tight delivery cycle, because there is no need to manufacture the mold, time or cost in advance during the processing process. In addition, in any case, die casting usually requires machining as an auxiliary operation. Post machining is used to achieve certain surface finishes, drill and tap holes, and to meet the strict tolerances of cast parts that fit with other parts in the assembly. And the post-processing needs to customize the fixture, which is very complicated. CNC machining can also produce higher quality parts. You can be more confident that every part will be consistently manufactured within your tolerance requirements. CNC machining is naturally a more accurate manufacturing process, and there is no risk of defects in the casting process, such as pores, dents and improper filling. In addition, casting complex geometry requires more complex molds, as well as additional components such as cores, sliders, or inserts. All of these add up to a huge investment in cost and time even before production begins. Not only complex parts are more meaningful to CNC machining. For example, CNC machines can easily manufacture flat plates by machining stock materials to the required size and thickness. But casting the same metal plate can easily lead to filling, warping or sinking problems. How to transform casting design into CNC machining design If you decide to redesign the part to make it more suitable for CNC machining, several key adjustments are required. You must consider the draft angle, groove and cavity, wall thickness, key dimensions and tolerances, and material selection. Remove draft angle If you initially considered casting when designing the part, it should include the draft angle. As with injection molding, the draft angle is very important so that the parts can be removed from the mold after cooling. During machining, the draft angle is unnecessary and should be removed. The design including draft angle needs a ball end milling cutter to process and increase your overall processing time. Extra machine time, extra tools and extra tool changing operations mean extra costs - so save some money and give up the draft angle design! Avoid large and deep grooves and hollow cavities In casting, shrinkage cavities and hollow cavities are usually avoided, because thicker areas are often poorly filled and may lead to defects such as depressions. These same functions require a long time to process, which will generate a lot of waste materials. Moreover, since all the forces are on one side, once the part is released from the fixture, the stress in the deep cavity will cause warpage. If grooves are not a critical design feature, and if you can afford extra weight, consider filling them, or adding ribs or gussets to prevent warping or deformation. The thicker the wall, the better Again, you need to consider the wall thickness. The recommended wall thickness for castings depends on structure, function and material, but is usually relatively thin, ranging from 0.0787 to 0.138 inches (2.0 to 3.5 mm). For very small parts, the wall thickness can be even smaller, but the casting process needs to be fine tuned. On the other hand, CNC machining has no upper limit on wall thickness. In fact, thicker is usually better, because it means less processing and less material waste. In addition, you can avoid any risk of warping or deflection that may occur to thin-walled parts during processing. Strict tolerance Casting usually cannot maintain strict tolerances like CNC machining, so you may have made concessions or compromises in casting design. With CNC machining, you can fully realize your design intent and manufacture more accurate parts by eliminating these compromises and implementing stricter tolerances. Consider using a wider range of materials Last but not least, CNC machining offers a wider choice of materials than casting. Aluminum is a very common die casting material. Zinc and magnesium are also commonly used in die casting. Other metals, such as brass, copper and lead, require more special treatment to produce high-quality parts. Carbon steel, alloy steel and stainless steel are rarely die cast because they are easy to rust. On the other hand, in CNC machining, more metals are suitable for machining. You can even try to make your parts with plastic, because there are many plastics that can be processed well and have useful material properties.

