Injection Molding — How to Manufacture Successfully in China

  • The different plastic injection molding techniques
  • The equipment used
  • Common defects and how to fix them

How it works


  • Advantages: Lower cost per part, lower tooling and maintenance costs, accommodates a wider variety of plastics as heat sensitivity of the raw material is not a factor, more flexibility in terms of design changes, faster color change.
  • Disadvantages: Slower production cycle, lower production volume, higher waste rate, requires removal of excess plastic, limitations in part design and size.
  • Suitable for: Electronic casings, keyboards.
  • Advantages: Faster production cycle, lower waste rate, requires less pressure, less post-production work (because removal of excess plastic, regrinding and recycling are not required), capable of producing higher volumes and larger parts.
  • Disadvantages: High tooling, maintenance and inspection expenses, higher cost per part, heat sensitive plastic resins might experience slight color changes.
  • Suitable for: Bottle caps, mobile phone cases, beauty products packaging.
Plastic injection molding is great for production in vast quantities


  • Single cavity, multi cavity and family molds: A single cavity mold, as its name suggests, has one cavity. A multi cavity mold has more than one cavity of the same part while a family mold has multiple cavities of different parts.
  • Hot runner and cold runner molds: Molds are also differentiated on the basis of the technique used — hot runner versus cold runner. Choosing between the two depends on the choice of plastic raw material.
  • Two-plate and three-plate molds: The more common of the two, the two-plate mold has two plates — one moving and one fixed — divided by the parting line. (The parting line is the line of separation where the two mold halves meet). Such a mold is also called a single parting surface mold. It has a simple design, is cheaper to tool and has a shorter cycle. However, it is not the best option for a multi cavity mold. A three-plate mold or double parting surface mold, on the other hand, is split into three sections with two parting lines. Its complicated design makes it suitable for large parts that require multiple gates as well as parts that cannot have gate marks along their side walls. A three-plate mold automatically eliminates the gate removal process and is also a cheaper alternative to a hot runner mold. However, its complex design makes it more expensive to tool, it has a longer injection time, it requires a higher injection pressure, and it creates more waste.
  • Unscrewing mold: This is a unique mold with a rotating threaded core that is used to make parts with a screw function, such as bottle caps, nuts and bolts. After the plastic sets in the cavity, the core unscrews and forms the threads — which can be internal or external — on the part.

Common defects and their fixes

  • Warping/shrinkage: If the part doesn’t cool uniformly, one section of it might shrink faster and get distorted or warped. Solutions: Maintain a precisely calculated cooling rate, adjust wall thickness, increase injection pressure.
  • Flash: A flash is a thin layer of solidified plastic on the product along the parting line, caused by molten plastic seeping out of the mold. Solutions: Lower injection pressure and/or speed, adjust clamp force to ensure the mold halves are tightly secured.
  • Bubbles: These are bulges on the surface of the molding, caused by trapped air or gas formation in the plastic. Solutions: Lower injection temperature and speed, make the gates larger to reduce trapped air; remove moisture from the raw material and lower barrel temperature to reduce gas formation.
  • Ejector marks: Small indentations on the part formed during ejection. Solutions: Lower ejection speed, allow adequate cooling.
  • Discoloration: These are color variations commonly caused by leftover plastic in the distribution system from an earlier cycle. Solution: Thoroughly flush the distribution system between cycles.
  • Short shot: When the shot doesn’t completely fill the mold cavity, it is called a short shot. Solutions: Remove gate blockages, widen the gates, increase melt temperature.




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