Shrinkage

1. What is shrinkage?
2. What are the units of shrinkage?
3. When does shrinkage occur?
4. What is the significance of shrinkage?
5. What happens if the molded parts shrink unequally?
6. What are the causes of variation in molded parts or shrinkage?
7. Which polymers have high or low shrinkage?
8. What are the methods to determine shrinkage?
9. What are the shrinkage values of various plastics?

What is shrinkage?

The shrinkage of plastics signifies the volume contraction of polymers. This occurs during the cooling step of the processing of polymers. The contraction is partly due to the difference in the density of polymers from the melt state and the cooled, rigid state.

What are the units of shrinkage?

Shrinkage is a rate, so it is expressed in percent, %.

When does shrinkage occur?

Most of the shrinkage occurs in the mold while cooling. A small amount of shrinkage occurs after ejection as the molded part continues to cool. After that, the part may continue to shrink very slightly until the temperature and moisture content stabilize.

What is the significance of shrinkage?

The amount of shrinkage needs to be accurately predicted. It is an important property to consider while designing plastic parts with:

• Optimized dimensional stability.
• High tolerances to ensure finished and assembled products function properly.

For example, critical dimensioning is important for a part that supports internal electrical components to make sure product’s operation. If a plastic part carrying a circuit board changes size with age, it can cause one or more circuits on the board to crack. This causes intermittent or complete failure.

What happens if the molded parts shrink unequally?

Unequal shrinkage is called warpage. If the regions of the part shrink unequally, they create stresses within the part. These stresses depend on part stiffness that may cause the part to deform or change shape. This leads to cracks in parts during long-term use.

The shrinkage of molded plastic parts can be as much as 20% by volume when measured at the processing and ambient temperature. This volume contraction of polymers often leads to wrapped parts and dimension differences. These changes occur between manufactured parts and the mold. In the extrusion processing technique, we use the die instead of mold.



Check out the video on how to avoid shrinkage and warpage

What are the causes of variation in molded parts or shrinkage?

The shrinkage rate is strongly depending on:

Factors	Examples
Polymer composition and material properties	PVT, thermal Properties
Processing conditions applied	Temperature, pressure, flow rate, etc.
Part design and geometry	Wall thickness, gate location, mold constraints

Polymer Composition Based on Crystallinity

Semi-crystalline polymers always show a higher shrinkage than amorphous polymers. This is because on cooling parts of their macromolecular chains are re-arranged. This forms a crystallite that is a well-organized structure. This leads to less space needed for the same number of atoms.

However, a slow rate of crystallization or a low degree of total crystallization has the effect of reducing shrinkage. This thereby reducing warpage in semi-crystalline polymers. Some examples of semi-crystalline polymers are Polybutylene terephthalate or Polypropylene.

Also, the presence of side chains leads to high degrees of chain entanglements in highly branched polymers. This inhibits the ability of molecules to fit into a developing crystal structure. Further, lowering rapid crystallization.

Nucleated resin grades show higher amounts of shrinkage. This is the same as copolymers and homopolymers.

Molecular Weight

The degree of shrinkage is also influenced by molecular weight. HMW resins show a higher viscosity on filling and a higher pressure drop in the tool cavity during filling. Higher packaging pressure must be used to compensate for the cavity pressure drop. Otherwise, the lower pressure melt will result in higher shrinkage in the final part.

Learn how to calculate the molecular weight »

Fillers and Fibers

Fillers and fibers are generally added in plastics to modify properties such as:

• stiffness,
• creep resistance, etc.

Filler and fiber reinforcements in composites result in shrinkage that varies from the virgin polymer.

Most fillers and fibers have a relatively low coefficient of thermal expansion. Hence, when a part cools down during processing, they tend to shrink significantly. The reduction in shrink is approximately proportional to their concentration.

Polymers filled with long glass fibers shrink less along the direction in which fibers align. The shrinkage occurs in the flow direction compared to the transverse direction. Recycled fiber-reinforced polymers exhibit different mold-shrinkage characteristics than those of virgin resin.

