There are many varieties of thermoplastics, even same variety has different use and process characteristics due to different ratios of resin molecules and additives. In addition, in order to change characteristics of original varieties, various chemical methods such as copolymerization and cross-linking are commonly used to introduce a certain percentage of other monomers or polymers into original resin structure, so as to change structure of original resin and become a modified product with new improved physical properties and processability.
For example, ABS is a modified copolymer after second and third monomers such as acrylonitrile and butadiene are introduced into polystyrene molecule, which can be regarded as modified polystyrene, which has better comprehensive performance and process characteristics than polystyrene.

 

Due to variety and complex properties of thermoplastics, even same type of plastics are only used for injection molding and extrusion. Therefore, this article mainly introduces various thermoplastics for injection molding.

Form and calculation of thermoplastic molding shrinkage. As mentioned above, factors that affect thermoplastic molding shrinkage are as follows:
1. Plastic varieties. In thermoplastic molding process, due to volume change caused by crystallization, internal stress is strong, residual stress frozen in plastic part is large, and molecular orientation is strong. 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 larger than that of thermosetting plastics.
2. When plastic part is formed, molten material contacts surface of cavity and outer layer immediately cools 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 greatly. In addition, presence or absence of inserts, layout and quantity of inserts directly affect flow direction, density distribution and shrinkage resistance, so characteristics of plastic parts have a greater impact on shrinkage size and directionality.
3. Factors such as form, size and distribution of feeding port directly affect direction of material flow, density distribution, pressure-holding, feeding effect and molding time. Direct feeding port and feeding port with large cross-section (especially thicker cross-section) have small shrinkage but large directionality. Close to feed port or parallel to direction of material flow, shrinkage is large.
4. Molding conditions. Mold temperature is high, molten material cools slowly, density is high, and shrinkage is large, especially for crystalline material, shrinkage is larger due to high crystallinity and large volume change. Mold temperature distribution is also related to cooling and density uniformity inside and outside plastic part, which directly affects shrinkage and direction of each part.
In addition, holding pressure and time also have a great influence on contraction, contraction is small but direction is large 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 appropriately reduced, material temperature is high, shrinkage is large, but directionality is small. Therefore, adjusting mold temperature, pressure, injection speed and cooling time and other factors during molding can also appropriately change shrinkage of plastic parts.
When designing mold, according to shrinkage range of various plastics, wall thickness and shape of plastic part, form size and distribution of feeding port, shrinkage rate of each part of plastic part is determined according to experience, then cavity size is calculated. For high-precision plastic parts and when it is difficult to grasp shrinkage rate, following methods should be used to design mold:
①Outer diameter of plastic parts should be taken as a smaller shrinkage rate, and inner diameter should be taken as a larger shrinkage rate, so as to leave room for correction after mold trial.
②Mold test determines form, size and molding conditions of gating system.
③Plastic parts to be post-processed are post-processed to determine dimensional change (measurement must be done after 24 hours after demoulding).
④ Correct mold according to actual shrinkage.
⑤ Retry mold and change process conditions to slightly correct shrinkage value to meet requirements of plastic parts.

1. Fluidity of thermoplastics can generally be analyzed from a series of indices such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (process length/plastic wall thickness). Small molecular weight is small, molecular weight distribution is wide, molecular structure regularity is poor, melt index is high, spiral flow length is long, apparent viscosity is small, and fluidity is good if flow ratio is large. According to mold design requirements, fluidity of commonly used plastics can be roughly divided into three categories:
①Good fluidity Nylon, polyethylene, polystyrene, polypropylene, cellulose acetate, poly(4) methyl pentylene;
②Fluidity is medium polystyrene series resin (such as ABS, AS), plexiglass, polyoxymethylene, polyphenylene ether;
③Poor fluidity Polycarbonate, rigid PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastic.
2. Fluidity of various plastics also changes due to various molding factors. Main influencing factors are as follows:
①The higher temperature, the higher fluidity of material, but different plastics are also different. Polystyrene (especially impact-resistant and high MFR value), polypropylene, nylon, plexiglass, modified polystyrene (such as The fluidity of plastics such as ABS, AS), polycarbonate, and cellulose acetate varies greatly with temperature. For polyethylene and polyoxymethylene, temperature increase or decrease has little effect on its fluidity. Therefore, the former should adjust temperature to control fluidity during molding.
②When injection pressure increases, molten material will be greatly sheared and fluidity will also increase, especially polyethylene and polyoxymethylene are more sensitive, so injection pressure should be adjusted to control fluidity during molding.
③Form, size, layout, cooling system design, flow resistance of molten material (such as surface finish, thickness of forehearth section, cavity shape, exhaust system) and other factors directly affect flow of molten material in cavity. Actual fluidity in interior, if temperature of molten material is reduced and fluidity resistance is increased, fluidity will decrease.​​
When designing mold, a reasonable structure should be selected according to fluidity of plastic used. During molding, material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled to properly adjust filling situation to meet molding needs.

