Choosing the Correct Tool Cavitation

Injection molding is great way to produce lots of plastic parts very rapidly and economically. What OEM’s often overlook, however is one key question which could make this great method even better:

How many cavities should there be in injection molding tools making our parts?

Answering this question can be the key to getting the most return from the OEM’s production cost as well as the contract manufacturer’s machine time. Deciding how many cavities per tool is all about making tradeoffs between the price per part, production time, and initial tooling cost in order to reach the lowest total production cost for producing quality parts. Lake many other optimization challenges in manufacturing, it requires close collaboration between contract manufacturers and OEM’s, engineers and management.

An Opportunity Too Often Missed

Unfortunately, this optimization step is often skipped because OEM’s (and their injection molding contract manufacturing partners) don’t see the often large benefit of increasing the number of cavities per mold.

Instead, what typically happens is that the OEM sticks with the both the default of one cavity per mold and the specific tonnage machine the contract manufacturer suggests. From these defaults and the specs of the part itself, the parts per hour (PPH) is determined.

The OEM then projects the number of parts per year they need (formally referred to as “estimated annual usage” or EAU) and divides this EAU by the PPH to make sure that there are enough hours in the year to make all the parts the OEM needs in that year. If not, then increasing the cavities per mold is the obvious place to boost the PPH because it’s more cost effective than creating two or more separate molds so that more than injection molding machine can be simultaneously. Besides, the contract manufacturer may not have another machine which isn’t already being used by another customer.

This standard approach works great in a lot of scenarios, but a lot of OEMs don’t realize that even if a single cavity mold can meet the EAU requirement, they can still greatly benefit from increasing the cavitation. This is true even if those additional cavities require moving production to a slightly larger machine.

Let’s use a hypothetical example to illustrate why. Suppose you’re trying to make a part which has an EAU of 20,000 pieces and you’re using a single cavity tool in a 150 ton press. The cycle time to complete for one of these parts is 30 seconds, thus resulting in a PPH of 120.

If, however you changed that tool to a 2 cavity it can produce 240 PPH and you will see a reduction in the piece part price, mainly due to the fact that the machine time required to make each individual part has been cut in half. This dramatic reduction in machine time cost per piece more than makes up for the slightly increased hourly rate that might be incurred if it’s necessary to move up to a larger machine (such as a 200 ton or 225 ton), and the small increase in tool cost.

The Benefits of Increasing Cavitation

This example illustrates exactly why increasing the cavities per mold very often produces a net win for customers: large increases in the PPH can be achieved with small percentage increases in cost, resulting in a significant overall reduction in the price per piece. This price per piece is, after all, the key reason why OEMs choose injection molding for making their plastic parts to begin with. Utilizing multi-cavity molds allows is a way to further squeeze maximum return on production investment.

Another benefit to the increased PPH and reduced price per part made possible by multi-cavity tools is the decreased cost incurred by deliberately overshooting an EAU (especially if it was too conservative to begin with) and thus creating some excess “buffer” stock. This can actually save OEMs money since it’s often less expensive to make and store too many cheap parts than to not have enough of them and thus incur losses due not being able to make enough product to meet demand. In effect, using multi-cavity tools can increase revenue during unexpected spikes in demand, provided a bottleneck doesn’t appear further down the production pipeline.

A variation on the idea of multi-cavity tools is a family mold: one tool with many cavities, each of which is for a different part. Since all parts are made from the same material and at the same time, family molds are great for assemblies like electronics enclosures, provided that the injection molding parameters will work for every individual part (cycle time, pressure, cooling rate, etc . . .).

Increasing the number of cavities in your injection molding tool can boost your profit by producing more parts for less cost, but in order to reap those benefits, you and your contract manufacturer must recognize this opportunity during the design phase of the tooling. Providence Enterprise is a contract manufacturer who sees the opportunities for its customers in every aspect of tooling design and implementation.


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