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Process Characteristics And Properties Of Thermoplastics

2008/8/9 10:10:00 41740

热塑性塑料品种极多，即使同一品种也由于树脂分子及附加物配比不同而使其使用及工艺特性也有所不同。

In addition, in order to change the characteristics of the original varieties, commonly used copolymerization, cross linking and other chemical polymerization methods in the original resin structure to import a percentage of heterogeneous monomer or polymer equivalent resin, so as to change the structure of the original resin and become a new modified variety with new use and technological characteristics.

For example, ABS is imported polyacrylonitrile, butadiene and other heterogeneous monomers into polystyrene molecules, and then become modified copolymers, also known as modified polystyrene, which has superior application and technological characteristics than polystyrene.

Because there are many varieties and complex properties of thermoplastic plastics, even if the same kind of plastic is only used for injection or extrusion, this chapter mainly introduces all kinds of thermoplastic plastics for injection.

一、工艺特性

(1) shrinkage rate

The form and calculation of forming shrinkage of thermoplastic plastics, as mentioned above, affect the forming shrinkage of thermoplastic plastics as follows:

1. Because of the volume change of the crystalline form, the internal stress is strong, the residual stress in the plastic parts is large and the molecular orientation is strong and so on. Therefore, compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage range is wide and the direction is obvious. Moreover, the shrinkage after forming, shrinkage or annealing is generally higher than that of thermosetting plastics.

2. When the plastic parts are formed, the melt will contact with the surface of the cavity, and the outer layer will immediately cool down to form a low density solid shell.

Due to the poor thermal conductivity of plastics, the inner layer of plastic parts is cooled slowly to form a high density solid layer with large shrinkage.

Therefore, the wall thickness, slow cooling and high density thickness shrink greatly.

In addition, the layout of the inserts and inserts directly affects the direction of the material flow, the density distribution and the size of the shrinkage resistance. Therefore, the characteristics of the plastic parts have a greater influence on the shrinkage size and direction.

3, the form, size and distribution of feed inlet directly affect the direction of flow, density distribution, packing and shrinkage and forming time.

The section of the direct feed port and the feed port is large (especially the thick section), but the shrinkage is small, but the direction is large.

Large shrinkage occurs near the inlet or parallel to the material flow direction.

4, forming conditions, mold temperature is high, melt material cooling is slow, high density, large shrinkage, especially for crystalline materials because of high crystallinity, large volume changes, so shrinkage is greater.

The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic parts, which directly affects the shrinkage and direction of each part.

In addition, keeping pressure and time also has a greater effect on contraction. When the pressure is large and the time is long, the contraction is small but the direction is large.

The injection pressure is high, the viscosity difference between the melt materials is small, the interlaminar shear stress is small, and the elastic rebound is large after demoulding, so the shrinkage can also be reduced to a proper degree. The material temperature is high and the shrinkage is large, but the direction is small.

Therefore, the factors such as mold temperature, pressure, injection speed and cooling time can also change the shrinkage of plastic parts properly.

According to the shrinkage range of various plastics, the thickness and shape of plastic parts, the size and distribution of feed ports, the shrinkage rate of each part of plastic parts is determined according to experience, and then the cavity size is calculated.

For high precision plastic parts and difficult to grasp shrinkage, it is generally advisable to design moulds with the following methods:

(1) a smaller shrinkage rate is obtained for the outer diameter of the plastic part, and a larger shrinkage is obtained in the inner diameter, so that there is room for correction after the trial die.

(2) to determine the form, size and forming condition of gating system.

(3) after processing, the size of the plastic parts must be determined after processing (the time must be 24 hours after the mold is released).

(4) correct the mold according to the actual shrinkage condition.

(5) retry the mold and appropriately modify the technological conditions to slightly modify the shrinkage value to meet the requirements of plastic parts.