Thermoplastics are sold by unit weight (e.g., $/lb, €/kg). However, the material cost for a plastic component must consider component volume and density of the plastic. For thermoplastics, density is reported as ‘specific gravity.’
Specific gravity is the ratio of the density of the thermoplastic to the density of water (i.e., SGP = DP/DW). The density of water varies with temperature. For the purpose of specific gravity calculations, it is common to use the density at 4oC (39oF) since the water is at its highest density of 1,000 kg/m3 (62.4 lb/ft3). A plastic with a specific gravity of 1.0 is considered to be equally dense as water. Those with specific gravities greater than 1.0 have a density greater than water (i.e., sink). Those with specific gravities less than 1.0 have a density less and water (i.e., float).
The cost of a thermoplastic used in a component is determined by the following equation:
Volume of Component (VC) x specific gravity of plastic (SGP) x density of water (DW) x plastic unit price (PP)
Consider a rectangular plate with the following dimensions: 2 inches wide, 10 inches long, and 0.125 inches thick. VC = 2 inches x 10 inches x 0.125 inches = 2.5 in3 = 0.0036 ft3. If the plate is manufactured from high density polyethylene (HDPE), with a specific gravity of 0.95 and material price of $0.93/lb, the component cost is as follows:
CHDPE = VC x SGP x DW x PP = 0.0036 ft3 x 0.95 x 62.4 lb/ft3 x 0.93 $/lb = $0.198/part
If the same component is manufactured from polyvinyl chloride (PVC), with a specific gravity of 1.40 and material price of $0.84/lb, the component cost is as follows:
CPVC = 0.0036 ft3 x 1.40 x 62.4 lb/ft3 x 0.84 $/lb = $0.264/part
Although the unit price of PVC is lower than HDPE, the higher specific gravity results in a more expensive component using PVC compared to HDPE.
An abbreviated calculation can be used when comparing two thermoplastics for the same component to determine which will be more expensive. The volume of the component and density of water will be the same in both instances. Therefore, multiplying the specific gravity by the unit price for each provides a ‘density-adjusted’ unit price (PD) for comparison. In the above example, these adjusted unit prices are as follows:
PD-HPDE = 0.93 $/lb x 0.95 = 0.88 $/lb
PD-PVC = 0.84 $/lb x 1.40 = 1.17 $/lb