Commonly used thermosetting plastics are phenolic, amino (melamine, urea-formaldehyde) polyester, polydipropylene phthalate, etc. Mainly used in compression molding, extrusion molding, injection molding. Plastics such as silicone and epoxy resin are currently mainly used for low-pressure extrusion packaging of electronic components and casting molding.

 

After plastic part is taken out of mold and cooled to room temperature, property of dimensional shrinkage is called shrinkage. Since shrinkage is not only thermal expansion and contraction of resin itself, but also related to various forming factors, shrinkage of plastic parts after forming should be called forming shrinkage.
1. Form of forming shrinkage. Forming shrinkage is mainly manifested in following aspects:
(1) Shrinkage of line size of plastic part is due to thermal expansion and contraction, elastic recovery and plastic deformation when plastic part is demolded, which causes size of plastic part to shrink after it is demoulded and cooled to room temperature. Therefore, compensation must be considered in cavity design.
(2) Shrinkage Direction. Molecules are arranged according to direction during forming, so that plastic part presents anisotropy, shrinkage is large and strength is high along direction of material flow (ie, parallel direction), and shrinkage is small in direction perpendicular to material flow (ie, perpendicular direction) , low strength. In addition, due to uneven distribution of density and fillers in various parts of plastic part during molding, shrinkage is also uneven. Difference in shrinkage makes plastic parts prone to warping, deformation, and cracks, especially in extrusion and injection molding, where directionality is more obvious. Therefore, shrinkage direction should be considered when designing mold, and shrinkage rate should be selected according to shape of plastic part and the flow direction.
(3) When post-shrinkage plastic parts are formed, due to influence of forming pressure, shear stress, anisotropy, uneven density, uneven filler distribution, uneven mold temperature, uneven hardening, plastic deformation and other factors, a series of stresses are caused, which cannot all disappear in viscous flow state, so there are residual stresses in plastic parts when they are formed under stress. After demoulding, due to influence of stress tending to balance and storage conditions, residual stress changes and reshrinkage of plastic part is called post-shrinkage. Generally, plastic parts change the most within 10 hours after demoulding, and are basically finalized after 24 hours, but it takes 30-60 days for final stability. Generally, shrinkage of thermoplastics is larger than that of thermosetting, that of extrusion molding and injection molding is larger than that of compression molding.
(4) Post-processing shrinkage. Sometimes plastic parts need to be heat-treated after forming according to performance and process requirements, and size of plastic parts will also change after treatment. Therefore, post-shrinkage and post-processing shrinkage errors should be considered and compensated for high-precision plastic parts during mold design.
2. Calculation of shrinkage. Shrinkage of plastic parts can be expressed by shrinkage, as shown in formula (1-1) and formula (1-2).
(1-1) Q = (a-b)/b×100
(1-2) Q gauge = (c-b)/b×100
In formula:
Q real - actual shrinkage rate (%)
Q meter - calculate shrinkage rate (%)
a—unidirectional size of plastic parts at forming temperature (mm)
b —one-way size of plastic part at room temperature (mm)
c — Dimensions in one direction at room temperature (mm)
Actual shrinkage rate indicates actual shrinkage of plastic part, because difference between value and calculated shrinkage is very small, so Q meter is used as design parameter to calculate cavity and core size during mold design.
3. Factors affecting change of shrinkage rate. In actual molding, not only shrinkage rates of different types of plastics are different, but also shrinkage values of different batches of same type of plastic or different parts of same plastic part are often different. Main factors affecting change of shrinkage rate are as follows.
(1) Plastic Varieties. Various plastics have their own shrinkage ranges, same type of plastics has different shrinkage rates and anisotropy due to different fillers, molecular weights, and proportions.
(2) Characteristics of plastic parts. Shape, size, wall thickness, presence or absence of inserts, number and layout of inserts of plastic parts also have a great influence on shrinkage rate.
(3) Mold structure. Parting surface and direction of pressure of mold, form, layout and size of gating system also have a great influence on shrinkage rate and directionality, especially in extrusion and injection molding.
(4) Forming process. Extrusion molding and injection molding processes generally have a large shrinkage rate and obvious directionality. Preheating conditions, forming temperature, forming pressure, holding time, filler form and hardening uniformity all have an impact on shrinkage and directionality.
As mentioned above, mold design should be based on shrinkage range provided in instructions of various plastics, and according to shape, size, wall thickness, presence or absence of inserts, parting surface and pressure forming direction, mold structure, size and position of feed port, and forming process, shrinkage rate value is selected comprehensively. For extrusion or injection molding, it is often necessary to select different shrinkage rates according to shape, size, and wall thickness of each part of plastic part.
In addition, molding shrinkage is also affected by various molding factors, but it is mainly determined by type of plastic, shape and size of plastic part. Therefore, adjusting various forming conditions during forming can also appropriately change shrinkage of plastic parts.

