Shrinkage Rates: PP, PA, PE, and Acetal Comparison

What is the average shrinkage of polypropylene, polyamide, polyethylene and acetal?

Based on the retrieved documentation and general engineering knowledge, here are the typical shrinkage values for these common engineering plastics:

Average Shrinkage Values

Polyethylene (PE)

According to the documentation, HDPE (High-Density Polyethylene) exhibits particularly high shrinkage rates:

• HDPE: Can exceed 0.030 inches/inch (3.0%) [Source 1, 3]
• Directional differences up to 0.001 inches/inch between flow and transverse directions
• The documentation emphasizes this is among the highest shrinkage rates in common thermoplastics

Polypropylene (PP)

While not explicitly mentioned in the retrieved documents, polypropylene typically shows:

• Average shrinkage: 1.5-2.5%
• Similar to HDPE, exhibits significant directional differences
• Unfilled grades tend toward the higher end of this range

Polyamide (PA/Nylon)

The documentation mentions Zytel® (a DuPont polyamide brand) [Source 2] but doesn't provide specific shrinkage values. Typical values for polyamides are:

• Unfilled PA6/PA66: 1.0-1.5%
• Glass-filled grades: 0.3-0.8%
• Moisture content significantly affects final dimensions

Acetal (POM)

Not covered in the retrieved documentation, but typical values are:

• Homopolymer: 2.0-2.5%
• Copolymer: 1.8-2.2%
• Generally very consistent and predictable shrinkage

Key Design Considerations

The documentation emphasizes several critical points about shrinkage:

1. Directional Variation: Materials like HDPE show different shrinkage rates parallel vs. perpendicular to flow direction, with differences up to 0.001 inches/inch [Source 1, 3]

2. Process Compensation: The documents stress the importance of "windage compensation" in tooling design to account for these high shrinkage values, particularly for blow-molded parts [Source 3, 5]

3. Moisture Sensitivity: For hygroscopic materials like polyamides, the documentation recommends using electronic hygrometer testing before molding to ensure proper moisture levels [Source 2]

Practical Impact

For design purposes, always:

• Consult specific material datasheets for exact grades
• Account for processing conditions (mold temperature, cooling rate)
• Consider part geometry effects on shrinkage
• Add appropriate tolerances based on directional shrinkage differences

The documentation particularly warns about HDPE's extreme shrinkage (>3.0%) causing potential warpage and assembly issues if not properly compensated in the design phase [Source 1, 3].