The vast majority of plastic products are manufactured using injection molding. This is primarily due to the process’s high production rates and very low unit cost. As with any manufactured assembly, the tolerances are of critical importance. If they are incorrectly specified or controlled, the final parts will not fit together during assembly. These types of errors are especially problematic since the upfront cost of the mold is so significant. This article will describe how to control injection molding tolerances and ensure high quality through DFM (design for manufacturing) principles, material selection, tool design, and process control.
Even though it may be minimal, there is always variation when manufacturing parts. As such, it is important to define an acceptable range of deviations from the nominal size that still allows the part to function as expected. This becomes all the more important when multiple parts are being assembled.
If, for example, two flat parts need to be bolted together, the locational tolerance of the holes on each part must allow for the full range of possibilities. Even if one part is at its minimum tolerance and the other is at its maximum, they must still fit during assembly. This seems relatively simple in this case, but when multiple parts need to be assembled one part can cause the entire assembly to be non-functional. Tolerance analyses like the worst-case method, tolerance stack, and statistical analysis can be used to optimize injection molding tolerances of multi-part assemblies.
One of the most important methods of limiting warping, excessive shrinking, and part misalignment is to use DFM principles when designing a part. That’s best achieved by engaging with an injection molding service early on in the design process to prevent costly redesigns later in the design phase.
Injection-molded plastics can be manufactured from a wide range of resins. The selection of these materials is driven primarily by the application of the final product. Each resin has a different shrink rate. The graph below indicates some shrinkage rates for some common materials (Figure 1).
This shrinkage needs to be taken into account when designing the molding tool and is normally done by oversizing the tool dimensions by the percentage shrinkage of the material. If multi-material assemblies are required, the different shrinkage rates need to be designed for. Not designing in appropriate tolerances can result in parts that do not fit together, which is a costly mistake when it comes to injection molding.
Injection molding tolerances are driven primarily by the shrinkage of the material and the part geometry. Before the tool can be designed and manufactured, the material selection needs to be finalized. Tool design is highly dependent on the chosen material(s).
Once the material is selected, the tool is often oversized to account for the relevant material shrinkage. However, shrinkage is not consistent in all dimensions. Thicker parts have a different cooling rate than thinner parts, for example. As such, a complex part that has a mixture of thin and thick walls will have variable cooling rates. The resultant warping or sink can severely affect injection molding tolerances and assembly fit-ups. To limit these effects, toolmakers take into consideration the following factors when designing mold features.
Despite all the upfront design work and material considerations to optimize your part’s injection molding tolerances, there is still the possibility that the part will be out of tolerance when the first samples, often called T1 shots, are delivered. Once all of the above methods are incorporated, the next step to improving tolerance compliance is to tweak the process. Controlling the temperature, pressure, and holding time are some of the most common ways of increasing part quality. Once the ideal set of conditions are determined, the mold can create consistent parts with very little dimensional variability between parts.
In complex, multi-feature parts, it may be beneficial to embed pressure and temperature sensors into the tool in order to measure these parameters during manufacturing to allow for real-time feedback and process control. Consistently maintaining pressure and temperature in the tool can go a long way toward ensuring consistent tolerances.
Injection molding is an incredibly versatile manufacturing process for making consistent and durable plastic parts. But with this versatility comes considerable complexity. To ensure that your investment into the tooling is as risk-free as possible, contact a Xometry representative to discuss how best to design your parts to ensure successful products. The Xometry team is happy to provide design for manufacturability feedback and suggestions before the project gets underway. Proper planning for injection molding tolerances can prevent cost overruns and vastly improve product quality.