Material selection is another decision that must be made early in the design process. Different resins can produce different tolerances for the same part, so sometimes a tradeoff must be made between tolerance expectations and the physical properties of the resin. For example, a glass- or mineral-filled material can hold tighter tolerances than an unfilled material. Different materials also have different shrink rates—the higher the shrink rate, the less repeatable the tolerance.
Crystalline materials don’t hold tight tolerances as well as other materials because of their higher shrink rates and the potential for continued crystal growth. Polyethylene and acetal are good examples—these materials can continue to grow more crystalline, even at freezing temperatures. As a result, the overall structure gets tighter and more compact, shrinking the size of the part. This can continue to fluctuate over time.
Therefore, to create tight-tolerance parts, the injection molding environment must be designed to ensure that the highest state of crystallinity is achieved quickly. In general, high mold temperatures provide the best environment for crystal growth. Crystal growth can also be maximized by adding a nucleating agent to the melt, which provides sites throughout the melt that stimulate crystal growth.
Holding tight tolerances can be a challenge with most plastics because they have high thermal expansion rates (fillers help reduce this). Even though plastic parts can be held to tight tolerances in a climate-controlled environment, this doesn’t mean they will maintain these dimensions as the temperature changes. This must be considered when plastic parts are combined with other material types such as metals, or when the end use occurs in an environment that has big temperature swings.
Moisture absorption also affects tolerance. Hygroscopic materials—especially nylons—change in size as they absorb water and humidity levels fluctuate. Mechanical properties can change as well. A good example is using molded, unfilled nylon for the throttle linkage in boat engines. In these hot and damp conditions, the nylon linkage gets longer as it absorbs more heat and moisture, throwing off the timing of the carburetors. Kaysun Corporation solved this problem by making the linkage from aluminum rods with overmolded polybutylene terephthalate (PBT) snap-fit ends. Not only was the aluminum lighter in weight, it had a lower thermal expansion that also matched the thermal expansion of the engine block.
To find the best material for your injection molded product that maximizes tight tolerance and superior quality, be sure to collaborate with the Kaysun engineering team early in the design process to discuss performance expectations, the end-user environment, and a production process that maximizes manufacturability. This will ensure the highest possible performance and the least amount of shrinkage/variance over the lifecycle of your part or product.