Thermoplastic Process Characteristics and Characteristics (1)

In order to change the characteristics of the original varieties, various chemical polymerization methods, such as copolymerization and cross-linking, are used to introduce a certain percentage of dissimilar monomers or polymer equivalent resins into the original resin structure to change the structure of the original resin to become new Use and modification of process characteristics.

For example, ABS is a modified copolymer obtained by introducing a heterogeneous monomer such as acrylonitrile or butadiene into a polystyrene molecule. It can also be referred to as a modified polystyrene and is superior to polystyrene in its use. characteristic. Due to the variety of thermoplastics and their complex properties, even plastics of the same type are only used for injection or extrusion. Therefore, this chapter mainly introduces various injection thermoplastics.

First, the process characteristics

(I) Shrinkage

Thermoplastic molding shrinkage forms and calculations As previously mentioned, the factors that affect the shrinkage of thermoplastics are as follows:

1. Plastic type thermoplastic plastics have a volume change due to crystallization, high internal stress, large residual stress frozen in the plastic parts, and strong molecular orientation during the forming process. Therefore, the shrinkage rate is higher than that of thermoset plastics. Larger, wide range of shrinkage, obvious direction, in addition to the shrinkage after forming, annealing or shrinkage after conditioning treatment are generally larger than the thermosetting plastic.

2. When the plastic part is shaped, the molten material contacts the surface of the cavity and 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 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, whether there are inserts and inserts layout, the number has a direct impact on the direction of the material flow, density distribution and shrinkage resistance, etc., so the characteristics of the plastic parts have a greater impact on the shrinkage size and directionality.

3, feed inlet form, size, distribution These factors directly affect the direction of the material flow, density distribution, pressure packing action and forming time. The direct feed port and feed port have large cross-sections (especially thick cross-sections) with small shrinkage but large 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.

4. Molding conditions The mold temperature is high, the melt cooling is slow, the density is high, and the shrinkage is large. Especially for crystallization materials, 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, and 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. The pressure is large, and when 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, and the material temperature is high and the shrinkage is large, but the directionality is small. Therefore, when adjusting the mold temperature, pressure, injection speed and cooling time and other factors can also be appropriate to change the shrinkage of plastic parts. (To be continued)