Injection Molding Shrinkage

Shrinkage values for polypropylene (PP) molded parts can range from approximately 0.008" to approximately 0.025" per inch. This broad range of possible shrinkage values have been calculated from a multitude of injection molded parts having various thicknesses. Linear mold shrinkage is mostly dependent on the particular polypropylene resin type, the processing conditions used to make the plastic parts, the part design, and or including the material flow path direction that's partly imposed by gate location(s). In summary, shrinkage is a function of all of these variables. Thicker parts shrink more so, anisotropically, inch/inch, than do thinner parts. However, packing more weight of polymer into the geometry of an injection mold / molded part can generally yield slightly less shrinkage. Exceptions to conventional shrinkage rules lie with filled PP. Filler-modified PPs, i.e., calcium carbonate, talc, glass, etc., will not necessarily shrink proportionally or equally in relationship to any given weight % of filler that might be contained within the polypropylene material.

Rules-of-thumb:

• General purpose PPs, including Homopolymer Polypropylene, Random Copolymer, Polypropylene, Impact Copolymer Polypropylene, HPP, RCP, ICP, respectively, shrink slightly slower than nucleated or clarified PP.
• Nucleated PPs shrink faster and slightly more than general purpose PPs.
• Clarified PPs (C-HPP & C-RCP) shrink the quickest in injection molding operations; to approximately 98 % of final dimensions and slightly more so in finished dimensions. Generally speaking, over time, shrinkage in relationship to the various components of PP can result in different shrinkage amounts (inch/inch).

Text book references sometimes report and or illustrate that random copolymers and or clarified varieties of PP, could exhibit approximately 0.0015" – 0.0025” more shrinkage over the total part dimensions, vs. nucleated PPs. In particular instances some part designers, where critical dimensions are not high risk, will use shrinkage values of approximately 0.016" to approximately 0.018 inch / inch. However, it’s very risky to utilize such tight-ranged shrinkage for steel cuts without first pursuing in-depth Design of Experiments studies that have been substantiated on actual injection molding conditions. Where such critical shrinkage determinations are required, Mold Flow Plastics Labs, CAE Development / Technologies, Data Point Labs, etc. can be contracted to conduct coefficient of shrinkage tests which can yield more precise dimensional shrinkage conclusions for a given polypropylene resin and or polypropylene part.

Other significant control options include: material temperature, injection, mold and gate temperatures, time in the mold, product temperature at time of ejection. Premature ejection can cause high shrinkage rates and can cause warpage. The longer the cycle, the greater control and dimensional predictability.

Tradeoffs can occur between low to high mold temperatures, higher mold temperatures provide reduced stresses and better stability after forming. But, it results in longer cycle times. Colder mold temperatures result in faster cycle times. But, greater stress inducement is a resultant factor. The higher stresses can cause uncontrollable shrinkage as well as cracking (usually in the flow direction). Many of these factors are prevalent in thermoforming as well as injection molding.