Plastic Molding Method: Injection Molding

Nowadays, injection molding has become the most common method of manufacturing plastic parts, and it is ideal for mass production.

Many plastic accessories can be made by injection moldings, such as buckles, packaging, bottle caps, automotive parts and components, toys, mechanical parts (including gears), components of medical products, one-piece chairs and small tables, storage containers, etc.

In order to facilitate the molding process, engineers must be very careful to consider all molding conditions, below elements must be taken into account while designing:

  1. The material injected
  2. The material of the mold
  3. The desired shape and features of the part
  4. The properties of the injection molding machine


Heat the plastic material until it becomes fluid, deliver the molten material with a plunger or screw to the injection nozzle at the same time, and then inject the material into mold cavities; after the molded plastic article cooling down and curing, eject it out with an ejector pin.


  1. Can be used to manufacture plastic parts in complicated shapes or different material
  2. Suitable to be used for mass production because of the efficiency
  3. Fine dimensional stability
  4. Can be fully automatic
  5. With less material wasted


  1. Unavoidable results including parting lines, ejector pin marks, or gate marks are usually present on the finished products
  2. The cost of mold and equipment is high
  3. The overall structure of the mold is complicated, thus it's not easy to do further processing


The quality of the mold is one of the important parts during production, in order to have nice performance and benefit from the production, it is necessary to select suitable die steel.

Ideal die steel should have the following features:

  1. Easy to polished
  2. Corrosion resistance
  3. Crack resistance
  4. Transcalent
  5. Abrasion resistance

Here're two different kinds, including Plunger Injection Molding and Screw Injection Molding:

  1. Plunger Injection Molding

    The crucial factors during plastic injection include the pressure of the mold interior/exterior and the temperature of molten plastic. As the temperature increases, the viscosity of molten resin decreases and becomes more fluid, which means it can quickly fill up all parts of the mold cavity.

    The torpedo-like spreader in the injection machine leads to the spreading of plastic material; the molten plastic material can be evenly distributed and molded quickly.

    Plunger injection molding process


    Heat up the thermoplastic until it becomes fluid, and force the molten plastic into the mold cavity with pressure. After the mold has shaped the material, please wait a few times until the plastic molded article cooled down and cured; finally, eject it out with an ejector pin.

  2. Screw Injection Molding

    The screw injection molding is the enhanced type of plunger injection molding, contributing to the temperature cycling during injection molding.

    Screw injection molding process


    Place the material into the funnel, then drop it into the heated and rotating barrel. With the high temperature in the barrel, the material plasticized gradually and goes forward through the rotating screw, and the molten material accumulates till it achieves a fixed quantity, then the plunger goes down to make the screw stops rotating, meanwhile, the plunger raised by hydraulic pressure to inject the molten plastic into the mold cavity.

    Before the plastic material is injected into the cavity, to prevent polymerization in the cylinder, it is necessary to cool down the front side of the cylinder by cooling water and do not eject the plastic article out until it is totally cooled down.

※Ji-Horng Plastic Co., Ltd. is a plastic injection molding manufacturer that has specialized in injection mold tooling OEM/ODM services for over 30 years, providing complete professional injection molding services from design, drawing, molding, mass production, secondary operation to assembling, good quality guaranteed. If you are interested in our injection molding services, please kindly contact us for more information!