Factors Affecting Injection Molding Process (Part 2)

When designing the mold, the shrinkage rate of each part of the plastic part is determined based on the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the form, size, and distribution of the feeding port, and then the cavity size is calculated according to experience. When it is difficult to control the shrinkage rate of high-precision plastic parts, it is generally advisable to design molds using the following methods:

① Trial mold to determine the form, size, and forming conditions of the pouring system.

② The size changes of the plastic parts that need post-processing must be determined after post-processing (measurement must be taken 24 hours after demolding).

③ Adjust the mold according to the actual shrinkage situation.

④ Try molding again and adjust the shrinkage value slightly by changing the process conditions appropriately to meet the requirements of the plastic part. The flowability of thermoplastic materials can generally be analyzed based on a series of indices such as molecular weight, melt index, Archimedean helix flow length, apparent viscosity, and flow ratio (process length/plastic part wall thickness).

If the molecular weight is small, the molecular weight distribution is wide, the molecular structure regularity is poor, the melt index is high, the screw flow length is long, the apparent viscosity is small, and the flow ratio is large, then the flowability is good. For plastics of the same name, their instructions must be checked to determine whether their flowability is suitable for injection molding. According to the requirements of mold design, the flowability of commonly used plastics can be roughly divided into three categories: ① good flowability PA PE,PS,PP,CA, Poly (4) methylsiloxane: ② Medium flowability polystyrene resin series (such as ABS AS),PMMA,POM, Polyphenylene ether;

③ Poor liquidity PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastics.