Due to thermal expansion and cold shrinkage, elastic recovery and plastic deformation during the demoulding of plastic parts lead to the reduction of the size of plastic parts after demoulding and cooling at room temperature. In the contractile directional forming, molecules are arranged in the direction, so that the plastic part presents anisotropy. Along the direction of material flow (i.e. parallel direction), the plastic part shrinks greatly and has high strength. In the right Angle direction of material flow (i.e. vertical direction), the plastic part shrinks and has low strength. In addition, due to the plastic parts of the density and packing distribution is not uniform, so that the contraction is not even. The shrinkage difference makes the plastic parts easy to warp, deform and crack, especially in extrusion and injection molding, the directivity is more obvious. Therefore, shrinkage direction should be considered in mold design, and shrinkage rate should be selected according to the shape of plastic parts and flow direction.

Influenced by the factor of forming pressure, shear stress, anisotropy, density, packing distribution, mold temperature-hardening inhomogeneity, plastic deformation, and other factors, next-shrink plastic parts forming in the flow state can not all disappear, so plastic parts in the state of stress forming residual stress. After demoulding, due to the effect of stress tending to balance and storage conditions, the residual stress will change and the plastic parts will shrink again, which is called post shrinkage. In general, the plastic parts change the most within 10 hours after demoulding and basically set after 24 hours, but the final stability needs 30-60 days.

Thermoplastics usually have a greater post shrinkage than thermosetting plastics and a greater post shrinkage than extrusion and injection molding. After treatment shrinkage sometimes plastic parts according to the performance and process requirements, after forming need heat treatment, treatment will also lead to plastic parts size change. Therefore, the error of post shrinkage and posttreatment shrinkage should be considered and compensated in the design of the high precision mold.

Shrinkage rate can be used to calculate the forming shrinkage of plastic parts. The actual shrinkage rate represents the actual shrinkage of the plastic parts. Because the difference between its value and the calculated shrinkage is very small, the cavity and core size are calculated by taking the calculated shrinkage rate as the design parameter in the mold design. Their calculation formula is shown as follows:

The actual shrinkage Q (%) = (A-B) /B 100

The calculated shrinkage Q (%) = (C-B) /B 100

A: unidirectional dimension (mm) of plastic parts at forming temperature

B: unidirectional size of plastic parts at room temperature (mm)

C: unidirectional size of mold at room temperature (mm)

The factor that affects systolic rate change when actual forming, systolic rate of plastic with different type is not same, the different plastic with the same batch or the different part of same plastic piece that systolic value is also different. The main factors affecting the change of shrinkage rate mainly include:

  • plastic varieties. All kinds of plastics have their own shrinkage range, with the same kind of plastics due to different fillers, molecular weight, and proportion, the shrinkage rate, and anisotropy are also different.
  • characteristics of plastic parts. The shape, size, wall thickness, and the number and layout of inserts also have a great influence on the shrinkage rate.
  • mold structure. Mold parting surface and direction of pressure, casting system form, layout and size of the shrinkage, and direction of a greater impact, especially in extrusion and injection molding is more obvious.
  • forming process. Extrusion and injection molding processes generally have high shrinkage and obvious directivity. Preheating conditions, forming temperature, forming pressure, holding time, filling the form and hardening uniformity all have an influence on shrinkage rate and direction.

Plastic mold design should be based on the shrinkage range provided in the manual, and plastic parts shape, size, wall thickness, no insert, parting surface, and pressure forming direction, mold structure and feeding port form size and location, forming process and other factors to consider the selection of shrinkage value. During extrusion or injection molding, different shrinkage rates are usually selected according to the shape, size, and wall thickness of each part of the plastic part.

Forming shrinkage is also affected by other factors, such as the type of plastic, shape, and size of plastic parts. The forming characteristics of plastics are related not only to the varieties of plastics but also to the packing varieties, particle size, and particle uniformity.