Thermoplastics refer to plastics that have characteristics of heating to soften and cooling to harden. Most of plastics used in our daily lives fall into this category. It becomes soft and flows when heated, and becomes hard when cooled. This process is reversible and can be repeated.
Thermoplastic ≠ Thermoset plastic

 

It becomes soft and flows when heated, and becomes hard when cooled. This process is reversible and can be repeated.
Polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyoxymethylene, polycarbonate, polyamide, acrylic plastics, other polyolefins and their copolymers, polyiron, polyphenylene ether, chlorinated polyether, etc. are all thermoplastics.
Resin molecular chains in thermoplastics are all linear or branched, and there is no chemical bond between molecular chains, which softens and flows when heated. Process of cooling and hardening is a physical change.

 

It can soften and flow when it is heated for the first time. When heated to a certain temperature, a chemical reaction occurs, cross-linking is solidified and hardened. This change is irreversible. After that, when heated again, it can no longer become soft and flow.
It is precisely with this characteristic that molding process is performed, and plasticizing flow during the first heating is used to fill cavity under pressure, then solidify into a product of a certain shape and size. This material is called thermoset plastic.
Resin of thermosetting plastics is linear or branched before curing. After curing, chemical bonds are formed between molecular chains and become a three-degree network structure. Not only can it not be melted again, but it cannot be dissolved in solvents.
Phenolic, aldehyde, melamine formaldehyde, epoxy, unsaturated polyester, silicone and other plastics are all thermosetting plastics.
Mainly used for heat insulation, wear resistance, insulation, high-voltage resistance and other plastics used in harsh environments, most of which are thermosetting plastics. The most commonly used ones should be wok handles, high and low voltage electrical appliances.

1. Plastic types: In molding process of thermoplastics, due to volume change caused by crystallization, strong internal stress, large residual stress frozen in plastic part, strong molecular orientation and other factors, compared with thermosetting plastics, shrinkage rate is larger, shrinkage rate range is wide, and directionality is obvious. In addition, shrinkage after molding, annealing or humidity conditioning is generally greater than that of thermosetting plastics.
2. Characteristics of plastic part. When molten material is in contact with surface of cavity, outer layer is immediately cooled to form a low-density solid shell. Due to poor thermal conductivity of plastic, inner layer of plastic part is slowly cooled to form a high-density solid layer with large shrinkage. Therefore, wall thickness, slow cooling, and high-density layer thickness will shrink more. In addition, presence or absence of inserts and layout and quantity of inserts directly affect direction of material flow, density distribution and shrinkage resistance. Therefore, characteristics of plastic parts have a greater impact on shrinkage and directionality.
3. Factors such as form, size, and distribution of feed inlet directly affect direction of material flow, density distribution, pressure maintaining and shrinking effect and molding time. Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have less shrinkage but greater directivity, shorter feed ports with shorter width and length have less directivity. The ones that are close to feed inlet or parallel to direction of material flow will shrink more.
4. Molding conditions. Mold temperature is high, molten material cools slowly, density is high, and shrinkage is large. Especially for crystalline material, shrinkage is greater due to high crystallinity and large volume changes. Mold temperature distribution is also related to internal and external cooling and density uniformity of plastic part, which directly affects size and directionality of shrinkage of each part. In addition, holding pressure and time also have a greater impact on contraction, contraction is smaller but directionality is larger when pressure is high and time is long.
Injection pressure is high, viscosity difference of molten material is small, interlayer shear stress is small, and elastic rebound after demolding is large, so shrinkage can also be reduced by an appropriate amount. Material temperature is high, shrinkage is large, but directionality is small. Therefore, adjusting mold temperature, pressure, injection speed and cooling time during molding can also appropriately change shrinkage of plastic part.
When designing mold, according to shrinkage range of various plastics, wall thickness and shape of plastic part, size and distribution of inlet form, shrinkage rate of each part of plastic part is determined according to experience, and then cavity size is calculated.