Tight Tolerance Molding and Product Design

Posted by Matt Fehrmann on Mar 26, 2013 12:00:00 PM

blank.pngThere are plenty of factors that impact tight tolerance of injection-molded parts—all of which need to be controlled with precision to achieve those specifications. The greater the number of factors, the harder it is to achieve tight tolerances consistently. That’s why it is so important to consider tight-tolerance goals during the product design process—it is here where tight-tolerance factors can be minimized through design modifications, without impacting quality or performance. In fact, depending on tolerance needs, working with an experienced injection molder like Kaysun can actually tighten those tolerances even further if needed, by modifying the design, materials, or production process.

Product design is the single biggest factor in controlling tight tolerances. Making improvements during the design phase will not only achieve repeatable tight tolerances but also improve manufacturability, quality, and customer satisfaction, all while reducing costs.

Not many product designers have in-depth injection-molding experience—they know the basics, but that is usually not enough when it comes to factoring injection molding into the design and production process.

Sometimes companies contract a design house for their products—this can sometimes work, but more often than not the design house focuses on eye-appeal of the final product and cost, without enough consideration for ease of manufacturing and meeting specs.

An experienced injection molder like Kaysun (we have over 60 years of tight tolerance injection molding experience) truly understands the tolerance limits that are achievable and how to support product design to make these tolerances a reality. Knowing just the basics from a plastics-manufacturing standpoint often results in flawed designs—for example, die lock conditions, poor areas of fill with abrupt changes in wall thickness, sharp corners, and impossible tolerances. This doesn’t happen with an experienced injection molder.

Is tight tolerance even necessary? Is it required for performance, longevity, or appearance? Many designers automatically set a tolerance in the CAD drafting software and all dimensions are toleranced to that number, when in reality the product may not need such a tight tolerance. Achieving tight tolerance is a more expensive process, so if it not required, loosen it up to reduce production costs.

Physical part size affects tight tolerance. Large parts are difficult to hold to tight sizes consistently. Local areas with the large part, however, are easier to manage and can be dimensioned accordingly from a local datum. Plastic material manufacturers also have charts to assist with choosing appropriate tolerance based on part size and wall thickness.

Material type and environment influence plastic behavior, which affects tolerance. For example, plastics typically have large thermal expansion coefficients; as a result, tight-tolerance parts may have to be measured at a consistent temperature. Temperature is also something to consider during design—if the tight-tolerance part will be exposed to high/low temperature extremes in normal operation, where it will expand and contract, is tight tolerance even necessary?

Crystalline plastics tend to shrink more (move away from tolerance) as time goes on because they become more and more crystalline. This is especially true with acetal-type plastics. This shrinkage also occurs when plastics are exposed to an environment that promotes crystal growth, such as high temperatures. Also, if these plastics are molded in cold molds that don’t allow much initial crystal growth, the tendency for this growth is stronger than if a plastic was molded in a hot mold. These points all need to be considered when selecting materials and mold processing.

Other key design factors that affect tolerance are differences in part wall thickness (can create uneven crystallinity) and moisture (for example, some plastic parts—especially those made from nylon—lose some of their tight tolerance when they leave a tightly controlled atmosphere and are used in environments with a higher moisture content).

Be sure to select an experienced injection-molding partner to work with your design team early in the process and give it a key role in designing the product for tolerance, performance, and manufacturability. This will ultimately reduce cost, time to market, and yield a higher quality product.

Topics: Plastic Part Design