In this article, we will guide you to understand the manufacturing and industrial design considerations of various material options, and provide material suggestions for different product design goals, including glass and fiber filling materials for stronger parts, and silicone and polyurethane materials for flexible parts. How to get stronger parts: common packing types glass fibre The most common way to improve the mechanical properties of plastic materials is to add glass fiber. Glass fiber improves structural properties, such as strength and stiffness, and reduces shrinkage of parts. They are relatively cheap and can be added to most plastics. Glass filled resins can have different colors. In terms of disadvantages, glass fiber can make parts brittle and reduce impact strength. Glass fiber will also reduce the service life of the mold and wear the barrel and nozzle of the molding machine. Glass filled resin also increases the viscosity of the material, making the mold more difficult to fill. carbon fibre Carbon fiber filler can improve the mechanical properties of plastic materials. Carbon filled plastic parts have similar mechanical properties to glass filled plastic, but will make the parts stronger and lighter. Carbon fiber has conductivity, so carbon filled parts have better electromagnetic shielding performance. Carbon fiber can even improve structural properties, such as strength and stiffness, and reduce the shrinkage of parts more than glass fiber. The main disadvantage of carbon filled parts is that they are expensive. Like glass fiber, carbon fiber will make the parts brittle and reduce the impact strength; Reduce the service life of the mold and cause the wear of the barrel and nozzle of the molding machine. Carbon fiber also increases the viscosity of the material, making the mold more difficult to fill. Remember that for carbon filled materials, the part color is limited to black. Some resins also require very high mold temperatures, which may require expensive auxiliary equipment. Die design of fiber filled parts When the glass fiber or carbon fiber is compounded with the resin, the elastic modulus and tensile strength of the plastic will be significantly improved, so the plastic parts feel hard. This means that if a heavy load is applied to the plastic part, the plastic part will not easily deform. However, the impact strength will decrease and the plastic will feel fragile. The fluidity is low, and the contraction in the flow direction is smaller than that perpendicular to the flow direction. In mold design, it is difficult to determine the shrinkage rate according to the plastic flow direction of the gate. The CAD software only allows the user to set the shrinkage in the X, y and Z directions. This means that if the part size is large and the tolerance is tight, some dimensions may be out of tolerance. The solution is to ensure the safety of die steel by leaving more die steel than needed. After measuring the part, it is easy to remove the die steel from the die by CNC or EDM, but it is difficult to add the steel to the die. To do this, you need to weld the mold and then remove the steel, using CNC or EDM. In addition, welding will lead to mold deformation, which is not very good for mold life or part quality. For further mold modification, if the plastic part size is out of tolerance, some mold steel needs to be removed or added from the mold to change the shape or size of the mold. In order to avoid this step, CNC aluminum test mold provides a fast and cheap way to make molds, obtain plastic parts samples, and compare the key dimensions of plastic parts with printed products. If any critical dimension is out of tolerance, the production mold needs to be changed accordingly (the production mold will be made after the test mold). The purpose of testing the mold is to determine which dimensions will exceed the tolerance and which key features will work as designed. Once it is determined how different shrinkage in different flow directions will affect the size, the 3D model can be adjusted when making the hard tool. Filling materials wear the mold faster than unfilled plastic, so when using these materials, hardened steel must be used to make the core cavity and insert of the mold. The HDT (thermal deformation temperature) will also be higher, so the material can be used in a higher temperature environment. Which increases the difficulty of ultrasonic welding. In some cases, fibers will float on the surface of visible plastic parts, so most filled plastic parts are used for internal parts. In order to avoid this situation, the cavity of the mold can be textured. How to realize flexible parts: polyurethane (PU) and silicone Polyurethane (PU) and silicone materials provide different methods to realize soft parts. Pu uses compression molding and RTV mold, while silicone and TPU use injection molding. The main disadvantage of silicone is that it has flash. When the flash is cut or trimmed, there will always be residues. In addition, when injection molding silicon, the mold must be heated instead of the traditional process of heating the material. Injection molded TPU is easier to process and provides similar performance to silicon. Polyurethane (PU) Polyurethane (PU) is divided into two categories: thermosetting polyurethane (PU) and thermoplastic polyurethane (TPE). The main difference between the two is that thermosetting materials are crosslinked during processing and cannot be reused. On the other hand, thermoplastic polyurethane can be recycled. You can learn more about thermosetting and thermoplastic materials here. Thermosetting Pu is mainly used to manufacture prototypes through a process called polyurethane casting or room temperature vulcanization (RTV). Urethane casting uses a parent part covered by liquid silicon elastic material, which will harden at room temperature. Once the silicon hardens, the master is removed, resulting in a soft, flexible mold that can make copies of the master. Parts manufactured by this process range from 30A to 85D. In polyurethane casting process, burrs are inevitable. Usually, if the part is hard plastic, the