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Auditing a plastic injection molding process in China

by Renaud Anjoran

We are continuing with our series of articles about a specific process.

What is injection molding?

Injection molding as a process has been around since the early 1930s, but a lot has changed since those early days.

The injection mold machines have become sophisticated pieces of equipment with closed loop control systems monitoring every aspect of the process you can think of. Polymer development has become a complex science with blends available to cater for virtually every possible product imaginable.

The process

The process of injection molding is composed of 6 steps:

1. Clamping

The very basics of a mold machine consist of three elements, the mold, the clamping unit, and the injection unit. The clamping unit is what holds the two halves of the mold together during the injection process.

2. Injection

During the injection stage of the process, the raw plastic (technically known as polymer) is loaded into a hopper which is located at the top of the machine. From there the polymer is fed into the injection cylinder where it is heated until it reaches melting point.

The molten polymer is then pushed up to the end of the injection cylinder by the use of a specially designed rotating screw. Once there is enough molten polymer at the end of the cylinder, the screw forces it into the mold at a controlled speed and pressure.

3. Dwell time

The dwell time is basically a pause after the mold has been filled but with a bit of extra pressure behind the screw ensuring the mold and all the cavities are completely full.

4. Cooling

The injected polymer is then cooled so that it solidifies into the final shape of the mold.

5. Mold opening

Now it is time to open the mold. The clamping mechanism is released and the machine opens the two halves of the mold.

6. Part ejection

Once the mold is open, the molded part can be ejected. This is achieved through a plate moving in from the back of the mold and using dowel pins to push the part out of the mold. This is the last step within the cycle. After this the process starts all over again.

You can have a look at Wikipedia’s drawings and photos. They are much nicer than ours and (I hope) outside of any confidentiality commitment.

Advantages of using injection molding

There are good reasons why this technology is widely used:

  • Repeatable tolerances on high quantity production
  • Low labor and unit cost
  • Vast range of materials for different applications

And China is particularly competitive for several reasons:

  • Hundreds (thousands?) of mold makers, at all levels of quality and prices
  • Relatively low labor cost allows for simpler (and much cheaper) molds
  • Fierce competition between thousands of manufacturers that own injection molding machines

To be fair, there are also limitations:

  • Molds often cost between 50,000 and 150,000 RMB
  • The set up can be particularly complex
  • Parting lines are often visible, and the factory doesn’t always cut the gate marks very carefully
  • Designs are often limited by what can be done with a mold, and that’s a reason why so many people are getting excited about 3D printing

How our auditors check this process

Our engineers report back on detailed checklists that are organized in 18 sections:

  • Production Capacity
  • Technical Capability
  • Process Controls
  • QC & Inspection Capabilities
  • Setup Procedures
  • Process Parameters
  • Cycling Parameters
  • Preventative Maintenance
  • Preventative Maintenance effectiveness
  • Drying
  • Polymer Control
  • Polymer Delivery
  • Material Processing
  • Tool/Die Storage
  • Tool Design & Manufacturing Capabilities
  • Housekeeping
  • Tooling Efficiency

The objective for the buyer is to have a good idea about the risks associated with engaging a particular manufacturer. Very often, molds need to be custom-made and it represents a substantial investment. 

Maybe some readers can share other points to evaluate?

Renaud Anjoran has been managing his quality assurance agency (Sofeast Ltd) since 2006. In addition, a passion for improving the way people work has pushed him to launch a consultancy to improve factories and a web application to manage the purchasing process. He writes advice for importers on

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