Injection molding is a process used to make plastic parts manufacturing. To do this, you need to use a material that can melt and be injected into the mold.

There are many different types of materials that can be used in injection molding services, but some are more popular than others. In this article, we classify and explain plastic materials from different perspectives so that you can have a comprehensive understanding of plastic materials.

Commonly used plastics for injection molding

Plastic materials commonly used for manufacturing injection molding: ABS, PP, PET, PMMA, PE, PVC, PC, PS, etc. The characteristics of each plastic material are different, and the injection molding process is also different.

We need to choose different plastic materials according to the characteristics of the product, the use of the environment, and other aspects of the requirements.

These are the top six materials used throughout the industry: Polypropylene (PP) Acrylonitrile Butadiene Styrene (ABS) Polyamide (Nylon) High-Density Polyethylene (HDPE) Polycarbonate (PC) ABS + PC Blend (mostly used for electronics enclosures).

Classification of physical and chemical properties of resins

The most basic physicochemical properties of plastics are determined by the nature of the resin. Resins can be classified as natural and man-made resins, also known as synthetic resins.

Resins are all polymers, and these polymers have a unique internal molecular structure and an external molecular structure.

The internal structure of a polymer determines the most basic physicochemical properties of the polymer. And the external structure of the polymer determines the processing properties and the physical and mechanical properties of the polymer.

Classification of resin structural form

Polymers can be classified as amorphous (amorphous), semi-crystalline and crystalline according to the structural form between the chains after solidification. So there are also amorphous and crystalline plastics.

Crystalline plastics have the process of nucleation to grain generation during solidification, forming a certain body state. For example, PE, PP, PE, POM, etc. are all crystalline.

Amorphous plastics in solidification, there is no nucleus, and the grain growth process is only free macromolecular chain “freeze” such as PS, PVC, PMMA, PC, etc.

Classification of the reflection of resin to heat

According to the reflection of its plastic to heat, can be divided into thermoplastic and thermosetting plastic two categories: thermoplastic plastic is characterized by heating can be softened, cooling back to solid.

This reversible process can be repeated many times. For example PS, PVC, PE, PP, POM, etc.; while thermosetting plastics are characterized by the ability to turn into a plastic melt at a certain temperature.

But if you continue to raise the temperature, extending the heating time polymer internal will produce cross-linking and solidification. Can no longer use the heating method to make it soft to the original state, can not be repeated processing. Such as epoxy, furan, amino, phenolic, etc.

Classification of the use characteristics about various plastics

Plastics are usually classified into three types: general-purpose plastics, engineering plastics, and special plastics.

(1) General purpose plastics
Generally refers to the plastic with large production, wide use, good molding, and low price. There are five major varieties of general-purpose plastics, namely polyethylene (PE), polypropylene (PP), polyvinyl chloride (PTFE), polystyrene (PS), and ABS, of which are thermoplastics.

(2) Engineering plastics
Generally refers to the ability to withstand certain external forces, with good mechanical properties and high and low-temperature resistance, good dimensional stability, can be used as engineering structure of the plastic, such as polyamide, polysulfone, etc.

In engineering plastics are divided into two categories:

A: General engineering plastics
General engineering plastics include polyamide, polyacetal, polycarbonate, modified polyphenylene ether, thermoplastic polyester, ultra-high molecular weight polyethylene, methyl pentene polymer, vinyl alcohol copolymer, etc.

B: Special engineering plastics
Special engineering plastics have cross-linked non-cross-linked types. The cross-linked ones are polyamine-bis-maleimide, polytriazine, cross-linked polyimide, heat-resistant epoxy tree finger, etc. Non-cross-linked: polysulfone, polyethersulfone, polyphenylene sulfide, polyimide, polyether ether ketone (PEEK), etc.

(3) Special plastics
Generally refers to the special function, can be used in aviation, aerospace, and other special applications of plastics. Such as fluoroplastics and silicones have outstanding high-temperature resistance, self-lubricating and other special functions, reinforced plastics and foam with high strength, high buffering, and other special properties, these plastics belong to the category of special plastics.

A. Strong plastics:
Reinforced plastic raw materials in shape can be divided into granular (such as calcium plastic reinforced plastic), fibrous (such as glass fiber or glass cloth reinforced plastic), flake (such as mica reinforced plastic) three.

According to the material can be divided into cloth-based reinforced plastics (such as rag reinforced or asbestos-reinforced plastics), inorganic mineral-filled plastics (such as quartz or mica-filled plastics), fiber-reinforced plastics (such as carbon-fiber-reinforced plastics) three.

