Plastic Shrinkage Rates Reference Table (2025 Data)

Introduction to Mold Shrinkage

Mold shrinkage is the reduction in dimensions that occurs when molten plastic cools and solidifies in the injection mold cavity. This dimensional change is fundamental to injection molding design and directly impacts part quality, fit, and functionality. Understanding and compensating for shrinkage is essential for producing accurate, interchangeable parts.

Shrinkage rates vary significantly between polymer types, processing conditions, and part geometry. This comprehensive 2025 reference provides the most current shrinkage data for major engineering polymers, enabling precise mold design and process optimization.

The data presented here represents industry-standard values based on ASTM D955 testing methodology and manufacturer specifications for 2025 material grades. All values are expressed as a percentage of the mold cavity dimension.

Amorphous vs. Semi-Crystalline Polymers

Polymer shrinkage behavior is fundamentally determined by molecular structure and crystallization characteristics:

Amorphous Polymers

Characteristics: Isotropic shrinkage (uniform in all directions), lower shrinkage rates (0.4-0.8%), glass transition behavior, dimensional stability over temperature ranges.

Examples: ABS, PC, PS, PMMA, PVC, SAN

Shrinkage Pattern: Primarily thermal contraction with minimal crystallization effects

Semi-Crystalline Polymers

Characteristics: Anisotropic shrinkage (direction-dependent), higher shrinkage rates (1.0-3.0%), crystallization during cooling, orientation effects from flow, post-mold shrinkage.

Examples: PP, PE, PA6, PA66, PBT, PET, POM

Shrinkage Pattern: Thermal contraction + crystallization shrinkage + orientation effects

Factors Affecting Shrinkage Rates

Several processing and material factors influence the final shrinkage rate:

Processing Parameters

• Packing Pressure: Higher pressure = lower shrinkage (more material packed)
• Melt Temperature: Higher temperature = slightly higher shrinkage
• Mold Temperature: Higher mold temperature = lower shrinkage (better packing)
• Cooling Time: Longer cooling = lower post-mold shrinkage
• Hold Pressure Time: Critical for semi-crystalline materials

Part Design Factors

• Wall Thickness: Thicker sections = higher shrinkage
• Flow Length: Longer flow = higher orientation = anisotropic shrinkage
• Gate Location: Affects packing efficiency and orientation
• Ribs & Bosses: Different shrinkage rates vs. main walls

Material Factors

• Filler Content: Glass/mineral fillers reduce shrinkage
• Moisture Content: Affects crystallization in some polymers
• Regrind Level: Can increase shrinkage variability
• Material Grade: Different formulations have different shrinkage

Comprehensive Shrinkage Rates Table (2025)

The following table provides comprehensive shrinkage rate data for major engineering polymers. All values are based on 2025 material specifications and standard processing conditions (melt temp: recommended, mold temp: 60-80°C, packing pressure: optimal).

