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.