B. Foam:
Foam can be divided into rigid, semi-rigid, and soft foam three.

Hard foam is not flexible, compression hardness is very large, only to reach a certain stress value to produce deformation, stress relief can not be restored to its original state.

Soft foam is flexible, compression hardness is very small, easy to deform, stress relief can be restored to the original state, residual deformation is small.

The flexibility and other properties of semi-rigid foam are between the rigid and soft foam.

What are the common plastics?

(1) Polyolefin
Polyolefin is the general name of an olefin polymer, generally refers to the homopolymer and copolymer of ethylene, propylene, butylene.

The 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 (UHMWPE), and chlorinated polyethylene (CPE).

Chlorinated polyethylene (CPE); ethylene-propylene copolymer, ethylene vinyl acetate copolymer (EVA); polypropylene (PP), chlorinated polypropylene (PPC), reinforced polypropylene (RPP) Reinforced polypropylene (RPP) polybutylene (PB), etc.

The primary uses of polyethylene are plastic bags, plastic films, containers including bottles, and geomembranes.

(2) The Chloride (PVC)
Polyvinyl chloride for injection molding is a suspension polymerization product, and there are compact and sparse types according to its particle form.

Modified varieties of polyvinyl chloride are: chlorinated polyvinyl chloride (CPVC), vinyl chloride – vinyl acetate copolymer, vinyl chloride – vinyl chloride copolymer (PVDC)

Vinyl chloride – ethylene-propylene rubber graft copolymer, cold-resistant PVC that is a copolymer of vinyl chloride and maleic anhydride.

There are two types of PVC for manufacturing injection molding: one is the wet blend granulation, that is, a variety of additives. Stabilizer. processing aids. lubricants. Impact modifier. Compound stabilizers, etc. are mixed and extruded into pellets. The other is a dry blend of non-pelletized powdered PVC.

(3) Styrene-based resins
Styrene-based resins are the general term for the homopolymer and copolymer resins of styrene. In recent years, to improve its brittleness and low heat-resistant temperature shortcomings.

The use of rubber and other blends and grafting methods to develop a series of modified varieties. Such as with acrylonitrile, butadiene, a-methyl styrene, methacrylate

Maleic anhydride and other binary copolymers can improve chemical resistance and brittleness; copolymers with acrylonitrile butadiene ABS is a very good impact toughness and processing properties of engineering plastics.

ABS does not have good chemical resistance, however, and should not be used in applications requiring electrical insulation or UV resistance.

At present, styrene plastics have a general-purpose grade, foam grade, impact grade, and AS, ABS, etc. AS has general-purpose grade AS (I) and heat-resistant grade AS (II).

(4) Acrylics
Acrylic plastics usually include polymethylmethacrylate (PMMA) commonly known as plexiglass, and fiber polymer acrylonitrile. These are polymers derived from acrylic acid.

PMMA for injection molding grade made of suspension polymerization, there are general-purpose grade heat-resistant grade and high flow grade.

(5) Amide resin poly
Amide resin poly, also known as nylon (PA) is one of the early varieties of engineering plastics when used as a fiber called spandex. We have PA6, PA610, PA612, PA66, PA1010, and high carbon nylon.

PA66 and elastic graft blended super-tough PA and aromatic polyamide.

(6) Linear polyesters
In the polymer links containing lipid or ether chains, no branched chains and cross-linked structure of the resin is collectively referred to as linear polyester or linear polyether.

Domestic production is polycarbonate bisphenol A-type (PC), modified polycarbonate, polyethylene terephthalate (polyester, PET) polybutylene terephthalate (PBT) polypropylenes (bisphenol A-type), polyformaldehyde (POM), etc.

PC is an amorphous thermoplastic polymer, pure PC has good overall performance, but is easy to stress cracking, wear-resistance, and poor liquidity, the current use of PE, ABS, PS, PMMA, and its blending to overcome the above defects.

PET is mostly used as fiber and less used for film, while the glass fiber reinforced (FRPET) is mostly used for injection molding, PBT and PET are both crystalline thermoplastic linear polyesters.

Polyarylenes (Bisphenol A), which is similar to PC amorphous engineering plastics

There are two types of polyformaldehyde (POM), homopolymer and copolymer, both of which are crystalline polymers. Homopolymer than copolymer POM thermal stability is poor processing temperature range is narrow.

In addition, there are also oil-containing POM which is a copolymer of POM with liquid lubricant and stearate surfactants. Oil-containing POM has a small coefficient of friction.

Right Material is not easy to transport so commonly used slotted barrel injection molding machine for production.