Polymer	Grade/Type	Shrinkage Range (%)	Typical Value (%)	Notes
Polypropylene (PP)	Homo-polymer	1.0 - 2.5	1.5 - 2.0	Higher with increased crystallinity
Polypropylene (PP)	Copolymer	1.2 - 2.8	1.8 - 2.3	Ethylene content affects shrinkage
Polypropylene (PP)	30% Glass Filled	0.3 - 0.8	0.4 - 0.6	Anisotropic due to fiber orientation
Polyethylene (PE)	HDPE	1.5 - 3.0	2.0 - 2.5	Density affects crystallization
Polyethylene (PE)	LDPE	1.0 - 2.5	1.5 - 2.0	Lower density = lower shrinkage
Polyethylene (PE)	LLDPE	1.2 - 2.8	1.8 - 2.3	Linear structure affects flow
Acrylonitrile Butadiene Styrene (ABS)	General Purpose	0.4 - 0.7	0.5 - 0.6	Isotropic shrinkage pattern
Acrylonitrile Butadiene Styrene (ABS)	High Impact	0.4 - 0.8	0.5 - 0.7	Rubber content slightly increases shrinkage
Acrylonitrile Butadiene Styrene (ABS)	20% Glass Filled	0.2 - 0.5	0.3 - 0.4	Fiber reinforcement reduces shrinkage
Polycarbonate (PC)	General Purpose	0.5 - 0.8	0.6 - 0.7	High mold temperature reduces shrinkage
Polycarbonate (PC)	20% Glass Filled	0.2 - 0.5	0.3 - 0.4	Glass fibers constrain shrinkage
Polycarbonate (PC)	Flame Retardant	0.5 - 0.9	0.6 - 0.8	Additives may affect shrinkage
Polyamide 6 (PA6)	Unfilled	0.8 - 1.5	1.0 - 1.3	Hydrolysis affects dimensional stability
Polyamide 6 (PA6)	30% Glass Filled	0.3 - 0.8	0.4 - 0.6	Fiber orientation causes anisotropy
Polyamide 6 (PA6)	Mineral Filled	0.5 - 1.0	0.7 - 0.9	Mineral fillers reduce but don't eliminate
Polyamide 66 (PA66)	Unfilled	0.8 - 1.6	1.1 - 1.4	Higher crystallinity than PA6
Polyamide 66 (PA66)	33% Glass Filled	0.3 - 0.9	0.4 - 0.7	Common engineering grade
Polybutylene Terephthalate (PBT)	Unfilled	0.8 - 1.6	1.2 - 1.4	Rapid crystallization
Polybutylene Terephthalate (PBT)	30% Glass Filled	0.2 - 0.6	0.3 - 0.5	Low shrinkage for precision parts
Polyethylene Terephthalate (PET)	Unfilled	0.2 - 0.8	0.3 - 0.6	Drying critical for consistency
Polyethylene Terephthalate (PET)	30% Glass Filled	0.1 - 0.4	0.2 - 0.3	Very low shrinkage applications
Polyoxymethylene (POM)	Homo-polymer	1.8 - 2.5	2.0 - 2.3	High crystallinity material
Polyoxymethylene (POM)	Copolymer	1.5 - 2.2	1.8 - 2.0	Better thermal stability
Polyoxymethylene (POM)	20% Glass Filled	0.5 - 1.2	0.7 - 1.0	Reduced shrinkage vs. unfilled
Polystyrene (PS)	General Purpose	0.3 - 0.7	0.4 - 0.6	Low shrinkage, good dimensional stability
Polystyrene (PS)	High Impact (HIPS)	0.3 - 0.8	0.4 - 0.7	Rubber content slightly increases shrinkage
Polymethyl Methacrylate (PMMA)	General Purpose	0.2 - 0.6	0.3 - 0.5	Very low shrinkage, excellent optics
Polyvinyl Chloride (PVC)	Rigid	0.2 - 0.6	0.3 - 0.5	Thermal expansion affects dimensions
Polyvinyl Chloride (PVC)	Plasticized	0.8 - 2.0	1.0 - 1.5	Plasticizer migration causes changes
Styrene Acrylonitrile (SAN)	General Purpose	0.3 - 0.7	0.4 - 0.6	Similar to PS but chemical resistant
Thermoplastic Elastomer (TPE)	SBS/SEBS	0.8 - 2.0	1.0 - 1.5	Soft grades have higher shrinkage
Thermoplastic Elastomer (TPE)	TPU	0.5 - 1.2	0.7 - 1.0	Polyester vs. polyether affects shrinkage

Shrinkage Calculation Formulas

Mold dimensions must be calculated to compensate for shrinkage. The basic formula is:

Basic Mold Dimension Formula

Mold Dimension = Part Dimension × (1 + Shrinkage Rate)

Where shrinkage rate is expressed as a decimal (e.g., 0.02 for 2% shrinkage)

Anisotropic Shrinkage Considerations

For semi-crystalline polymers, shrinkage varies by direction:

• Flow Direction: Lower shrinkage due to molecular orientation
• Cross-Flow Direction: Higher shrinkage perpendicular to flow
• Through-Thickness: Highest shrinkage across wall thickness

Differential Shrinkage Formula

Shrinkage Factor = 1 + (S_flow + S_cross + S_thickness) / 3

Where:

• S_flow = shrinkage in flow direction
• S_cross = shrinkage perpendicular to flow
• S_thickness = shrinkage through thickness

Post-Mold Shrinkage Estimation

For semi-crystalline polymers, additional shrinkage occurs after ejection:

Post-Mold Shrinkage (%) = Initial Shrinkage × (1 - exp(-t/τ))

Where:

• t = time after molding
• τ = relaxation time constant (material specific)

Post-Mold Shrinkage Behavior

Many polymers continue to shrink after ejection from the mold. This post-mold shrinkage is particularly significant for semi-crystalline polymers.

