What are the factors that affect the molding shrinkage of thermoplastics?
Due to the volume change in the formation of crystallization from the thermoplastic plastic molding process, the internal stress is high, the residual stress in the plastic part is frozen, and the molecular orientation is strong. Therefore, the shrinkage rate is higher than that of the thermosetting plastic. , Wide range of shrinkage, obvious direction, in addition to shrinkage after molding, annealing or shrinkage after conditioning treatment are generally larger than thermoset plastics.
When the plastic parts are molded, the outer layer of the molten material contacts the surface of the cavity and 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 is slowly cooled to form a high density solid layer with a large shrinkage. Therefore, the wall thickness, the slow cooling, and the high density layer thickly shrink.
In addition, the presence or absence of inserts and inserts layout and quantity have a direct impact on the material flow direction, density distribution and shrinkage resistance, etc. Therefore, the characteristics of plastic parts have a great influence on the shrinkage size and directionality.
Feeding port form, size, distribution of these factors directly affect the material flow direction, density distribution, pressure packing action and molding time. The direct feed port and feed port have large cross-sections (especially thick cross-sections) with small shrinkage but high directivity, and small feed port widths and short lengths. Parallel to the feed port or parallel to the direction of the material, the shrinkage is large.
Molding conditions The mold temperature is high, the melt cooling is slow, the density is high, and the shrinkage is large. Especially for the crystallization material, the crystallinity is high and the volume change is large, so the shrinkage is greater. 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 shrinkage of each part.
In addition, the holding pressure and time also have a great influence on the shrinkage. When the pressure is large and the time is long, the shrinkage is small but the directionality is large. The injection pressure is high, the melt viscosity is small, the interlaminar shear stress is small, and the elastic rebound after demolding is large. Therefore, the shrinkage can also be reduced appropriately, the material temperature is high, and the shrinkage is large, but the directionality is small. Therefore, adjusting mold temperature, pressure, injection speed, and cooling time during molding can also change the contraction of plastic parts.
The mold design depends on the shrinkage range of various plastics, the wall thickness and shape of the plastic parts, the size and distribution of the feed port, the shrinkage rate of each part of the plastic parts according to experience, and the size of the cavity.
For high-precision plastic parts and difficult to grasp the shrinkage rate, it is generally advisable to use the following method to design the mold:
1. Take a small shrinkage rate for the outer diameter of the plastic part, and take a larger shrinkage rate for the inner diameter so as to leave room for correction after the trial mode.
2. The test mold determines the form, size, and molding conditions of the gating system.
3. After the post-processing of plastic parts to determine the size of the change (measurement must be 24 hours after demoulding).
4, according to the actual contraction correction mold.
5. Retest the mold and change the process conditions appropriately. Slightly modify the shrinkage value to meet the plastic parts requirements