(7) Fluoroplastics
Fluorine plastic varieties are polytetrafluoroethylene (PTFE), polytetrafluoroethylene, and hexafluoroacrylic acid copolymer (FEP), tetrafluoroethylene (PCTFE), polyvinylidene fluoride (PVDF) polyvinyl fluoride (PVF), etc.

The main difference between PCTFE and PTFE from the molecular structure is the presence of chlorine atoms, which breaks the symmetry of PTFE and reduces the macromolecular chain stacking, making it more flexible.

PCTFE is more sensitive to heat and easily decomposes at high temperatures. Polyvinylidene fluoride (PVDF), is a white powdered, crystalline thermoplastic resin.

(8) Cellulosic plastics
Cellulosic plastics are made from cellulose resins produced by the action of natural cellulose and inorganic or organic acids plus plasticizers. Cellulose is the oldest semi-synthetic thermoplastic.

Commonly used are cellulose nitrate cellulose acetate, cellulose acetate butyrate, and cellulose acetate for injection molding material is the main one.

(9) High-temperature resistant resins
These are polysulfone, polyaryl sulfone, polyphenylene ether sulfone, polyphenylene sulfide, polyphenylene ether, polyimide. These polymers contain arylene groups or heterocyclic structures in the main chain of the molecule, so they have high-temperature resistance, radiation resistance, and both.

They are resistant to radiation and have high impact strength and dimensional stability.

Polysulfone (PSF), bisphenol A polysulfone is a linear thermoplastic polymer that has a formal structure but remains in an amorphous structural form. Polysulfone has a higher viscosity and is more dependent on temperature than on shear rate.

This is in contrast to polyethylene, which is similar to polycarbonate. In plastic injection molding service, when the shear rate is low, the effect of temperature on the swelling effect is not significant.

Polyphenylene ether sulfone (PES), which does not contain aliphatic groups in its molecular structure, is better for heat resistance and oxygen resistance. It can be used for a long time in the range of 180~200 degrees, and the melting temperature is 50~350 degrees.

Polyphenylene ether (PPO), PPO, and many other thermoplastics are different; the rheological properties of the melt are close to those of a Newtonian fluid, and the viscosity does not depend significantly on the shear rate. Also used for injection molding are modified polyphenylene ethers and chlorinated polyethers.

Polyphenylene sulfide (PPS, Retten), a new type of engineering plastic it has excellent overall performance, is currently the best village material for journals and bearings.

Direct processing difficulties therefore must be cross-linked pretreatment to improve the fluidity. The plastic Injection molding material of PSS is very similar to HDPE, but the difference is that PSS requires a higher molding. The temperature is higher: at this 343 degrees, the flowability is equivalent to that of HDPE.

Thermoplastic Rubber (TPR), in other words elastomer, is another injection moulding material. It contains a mix of plastic parts and rubber. Used for automotive parts such as wires and cable insulation and other applications as a home appliance.

Thermoplastic Polyurethane (TPU) TPU has many different properties such as elasticity, transparency and resistances. Its traits are its soft and hard segments. The main use for this type of plastic is cases for mobile phones as well as keyboard protectors and footwear.

What are the common fillers?

Plastic can be pure resin or a mixture with various additives, the resin acts as a binder. The purpose of adding additives to the mixture is to improve the physical and mechanical properties of pure resin, improve processing performance, improve poor resistance or save resin at low cost.

Commonly used fillers for manufacturing injection molding materials are general fillers, metal fillers, organic fillers, short fiber fillers, and long fiber fillers.

Adding these filters can reduce the cost of custom injection molding products, improve economic efficiency can improve the physical and mechanical properties, chemical properties, and photoelectric properties; can improve processing performance, rheological properties, reduce viscosity, improve the role of dispersion.

General fillers are limestone, calcium carbonate, talc, calcium silicate, mica, aluminum hydroxide, calcium sulfate, agricultural by-products, etc.

Organic filler is the main filler in plastic products, there are natural materials and synthetic materials, including wood, wood flour, shellbark of beard kernel, cotton plant cellulose, etc.; synthetic materials are recycled cellulose, including artificial fabrics, polyacrylonitrile fiber, nylon fiber, polyester fiber, etc.

Some fillers added to the plastic injection molding materials need to be treated with surface modifiers, the treatment process follows the theory of interfacial chemistry, filler and polymer surface wetting theory of acid-base interaction theory, and mixing theory to give the material some excellent properties.

Currently commonly used surface modifiers are silane coupling agent, titanate coupling agent, silicone treatment agent, etc. These surface modifiers plus can further improve the filler effectiveness.