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the injection molding process you should know

An injection polymer molding process involves filling a relatively cold, negative mold with a predetermined quantity of heated and plasticized material by injecting it under pressure. Upon solidifying, the injected material is cooled further and the mould is opened to remove the product. A horizontal plunger type injection moulding machine illustrates the main steps of the process. Following are the steps that lead from the granule to the finished product, as illustrated in the diagrams:

  • Feed hopper

Above the injection unit, the granulate is placed in the hopper of the measuring or dosing unit. Predetermined quantities of material are thrown into the plunger from the hopper. The plunger pushes this charge forward so that it is deposited on top of the injection plunger, and then after it has been withdrawn, directly in front of it. The injection plunger forces the material into the heating cylinder as it advances through the plunger guide.

An electric resistance-type heater band heats the outside of the cylinder so as to melt the thermoplastic material. By utilizing a spreader located inside the cylinder, the material is even heated while passing through it. It pushes the material against the cylinder’s inner hot wall, known as a spreader or torpedo.

  • The nozzle of the injection machine

The material is forced through successive strokes of the plunger until, at the front of the cylinder, it becomes a homogeneous melt, ready for injection. The injection nozzle forces the melt into the mould. Molds are essentially two parts that are clamped or bolted together in the injection machine. One side is mounted on the stationary platen, while the other side is mounted on the moving platen. The two halves are known as A, B plates, while the speed at which they are injected is regulated to fit the material and components.

  • cavity filling and ejection

Sprue bush is where material passes through after being injected into the nozzle or fixed side of the mould. The melt is distributed into the cavities through interconnected channels or runners. When the cooling period ends, the mould is opened and the components (T) are ejected through ejector pins, which are engaged by the ejector tail hitting the stroke limiting screw as the mould reaches its fully opened position. Accordingly, the injection moulding process can be divided into five stages:

  1. A measurement of the amount of granulate to be placed in the mold cavity or cavities, in front of the injection plunger.
  2. A melting cylinder is used to melt the material to an injectable consistency.
  3. The melted material is injected into a closed, cold mold.
  4. The injected material must be cooled until it solidifies.
  5. The mould is opened and the product is removed.

 Steven Cheng is a  China injection molding expert with Topworks Plastic Mold, a company that offers comprehensive services from design to production. For customers, Topworks offers a one-stop for injection molding of polycarbonate and ABS plastic part. As a result of the company’s extensive experience in the plastics industry, it provides customers with prompt responses and high-quality service.