Not taking the time to properly determine shrink rate can have a big impact on the quality of the final product—including its geometry, performance, and appearance.
Material suppliers typically provide information on the shrink rate of their materials. This number is usually based on ASTM Standard D955 and a .125-inch thick plaque with a specific gating size and location. Although this is a good place to start, this value is typically not accurate enough for many products, especially critical, highly complex parts.
Wall thickness of the part, mold gate size, and processing conditions such as packing pressure and mold temperature are major factors in determining shrinkage of the part. For example, thinner wall sections cool faster, resulting in less shrinkage. Larger gates will result in longer packing time, also providing less shrinkage.
Shrink rates also vary according to flow direction. After gate locations are selected, it is important to analyze the part to determine the basic direction of flow. On long, narrow parts—gated at the end, for instance—flow will essentially parallel the length of the part. In these cases, the “in-flow” shrink rate is used for this dimension of the part. The “cross-flow” shrinkage is used for dimensions that are perpendicular to the long dimension. For parts with random fill directions, an average of the in-flow and cross-flow directions can be used. For parts with critical dimensions, prototyping is the safest option (the mold can be also left steel-safe so that critical areas can be “tweaked” if needed after a production molding process is established).
Other key considerations for determining shrink rate are:
- Mold flow analysis. It is important to have a gate location that provides the most uniform fill and plastic pressure within the cavity. Since shrinkage is proportional to the pressure, having a large pressure drop throughout the part will provide varying shrink rates. This could result in low shrinkage on the end of the part with high pressure, and high shrinkage on the other side of the part with the low pressure.
- Sprue/runner and gate size. Any premature freeze-off of these items could increase the variation of the packing of the part, resulting in inconsistent shrink rates.
- Prototype mold. The best method for predicting shrinkage is building a prototype mold with the exact cooling and gating as the final production mold—this way the shrink rate can be exactly determined for the part. An alternative to a prototype mold is running the chosen material in a similar mold, with a similar wall section and gate scheme. Measurements taken from the mold steel and molded parts can be used to determine a “likely” shrink rate, based on a similar part.
Shrink rate should definitely be discussed during design-for-manufacturability meetings. Most customers are surprised to learn how many process variables (wall thickness, plastic flow direction, fillers, etc.) can impact shrinkage. It pays to make the best determination for shrink rates up front to eliminate any surprises during the cooling stages.