Because of the volume change caused by crystallization, strong internal stress, large residual stress frozen in plastic parts and strong molecular orientation, the shortening rate of thermosetting plastics is larger than that of thermosetting plastics. The shortening rate of thermosetting plastics is planned broadly and directionally remarkable. The shortening rate of other thermosetting plastics after shortening, annealing or moisture adjustment is also generally higher than that of thermosetting plastics. Large sex plastic.

The melt material and the outer surface of the cavity touch the outer layer and then cool down to form a low density solid shell. Because of the poor thermal conductivity of plastics, the inner layer of plastic parts is slowly cooled and a high density solid layer is formed. So wall thickness, slow cooling and high density layer thickness are shortened greatly. Otherwise, the layout and quantity of inserts and inserts have a direct impact on the direction of material flow, density distribution and the size of reducing resistance, so the characteristics of plastic parts have a greater impact on the size and direction of shortening.

The method, specification and distribution of inlet and outlet of automatic moulding machine directly affect the direction of material flow, density distribution, pressure holding and feeding effect and moulding time. When the cross section of the direct inlet and the inlet is large (especially the thicker cross section), the directivity of the direct inlet and the inlet is small, while the width and length of the inlet are short. Close to the feed inlet or parallel to the direction of the material flow will shorten greatly.

Mold temperature is high, melt material cooling is slow, density is high, shortening is large, especially for crystalline material because of high crystallinity and large volume change, so shortening is greater. Mold temperature distribution is also related to internal and external cooling and density uniformity of plastic parts, which directly affects the size and direction of each part shortening. Otherwise, persistent pressure and time also have a greater impact on shortening, while those with high pressure and long time are smaller but more directional. The injection pressure is high, the viscosity difference of molten material is small, the interlaminar shear stress is small, and the elastic rebound after demoulding is large, so the shortening can also be reduced appropriately, the material temperature is high, the shortening is large, but the directivity is small. Therefore, adjusting the factors such as mould temperature, pressure, injection speed and cooling time can also appropriately change the shortening of plastic parts.

According to the shortening plan of various plastics, the wall thickness and shape of plastic parts, the method of inlet and outlet and distribution, the shortening rate of each part of plastic parts is recognized according to the curriculum vitae, and then the cavity specification is calculated. When it is difficult to grasp the shortening rate of high precision plastic parts, the following methods should be used to plan the die:

(1) The outer diameter of the plastic parts should be shortened at a smaller rate and the inner diameter should be shortened at a larger rate so as to retain the level of correction after the test.

(2) The method, specification and forming conditions of the accreditation gating system of the trial die.

(3) The plastic parts to be post-processed should be modified by the post-processing acceptance criteria (24 hours after the demoulding is needed for measurement).

(4) Mould modification according to practice shortening.

_Re-test the die and modify the process conditions appropriately to shorten the value slightly to meet the requirements of the plastic parts.

The activity of thermoplastics can be analyzed by a series of indexes such as molecular weight, melting index, active length of Archimedes helix, reflecting viscosity and activity ratio (flow length/wall thickness). Low molecular weight, wide molecular weight distribution, poor regularity of molecular structure, high melting index, long snail activity length, low viscosity, high activity ratio, good activity. For the same name of plastics, it is necessary to check its instructions to determine whether its activity is suitable for injection moulding.

According to the requirements of mould planning, the activity of common plastics can be roughly divided into three categories:

(1) PA, PE, PS, PP, CA and poly (4) methyleuropiene with good activity:

(2) Active medium polystyrene series resins (such as ABS, AS), PMMA, POM and polyphenyl ether;

(3) PC, rigid PVC, polyphenyl ether, polysulfone, polyaromatic sulfone and fluoroplastics with poor mobility.

The activity of various plastics also varies with the forming elements. The main factors affecting the activity of plastics are as follows:

The activity of PS, PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics changed greatly with temperature. For PE, POM and temperature increase or decrease have little effect on their activity. Therefore, the former should schedule the temperature to control the activity in the forming process.

(2) When the pressure of injection moulding increases, the shearing force and the activity of melt material increase, especially PE and POM. Therefore, the injection pressure should be dispatched to control the activity.

(3) The method, specification, layout, cooling system planning of the pouring system of the die structure, the factors such as the smoothness of the die surface, the thickness of the material cross section, the shape of the die cavity and the exhaust system directly affect the practical activity of the molten material in the die cavity. When the temperature of the molten material decreases, the activity of the molten material decreases when the temperature of the molten material decreases and the resistance of the molten material is added. The rational structure should be selected according to the activity of the plastics used in the die planning. When forming, the material temperature can also be controlled. Mold temperature, injection pressure and injection speed should always be properly controlled to satisfy the forming requirements.

Crystalline thermoplastics can be divided into crystalline plastics and amorphous plastics according to their non-crystallization during condensation. The so-called crystallization phenomenon refers to the phenomenon that the molecule moves independently and in disorder from melting to condensation, and becomes the molecule intermittently and freely, in a slightly fixed position, and has a tendency to make the molecule placed into a regular typing.

As a criterion for judging the appearance of these two types of plastics, the transparency of thick-walled plastic parts can be determined. Generally, crystalline materials are not.