Plastic can be pure resin or a mixture with various additives. Resin acts as a binder. Purpose of added additives is to improve physical and mechanical properties of pure resin, improve processing performance or save resin.

 

Therefore, the most basic physical and chemical properties of plastics are determined by properties of resins. Resins can be divided into natural resins and artificial resins, the latter of which are also called synthetic resins.
Resins are all polymers, which have unique molecular internal structures and molecular external structures. Internal structure of polymers determines the most basic physical and chemical properties of polymers; while external structure of polymers determines processing performance and physical and mechanical properties of polymers.
Polymers can be divided into non-crystalline (amorphous), semi-crystalline and crystalline types according to structural morphology between chains after solidification. Therefore, plastics can also be divided into amorphous and crystalline types.
When crystalline plastics solidify, there is a process of generation from crystal nuclei to crystal grains, forming a certain body state. For example, PE, PP, PA, POM, etc. are all crystalline.
When amorphous plastics solidify, there is no growth process of crystal nuclei and grains, but only free macromolecular chains are "frozen", such as PS, PVC, PMMA, PC, etc.
According to response of plastics to heat, they can be divided into two categories: thermoplastic plastics and thermosetting plastics: Characteristic of thermoplastic plastics is that they can soften when heated and return to solid state when cooled. This reversible process can be repeated many times. For example: PS, PVC, PA, PP, POM, etc.; while characteristic of thermosetting plastics is that they can be transformed into plastic melt at a certain temperature, but if temperature is continued to increase and heating time is prolonged, cross-linking will occur inside polymer and solidify. It can no longer be softened to its original state by heating, and cannot be repeatedly processed. For example: epoxy, furan, amino, phenolic, etc.
Commonly used plastics
(1) Polyolefins. Polyolefins are general term for olefin polymers, generally referring to homopolymers and copolymers of ethylene, propylene, and butene.
Main varieties are: low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), ultra-high molecular weight polyethylene (UHMPE), chlorinated polyethylene (CPE), ethylene-propylene copolymer, ethylene-vinyl acetate copolymer (EVA), polypropylene (PP), chlorinated polypropylene (PPC), reinforced polypropylene (RPP), polybutene (PB), etc.
(2) Vinyl chloride (PVC) Polyvinyl chloride for injection molding is a suspension polymerization product, its particle shape has compact type and loose type.
Modified varieties of polyvinyl chloride include: chlorinated polyvinyl chloride (CPVC), vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer (PVDC), vinyl chloride-ethylene-propylene rubber graft copolymer, cold-resistant PVC is a copolymer of vinyl chloride and maleic anhydride.
There are two types of PVC for injection molding: one is wet mixing granulation, that is, various additives are added. Stabilizer, processing aid, lubricant, impact modifier, composite stabilizer, etc. are mixed and extruded into granules. The other is dry mixed material without granulation of powdered polyvinyl chloride.
Styrene resin. Styrene resin refers to general term for homopolymer and copolymer resins of styrene. In recent years, in order to improve its shortcomings of brittleness and low heat resistance temperature, a series of modified varieties have been developed by blending and grafting with rubber, etc.
For example, binary copolymers with acrylonitrile, butadiene, a-methylstyrene, methyl methacrylate, maleic anhydride, etc. can improve heat resistance and brittleness; ABS, a copolymer with acrylonitrile butadiene, is an engineering plastic with good impact toughness and processing performance.
At present, styrene plastics include general grade, foaming grade, impact grade and AS, ABS, etc. AS has general grade AS (I) and heat-resistant grade AS (II).
(4) Acrylics. Acrylic plastics usually include polymethyl methacrylate (PMMA), commonly known as plexiglass, and fiber polymer acrylonitrile. These are all polymers derived from acrylic acid.
PMMA for injection molding is made by suspension polymerization and is available in general-purpose, heat-resistant and high-flow grades.
(5) Amide resin. Amide resin, also known as nylon (PA), is one of the earliest engineering plastics. When used as fiber, it is called nylon. China has PA6, PA610, PA612, PA66, PA1010, high-carbon nylon, super-tough PA blended with elastic grafting, and aromatic polyamide, etc.
(6) Linear polyesters. Resins containing lipid chains or ether chains in polymer chain segments but without branching and cross-linking structures are collectively referred to as linear polyesters or linear polyethers. Domestic products include: polycarbonate bisphenol A type (PC), modified polycarbonate, polyethylene terephthalate (polyester, PET), polybutylene terephthalate (PBT), polyaromatic resin (bisphenol A type), polyoxymethylene (POM), etc.

