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.
