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Introduction to Injection Molding
2012-04-16

Injection molding is considered one of the most common plastic part manufacturing processes. It can be used for producing parts from both thermoplastic and thermoset polymers. The process usually begins with taking the polymers in the form of pellets or granules and heating them to the molten state. The melt is then injected/forced into a chamber formed by a split-die mold. The melt remains in the mold and is either chilled down to solidify (thermoplastics) or heated up to cure (thermosets). The mold is then opened and the part is ejected.


A Typical Injection Molding Process

In spite of the relatively expensive tooling cost, injection molding remains the most popular manufacturing process for plastic materials in mass production, thanks to its low operational cost, high throughput, and the flexibility to make parts with complex shapes.

Polymers commonly used for injection molding include:

Polystyrene (PS)

Polystyrene, also known as PS, has been mass-produced since 1930.

It is a thermoplastic polymer. Applications: Used for inexpensive packaging materials, pens, safety razors, flatware, and CD jewel boxes. In Styrofoam format, PS is used to make high-throughput, thin-walled, easy-to-mold parts where structure is not paramount.

General properties: A material not structurally strong but easy to mold, it is inexpensive, machines well, and possesses excellent transparency.

Trade Name: Novacor 555

Acrylonitrile-Butadiene-Styrene, also known as ABS, has been mass-produced since 1960's.

It is a thermoplastic.

Applications: Used in electronic housings, auto parts, consumer products, pipe fittings, waste pipes, computer housings (electroplated on the inside), and automotive interior and exterior trim.

General properties: It is tough, hard and rigid. Good chemical resistance and dimensional stability; creep resistance, electroplatable, moderate strength, inexpensive. Tendency to stress crack.

Trade Name: Cycolac, Lustran.

  • Polyamide (PA)

Polyamide, also known as PA, or Nylon, has been mass-produced since 1935.

It is a thermoplastic polycondensate.

Applications: Used to make high-lubricity parts (e.g. bearings, blow moldings, and clothing fabric).

General properties: It has high lubricity and moderate strength. It is tough, inexpensive, and has poor dimensional stability due to water absorption (hygroscopic nature).

Trade Name: Ultramid, Zytel.

  • Polypropylene (PP)

Polypropylene, also known as PP, has been mass-produced since 1950's.

It is a thermoplastic, (crystalline), polymer.

Applications: Used to make medical syringes, beakers, and parts for auto interiors.

General properties: It possesses high lubricity, high resistance to flexing (excellent for living hinges), excellent dielectric strength and chemical resistance, good impact strength, and high solvent resistance. It is inexpensive and electroplatable. It is difficult to paint, print on, or bond to. It will be attacked by fuming nitric acid and degraded by UV and ionizing radiation.

Trade Name: Marlex HGL, Fortilene 1800 series (FDA materials).

  • Polyethylene (PE)

Polyethylene, also known as PE, has been mass-produced since 1939.

It is a thermoplastic polymer.

Applications: The largest volume commodity plastic, PE is used in blow-molded beverage bottles, auto gas tanks, and extruded pipe.

General properties: It has good toughness at low temperatures and is inexpensive.

Trade Name: Marlex, Alathon, Hostalen.

  • Polyvinylchloride (PVC)

Polyvinylchloride, also known as PVC, has been mass-produced since 1938.

It is a thermoplastic polymer.

Applications: Used in pipes and fittings, wire and cable insulation, extruded film and sheet, and medical applications.

General properties: Its flexible grades are easy to process. Its rigid grades have a high dielectric strength, outdoor stability, chemical resistance, good moisture stability, and low cost. Both have low heat resistance.

Trade Name: Geon, Viclon.

  • Other short fiber reinforced plastics
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