Injection molding shrinkage
There are 7 key points should be considered in the setting of injection molding technology such as shrinkage, fluidity, crystallinity, heat-sensitive plastics and easily hydrolyzed plastics, stress cracking and melt fracture, thermal performance and cooling rate, and moisture absorption . The following we would like to share the factors which make shrink effect in the injection molding process.
1. Shrinkage rate
The factors that affect thermoplastic injection molding shrinkage are as follows:
- Plastic types Thermoplastic plastics are also subject to volume changes caused by crystallization, strong internal stress, large residual stress frozen in the plastic parts, and strong molecular orientation during the molding process. Therefore, compared with thermoset plastics, the shrinkage rate is Larger, the injection molding shrinkage range is wide, and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity treatment is generally greater than that of thermosetting plastics.
- The characteristics of the plastic part. When the molten material is in contact with the surface of the cavity, the outer layer is immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part is slowly cooled to form a high-density solid layer with large shrinkage. Therefore, the wall thickness, slow cooling, and high-density layer thickness will shrink more. In addition, the presence or absence of inserts and the layout and quantity of inserts directly affect the direction of material flow, density distribution and shrinkage resistance, so the characteristics of plastic parts have a greater impact on shrinkage and directionality.
- Factors such as the form, size and distribution of the feed inlet directly affect the direction of material flow, density distribution, pressure maintaining and shrinking effect and molding time. Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have small shrinkage but large directionality, and short feed ports with short width and length have small directivity. The ones that are close to the feed inlet or parallel to the direction of the material flow will shrink more.
- Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for the crystalline material, the shrinkage is greater due to high crystallinity and large volume changes. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic part, which directly affects the size and direction of the shrinkage of each part.
In addition, holding pressure and time also have a greater impact on contraction, and the contraction is smaller but the directionality is greater when the pressure is high and the time is long. The injection pressure is high, the viscosity difference of the molten material is small, the interlayer shear stress is small, and the elastic rebound after demolding is large, so the shrinkage can also be appropriately reduced. The material temperature is high, the shrinkage is large, but the directionality is small. Therefore, adjusting mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of plastic parts.
When designing the mold, according to the injection molding shrinkage range of various plastics, the wall thickness and shape of the plastic part, the size and distribution of the inlet form, the shrinkage rate of each part of the plastic part is determined according to experience, and then the cavity size is calculated.
For high-precision plastic parts and when it is difficult to grasp the shrinkage rate, the following methods should generally be used to design the mold:
- Take a smaller shrinkage rate for the outer diameter of the plastic part, and a larger shrinkage rate for the inner diameter to leave room for correction after the test mold.
- Trial molds determine the form, size and molding conditions of the gating system.
- The plastic parts to be post-processed are subjected to post-processing to determine the size change (measurement must be 24 hours after demolding).
- Correct the mold according to the actual shrinkage.
- Retry the mold and appropriately change the process conditions to slightly modify the shrinkage value to meet the requirements of the plastic part.