Cavitation

Definition: 1: a depression or sometimes the set of matching or associated depressions in a mold that forms the outer surfaces of molded articles. 2: plural multiple cavities; one equals single cavity, two would be two cavity...etc.

One of the main issues with injection molding is the cost of the mold. As the old commercial goes you get what you pay for, plain and simple. But the issue to be explored is what our cost per unit is as it relates back to the mold itself.

One of the first factors to be considered is what will be the total production required from this tool? This would possibly than equate to a standard for tool build, but not always. If only 1 million parts were required a single cavity would need to support over a million shots while a 4 cavity would only need to support slightly over two hundred and fifty thousand shots. In both cases the million parts could be produced.  

In addition to how many parts are needed total out of the tool. What are the estimates for parts per year, per month and or per week? This helps in selecting cavitation, as with cycle time estimates calculation are made resulting in data that shows to produce the product requirements a high cavitation mold is necessary, or multiple tools are necessary. The worst possible issue is that when demand increases you are not able to maintain or supply to all parties. Another issue is in filling the pipeline with product for launch, that additional tools be considered to meet demand.

So if a mold is built at a cost of $30,000 with only one cavity, the equation is divide the $30,000 (cost of mold) by how many parts are to be produced and that is the cost per part for the mold. If only 30,000 parts were produced out of the mold than $1.00 per unit is the cost of the mold. If we double production than it is $0.50 per part cost for the mold.

As production numbers go up the price per part related back to tooling is lower and lower. Thus when looking at cavitation the tooling cost increases as cavitation goes up, the factor may be 1.5 (when doubling cavitation) for example so a 2 cavity costs $45,000 and a 4 cavity $67,500 and so on. (only used for explanation)

The above chart shows the declining cost per unit, and should be one of many processes to run through. As can be seen the increases in tooling become less pronounced per part as numbers increase.  This should not be a deterrent to building high cavitation tools because in most accounts production per hour is increased enough (decrease in machine hours) to offset this higher cost and still achieve a lower cost per unit. (a subject for another discussion)

The point to be made is that tooling / cavitation costs are important but are often misunderstood. They should be related back to cost per unit for the tool cost. It is a capital expense and one that has always been debated or as my experience has been "can't you build it cheaper." There needs to be a balance between cavitation, parts needed and time allowed to produce those parts.

This than brings up the hidden cost in the tool cavitation numbers and that is maintenance. What is the cost to maintain the tool? How is it amortized over the part cost and or cost to build the tool? This can only be done after the fact, but for the above example let's state that for every 10,000 shots a PM must be done at a cost of $1000. (for the single cavity)

If production is 30,000 units that equates to $3000 added to our tool, or an additional $0.10 per unit cost. If in fact we double our production to 60,000 units we than add in $6,000 for PM cost and we still have the $0.10 per unit as it is a fixed cost, so instead of $0.50 per unit cost for tooling we have $0.60 per unit, $0.10 for maintenance and $0.50 variable due to product produced. As cavitation increases and we were to use the same factor on maintenance as a tool build of 1.5 each time the cavitation doubled the cost would be

Basically as our cavities increase, though the maintenance cost increase, the volume of parts produced per shot ultimately causes a decrease per unit charged backed to the cost of our tool. Combing both the PM cost and cost to build per part at a million part level equals a $0.13 per part for a single cavity, $012 per part for a 2 cavity and $0.124 per part for a 4 cavity.

Note that in most areas this cost may not even be considered, or might have a separate line item. Further maintenance may be at higher shot numbers which than lower the costs, but it is a factor in considering cavitation models.

Now here is the kicker, if the molder builds the mold and runs the production for the product in most cases they pick up the cost of maintenance as it was built into the tool build and or part costing. If the tool was built by party one and shipped to party two to run for the OEM, the maintenance is now most likely the responsibility of the OEM.

The bottom line is how many cavities do we need to create within the mold to produce the parts necessary in an economical way? Another side is if we build an inferior tool and have higher maintenance costs is it acceptable to our overall product cost or more importantly who is picking up the cost and whose bottom line is it effecting?