flash can be trimmed manually, and the scar can be sanded with sandpaper, so it is not obvious. However, when the parts are as soft as PU, the burrs cannot be easily removed. Pu has better wear resistance than thermoplastic elastomer (TPE) and polyvinyl chloride (PVC), so it can be used to manufacture castors and soles. Thermoplastic polyurethane parts can be injection molded, so the parting line can be very precise (no burrs). The hardness of thermoplastic polyurethane ranges from 65A to 85D, so the resin can be as soft as rubber and as hard as hard plastic. Thermoplastic polyurethanes are commonly used for overmolding, such as jacks for manufacturing electronic wires. Compared with the flexible cord made of PVC or TPE, the flexible cord made of thermoplastic PU material has better elasticity and bending test results. silica gel Silica gel is a thermosetting resin, so it has good heat resistance and weather resistance. There are three manufacturing methods for silicone parts: RTV casting, compression molding or liquid silicone injection. Silica gel cannot be reprocessed or recycled. Manufacturing flexible parts As mentioned above, polyurethane casting is the most commonly used method for prototyping using soft materials. The hardness is about shore a 40-50. However, only a limited number of samples can be made from polyurethane molds. Compression molding is usually used for mass production of ordinary silicone parts. Burrs are unavoidable and must be trimmed manually. Customers can still see scars with thicknesses from most heat compression thicknesses exceeding 0.2 mm. Few factories can produce a thickness of 0.1 mm. Generally, the compression molding cycle is several minutes. The die material is usually steel with many cavities to improve production efficiency. When designing silicone parts, it is not necessary to follow the rule that the rib / nominal wall thickness ratio is less than or equal to 0.6. In most cases, even if there is undercut, the side action is not used in the tool, and can be manually selected from the tool. Liquid silicone injection is a very similar process to injection molding, but the difference is that the mold is heated to high temperature. Usually, the lead time is longer than injection molding, and the parts can be as detailed as injection molding parts, which means that there are no burrs or the burrs are very thin. The following figure shows typical samples with different hardness: Other material considerations for injection molding: fluidity (viscosity) When selecting materials, the fluidity of materials must be considered. For very thin-walled parts or large parts, fluidity is very important. Different types of resins have different fluidity. There are many different grades of a resin; For example, ABS has general grade, high flow grade and high impact grade. There are many kinds of ABS materials, which have different mechanical properties and prices. Some types of ABS are very suitable for manufacturing parts with high gloss finish; Some models are ideal for making electroplated parts; Some have good fluidity and are used to manufacture thin-walled parts or large-sized parts. Generally, for the same resin of different grades, the higher the fluidity, the lower the mechanical properties. The melt index (MI) represents the fluidity of the resin. Good fluidity resin can be used to manufacture thin-walled plastic parts, such as cell phone battery cases, or large plastic parts, such as baby bathtubs. Resins with good fluidity: LCP, PA, PE, PS, pp. Medium flow resin: ABS, as, PMMA and POM. Resins with poor fluidity: PC, PSF and PPO. machine design Engineering performance considerations determine which type of material should be used. Glass filled resins are best suited for rugged components that require wear resistance and strength, such as computer housings, toys and other consumer goods. In contrast, unfilled materials, such as ABS or polycarbonate, are most suitable for decorative parts that do not require special strength. Polypropylene or polyethylene is an ideal design for containers or parts with movable hinges. dimensional stability When designing a plastic part, you need to consider the accuracy of the fitting between the part and other parts. In order to fit accurately, it is important to select plastics with good dimensional stability, such as PC, ABS or POM. In this case, PA and PP are not a good choice, because shrinkage, strength and flexibility will be unfavorable to the part design, which needs to cooperate with other parts. However, in the case where PA or PP must be used, a nucleating agent may be added to the resin to improve dimensional stability. impact strength Impact strength represents the toughness of a material - when the impact strength is low, it is brittle. Generally, the impact strength of recycled plastics is lower than that of untreated resins. When glass fiber and carbon fiber are compounded with resin, the impact strength is lower, but the load and wear strength are higher. When a new plastic part is designed, it is important to consider what kind of force will be loaded on the part, how large the force is, and the frequency of the force. For example, handheld electronic products may fall, so the shell material of the product should be PC or PC / ABS. PC plastic has almost the highest impact strength among ordinary engineering plastics. Weather resistance and UV resistance linearity When the plastic is used outdoors, the plastic parts shall have good weather resistance and UV resistance. ASA is a kind of resin with good weather resistance and UV resistance. Its mechanical properties are similar to ABS. When another resin must be used, it is optional to add ultraviolet stabilizer and weather resistant agent to the resin. However, any plastic resin shall be thoroughly tested before use to ensure that it meets the product requirements. Temperature precautions It is also important to consider the temperature when selecting the resin. When the engine is working, the temperature in the engine housing is about 70 ℃ - 90 ℃, so all materials in the engine housing should be able to withstand this temperature.

