Part Details Review

Mold shrinkage, listed as length-per-unit-length values or as percentages, assumes room-temperature measurements. Many processing and design factors determine the amount of shrinkage for a given application. Use published shrinkage information with caution as it is tested under laboratory conditions that may not reflect your specific part geometry or processing environment. Consider the following when addressing shrinkage:

Cooling rate and mold temperature can affect the level of crystallinity and shrinkage in semicrystalline resins;
Thick-wall sections cool more slowly and tend to shrink more than thin-wall sections;
Fiber-filled materials typically exhibit much less shrinkage in the flow direction;
Mixed orientation typically leads to shrinkage ranging between published flow and cross-flow values; and
Shrinkage varies with the level of packing.

Packing forces additional material into the mold to compensate for volume reduction, lowering shrinkage. Packing typically decreases and shrinkage increases further from the gate, particularly in distant thick-wall sections. The mold constrains the part and prevents significant dimensional change until after part ejection. The type and duration of this constraint can affect net shrinkage between part features. Long cycle times constrain the part in the mold longer and reduce initial shrinkage, but can induce stresses that lead to additional shrinkage over time as the stresses relax.

Many factors can affect the level of shrinkage. You can usually obtain the most accurate shrinkage values for new molds by calculating the actual shrinkage in existing molds producing similar parts sampled in the same material.

Published shrinkage data represents the typical range of shrinkage based on laboratory conditions. Applying this data to a specific part and mold requires a combination of engineering judgment and educated guess. Tend toward the lower end of the range for parts thinner than 0.100 inch, and for highly constrained features such as the distance between holes. Anticipate flow orientation in glass-filled parts and apply the flow and cross-flow shrinkage values appropriately. Areas of random orientation will tend to shrink at a level midway between the flow and cross-flow values. Consider designing critical features and dimensions "steel safe" to simplify modifications to correct for errors in shrinkage prediction.

[ Close This Window ]