Seven factors should be considered in the setting of injection moulding process, such as shortening rate, activity, crystallinity, thermosensitive plastics and hydrolyzable plastics, stress cracking and melt splitting, thermal function, cooling rate and moisture absorption.

I. Shortening Rate

As mentioned above, the factors affecting the shortening of thermoplastic moulding are as follows:

1. In the process of forming thermoplastic plastics, there are still some factors, such as volume change from crystallization, strong internal stress, large residual stress frozen in plastic parts and strong molecular orientation. Therefore, compared with thermosetting plastics, the shortening rate is larger, the shortening rate is planned broadly and directionally remarkable, and the shortening rate after other moulding, annealing or humidity adjustment treatment is generally also higher than that after heat treatment. Solid plastics are large.

2. 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.

3. The factors of inlet and outlet method, specification and distribution directly affect the direction of material flow, density distribution, pressure holding and feeding effect and forming 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.

4. Mold temperature is high, melt material cooling is slow, density is high, shortening is big, especially for crystalline material because of high crystallinity and large volume change, so shortening is bigger. Mold temperature distribution is also related to surface 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 of mould temperature, pressure, injection speed and cooling time can also change the shortening of plastic parts.

According to the shortening planning of various plastics, the thickness and shape of the plastic parts, the method of inlet and outlet and the distribution, the cavity specification is calculated according to the shortening rate of each part of the plastic parts admitted through experience.

When it is difficult to grasp the shortening rate of high precision plastic parts, the following methods should be used to plan the die:

For plastic parts, smaller reduction rate of outer diameter and larger reduction rate of inner diameter are adopted, so as to retain the stage of correcting after testing.

The method, specification and forming conditions of the confession gating system of the trial die.

The plastic parts to be reprocessed are changed by the post-processing admission specification (24 hours after demoulding is needed for measurement).

Correct the mould according to the shortening of practice.

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

II. Activity

1. The activity of thermoplastics can be analyzed by a series of indexes such as molecular weight, melting index, active length of Archimedes helix, apparent 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 apparent viscosity, high activity ratio, good activity. For the same name of plastics, it is necessary to check its specifications to determine whether its activity is suitable for injection molding.

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

Good activity PA, PE, PS, PP, CA, poly (4) methyl europiene;

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

Poor activity PC, rigid PVC, polyphenyl ether, polysulfone, polyaromatic sulfone, fluoroplastics.

2. 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 methods, specifications, settling, cooling system planning, melt resistance (such as surface finish, material cross-section thickness, cavity shape, exhaust system) and other factors directly affect the practical activity of molten material in the cavity. When the melt temperature is increased, the activity resistance will decrease. The rational structure should be selected according to the activity of the plastics used in the die planning.

When forming, material temperature can also be controlled. Mold temperature, injection pressure and injection speed should always be properly controlled to satisfy the forming requirements.

3. Crystallinity