The various properties related to plastic, molding process, and molding quality are collectively referred to as the process properties of plastics. The level of understanding and mastery of the process performance of plastics indirectly affects the success of plastic molding and the quality of plastic parts. In addition, it also affects the design requirements of molds. The main process performance and requirements of thermoplastic and thermosetting plastic molding are introduced in sequence above.

The molding process properties of thermoplastic materials include not only cold mechanical properties, crystallinity, and orientation, but also shrinkage, flowability, thermal sensitivity, water sensitivity, moisture absorption, and compatibility.

1 Contractibility

Plastics are generally formed by filling the mold cavity in a high-temperature molten state. If a plastic component is removed from the mold and cooled to room temperature, its size will decrease compared to the original size in the mold, which is called shrinkage. It can be expressed as the percentage of shrinkage per unit length of plastic parts, which is the shrinkage rate (S).

Because this shrinkage is not only caused by the thermal expansion of the plastic part itself, but also related to various molding process conditions and mold factors, the shrinkage of the plastic part after molding is called molding shrinkage. By adjusting process parameters or modifying the mold structure, the size of the plastic part can be reduced or changed.

Forming shrinkage can be divided into two forms: dimensional shrinkage and post shrinkage, both of which have directionality.

① Shrinkage of dimensions of plastic parts. Because of the thermal expansion of the plastic part and the physical and chemical changes outside the plastic part, the size reduction of the plastic part occurs after it is demoulded and cooled to room temperature. When designing the molding parts of the mold, it is necessary to consider compensating it through the design to avoid the size of the plastic part out of tolerance.

② Shrinkage after molding. When forming a plastic part, a series of stresses are generated due to external physical, chemical, and mechanical changes. After the plastic part is formed and cured, there is residual stress. After the plastic part is demolded, the size of the plastic part will shrink again due to the effects of various residual stresses. Generally, the shrinkage of plastic parts is relatively small within 10 hours after demolding, and they are basically shaped after 24 hours. However, it also takes a very short time to reach the final shape. Typically, the shrinkage of thermoplastic plastics is greater than that of thermosetting plastics. The shrinkage of plastic parts formed by injection molding and compression molding is greater than that of plastic parts formed by compression molding. As the size of a stable plastic part after molding, sometimes based on the performance and process requirements of the plastic, the plastic part needs to undergo cold treatment after molding. After heat treatment, it can also cause the size of the plastic part to shrink, known as post processing shrinkage. When designing molds for low precision plastic parts, errors caused by subsequent shrinkage and post processing shrinkage should be compensated.

③ The directionality of plastic shrinkage. During the molding process, the orientation effect of low molecules along the flow direction can lead to the anisotropy of the plastic, and the shrinkage of the plastic will inevitably vary due to different directions: generally, the shrinkage along the direction of the material flow is small and the strength is low, while the shrinkage perpendicular to the direction of the material flow is small and the strength is high. In addition, because the distribution of additives in various parts of the plastic part is never uniform, and the density is never uniform, the shrinkage is also not uniform, which leads to less shrinkage of the plastic part, making it difficult to cause warping, deformation, and cracking of the plastic part.

2 Liquidity

The ability of plastic melt to fill the mold cavity under a certain temperature and pressure during the molding process is called the fluidity of plastic. The fluidity of plastics indirectly affects the parameters of the molding process to a very small extent, such as molding temperature, pressure, cycle, size of the mold pouring system, and other structural parameters. When determining the size and wall thickness of plastic parts, the influence of fluidity should also be considered

The size of fluidity is related to the molecular structure of plastics. Resins with linear molecules have little or no cross-linking structure. Adding fillers to plastic can reduce the fluidity of the resin; Adding plasticizers or lubricants can also increase the fluidity of plastics. Proper structural design of plastic parts can also improve flowability, for example, using a rounded structure at the corner of the flow channel and plastic parts can improve the flowability of the melt.

