Why Using Cost-per-Pound to Calculate Purge Costs Is Bad Math

Ask whoever buys purging compound this simple question: Are they including time in their calculations?

Hopefully, your plant manager and engineering staff have a lot of input when it comes to purging your machines. Yet, more often than not, I am sitting across the desk from the purchasing manager whose sole task is to look at a spreadsheet and see where he can drive down costs.

The cost of shop rags is completely different than the price of purging compound.

You need to factor in time.

How long is the purge? How long does it take for the next shot to meet QA approval? How long is the machine out of service before you are in full production?

...tick, tock

Let’s do the math, right

Some molders just use resin to clear the barrel. Some use purging compounds. But they all require time. It takes time to get out all the old resin and color. It takes time to be clear of specs and streaks. So, if it takes 35 minutes using inexpensive resin to purge, and 10 minutes to use a purging compound, which is the better deal?

And if one purging compound costs $2.35/lb and another costs $4.12/lb, which is the better buy.

Time will tell.

Do a little math to include the amount of time a purge takes in each instance, the cost/minute of machine downtime, and how much purging compound or resin is required to get a clear and move into full production.

You can get even more precise if you include the cost of labor. Idle staff is a cost as well.

Make it simpler and use our calculator

Slide has made this a lot easier. There is an online tool for you to plug in your numbers, even rough numbers, to see how much you currently spend on a single machine’s purge. You can even run the numbers of an alternative purge using a different purging compound or purging method, and see how much you might.

The calculator takes time, cost/lb of materials, cost of machine downtime, etc. into account.

Using this method, molders have found a savings of hundreds of thousands of dollars per year when they add up the savings for all of their machines.

So use the calculator, print out the results and walk them over to the purchasing agent. That is a number they can understand.

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