Watch Course: Efficient Optimization of Fillers in Thermoplastics

Pigments

Pigments, in general, lead to an increase in shrinkage. They promote shrinkage by acting as a nucleating agent. The use of pigments tends to increase the cross-flow shrinkage in semi-crystalline polymers. The presence of pigments in polymers can affect crystallization and, hence, mold shrinkage.

• Organic pigments provide crystalline nuclei from which crystals grow. In comparison to neat polymers, a high amount of crystallinity in the pigmented resins can be seen due to:
  
  
  • earlier initiation of crystallization and
  • more rapid crystallization.


• Inorganic pigments cause the same type of shrinkage change to a less degree.

Check out the video on how to design better plastic part design
(Source: CADimensions, Inc.)

Time and Stress – Dimensional Stability

In plastics, the rate of dimensional change is determined by the stress level and the temperature at which the part is held under stress. At increasing times, the part under load will deform in response to the applied load.

Loss of fluids such as plasticizers loss due to migration or boil-off with time also causes shrinkage and increases brittleness.

Further excessive shrinkage beyond the acceptable level can be caused by:

• Low injection pressure
• Short pack-hold time or cooling time
• High melt temperature
• High mold temperature
• Low holding pressure

Which polymers have high or low shrinkage?

• Polymers with high shrinkage – View Product List
• Polymers with low shrinkage – View Product List

In higher shrink materials such as acetal and nylon, the post-mold shrinkage can be significant.

What are the methods to determine shrinkage?

The methods to determine the mold shrinkage of thermoplastics and thermosets include:

• ASTM D955 - Standard Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics. This standard covers the measurement of specimen shrinkage for injection and compression molding.
• ISO 294-4 - Plastics — Injection Molding of Test Specimens of Thermoplastic Materials — Determination of Molding Shrinkage
• ISO 2577 - Plastics — Thermosetting molding materials — Determination of shrinkage

What are the shrinkage values (%) of various plastics?

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A-C     |      E-M     |      PA-PC     |      PE-PL     |      PM-PP     |      PS-X