When you finish the CNC machining of the parts, your work is not finished. These original components may have unsightly surfaces, may not be strong enough, or may only be part of one component, which must be connected with other components to form a complete product. After all, how often do you use equipment made up of individual parts? The key point is that the post-processing process is necessary for a series of applications. Here we introduce some precautions to you so that you can choose the correct secondary operation for your project. In this three part series, we will introduce options and considerations for heat treatment process, surface treatment and hardware installation. Any or all of these may be required to transition your part from a machined state to a customer ready state. This article discusses heat treatment, while the second and third parts examine surface treatment and hardware installation. In this three part series, we will introduce the heat treatment process, finishing and hardware installation options and considerations. Any or all of these may be necessary to change your part from a machined state to a customer ready state. This paper discusses heat treatment. Heat treatment before or after processing? Heat treatment is the first operation to be considered after processing, and it can even be considered to process preheating materials. Why use one method instead of the other? The order in which heat treatment and machining metals are selected may affect the material characteristics, machining process and tolerances of the parts. When you use materials that have been heat treated, this will affect your processing - the harder materials have a longer processing time and faster tool wear, which will increase the processing cost. Depending on the type of heat treatment applied and the depth below the affected surface of the material, it is also possible to cut off the hardened layer of the material and first destroy the purpose of using the hardened metal. The machining process may also generate enough heat to increase the hardness of the workpiece. Some materials, such as stainless steel, are more susceptible to work hardening during machining, and extra care is required to prevent this. However, there are some advantages in choosing metals that have been preheated. For hardened metals, your parts can maintain tighter tolerances, and it is easier to purchase materials because pre heat treated metals are readily available. Moreover, if the processing is completed, the heat treatment will add another time-consuming step in the production process. On the other hand, heat treatment after machining enables you to better control the machining process. There are many types of heat treatment, and you can choose which type to use to obtain the required material properties. The heat treatment after machining can also ensure that the heat treatment effect of the part surface is consistent. For the materials that have been preheated, the heat treatment may only have a certain depth of influence on the materials, so the machining may remove the hardened materials in some places and not in other places. As mentioned earlier, the post-processing heat treatment increases the cost and lead time because this process requires additional outsourcing steps. Heat treatment may also lead to warpage or deformation of parts, thus affecting the tight tolerance obtained during machining. heat treatment Generally, heat treatment will change the material properties of metals. In general, this means increasing the strength and hardness of the metal so that it can withstand more extreme applications. However, some heat treatment processes, such as annealing, actually reduce the hardness of the metal. Let's look at different heat treatment methods. sclerosis Hardening is used to make metal harder. The higher hardness means that the metal is less likely to be dented or marked upon impact. Heat treatment also increases the tensile strength of the metal, which is the force of material failure and fracture. The higher strength makes the material more suitable for certain applications. In order to harden the metal, the workpiece is heated to a specific temperature higher than the critical temperature of the metal, or a point at which its crystal structure and physical properties change. The metal is maintained at this temperature and then quenched and cooled in water, brine or oil. The quenching fluid depends on the specific alloy of the metal. Each quenchant has a unique cooling rate, so it is selected according to the cooling rate of the metal. Surface hardening Case hardening is a type of hardening that affects only the outer surface of a material. This process is usually completed after processing to form a durable outer layer. The hardening depth can be changed by modifying the process parameters Precipitation hardening Precipitation hardening is a process for specific metals with specific alloying elements. These elements include copper, aluminum, phosphorus and titanium. When the material is heated for a long time, these elements precipitate in the solid metal or form solid particles. This will affect the grain structure and increase the strength of the material. annealing As mentioned earlier, annealing is used to soften the metal, as well as to release stress and increase the ductility of the material. This process makes the metal easier to process. To anneal the metal, the metal is slowly heated to a certain temperature (higher than the critical temperature of the material), then maintained at that temperature, and finally cooled very slowly. This slow cooling process is accomplished by burying the metal in the insulating material or holding it in the furnace as the furnace and the metal cool down. Stress relief of large plate processing Stress relief is similar to annealing, that is, the material is heated to a certain temperature and cooled slowly. However, in the case of stress relief, the temperature is lower than the critical temperature. The material is then air cooled. This process can eliminate the stress caused by cold working or shearing, but does not significantly change the physical properties of the metal. Although the physical properties do not change, eliminating this stress during further processing or part use helps to avoid dimensional changes (or warpage or other deformation). tempering When the metal is tempered, it needs to be heated to a point below the critical temperature and then cooled in air. This is almost the same as stress relief, but the final temperature is not as high as stress relief. Tempering increases toughness while maintaining most of the hardness of the material added by the hardening process. Last thought Heat treatment of metals is often necessary to achieve the physical properties required for a particular application. Although heat treatment of materials before milling can save the overall production time, it will increase the processing time and cost. At the same time, the processed heat-treated parts make it easier to process materials, but add additional steps to the production process.