When comparing two materials for an application, the true cost of a polymer is not limited to the price per pound. As any molding or extrusion shop knows, there can be hidden costs in the complexity to run, scrap rate, tooling changes, special processing equipment, and many other factors. One aspect to consider that is often overlooked is the specific gravity of the product(s) in question.
To get started, let’s look at the definition of specific gravity. The dictionary defines specific gravity as the ratio of the density of a substance (in our case, plastics) to the density of a standard, usually water. Now in the case of water, the density is roughly 1 gram/ml. As such, when comparing a material’s ratio of density to 1 g/cm3, the specific gravity will be the same as the material’s density. For instance, if a 30% glass-filled polypropylene homopolymer has a density of 1.13 g/cm3, then the specific gravity of that compound would be 1.13 as well (specific gravity is a unitless value). Specific gravity is commonly seen on plastics’ datasheets, so we mention this description. However, density and specific gravity can be used interchangeably for most intents and purposes since both represent how much plastic you get for a set volume of material.
This begins to affect cost because we buy and sell plastic raw materials by the pound rather than its volume. Specific gravity can be used to show how far a set weight of material will go in terms of how many parts it can produce. Roughly stated, lighter materials can make more parts per bag or box of raw material used.
For instance, let’s use the even number of a proposed part with a volume of 100 cm3 where we have the option to use HDPE or acetal (POM) for the same job. If we have a 25kg bag of HDPE with a density of 0.953 g/cm3, it will yield approximately 221 parts from that bag. The math for this is as follows:
25kg bag = 25,000g of HDPE. At 0.953 g/cm3 that equals 26,232.9 cm3.
Since each part is only 100 cm3, you would be able to create 262 parts.
If we were to produce the part in acetal, the specific gravity would jump up to 1.41 g/cm3. That would adjust the math to the following:
25kg bag = 25,000g of POM. At 1.41 g/cm3 that equals 17,730.4 cm3.
Since each part is only 100 cm3, you would be able to create 177 parts.
Therefore, if we made the parts from HDPE rather than POM, you would create roughly 48% more parts using one 25kg bag of material. This equation works easily when comparing two materials of the same price, but what about when they are different or arguably more expensive?
Say you are comparing two materials, but one is 5% more expensive. If the more expensive material has a specific gravity that is lower by 5%, then the two materials are functionally equivalent in cost per part. If the more expensive material is 10% lower in specific gravity, it is actually a cost savings per part to use the “more expensive” material per lb.
When comparing materials for a new job, it is always pertinent to compare the specific gravity for potential savings.
The Engineering Team here at Chase Plastics is ready and willing to walk you through this analysis if needed or offer lighter materials for savings opportunities when possible. Give us a call at 844-411-2427 or send an email to firstname.lastname@example.org to get support on any of your technical needs today!
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