Perhaps you’ve heard high quality product should have “tight tolerance,” but do you really know what level of tolerance is suitable for your product? Depending on the end use of the product, tight tolerances may or may not be necessary for the product. Most consumer type products do not require more than the standard tolerance, because the severity of consequence for their failure is low. Automotive, industrial, or other functional parts, on the other hand, may undergo catastrophic failure or severe under-performance should the molded part not be within a very tight tolerance. The benefit of knowing the proper tolerance to choose for your product can equate to significant cost savings and production time saving for your project.
Tight Tolerance: Injection Molded Rubber
Tight Tolerance: Injection Molded Plastic
|±.005 inches||±.002 inches|
The biggest deciding factor in achieving a tight tolerance is the part design. The geometry of the part, overall size, and wall thickness can all affect the tolerance control. If the overall part has very large dimensions, greater shrink will occur and the more difficult it will be to maintain a tight tolerance. That is why it is easier to maintain tight tolerances in smaller areas of a part. A part with thick walls may have varying shrink rates in its thicker sections making it difficult to maintain a very tight tolerance throughout.
Plastics typically go through a high rate of expansion per temperature degree; so when designing a part, the temperature during the product’s application must be considered. If the part will be exposed to very high or low temperatures during its application, the part will be contracting and expanding naturally. In extreme temperature applications, a tight tolerance may not even be necessary.
Maintaining a tight tolerance can be a challenge with plastics, but it is possible with the use of fillers to be able to reduce their thermal expansion rate.
Materials have different shrink rates and therefore different tolerance capabilities, so sometimes a trade-off is made in favor of either the preferred tolerance range or in the special properties of the material. The rule of thumb is – the higher the shrink rate, the less repeatable it is to keep the tolerance. There are two categories of materials which have their own distinct shrink rates: amorphous and crystalline.
Amorphous materials flow more easily when melted and does not go through any drastic volume change compared to crystalline materials, so amorphous materials actually have less shrinkage than crystalline materials. Fillers can also be added to increase or decrease the degree of shrinkage.
Crystalline materials are harder to hold their mold size. Because crystalline materials must go through a phase change from solid to fluid, their volume changes result in more variable dimensions. To be able to achieve tight tolerance parts from crystalline materials, the use of high mold temperatures is needed. Parts with thick walls can provide an insulated area for the material to crystallize. A chemical agent may also be added to the melt to stimulate crystal growth. Of interest, acetal and polyethylene are two common materials which can continue to grow crystals at even below room temperatures.
By far the most important step to take in deciding the most appropriate tolerance for molded products is clear communication between the part design team and the engineering team. The design team must consider how the product design and material may affect tolerance. And the engineering team must understand the application and intended lifecycle of the product in order to design the cost effective process to achieve the design team’s goals.