The shrinkage of injection molding
The shrinkage of injection molding
The factors affecting thermoplastic forming shrinkage are as follows:
The thermoplastic plastic molding process is due to the existence of crystallization of the shape of the volume change, strong internal stress, frozen in the plastic parts of the residual stress, molecular orientation, and other factors, so compared with thermosetting plastic shrinkage rate is larger, shrinkage rate range-wide, obvious direction, In addition, the shrinkage rate after molding, annealing or humidity treatment is generally larger than thermosetting plastics.
When forming molten material and cavity surface contact outer layer, plastic parts are formed immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of plastic slowly cools and forms a large shrinkage of the high-density solid layer. Therefore, wall thickness, slow cooling, and high-density layer thickness shrink greatly. In addition, there is no insert and insert layout. The number directly affects the direction of material flow, density distribution, and shrinkage resistance size, so the characteristics of plastic parts on the size of the contraction, the direction of the greater impact.
Feed inlet form, size, distribution of these factors directly affect the direction of material flow, density distribution, pressure holding, and feeding and molding time. Direct feeding mouth, feeding mouth section (especially thick section) is reduced, but the direction is large, feeding mouth width and length is short direction is small. Close to the inlet or parallel to the direction of the material flow is a large contraction.
Molding conditions high mold temperature, melting material cooling slow, high density, large shrinkage, especially for the crystallization material is due to high crystallinity, large volume change, so the shrinkage is larger. The mold temperature distribution is also related to the internal and external cooling and density uniformity of plastic parts, which directly affects the size and direction of the contraction of each part.
In addition, maintaining pressure and time also greatly influence contraction, and when stress is high and time is long, contraction but direction is great. The injection pressure is high, the melt viscosity difference is small, the interlayer shear stress is small, the elastic rebound after demoulding is large, so the shrinkage can also be reduced, the material temperature is high, the shrinkage is large, but the direction is small. Therefore, adjusting the mold temperature, pressure, injection speed and cooling time, and other factors can be appropriate to change the shrinkage of plastic parts.
Mold design according to the shrinkage range of various plastics, plastic wall thickness, shape, inlet form size, and distribution, according to experience to determine the shrinkage rate of various parts of the plastic, and then to calculate the size of the cavity.
For high-precision plastic parts and difficult to grasp shrinkage rate, it is generally appropriate to use the following method to design the mold:
- 1. The outer diameter of the plastic parts is smaller shrinkage, and the inner shrinkage is larger to leave room for modification after the mold trial.
- 2. Mold test determines the pouring system’s form, size, and molding conditions.
- To post-processing plastic parts after post-processing to determine the size change (measurement must be 24 hours after demounding).
- Modify the mold according to the actual shrinkage.
Try the mold again and can appropriately change the process conditions slightly modify the shrinkage value to meet the requirements of plastic parts.