Specific Gravity in TPE Selection
Selecting a Raw Material
Selecting a TPE material to be used in your application may involve many factors:
- Raw material cost
- Specific gravity
- Colorability
- Processability
- Hygroscopic vs non-hygroscopic
- Shrinkage
- Supplier’s history, size and location
- Customer service
- Technical support
- Quality, consistency, and delivery
GLS believes these factors all play a role in the decision process and should be used in your decision process.
However, when determining how to quote a specific application, there are three main components that go into typical cost estimates:
- Raw material cost
- Machine or conversion cost
- Overhead cost
The summation of these three component costs should help define why one product is better than the other product on pure economics. This TIPS sheet will focus on the raw material cost, specifically, how Specific Gravity plays a role in the overall economics.
Definition of Volume
Volume is defined as the cubic area of a part design. The TPE industry uses the standard units of cubic inches or cubic centimeters. To convert cubic inches to cubic centimeters, the conversion factor is 16.4 (in^3 x 16.4 = cm ^3 and 2.54 in = 1 cm).
An Example
Consider a part having the following design dimensions: 5” long x 3” wide and is about 0.100” thick. The part volume would be 1.5 in^3 (5x3x0.100) or 24.6 cm^3 (1.5 x 16.4). The part mass would be 24.6 cm^3 x Specific Gravity of TPE material.
Therefore, two TPE materials with different specific gravities will result in different part weights. For example, using GLS product XL9070 (SpGr of 0.89) and GLS product G7970 (SpGr of 1.18), both 70A TPEs, the mass of the part in grams is calculated as follows:
XL9070: 24.6 cm^3 x 0.89 = 21.8 g
G7970: 24.6 cm^3 x 1.18 = 29.0 g
For an annual number of units required of 500,000 parts per year, a buyer would require the following pounds of material excluding runner system and scrap (454 g/lbs conversion factor):
XL9070: 21.8 g / 454 g/lbs x 500,000 = 24,008 lbs
G7970: 29.0 g / 454 g/lbs x 500,000 = 31,938 lbs
If both products sold at price of $1.50/lbs, the total raw material cost of both materials would be:
XL9070: $1.50/lbs x 24,008 lbs = $36,012
G7970: $1.50/lbs x 31,938 lbs = $47,907
As a buyer, one can easily see how specific gravity of a TPE plays a major role in the total number of pounds required to fill the order and the total dollars that will be spent.
A simple rule of thumb to compare two materials is to multiply the $/lbs actual raw material cost by the material’s specific gravity. Choose the material with the lower cost; this cost is essentially on a lbs-volume basis and is used throughout the rubber and plastic industry.
