Commonly used thermosetting plastics are phenolic, amino (melamine, urea aldehyde) polyester, poly diallyl phthalate and so on. Mainly used in pressure molding, extrusion, injection molding. Silicone, epoxy resin and other plastics are mainly used for low pressure extrusion sealing electronic components and casting forming.

a, process feature

(a) shrinkage rate

removed from the mould after cooling to room temperature, size shrinkage this performance called contractility. Because the shrinkage is not only the thermal expansion and contraction of the resin itself, but also related to the forming factors, the contraction of the plastic parts after forming should be called the forming contraction.

1, the form of forming contraction forming contraction is mainly manifested in the following aspects:

(1) The linear size contraction of plastic parts due to thermal expansion and cold contraction, the elastic recovery of plastic parts when demoulding, plastic deformation and other reasons lead to the size reduction of plastic parts after demoulding cooling to room temperature, so the cavity design must be considered to be compensated.

(2) shrinkable directional forming molecules are arranged in the direction, so that the plastic parts show anisotropy, along the direction of material flow (that is, parallel direction) the contraction is large, high strength, and material flow right Angle direction (that is, vertical direction) is reduced, low strength. In addition, due to the uneven distribution of density and fillers in various parts of the plastic parts, the contraction is also uneven. The shrinkage difference makes the plastic parts prone to warping, deformation and cracks, especially in extrusion and injection molding, the direction is more obvious. Therefore, the mold design should consider the direction of shrinkage according to the shape of the plastic parts, the direction of the flow of the selected shrinkage rate is appropriate.

(3) After the shrinkage of plastic parts forming, due to the forming pressure, shear stress, anisotropy, uneven density, uneven filler distribution, mold temperature, hardening, plastic deformation and other factors, resulting in a series of stress effects, in the viscous flow state can not all disappear, so plastic parts in the stress state when forming residual stress. When the demoulding due to the stress trend balance and storage conditions, so that the residual stress changes and the plastic parts reshrink called post-contraction. Generally, the plastic parts change the most within 10 hours after the release, and the basic shape is set after 24 hours, but the final stability is 30-60 days. Usually, the shrinkage of thermoplastics is larger than that of thermosetting, and the extrusion and injection molding are larger than that of compression molding.

(4) post-processing shrinkage sometimes plastic parts according to the performance and process requirements, after forming heat treatment, after treatment will also lead to changes in the size of plastic parts. Therefore, the mold design of high-precision plastic parts should be considered after shrinkage and post-processing shrinkage error and compensation.

2, shrinkage calculation of plastic parts forming shrinkage can be expressed by the shrinkage rate, as shown in formula (1-1) and formula (1-2).

(1-1)Q real = (a-b) /b× 100

(1-2)Q meter = (c-b) /b× 100

formula:

Q real — Actual shrinkage (%)

Q meter — Calculated shrinkage (%)

a — Unidirectional dimension of plastic parts at forming temperature (mm)

b — Unidirectional size of plastic parts at room temperature (mm)

c — Mold at room temperature one-way size (mm)

Actual shrinkage to represent the actual shrinkage of plastic parts, because its value and the calculated shrinkage difference is very small, so the mold design with Q as the design parameters to calculate the cavity and core size.

3, the factors affecting the shrinkage rate change in the actual formation of not only different varieties of plastic shrinkage rate is different, and different batches of the same variety of plastic or the same plastic parts of the shrinkage value is often different, the main factors affecting the shrinkage rate change are as follows.

(1) A variety of plastic plastics have their own shrinkage range, the same kind of plastic due to fillers, molecular weight and ratio of different, then its shrinkage rate and anisotropy are different.

(2) Characteristics of plastic parts The shape, size, wall thickness, there are no inserts, the number and layout of inserts also have a great impact on the shrinkage rate.

(3) Mold structure mold parting surface and pressure direction, the form of the pouring system, layout and size on the shrinkage and direction of the greater impact, especially in extrusion and injection molding more obvious.

(4) Forming process extrusion, injection forming process generally shrinkage is large, obvious directivity. Preheating condition, forming temperature, forming pressure, holding time, filling form and hardening uniformity have an effect on shrinkage and directionality.

As mentioned above, the mold design should be based on the shrinkage range provided by the manual of various plastics, and according to the shape, size, wall thickness, there are no inserts, parting surface and pressure forming direction, mold structure and feed port form size and position, forming process and other factors to consider the selection of shrinkage value. When extrusion or injection molding, it is often necessary to select different shrinkage rates according to the shape, size, wall thickness and other characteristics of each part of the plastic.

In addition, the forming shrinkage is also affected by various forming factors, but it is mainly determined by the plastic variety, the shape and size of the plastic parts. Therefore, adjusting the forming conditions during forming can also appropriately change the shrinkage of plastic parts.

(2) liquidity

plastic under certain temperature and pressure filling mold ability is called liquidity. This is an important process parameter that must be considered in mold design. The fluidity is easy to cause excessive overflow, the filling cavity is not dense, the organization of the plastic parts is loose, the resin and the filler are separated and accumulate, the mold is easy to stick, the mold is difficult to remove and clean, and the hardening is premature. But small fluidity is insufficient filling, not easy to form, forming pressure.

