Molding Shrinkage of Injection Molded Products

During the injection molding process, plastics undergo volumetric changes. The dimensions of the demolded part are generally smaller than those of the mold cavity. This phenomenon is known as molding shrinkage and is expressed by the shrinkage rate (S):

S = [(D - M) / D] × 100%

in the formula of

• S = Molding shrinkage rate of the part (%)

• D = Mold cavity dimension (mm)

• M = Part dimension after demolding (mm)

Shrinkage during molding is volumetric, influenced by polymer crystallization, orientation, phase transitions, and other factors. For practical production calculations, linear shrinkage is often used for convenience. The shrinkage rates provided in typical references or books usually refer to shrinkage in a specific linear direction. The linear shrinkage rate (SL) and volumetric shrinkage rate ((Sv) can be approximately converted using the following formula:

Sv=3SL

After demolding, injection molded parts continue to shrink (free shrinkage) for a period, known as post-molding shrinkage. Typically, 90% of the shrinkage is completed within 6 hours after demolding. Therefore, measuring the molding shrinkage rate should be done after the part has fully cooled (generally 24 hours after demolding).

To produce injection molded parts with minimal shrinkage variation, the following are essential:

• Rational part design

• Well-engineered mold design

• An injection molding machine with good performance

• Appropriate process conditions

If any of these factors change, the measured shrinkage rate will also differ. Consequently, shrinkage rates found in different design manuals or references may vary. As an injection molding operator, it is crucial to comprehensively consider the impact of these factors on shrinkage. To accurately control molding shrinkage, the following technical measures can be employed:

Plastic Material

• Crystalline plastics generally exhibit higher shrinkage than amorphous plastics.

• For amorphous plastics, under stable holding pressure after gate sealing, shrinkage depends on the mold opening temperature.

• For crystalline plastics, attention must be paid to the relationship between volume and temperature under different injection pressures.

• For highly hygroscopic plastics, drying can control moisture and volatile content.

• Prefer materials with lower molecular weight (higher MFR) and narrow molecular weight distribution.

• Blending with reinforcements, fillers (masterbatch), or other additives can reduce shrinkage.

High additive, moisture, or volatile content can lead to significant shrinkage variation.

Part and Mold Design

• Avoid excessive wall thickness or concentration of thick sections in one area.

• Minimize significant differences between wall and rib thicknesses; redesign such areas for uniform wall thickness distribution (refer to section 10.1.1 for design methods).

• In mold design, gates located in thin sections, with small cross-sections or excessive length, can worsen shrinkage. Modify the mold to position gates in thick sections, increase gate cross-section, and shorten gate length.

Process Conditions

• Mold temperature has the greatest impact on shrinkage: higher mold temperature leads to greater shrinkage. If too high, reduce cooling temperature.

• Higher barrel temperature lowers melt viscosity, facilitating easier filling under the same injection pressure, resulting in denser parts and reduced shrinkage. Maintain a suitably high melt temperature during molding.

• Higher injection pressure, higher holding pressure, and longer holding time allow for material compensation before the gate solidifies, reducing shrinkage. If shrinkage is excessive, appropriately increase these parameters.