Understanding how to determine injection molding costs can be beneficial in three ways:
- To determine if making the product will be profitable,
- To play “what-if ’ games by changing materials, cavitation, volumes, or press sizes to see if additional profit can be generated, and
- As a monitor to determine whether or not outside molding vendor costs are reasonable.
There are four basic items to be analyzed for determining molding costs: material, labor, machine rates, and toolings’ costs.
When calculating material costs, the volume, or cost per cubic inch is the factor to look for, not just the cost per pound of raw material.
If possible, it is wise to utilize regrind plastic, as the price is usually half the cost of virgin material.
The amount of regrind that can safely be utilized is usually 15% of the total by weight. However, tests have shown that 100% regrind can be just as good as 100% virgin concerning final physical properties.
Unless hot runner systems are being utilized, the amount of material that makes up the runner and sprue should never be more than 15% of the total shot size, by weight. Following this rule of thumb will ensure that there is no more than 15% regrind material available for use.
Clamp tonnage determines the size of machine needed for molding. This is calculated by determining the projected area of the cavities and multiplying that number by a factor from 2 to 8, which is the number of tons required to keep the mold closed against injection pressure.
The machine hour rate, or MHR, fluctuates because of geographical location, with the Northeast and far West being the most expensive areas in the U.S.
The total molding cycle is primarily dependent upon the cooling portion of that cycle.
Tooling costs are not normally amortized over the life of a product, but rather are paid for up front in a lump sum.