The factors affecting the shrinkage of thermoplastics are as follows:
1. Plastic type:
During the molding process of thermoplastics, there are still some factors such as the volume change due to crystallization, strong internal stress, large residual stress frozen in the plastic part, strong molecular orientation, etc., so compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage rate range is wide, and the directivity is obvious. In addition, the shrinkage rate after external molding, annealing or humidity conditioning treatment is generally larger than that of thermosetting plastics.
2. Plastic part characteristics:
When the molten material contacts the surface of the mold cavity, the outer layer immediately cools to form a low density solid shell. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part cools slowly to form a high-density solid layer with large shrinkage. Therefore, those with 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 material flow direction, density distribution and shrinkage resistance. Therefore, the characteristics of plastic parts have a greater impact on the shrinkage size and direction.
3. Feed inlet type, size and distribution:
These factors directly affect the direction of material flow, density distribution, pressure holding and feeding effect and molding time. The direct feed inlet and feed inlet with large section (especially thick section) have small shrinkage but large directivity, while the feed inlet with short width and length has small directivity. Those close to the feed inlet or parallel to the direction of material flow will have large shrinkage.
4. Forming 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 larger because of the high crystallinity and large volume change. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic parts, which directly affects the size and direction of the shrinkage of each part.
During mold design, the shrinkage rate of each part of the plastic part shall be determined based on experience according to the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the form, size and distribution of the feed inlet, and then the cavity size shall be calculated.
For high-precision plastic parts and when it is difficult to master the shrinkage rate, the following methods should be generally used to design the mold:
① The outer diameter of plastic parts shall have a smaller shrinkage rate, and the inner diameter shall have a larger shrinkage rate, so as to leave room for correction after mold testing.
② The mold test determines the form, size and molding conditions of the gating system.
③ The plastic parts to be post-treated shall be subject to post-treatment to determine the size change (the measurement must be made 24 hours after demoulding).
④ Correct the mold according to the actual shrinkage.
⑤ Try the mold again and modify the shrinkage value slightly by changing the process conditions appropriately to meet the requirements of the plastic part.
Post time: Dec-06-2022