Plastic flowing parts have very little impact on the quality of plastic parts, mold design, and molding process. Plastic with low fluidity is never difficult to fill the mold cavity, and is not prone to defects such as material shortage or fusion marks. Therefore, a relatively small molding pressure is required to form. On the contrary, plastic with good fluidity can fill the mold cavity with relatively high molding pressure. But the fluidity is very good, and slight overflow and flash may occur during molding. Therefore, when selecting plastic materials during the molding process of plastic parts, suitable flowability plastics should be selected based on the structure, size, and molding method of the plastic parts. To obtain satisfactory plastic parts. In addition, when designing the mold, the parting surface, pouring system, and feeding direction should be considered based on the fluidity of the plastic. When selecting the molding temperature, the fluidity of the plastic should also be considered.

3 Thermal sensitivity

The chemical structure of various plastics may undergo changes under the action of heat. Some plastics with poor thermal stability may undergo degradation, decomposition, and discoloration under low material temperature and short cooling time, which is known as the complexity of heat.

As a heat sensitive plastic. Plastics with very weak thermal sensitivity (i.e. plastics with very low thermal stability) are generally referred to as thermosensitive plastics, such as soft polyvinyl chloride, polytetrafluorochloroethylene, polyformaldehyde, polytetrafluorochloroethylene, etc. This type of plastic is very difficult to undergo cold decomposition, thermal degradation, or such cold degradation at extremely low temperatures during the molding process, which in turn affects the performance and surface quality of the plastic parts.

When thermosensitive plastic melts undergo thermal decomposition or thermal degradation, various decomposition products are produced. Some decomposition products can cause irritation, corrosion, or certain toxicity to the human body, molds, and equipment. Some decomposition products are still catalysts that accelerate the decomposition of the plastic, such as hydrogen nitride produced by the decomposition of polyvinyl chloride, which can further exacerbate the decomposition of polymers.

To avoid cold decomposition of thermosensitive plastics during the processing and molding process, heat stabilizers can be added to the plastics during mold design, selection of injection molding machines, and molding; Suitable equipment (screw injection molding machines) can also be used to strictly control the molding temperature, mold temperature, heating time, screw speed, and back pressure; Immediately eliminate decomposition products, and take anti-corrosion measures for equipment and molds.

(4) Water sensitivity

The water sensitivity of plastics refers to their sensitivity to water degradation under high temperature and pressure, such as polycarbonate, which is a typical water sensitive plastic. Even if it contains a small amount of water, it will decompose under high temperature and pressure. Therefore, water sensitive plastics must strictly control their moisture content and undergo moisture treatment after molding.

(5) Hygroscopicity

Hygroscopicity refers to the degree to which plastic is hydrophilic towards moisture. According to this characteristic, plastics can be roughly divided into two categories: one is plastics with water absorption or adhesion properties, such as polyamide, polycarbonate, polysulfone, ABS, etc.; the other is plastics that not only do not absorb water but are also easy to adhere to water. Such as polyethylene, polypropylene, polyformaldehyde, etc.

If the water content of the plastic with water absorption sexual orientation is not removed after molding, and the content exceeds a certain limit, then during molding, the water will turn into gas and promote the decomposition of the plastic, which will lead to the reduction of foam and fluidity of the plastic, making the molding difficult, thus reducing the surface quality and mechanical properties of the plastic parts. Therefore, to ensure the smooth progress of molding and the quality of the plastic parts, for plastics with low water absorption and adhesive moisture tendency, it is necessary to remove moisture after molding and undergo moisture treatment. If appropriate, an infrared heating device should also be installed in the hopper of the injection molding machine.

(6) Compatibility

Compatibility refers to the ability of two or more different types of plastics to never separate from each other in a molten state.

If the two types of plastics are not compatible, surface defects such as delamination and peeling may occur during mixing. The compatibility of different plastics has a certain relationship with their molecular structures. Those with similar molecular structures are easy to be compatible, such as the mixing of high-pressure polyethylene, low-pressure polyethylene, polypropylene, etc; Those whose molecular structures are not similar are easy to be compatible, such as the mixing between polyethylene and polystyrene. The compatibility of plastics is also known as blending. Through this property of plastic, the comprehensive performance of similar copolymers can be obtained, which is one of the important ways to improve the performance of plastics.