Polymer Name	Min Value (%)	Max Value (%)
ABS - Acrylonitrile Butadiene Styrene	0.70	1.60
ABS Flame Retardant	0.30	0.80
ABS High Heat	0.40	0.90
ABS High Impact	0.40	0.90
ABS/PC Blend - Acrylonitrile Butadiene Styrene/Polycarbonate Blend	0.50	0.70
ABS/PC Blend 20% Glass Fiber	0.20	0.30
ABS/PC Flame Retardant	0.30	0.60
Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (High Flow)	0.80	1.00
Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (Standard Flow)	0.80	1.00
Amorphous TPI, High Heat, High Flow, Transparent, Lead-Free Solderable (High Flow)	1.00	1.20
Amorphous TPI, High Heat, High Flow, Transparent, Lead-Free Solderable (Standard Flow)	1.00	1.20
Amorphous TPI, Moderate Heat, Transparent	0.50	0.70
Amorphous TPI, Moderate Heat, Transparent (Food Contact Approved)	0.50	0.70
Amorphous TPI, Moderate Heat, Transparent (Mold Release grade)	0.50	0.70
Amorphous TPI, Moderate Heat, Transparent (Powder form)	0.50	0.70
ASA - Acrylonitrile Styrene Acrylate	0.40	0.70
ASA/PC Blend - Acrylonitrile Styrene Acrylate/Polycarbonate Blend	0.30	0.70
ASA/PC Flame Retardant	0.40	0.80
ASA/PVC Blend - Acrylonitrile Styrene Acrylate/Polyvinyl Chloride Blend	0.30	0.70
CA - Cellulose Acetate	0.30	1.00
CAB - Cellulose Acetate Butyrate	0.20	0.90
Celllulose Diacetate-Pearlescent Films	1.00	1.50
Celllulose Diacetate-Matt Film	1.00	1.50
CP - Cellulose Proprionate	0.10	0.90
CPVC - Chlorinated Polyvinyl Chloride	0.30	0.70
ETFE - Ethylene Tetrafluoroethylene	3.00	4.00
EVA - Ethylene Vinyl Acetate	0.40	3.50
FEP - Fluorinated Ethylene Propylene	3.00	6.00
HDPE - High Density Polyethylene	1.50	4.00
HIPS - High Impact Polystyrene	0.20	0.80
HIPS Flame Retardant V0	0.30	0.60
LCP - Liquid Crystal Polymer	0.10	0.60
LCP Carbon Fiber-reinforced	0.10	0.50
LCP Glass Fiber-reinforced	0.10	0.40
LCP Mineral-filled	0.10	0.50
LDPE - Low Density Polyethylene	2.00	4.00
LLDPE - Linear Low Density Polyethylene	2.00	2.50
MABS - Transparent Acrylonitrile Butadiene Styrene	0.40	0.70
PA 11 - (Polyamide 11) 30% Glass fiber reinforced	0.50	0.50
PA 11, Conductive	0.70	2.00
PA 11, Flexible	1.40	1.80
PA 11, Rigid	0.70	2.00
PA 12 (Polyamide 12), Conductive	0.70	2.00
PA 12, Fiber-reinforced	0.70	2.00
PA 12, Flexible	0.70	2.00
PA 12, Glass Filled	0.70	2.00
PA 12, Rigid	0.70	2.00
PA 46 - Polyamide 46	1.50	2.00
PA 46, 30% Glass Fiber	0.30	1.30
PA 6 - Polyamide 6	0.50	1.50
PA 6-10 - Polyamide 6-10	1.00	1.30
PA 66 - Polyamide 6-6	0.70	3.00
PA 66, 30% Glass Fiber	0.50	0.50
PA 66, 30% Mineral filled	0.60	1.00
PA 66, Impact Modified, 15-30% Glass Fiber	0.20	0.60
PA 66, Impact Modified	1.20	3.00
PAI - Polyamide-Imide	0.60	1.00
PAI, 30% Glass Fiber	0.10	0.30
PAI, Low Friction	0.10	0.50
PAN - Polyacrylonitrile	0.20	0.50
PAR - Polyarylate	0.90	1.20
PARA (Polyarylamide), 30-60% glass fiber	0.10	0.40
PBT - Polybutylene Terephthalate	0.50	2.20
PBT, 30% Glass Fiber	0.20	1.00
PC (Polycarbonate) 20-40% Glass Fiber	0.10	0.50
PC (Polycarbonate) 20-40% Glass Fiber Flame Retardant	0.10	0.50
PC - Polycarbonate, high heat	0.70	1.00
PC/PBT Blend - Polycarbonate/Polybutylene Terephthalate Blend	0.60	1.10
PCTFE - Polymonochlorotrifluoroethylene	0.50	1.50
PE - Polyethylene30% Glass Fiber	0.20	0.60
PEEK - Polyetheretherketone	1.20	1.50
PEEK 30% Carbon Fiber-reinforced	0.00	0.50
PEEK 30% Glass Fiber-reinforced	0.40	0.80