Ability of a plastic to fill a cavity at a certain temperature and pressure is called fluidity. This is an important process parameter that must be considered in mold design. High fluidity can easily lead to excessive flash, insufficient filling cavity, loose structure of plastic parts, separate accumulation of resin and filler, easy sticking to mold, difficulty in demoulding and cleaning, and premature hardening. However, if fluidity is small, filling is insufficient, it is not easy to form, and forming pressure is high.
Therefore, fluidity of selected plastic must be compatible with requirements of plastic part, forming process and forming conditions. When designing mold, gating system, parting surface and feeding direction should be considered according to flow performance. Thermoset fluidity is usually expressed in Lasig flow (in millimeters). The larger value, the better fluidity. Each type of plastic is usually divided into three different levels of fluidity for different plastic parts and forming processes.
Generally, when area of plastic parts is large, there are many inserts, core and inserts are weak, complex shape with narrow deep grooves and thin walls is not good for filling, plastics with better fluidity should be used. Plastics with a Rasig fluidity of 150mm or more should be used for extrusion molding, and plastics with a Rasig fluidity of 200mm or more should be used for injection molding.
In order to ensure that each batch of plastics has same fluidity, method of combining batches is often used to adjust in practice, that is, same type of plastics with different fluidity are used together, so that fluidity of each batch of plastics can compensate each other to ensure quality of plastic parts. Lasig fluidity values of commonly used plastics are shown in Table 1-1, but it must be pointed out that injection fluidity of plastics is not only determined by type of plastic, but also often affected by various factors when filling cavity, which changes ability of plastic to actually fill cavity.
Such as fine and uniform particle size (especially round pellets), high humidity, high moisture content and volatile matter, proper preheating and forming conditions, good surface finish of mold, and proper mold structure are all conducive to improving fluidity. Conversely, poor preheating or forming conditions, poor mold structure, large flow resistance, or long plastic storage period, overdue period, high storage temperature (especially for amino plastics), etc. will lead to decline of actual flow performance of plastic filling cavity and cause poor filling.

Specific volume is volume occupied by each gram of plastic (in centimeters per gram). Compression rate is ratio of volume or specific volume of plastic powder to plastic part (value is always greater than 1). They can both be used to determine size of compression mold charge chamber. A large value means that volume of charging chamber must be large, and at the same time it means that plastic powder is filled with a lot of air, exhaust is difficult, molding cycle is long, and productivity is low. Opposite is true if specific volume is small, it is beneficial to ingot pressing and pressing. Specific volume of various plastics is shown in Table 1-1. However, specific volume value often has errors due to particle size of plastic and unevenness of particles.

 

Thermosetting plastics transform into a plastic viscous flow state under heating and pressure during forming process, then fluidity increases to fill cavity, and at the same time condensation reaction occurs, crosslinking density continues to increase, fluidity decreases rapidly, melt gradually solidifies . When designing mold, for materials with fast hardening speed and short flow state, care should be taken to facilitate loading, loading and unloading inserts, selection of reasonable forming conditions and operations to avoid premature hardening or insufficient hardening, resulting in poor molding of plastic parts.
Hardening speed can generally be analyzed from holding time, which is related to plastic type, wall thickness, shape of plastic part, and mold temperature. But it is also affected by other factors, especially related to preheating state. Proper preheating should maintain condition that plastic can exert its maximum fluidity, and increase its hardening speed as much as possible. Generally, preheating temperature is high and time is long (within allowable range), hardening speed will be accelerated, especially if pre-compacted billet is preheated by high frequency, hardening speed will be significantly accelerated. In addition, high forming temperature and long pressing time will increase hardening speed. Therefore, hardening speed can also be properly controlled by adjusting preheating or forming conditions.
Hardening speed should also meet requirements of forming method. For example, injection and extrusion molding should require slow chemical reaction and slow hardening during plasticization and filling, should maintain a flow state for a long time, but should harden quickly under high temperature and high pressure after filling cavity.

All kinds of plastics contain different levels of moisture and volatile matter content. When there is too much, fluidity increases, easy to overflow, long retention time, increased shrinkage, prone to ripples, warping and other disadvantages, which affect mechanical and electrical properties of plastic parts. However, when plastic is too dry, it will also lead to poor fluidity and difficulty in forming. Therefore, different plastics should be preheated and dried as required. For materials with strong hygroscopicity, especially in wet seasons, even preheated materials should prevent re-absorption.
Since various plastics contain moisture and volatiles of different components, and condensed moisture will occur during condensation reaction, these components need to be turned into gas and discharged out of mold during molding. Some gases have a corrosive effect on mold and are irritating to human body. For this reason, when designing mold, we should understand characteristics of various plastics, and take corresponding measures, such as preheating, chrome plating of mold, opening exhaust grooves or setting an exhaust process during forming.

In mold design, it is necessary to master forming characteristics of plastic used and process characteristics during forming.
Forming characteristics of various plastics are not only related to types of plastics, but also related to types of fillers contained, particle size and particle uniformity. Fine material has good fluidity, but it is not easy to preheat evenly, it is difficult to discharge if it is filled with air, heat transfer is poor, and forming time is long.
Coarse plastic parts are dull and prone to uneven surfaces. Too thick and too fine will directly affect specific volume and compressibility, and volume of mold feeding chamber. If particles are uneven, formability will be poor and hardening will be uneven. At the same time, it is not suitable to use volumetric method for feeding.