 

PC is an amorphous thermoplastic polymer. Although pure PC has good comprehensive properties, it is prone to stress cracking, poor wear resistance and fluidity. Currently, PE, ABS, PS, and PMMA are mostly used to blend with it to overcome above defects.
Most of PET is used as fiber and a small part is used for film, while glass fiber reinforced (FRPET) is mostly used for injection molding. PBT and PET are both crystalline thermoplastic linear polyesters.
Polyarylate (bisphenol A type), which is an amorphous engineering plastic similar to PC, has two types of polyoxymethylene (POM) homopolymer and copolymer, both of which are crystalline polymers. Homopolymer has poorer thermal stability than copolymer POM and has a narrower processing temperature range.
In addition, there is oil-containing POM, which is a copolymer of liquid lubricating oil and stearate surfactants added to POM. Oil-containing POM has a small friction coefficient and material is not easy to transport, so it is often produced by an injection molding machine with a slotted barrel.
(7) Fluoroplastics. Fluoroplastics include: polytetraethylene (PTFE), polytetraethylene and hexafluoropropylene copolymer (FEP), trifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), etc.
From perspective of molecular structure, main difference between PCTFE and PTFE is presence of chlorine atoms, which destroys symmetry of PTFE, reduces stacking of macromolecular chains, and increases its flexibility. PCTFE is sensitive to heat and easily decomposes at high temperatures. Polyvinylidene fluoride (PVDF) is a white powdery, crystalline thermoplastic resin.
(8) Cellulose plastics. Cellulose plastics refer to cellulose resins produced by reaction of natural cellulose with inorganic or organic acids and addition of plasticizers. Cellulose is the oldest semi-synthetic thermoplastic plastic. Commonly used ones include cellulose nitrate, cellulose acetate, and cellulose acetate butyrate. Cellulose acetate is mainly used for injection molding.
(9) High temperature resistant resins. These include polysulfone, polyarylsulfone, polyphenylene ether sulfone, polyphenylene sulfide, polyphenylene ether, and polyimide. Since these polymers contain arylene or heterocyclic structures in main chain of molecule, they are resistant to high temperature and radiation, have high strength and dimensional stability.
Polysulfone (PSF), bisphenol A polysulfone is a linear thermoplastic polymer. Although it has a regular structure, it is still an amorphous structure. Viscosity of polysulfone is relatively large and its dependence on temperature is greater than that on shear rate. This is opposite of polyethylene and similar to polycarbonate. In injection molding, when shear rate is low, effect of temperature on its expansion effect is not obvious.
Polyphenylene ether sulfone (PES) does not contain aliphatic groups in its molecular structure, so it has good heat resistance and oxidation resistance. It can be used for a long time in the range of 180~200 degrees, and melting temperature is 50~350 degrees.
Polyphenylene ether (PPO), PPO is different from many other thermoplastics; rheological properties of melt are close to Newtonian fluids, and viscosity is not significantly dependent on shear rate. Modified polyphenylene ether and chlorinated polyether are also used for injection molding.
Polyphenylene sulfide (PPS, Lei Teng) is a new type of engineering plastic. It has excellent comprehensive properties and is currently the best material for journals and bearings. PSS raw powder has great fluidity after melting, and it is difficult to process directly. Therefore, it must be pre-treated by cross-linking to improve fluidity. There are two types of injection molding: powder and granular. Injection molding of PSS is very similar to that of HDPE. Difference is that PSS requires a higher molding temperature: at 343 degrees, its fluidity is equivalent to that of HDPE.