PEI - Polyetherimide	0.70	0.80
PEI, 30% Glass Fiber-reinforced	0.20	0.40
PEI, Mineral Filled	0.50	0.70
PEKK (Polyetherketoneketone), Low Crystallinity Grade	0.004	0.005
PESU - Polyethersulfone	0.60	0.70
PESU 10-30% glass fiber	0.20	0.30
PET - Polyethylene Terephthalate	0.20	3.00
PET, 30% Glass Fiber-reinforced	0.20	1.00
PET, 30/35% Glass Fiber-reinforced, Impact Modified	0.20	0.90
PETG - Polyethylene Terephthalate Glycol	0.20	1.00
PE-UHMW - Polyethylene -Ultra High Molecular Weight	4.00	4.00
PFA - Perfluoroalkoxy	3.00	5.00
PHB - Polyhydroxybutyrate	1.20	1.60
PI - Polyimide	0.20	1.20
PLA,injection molding	0.30	0.50
PMMA - Polymethylmethacrylate/Acrylic	0.20	0.80
PMMA (Acrylic) High Heat	0.20	0.80
PMMA (Acrylic) Impact Modified	0.20	0.80
PMP - Polymethylpentene	1.60	2.10
PMP 30% Glass Fiber-reinforced	0.30	1.20
PMP Mineral Filled	1.40	1.70
Polyamide 66 (Nylon 66)/Carbon Fiber, Long, 30 % Filler by Weight	0.30	0.30
Polyamide 66 (Nylon 66)/Carbon Fiber, Long, 40 % Filler by Weight	0.30	0.30
Polyamide 66 (Nylon 66)/Glass Fiber, Long, 40 % Filler by Weight	0.30	0.30
Polyamide 66 (Nylon 66)/Glass Fiber, Long, 50 % Filler by Weight	0.30	0.30
Polyamide 66 (Nylon 66)/Glass Fiber, Long, 60 % Filler by Weight	0.30	0.30
PP Homopolymer, Long Glass Fiber, 30% Filler by Weight	0.40	0.40
PP Homopolymer, Long Glass Fiber, 40% Filler by Weight	0.30	0.30
PP Homopolymer, Long Glass Fiber, 50% Filler by Weight	0.30	0.30
POM - Polyoxymethylene (Acetal)	1.80	2.50
POM (Acetal) Impact Modified	1.00	2.50
POM (Acetal) Low Friction	1.80	3.00
POM (Acetal) Mineral Filled	1.50	2.00
PP - Polypropylene 10-20% Glass Fiber	0.30	1.00
PP, 10-40% Mineral Filled	0.60	1.40
PP, 10-40% Talc Filled	0.90	1.40
PP, 30-40% Glass Fiber-reinforced	0.01	1.00
PP (Polypropylene) Copolymer	2.00	3.00
PP (Polypropylene) Homopolymer	1.00	3.00
PP, Impact Modified	2.00	3.00
PPA - Polyphthalamide	1.50	2.20
PPA – 30% Mineral	1.00	1.20
PPA, 33% Glass Fiber-reinforced	0.50	0.70
PPA, 33% Glass Fiber-reinforced – High Flow	0.74	0.76
PPA, 45% Glass Fiber-reinforced	0.10	0.30
PPE - Polyphenylene Ether	0.50	0.80
PPE, 30% Glass Fiber-reinforced	0.10	0.40
PPE, Flame Retardant	0.60	1.00
PPE, Impact Modified	0.60	1.00
PPE, Mineral Filled	0.30	0.70
PPS - Polyphenylene Sulfide	0.60	1.40
PPS, 20-30% Glass Fiber-reinforced	0.20	0.50
PPS, 40% Glass Fiber-reinforced	0.20	0.50
PPS, Conductive	0.30	1.00
PPS, Glass fiber & Mineral-filled	0.30	0.70
PPSU - Polyphenylene Sulfone	0.70	0.70
PS (Polystyrene) 30% glass fiber	0.20	0.20
PS (Polystyrene) Crystal	0.10	0.70
PS, High Heat	0.20	0.70
PSU - Polysulfone	0.70	0.70
PSU, 30% Glass finer-reinforced	0.10	0.60
PSU Mineral Filled	0.40	0.50
PTFE - Polytetrafluoroethylene	3.00	6.00
PTFE, 25% Glass Fiber-reinforced	1.80	2.00
PVC (Polyvinyl Chloride), 20% Glass Fiber-reinforced	0.10	0.20
PVC, Plasticized	0.20	4.00
PVC, Plasticized Filled	0.80	5.00
PVC Rigid	0.10	0.60
PVDC - Polyvinylidene Chloride	0.50	2.50
PVDF - Polyvinylidene Fluoride	2.00	4.00
SAN - Styrene Acrylonitrile	0.30	0.70
SAN, 20% Glass Fiber-reinforced	0.10	0.30
SMA - Styrene Maleic Anhydride	0.40	0.80
SMA, 20% Glass Fiber-reinforced	0.20	0.30
SMA, Flame Retardant V0	0.50	0.50
TPS, Injection General Purpose	0.60	1.50
TPS, Water Resistant	0.60	0.90
XLPE - Crosslinked Polyethylene	0.70	5.00