How to define number of cavities in a plastic injection mold?

When it comes to cavity number for a plastic injection mold, there is a lot to be considered by injection molder though usually the number of cavities is decided by the customer.  Please note the following:

 

  • The most important thing to consider is the size of the part and how complext. Does the part have a lot of slides,lifters, core pulls etc.
  • Size of press that the mold is going to run in, how many tons in the press rated for, what’s the platen size, and what’s the shot size.
  • What is total quantity of the part? all you must consider delivery time on the parts. If you have a tight just in time delivery schedule that will also determine the amount of cavities needed to support the timing.
  • What is the target cycle time?
plastic injection mold
plastic injection mold
  • How is the resin going to be distributed within the mold? surface runner, insulated runner, hot runner, three plate.

6) For a conventional runner mold producing the same or similar parts, cavitation should be a number that is a multiple of 2 IE: 2, 4, 8, 16, 32, 64, 128… That way you can utilize balanced “H” runners.. If using a hot runner, this rule does not always apply.

7) Lastily How many parts do you expect to sell ?  We built a few molds for BMW auttomotive parts before and the quantity is only 200 for an entire year.  So each mold is structured with single cavity

 

Please bear one rule in mind that not the bigger quantity, the unit price will be cheaper. When you increase the number of cavities in a mold, the more difficult it becomes to handle in the process and maintain stable level of quality. We had several instances where it was more economical to run two 2 cavity molds, rather than one 4 cavity mold. If one of the molds is down for maintenance or repairs, the other one can still be running, instead of one big mold being out of service and the production has to shut down. Mold shop should consider about this!

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