Therefore, the fluidity of the plastic must be adapted to the requirements of the plastic parts, the forming process and the forming conditions. Mold design should be based on the flow performance to consider the pouring system, parting surface and feed direction and so on. Thermoset plastic flow is usually expressed in terms of lassig flow (measured in millimeters). The value is large, and the fluidity is good. Each variety of plastic is usually divided into three different levels of fluidity for different plastic parts and forming processes.

General plastic parts area is large, insert more, core and insert weak, there are narrow deep slots and thin walls of the complex shape to fill unfavorable, should be used better fluidity of plastic. When extrusion molding should be selected Lassig fluidity of more than 150mm plastic, the application of Lassig fluidity of more than 200mm plastic injection molding.

In order to ensure that each batch of plastics have the same fluidity, in practice, the commonly used and batch method to adjust, that is, the same variety of plastics with different liquidity to be used, so that each batch of plastic liquidity to compensate each other to ensure the quality of plastic parts. The Lassig fluidity value of commonly used plastics is detailed in Table 1-1, but it must be pointed out that in addition to the injection of plastic depends on the variety of plastic, it is often affected by various factors when filling the cavity, so that the actual ability of plastic to fill the cavity changes.

Such as fine particle size (especially round particles), high humidity, moisture and volatiles, preheating and forming conditions are appropriate, mold surface finish is good, mold structure is appropriate, etc., are conducive to improving fluidity. On the contrary, poor preheating or forming conditions, poor mold structure flow resistance or plastic storage period is too long, extended, high storage temperature (especially for amino plastics) will lead to the actual flow performance of the plastic filling cavity is reduced, resulting in poor filling.

(3) the specific volume and compression rate

volume for each gram plastic occupied volume meter (with 3 cm/g). The compression rate is the ratio of the volume or specific volume of the plastic powder and the plastic parts (its value is greater than 1). Both of them can be used to determine the size of the die charging chamber. The large value requires that the volume of the charging chamber be large, and at the same time shows that the plastic powder is inflated more, the exhaust is difficult, the forming cycle is long, and the productivity is low. The specific volume is small, on the contrary, and is conducive to the ingot and pressing. See Table 1-1 for the specific volume of various plastics. However, the specific volume value is also often due to the size of the plastic particle size and particle heterogeneity and error.

(4) hardening characteristics

Thermosetting plastics in the forming process under heating pressure into a plastic viscous flow state, with the increase of fluidity filling cavity, at the same time condensation reaction, crosslinking density continues to increase, the fluidity rapidly decreases, and the melting material gradually solidifies. Mold design for the fast hardening speed, keep the flow state of short material should pay attention to easy loading, loading and unloading inserts and select reasonable forming conditions and operation, so as to avoid premature hardening or insufficient hardening, resulting in poor forming of plastic parts.

The hardening speed can generally be analyzed from the holding time, which is related to the variety of plastic, wall thickness, shape of plastic parts and mold temperature. But also by other factors and change, especially related to the preheating state, the appropriate preheating should be maintained so that the plastic can play the maximum flow conditions, as far as possible to improve its hardening speed, the general preheating temperature is high, the time is long (within the allowable range) the hardening speed is accelerated, especially the prepressing billet by high frequency preheating the hardening speed is significantly accelerated. In addition, the forming temperature is high, the pressure time is long, the hardening speed will also increase. Therefore, the hardening rate can also be adjusted preheating or forming conditions to be properly controlled.

hardening speed should also be suitable for forming method requirements, such as injection, extrusion molding should be required in plasticizing, filling chemical reaction slow, slow hardening, should be maintained for a long time flow state, but when full of cavity after high temperature, high pressure should be fast hardening.

(five) Moisture and volatile matter content

Various plastics contain different degrees of moisture, volatile matter content, too much fluidity, easy to overflow, hold a long time, shrinkage increase, prone to ripple, warping and other ills, affecting the mechanical and electrical properties of plastic parts. However, when the plastic is too dry, it will also lead to poor fluidity and forming difficulties, so different plastics should be preheated and dried according to the requirements, and the material with strong hygroscopic properties, especially in the wet season, even the material after preheating should be prevented from hygroscopic again.

Due to various plastics contain different components of water and volatiles, at the same time in the condensation reaction to occur condensation of water, these components need to be formed into a gas discharge mold, some gases have a corrosive effect on the mold, the human body also has a stimulating effect. To this end, we should understand the characteristics of various plastics in the mold design, and take corresponding measures, such as preheating, chrome plating of the mold, opening the exhaust tank or forming the exhaust process.

Two, forming characteristics

In the mold design must master the forming characteristics of the plastic and the process characteristics of the forming.

The forming characteristics of various plastics are related to various plastic varieties, and are also related to the variety of fillers contained and the particle size and particle uniformity. Fine material fluidity is good, but the preheating is not easy to uniform, the air is not easy to discharge, the heat transfer is poor, the forming time is long.

Coarse plastic parts are not shiny, easy to occur surface uneven. Too thick, too fine also directly affect the specific volume and compression rate, mold charging chamber volume. The particles are not uniform, the formability is not good, the hardening is not uniform, and it is not suitable to use the capacity method of feeding.