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. 

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.

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 is operational. 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. 

 

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. 

 

The shrinkage rate is strongly depending on:

 

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.

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

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