Material Selection Based on Processing Costs

some thoughts on selecting plastic materials for the lowest possible manufacturing cost

Material selection based on costEveryone is concerned about costs these days. Whether labor rates, tooling costs, machine prices, utility rates, real estate taxes, each affects the final part cost of anything manufactured. The basic equation is simple:

Part Cost = Material Cost + Processing Cost

Some companies reduce costs by buying materials at the lowest possible price. They will negotiate global supply contracts, conduct blind auctions, and basically do everything they can to hammer their suppliers for the lowest possible price.

Other companies pursue cost reduction through implementation of a set of processes called Lean Manufacturing. There are some basic ideas that govern these processes: reduce the number of parts, make part each easier to manufacture, simplify the assembly steps, integrate discrete parts into robust sub-assemblies, reduce waste at each and every opportunity, etc. They often go by names such as Design for Assembly, Design for Manufacture, or some other type of Design for X (sometimes designated DfX ). Most Lean Manufacturing methods focus on processing costs, which is a key aspect of mass production.

Process Cost
(per part)
= Cost of Process
(per unit of time)
x Process Time
(per part)

For plastic parts, the process steps typically include molding, handling, and assembly, but can also involve printing, painting, plating, sterilizing, etc. The cost of each step depends on a number of factors, including many of the items mentioned above.

As a design engineer, I am often constrained in my choice of materials by the specific requirements of the project. (Did I say often? I meant to say almost always).  But I usually do have a say about what material(s) can be used in my design. And if I can recommend a material that reduces the time involved in any given process – effectively reducing the cost of the parts – I am saving my client money.

To help with this, I find it useful to translate abstract numbers about processing costs into simple, common sense things I can relate to – usuallly involving dollars and cents. (or as my mother used to say, dollars and sense).

As an example, the following hourly rates as translated into common cents:

$3.60 / hr = a penny every ten seconds

$36 / hr = a penny per second

$360 / hr = a dime a second

$60 / hr = a dollar per minute

$120 / hr = 2 dollars a minute

Using the above numbers, if I can choose a material for my design that processes a total of three seconds faster – saving three cents per part (or more, depending on the number of parts per cycle) – I will save my client money – even if the raw material costs two cents more.

So, the next time you are about to choose a material, consider the processing costs.

It only makes sense.

 


thermoplastic material selection
This article is based on content from a paper presented at the Society of Plastics Engineers 2014 Annual Technical Conference. That paper was also used to formulate a chapter in Eric’s newest book, Thermoplastic Material Selection: A Practical Guide, which was published in May of 2105 by Elsevier, and is available from Amazon.

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