In continuing our series on a good part, last month we listed what we wanted from our material. Thus by use of a good data base on materials you should be able to search and come up with the right materials to look at.  Choices are made on the ranking of the important properties you listed. Ranges may need to be established as to where you need to be.

Sometimes people may not know what properties or numbers to put on the properties. This is where comparable materials come to play. Say for flexibility someone doesn’t know the flex modulus, but wants something close to a coffee can lid, thus looking up LDPE or LLDPE up and using that would be a starting point. .

Now than one factor that always grabs the details at this point is price per pound. I have as yet not found a program that will give this to me, though through experience and all I have an idea of where cost might be.   If in fact you printed, copied the last article and look at 4.12 on the list you will note that cost is listed. Since this may not be clear, costs are important, if someone knows the volume of the part than they can figure how much the part weighs and thus the cost of material per part. Understand that materials have a specific gravity and that they are different and thus cause parts to weigh differently. All in all the point of this is how important is it to you, and or the part?

To do this calculation we either need cubic inches or cubic centimeters for the volume.

To calculate the weight for the part:

(Cubic inches) multiplied by (16.35) multiplied by (specific gravity of material) = weight in grams of part or volume. Since there are 454 grams to a pound you can than calculate what is the weight of the part.

In doing the calculation do not forget the volume of the runner system, and the approximate waste when starting and stopping the job. But we are getting ahead here. The over all fact is to know that the material at its approximate price can work in the part, after of course it meets the needs for the application. If in fact the cost for the part has to be such that only a commodity materials would work and you need flex modulus of 500,000 psi than why continue.
In any case the deciding case for the selection does in most cases come down to price per pound.  So if in fact you work out this calculation first you may save yourself a lot of time. But for now we have finally decided which material we will use and from that we need to progress to a tool design for that material.

Thanks for the time, more next time.

Steven L Silvey
Sr. Technical Service
GP-ADC
Slsilvey@ashland.com