Plastic Molding in China
"Injection Molding" is used to make three dimensional shapes with great detail. The material is placed in the hopper of an injection molding machine where it is fed into a chamber to be melted. The melting is achieved by conducting heat into the material in a "Plunger" machine, while the material is primarily heated by shearing or mechanically working the material in a "Screw" machine. Several shots of material are being heated and held in the injection unit. The maximum volume of material a machine can inject in a single shot determines its shot capacity. The capacity is given in ounces of a material.
Once melted the material is forced, under pressure, into the mold where it conforms to the shape of the cavity. The mold is temperature controlled, usually by circulating temperature controlled water through it. Once the part is cooled, the mold is opened and the part removed. The mold is then closed and ready for the next shot. The mold is clamped shut while the material is being injected in to the cavity since the cavity pressure may be as much as 5,000 psi. The clamp is sized by the "Tonnage" it holds. Injection molding machines will be referred to by its shot size in ounces and its tons of clamping ability. An example would be a 6 oz, 80 Ton machine.
The molds are most often made out of hardened steel and carefully finished. They may also be made out of prehard steel, aluminum, epoxy, etc. The type of mold material selected depends on the number of parts to be made and the plastic material to be used. Parts are often machined to test the shape and function of a part before a mold is built.