Choosing Plastics – Price per Part, not Price per Pound

I’m going to discuss two situations from my career where an obsession with the price per pound of the resin was evident, rather than the much-more-relevant price per part.  I’ll conclude with some points to ponder when choosing plastics for cost minimization, whether in a new application or when considering substituting one resin for another in an existing product.

The first situation was my job straight out of graduate school.  I’d landed at the engineering center for M.A. Hanna Resin Distribution, now PolyOne.  I’d come to this particular project quite late, but still managed to contribute a little, and to see the end-game play out.

The situation was that a nationally-known maker of home laundry-drying racks, er, exercise equipment 🙂 who wanted a plastic base platform underneath the treadmill of their latest design.  Their purchasing manager had heard of a material we had available: recycled polypropylene battery trays.  It was cheaper than dirt – if I recall, something like 20 cents a pound.  It also had the engineering properties of dirt.  Low stiffness, low strength, low modulus of elasticity, and being recycled with varying feedstock, it processed inconsistently with predictable quality and consistency issues.  But it was cheap.

So despite our concerns about this material’s suitability for the application, we had developed a design that met their specifications.  It was heavy, with many deep ribs and thick sections necessary to meet the structural, deflection, and impact requirements.  Because of these design features, we estimated its cost per part as fairly high despite the low cost of the base material.

As an experiment, we picked a prime engineering resin; I recall it being a PC/PET blend with 10% glass fiber reinforcement.  At close to 10x the per-pound material cost of the original material, it seemed like a non-starter.  But we plugged the different loading scenarios into the CAD program’s design optimizer (at this employer I used Pro-Engineer), with parameters such as the number, depth, and thickness of ribs, the thickness of the base platform’s flat area, and so on.  Setting the objective function to minimize the part volume while still meeting all the different loading scenarios, we let slip the dogs of optimization to try different design iterations.

Lo and behold, because we were using a prime resin with much higher strength, rigidity, and impact resistance, the reduction in the thicknesses, depths, and number of ribs resulted in a part that was so much lighter than the original design that the reduction in weight more than compensated for the higher per-pound price.  Additionally, there were three other cost benefits; one we could approximate, and the other two were an arm-waving savings we couldn’t quantify, but which were definitely something that needed to be considered.

First, because the mass of material was so much lower, and the wall thicknesses so dramatically thinner, the estimated cycle time per part was vastly reduced because the part could be cooled more quickly; this meant – going from memory – something like a 30% increase in the number of parts per hour.  Even if the material cost had been a wash from one to the other, this added in a second advantage for the better material.

Second, the because of the weight reduction, shipping costs would be lower because of the reduced weight and lower volumetric part envelope, which resulted in a greater packing density per shipping container.  We couldn’t quantify this in any meaningful way, but it was certainly something to point out.  And third, the lower number of ribs, thinner sections, and overall shallower design meant less machining of tool steel, for an unknown but definite advantage in tooling cost and timing coming from the more expensive material.

But the project was killed.  In our presentation to our customer we had two columns for the two materials.  And we made the mistake of having, right under the two material names, the price per pound.  Our customer’s purchasing manager never got past those two numbers to the nitty-gritty where the part with a more expensive resin was actually less costly, with other benefits to boot.

The second example comes from when I was at Ford Motor Company in Sandusky, Ohio; specifically, injection molded nylon housings for air cleaners.  There were two suppliers (“A” and “B”) who continually vied for the business – we bought millions of pounds of plastic a year just for this application family.  Big bucks were at stake.

Our purchasing person was obsessed with price-per-pound.  Company A had had a cent-or-two advantage, and this was the supplier they wanted to use.  But Company B had three advantages, and I (and others!) wanted to use them preferentially.

First, the densities were different.  Company B’s material was lighter; even though it was marginally more expensive per pound, since the mold’s cavity had the same volume of plastic used the relevant parameter was not cost-per-pound but cost-per-cubic-inch.  Company B’s material, on that basis, was actually roughly a cent per part cheaper.

Company B’s material also processed marginally faster.  Because it was slightly less dense, and had a fractionally-higher thermal conductivity, it would cool faster after injection, leading to a couple of extra parts per hour.

And they had one final advantage: Company B was much more responsive.  Faced with a service request call because of a production issue, Company A’s response was, typically, to schedule a visit within a week or so.  Company B?  You’d call with a problem and they’d be there the next day – live and in person – to help you figure things out.

Presenting these arguments convinced Purchasing to go with Company B.

From these two examples I’d like to abstract out some lessons which I hope are useful:

  1. In the design phase of a project, use your CAD program’s optimization feature to minimize the volume of material used under the different loading scenarios.  It’s very likely that the cheapest material, which probably has lesser properties than a more expensive one, may require more material than the expensive one; the reduction in material used per part might well overcome the more expensive material’s price per pound sticker shock.
  2. Cycle time is important too.  A higher-cost material, having better structural properties, can reduce cycle times by having thinner walls, which cool faster; this adds more parts per hour into the cost equation.
  3. Depending on the part’s functionality, a higher-end resin may require less structural features like ribs and gussets, as well as those features being smaller – resulting in a less expensive tool delivered faster because of reduced machining requirements.
  4. When considering swapping out one material for another in an existing mold, consider these two “lumped parameters” as rough first-pass screening tools; these two will interact, and your internal labor cost will be necessary in factoring out which one is more important (remember – these are presented as screening tools only, not definitive factors – you need to do a proper analysis based on your own situation!):
    1. Multiple price per pound times density to get price per volume.  Using this parameter, the lower cost part will come from the material with the lower value.
    2. Multiply density times heat capacity and divide by the thermal conductivity.  Using this parameter, the lower cost part will come from the material with the lower value.
  5. When presenting alternative materials, especially to non-engineers, put the estimated price per part right at the top of the two columns comparing the alternatives – above any other data.  Get into the details of price per pound, wall thickness, cycle times, etc., later to support your cost per part estimate.  Remember that, at the end of the day, what matters is cost per part.  So put that first!
  6. Recycled materials are not necessarily bad materials per se, and can often offer substantial cost advantages – but unless the supplier takes extreme care in the recycling and pelletizing operation they may have the requisite physical properties while processing less consistently from lot to lot.  This inconsistency may end up being more trouble than the lower material cost is worth.  (This was seen in a third case at Ford, not discussed above, where we considered recycled polycarbonate from CDs instead of virgin material; the inconsistent processing and increased scrap eliminated any material cost advantage.)
  7. Service and response time matter.  When you have a problem you need help now, not in a week or so.  Lost production can cost you a lot, in scrap costs and in OT required to make up lost production, as well as in your reputation with your customer (and possible penalties they may charge you for not meeting their schedule).  That responsiveness is worth an added margin to the raw material cost – especially in these days of lean manufacturing and minimal safety stock which could otherwise insulate your customer from your production hiccups.

Update: Thanks to Deepak Ramanathan, who pointed out the potential that a lighter part – depending on other functional requirements, of course – might also be moldable on a smaller machine, thus reducing costs in another way!


© 2014, David Hunt, PE

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