Time-Dependent Shrinkage

• Initial (0-24 hours): 20-40% of total post-mold shrinkage
• Short-term (1-7 days): 50-70% of total post-mold shrinkage
• Long-term (weeks-months): Final stabilization

Environmental Factors

• Temperature: Higher temperatures accelerate shrinkage
• Humidity: Affects hygroscopic polymers (PA, PBT)
• Stress Relaxation: Internal stresses relax over time

Critical Polymers for Post-Mold Shrinkage

Polymer	Post-Mold Shrinkage (%)	Time to Stabilization
PP Homo-polymer	0.1 - 0.3	2-4 weeks
PA6	0.2 - 0.5	1-3 weeks
PA66	0.3 - 0.6	2-4 weeks
PBT	0.1 - 0.3	1-2 weeks
POM	0.2 - 0.4	3-6 weeks

Mold Design Compensation Strategies

Effective shrinkage compensation requires understanding part geometry and material behavior:

Wall Thickness Compensation

Shrinkage increases with wall thickness. Compensation factor:

K_thickness = 1 + S × (1 + 0.01 × (h - h_ref))

Where:

• S = base shrinkage rate
• h = actual wall thickness
• h_ref = reference thickness (2-3mm)

Flow Length Considerations

Long flow lengths cause molecular orientation and differential shrinkage:

• Short flows (L/t < 50): Isotropic shrinkage
• Medium flows (L/t = 50-150): Moderate anisotropy
• Long flows (L/t > 150): Significant differential shrinkage

Rib and Boss Design

Ribs shrink differently than main walls due to different cooling rates:

• Rib Shrinkage: 10-20% higher than adjacent walls
• Boss Shrinkage: 5-15% lower due to better packing
• Design Rule: Use draft angles to accommodate differential shrinkage

Processing Parameters Optimization

Processing conditions significantly affect final shrinkage:

Packing Pressure Optimization

Insufficient packing pressure leads to excessive shrinkage. Guidelines:

• Amorphous polymers: Pack to 95-98% of theoretical density
• Semi-crystalline polymers: Pack to 98-99% of theoretical density
• Pressure profile: High initial pressure, gradual reduction

Mold Temperature Control

Higher mold temperatures reduce shrinkage by allowing better packing:

• ABS/PC: 80-100°C for minimum shrinkage
• PA/PBT: 90-120°C for crystallization control
• PE/PP: 40-60°C to balance cooling and shrinkage

Cooling Time Optimization

Adequate cooling time ensures dimensional stability:

• Thin walls (< 2mm): 10-20 seconds cooling time
• Medium walls (2-4mm): 20-40 seconds cooling time
• Thick walls (> 4mm): 40-80 seconds cooling time

Troubleshooting Shrinkage Issues

Common shrinkage-related problems and solutions:

Excessive Shrinkage

• Cause: Low packing pressure, short hold time, low mold temperature
• Solution: Increase packing pressure by 10-20%, extend hold time, raise mold temperature
• Tederic Tip: Use closed-loop pressure control for consistent packing

Differential Shrinkage

• Cause: Non-uniform cooling, improper gate location, long flow lengths
• Solution: Optimize cooling layout, relocate gates, add conformal cooling channels
• Tederic Tip: Utilize variotherm mold temperature control for uniform shrinkage

Post-Mold Dimensional Changes

• Cause: Inadequate crystallization, moisture absorption, stress relaxation
• Solution: Increase cooling time, ensure proper drying, use stress-relieving annealing
• Tederic Tip: Implement post-mold dimensional measurement and feedback control

Inconsistent Shrinkage

• Cause: Material variations, temperature fluctuations, machine inconsistencies
• Solution: Use consistent material lots, stabilize process temperatures, calibrate machine
• Tederic Tip: Employ Industry 4.0 sensors for real-time shrinkage monitoring

Summary & Key Takeaways

Shrinkage compensation is critical for producing dimensionally accurate injection molded parts. The 2025 data provided here represents the most current industry standards for major engineering polymers.

Key Points:

• Amorphous polymers: 0.2-0.8% shrinkage, isotropic behavior
• Semi-crystalline polymers: 0.8-3.0% shrinkage, anisotropic behavior
• Filled polymers: 0.1-1.0% shrinkage, reduced by fiber reinforcement
• Processing factors: Packing pressure most critical for shrinkage control
• Post-mold shrinkage: Significant for semi-crystalline materials (weeks to stabilize)

Mold Design Formula: Mold Dimension = Part Dimension × (1 + Shrinkage Rate)

Always verify shrinkage rates with your specific material supplier, as formulations can vary. Use this reference table as a starting point for mold design and process development.

Tederic Advantage: Our advanced injection molding machines with closed-loop controls and variotherm capabilities ensure consistent shrinkage and dimensional accuracy across all polymer types.