Your Guide to Injection Molding

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Jack Lander

By Jack Lander

When it comes to tooling parts, three main principles apply:

  • Tooling is available in a range of options based on quality and cost.
  • Each option fits a total anticipated lifetime quantity of parts to be produced.
  • Each option affects the cost of the parts it produces and the cost of the tooling (the mold).

To illustrate the first principle, from least to most expensive, the main mold options are:

1)      Silicone rubber mold: produces from 1 to 100 pieces; costs about $500.

2)      Epoxy mold: 50 to 1,000 pieces; costs about $1,000.

3)      Aluminum mold: up to 200,000 pieces; costs about $5,000 and up.

4)      Steel mold: up to two million pieces; costs about $20,000 and up.

5)      Multi-cavity aluminum or steel mold: number of pieces is a multiple of the number of cavities.

These mold-life quantities and prices are examples only, and both may vary greatly depending on size and complexity of your part. Each part has its own characteristics that determines its cost and the cost of the mold from which it is made.

Silicone rubber molds are used for casting a two-part liquid plastic, such as urethane, without pressure – there’s no injection molding involved. The process consists of making a plastic model using stereolithography or a similar process, and then pouring a two-part liquid silicone rubber around the model at room temperature. The liquid plastic sets up chemically, similar to epoxy glue. This process can be used to produce prototypes or a limited quantity of parts – perhaps 100 or so – for samples or testing.

An epoxy mold deteriorates fairly quickly due to the heat and wear of the molten plastic injected into it under high pressure. But for a pilot run of a few hundred parts to satisfy initial orders, epoxy may suit your needs.

For many projects, aluminum is the best choice. If you change the design of your part, you’ll have to change the mold, and aluminum molds are cheaper to alter than steel molds. Aluminum molds generally are good for a run of 100,000 parts. Many inventors overestimate the lifetime sales volume of their product, and find that an aluminum mold is entirely adequate.

A steel mold is the most typical production mold, the most durable, and the most expensive.

Both aluminum and steel molds may be designed for more than one cavity – the hollow section into which the molten plastic is injected. A four-cavity mold takes essentially the same amount of time to produce four parts as a single-cavity mold takes to produce a single part.  Thus, the machine cycle cost per part is cut in four.

The cost of the plastic per part is constant, of course, and added to the machine cycle cost regardless of the number of cavities. Molds for small parts may have any number of cavities. I once purchased a 100 cavity mold for a small cylindrical part.

A four-cavity mold does not cost four times as much as a single cavity mold. Economies in machining are present. A four-cavity mold may cost around two-and-a-half times the cost of a single-cavity mold.

To determine the most profitable number of cavities, weigh part cost-savings against incremental investment. For example, if a four-cavity mold enables you to save 30 cents per part over a two-cavity mold, and your two-year total forecast usage is 50,000 parts, your savings for the two years will be $15,000. Now, if the four cavity mold costs $13,000 more than the two-cavity mold, the added cavities appear to be a justifiable investment. Generally, corporations require their investments to be paid off from savings in two years.

And knowing what corporations require can increase your odds of success.

3 Responses to “Your Guide to Injection Molding”

  1. I am trying to get an injection mold cost on a shallow plastic chute which is 25″wX14″hX12″d. Wall thickness .08″ Who should I go to get the best price on an aluminum mold as compared to a steel mold in China. Also looking to have prototypes made prior to making large commitment in the range 10 to 50 units.
    Thanks
    Tom
    If you click on the title bar of my website a 3.5 minute demo will appear.

  2. [...] Read the full article on InventorsDigest.com » [...]

  3. Another cost saving idea is to build a single cavity mold in a larger mold base. The cost of adding the additional cavities is substantially less than the cost of building a whole new tool when you have underestimated product demand.

    Also, this approach gives the ability to make changes to the tool that effect only 1 cavity not the entire potential of the base.

    Dave
    Hitech-plastics.com

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