Thermoplastics can be divided into two categories: crystalline plastics and non-crystalline (also known as amorphous) plastics according to fact that there is no crystallization during condensation.
So-called crystallization phenomenon is a phenomenon that when plastic changes from molten state to condensation, molecules move independently, plastic is completely disordered from molten state to condensing state, molecules stop free movement, according to a slightly fixed position, and there is a tendency for molecular arrangement to become a normal model.
As standard for judging appearance of these two types of plastics, it depends on transparency of thick-walled plastic parts. Generally, crystalline materials are opaque or translucent (such as polyoxymethylene, etc.), and amorphous materials are transparent (such as plexiglass, etc.) . But there are exceptions, such as poly (4) methyl pentylene is a crystalline plastic but has high transparency, ABS is an amorphous material but not transparent.
When designing a mold and selecting an injection molding machine, following requirements and precautions for crystalline plastics should be noted:
①Heat required for material temperature to rise to molding temperature is large, and equipment with large plasticizing capacity should be used.
②Heat released during cooling is large, so it should be fully cooled.
③Specific gravity difference between molten state and solid state is large, molding shrinkage is large, shrinkage holes and pores are prone to occur.
④Fast cooling, low crystallinity, small shrinkage and high transparency. Degree of crystallinity is related to wall thickness of plastic parts. Wall thickness is slow to cool, crystallinity is high, shrinkage is large, and physical properties are good. Therefore, crystalline material should control mold temperature as required.
⑤ Significant anisotropy and large internal stress. After demolding, uncrystallized molecules tend to continue to crystallize and are in a state of energy imbalance, which is prone to deformation and warpage.
⑥Crystallization temperature range is narrow, and it is easy to inject unmelted powder into mold or block feeding port.

Heat sensitivity means that some plastics are more sensitive to heat, when heating time is long at high temperature or cross-section of feed port is too small, shearing action is large, material temperature is increased, it is prone to discoloration, degradation, and decomposition. Plastics with this characteristic are called heat-sensitive plastics.
Such as rigid polyvinyl chloride, polyvinylidene chloride, vinyl acetate copolymer, polyoxymethylene, polychlorotrifluoroethylene, etc. When heat-sensitive plastics are decomposed, by-products such as monomers, gases, and solids are produced. In particular, some decomposed gases are irritating, corrosive or toxic to human body, equipment and molds.
Therefore, attention should be paid to mold design, selection of injection molding machines and molding. Screw injection molding machines should be selected. Section of gating system should be large. Molds and barrels should be chrome-plated, and there should be no dead corners. Stabilizer, weakening its heat-sensitive properties.
Even if some plastics (such as polycarbonate) contain a small amount of water, they will decompose under high temperature and high pressure. This property is called easy hydrolysis, which must be heated and dried in advance.

Some plastics are sensitive to stress, are prone to internal stress during molding, are brittle and easy to crack. Plastic parts will crack under action of external force or solvent. To this end, in addition to adding additives to raw materials to improve crack resistance, attention should be paid to drying raw materials, molding conditions should be reasonably selected to reduce internal stress and increase crack resistance. A reasonable shape of plastic parts should be selected, and measures such as inserts should not be set to minimize stress concentration.
When designing mold, demolding slope should be increased, a reasonable feeding port and ejector mechanism should be selected. During molding, material temperature, mold temperature, injection pressure and cooling time should be properly adjusted to avoid demoulding when plastic parts are too cold and brittle. After molding, plastic parts should also be post-treated to improve crack resistance, eliminate internal stress and prohibit contact with solvents.
When polymer melt with a certain melt flow rate passes through nozzle hole at a constant temperature and its flow rate exceeds a certain value, obvious transverse cracks on melt surface are called melt fracture, which damages appearance and physical properties of plastic parts. Therefore, when choosing a polymer with a high melt flow rate, cross-section of nozzle, runner, and feed port should be increased, injection speed should be reduced, and material temperature should be increased.

Various plastics have different thermal properties such as specific heat, thermal conductivity, and thermal deformation temperature. When plasticizing with a high specific heat, a large amount of heat is required, an injection molding machine with a large plasticizing capacity should be selected. Cooling time of plastic with high heat distortion temperature can be short and demoulding is early, but cooling deformation should be prevented after demoulding. Plastics with low thermal conductivity have a slow cooling rate (such as ionomers, etc.), so they must be fully cooled to enhance cooling effect of mold.
Hot runner mold is suitable for plastics with low specific heat and high thermal conductivity. Plastics with large specific heat, low thermal conductivity, low thermal deformation temperature and slow cooling rate are not conducive to high-speed molding, appropriate injection molding machines must be selected and mold cooling must be strengthened.
All kinds of plastics are required to maintain an appropriate cooling rate according to their types, characteristics and shape of plastic parts. Therefore, mold must be equipped with a heating and cooling system according to molding requirements to maintain a certain mold temperature. When material temperature increases mold temperature, it should be cooled to prevent plastic parts from deforming after demoulding, shorten molding cycle, and reduce crystallinity.
When plastic waste heat is not enough to keep mold at a certain temperature, mold should be equipped with a heating system to keep mold at a certain temperature to control cooling rate, ensure fluidity, improve filling conditions or control plastic parts to cool slowly, prevent uneven cooling inside and outside of thick-walled plastic parts, improve crystallinity.
For those with good fluidity, large molding area and uneven material temperature, according to molding conditions of plastic parts, sometimes heating or cooling is needed alternately, or local heating and cooling are used together. For this purpose, mold should be equipped with a corresponding cooling or heating system.

There are various additives in plastics, which make them have different degrees of affinity for water. Therefore, plastics can be roughly divided into two types: hygroscopic, adhering to water, non-absorbent and not easy to adhere to water. Water content in material must be controlled within allowable range. Otherwise, under high temperature and high pressure, water will become gas or hydrolysis will occur, which will cause resin foaming, decreased fluidity, poor appearance and mechanical properties. Therefore, hygroscopic plastics must be preheated by appropriate heating methods and specifications according to requirements, and also need to be irradiated with infrared rays during use to prevent re-hygroscopicity.