Jan 04 2009


Published by at 5:25 pm under Uncategorized

A mold has landed on your shipping dock complete with the ‘last(?) shot’ and the drawings.  You quote the refurbishment of the mold to the customer then begin to ogle the specs.  OOPS!  You can’t get the specified material because the resin company that originally supplied it is no longer producing this material.  Your customer, in his infinite wisdom, cleverly put “Or equivalent” after the material specification. Now what?
In this example let’s assume the original material was ABS.  To most customers ABS is plastic; being different from Polyethylene.  With this extensive knowledge of the world of plastics to them ABS is a material specification in and of itself needing no further explanation.  But ABS isn’t just a resin; it is a class of hundreds of resins (types and vendors). This concept is way beyond discussion with a customer.  More importantly there is never an exact equivalent but there may be a few choices that are close.

Material selection is really a balance of selecting the important properties a material offers to fit the product application.  Obviously this will almost always be a compromise of properties.  So we will have to choose the material the ‘best fits’ what is necessary.  Best Fits are generally done by ranking and comparing each property individually and then looking at the candidate materials to pick the one that most closely fills the needs.

First, do some research through the computer data bases and find the material you can no longer use.  Without trying to confuse the customer call him up and see if you can figure out what properties the material must possess that are important for the product’s function.  In the example the interview came out with the following properties:

Melt flow – Important because of the part’s geometry.  A higher melt flow would be desirable but a significantly lower one might cause filling problems.
Flex modulus, and Tensile strength – This part will see flex and tension.  The current properties seem to work.  It is unknown if a significant variation from the current levels will cause a product failure so being reasonably close is desirable.
Heat distortion temperature under load — This product does operate in a heat environment so its resistance to heat is definitely a concern
Cost – Buyers are always concerned with cost.  BUT you buy plastic in terms of weight ($/Lb or $/Kilo).  However you mold in terms of density (gr/cc or Lb/cubic-inch).  Material cost per part is really $/cc of $/cubic-inch.  Going for the low price may be a bad decision if the density is considerably higher. (For example – PVC is relatively inexpensive per pound BUT it is quite dense.  Therefore the dollar per part might be cheaper with an alternative material.)

The working spreadsheet can be found at this link, or you can point your browser to XXXX   Go to the spreadsheet (See DEMO on the link to the spreadsheet) and fill in the blue row labeled Properties that you feel are important.  In this case there are five although the sheet allows you six.  Go to your spec sheets/data bases and fill in the appropriate numbers for the old material’s properties in the yellow cells under Standards.

Again go Dumpster Diving back into the data bases and come up with candidates filling in the rows of yellow cells as you find candidate materials.

Now look vertically down the columns at each candidate’s individual property using the number “1” as most desirable and “5” as the least desirable when compared with the others. Do this for each property.

The spreadsheet will add up the scores and then rank them.  Obviously the lowest overall score will be the material is the closest “equivalent’ material because of its overall desirable rankings.

The result you get from this spreadsheet is a quintessential proof of the old rule “Garbage in equals Garbage out.”  Your results depend on the materials you’ve researched and the properties you choose.  In the example we might be able to get all the important properties by using a less expensive material that isn’t even ABS.  Or both you and your customer may have missed a property that is very important such as resistance to stress cracking by exposure to oil or grease.  The results of this spreadsheet can only be a recommendation for a substitute material and not a guarantee.  Since this is a scoring technique go back and look at your Best Choice’.  Let’s say it scored all 1’s in every property but HDUL, and in that category if failed unacceptably low.  While the spreadsheet may show you it is the best choice, because of this one unacceptable characteristic, this material must be eliminated from the candidate list.

Once you’ve made your choice for a new material, Mold a few parts ad the go through extensive side by side testing with the old material if it is possible.  Your new material must perform equal to or better than the old one on all fronts: Functional, Dimensional, and Cosmetic.  If it doesn’t; go back and redo the process until you find a material that meets all the requirements.  The proof you’ve made a good substitute is ONLY from end use testing.  If you don’t test you’ve made a huge mistake.

This spreadsheet can also be used to choose materials for new products.  If you want the Tensile of Nylon, the cost density ($/cc) of ABS, the heat resistance of PC,  and the ability to be paint/decorated (meaning solvent compatibility); you can easily plug in these properties as your Standard and then again go through this process.

“OR EQUIVALENT” is a sucker punch.  If the molder makes the material choice you’ve just become a party to a lawsuit if the product fails or financially responsible for the recall.  Any time you are faced with a material substitution as a molder; your work should stop with presenting the alternatives to the customer and have him make the final choice and update the specifications.

This is a simple spreadsheet.  In all fairness you can log onto IDES.COM and subscribe to their PROSPECTOR product that will do much the same thing going through their 78,000+ materials library.

This article and the spreadsheet will become part of the book ‘Tribal Knowledge’.  I’d love to have others contribute to it.  Go to the outline of the book, look at it and see if you’d like to give me a page or two on the Common Knowledge of the Molding Community.

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This article and spreadsheet are virtual.  Feel free to print out the article, download the spreadsheet, and give it to your customers and suppliers. You can use it to your advantage and not make the mistake of suggesting a material that might not work.  Or you can pass it along to your customer and hope he has an IQ higher than his shoe size with the ability to make the proper inputs.  You can also print everything out and feed it to the Office Gerbil.  Alternatively, you can ignore everything and allows you the Universal Equivalent Material philosophy of saying “Why not Nylon?” and then start blindly substituting other materials until you stumble across one that might work.  Using this approach you must also hope you don’t sound like a dork when you have to defend your position.

It’s your choice

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