Cost of Injection Molding Plastic Parts
The injection molding process is complex with hundreds of variables which influence cost. However, the most influential variables are material, molding machine time, and man-hours.
Material selection is an important factor in part cost. In all but the very lowest-volume parts the cost of the resin directly accounts for a set portion of the per-part cost. Since your parts remain constant in volume regardless of the number of cavities, the only ways to reduce this portion of the part cost is to choose an economical material or design the part to minimize volume.
Well-designed parts which minimize part volume by avoiding thick sections and maintaining constant wall thickness reduce not only the cost of materials but also another large factor in part price: molding machine time.
Molding Machine Time
Molding machine time, or cycle time, is the time required to close a mold, inject the material, cool the material, open the mold, eject the part, and, in the case of inserts, reset the mold.
Cycle times can be made faster by having well-designed parts with walls that maintain a uniform thickness and which are no thicker than necessary. Thick-walled parts take longer to cool due to the extra material and slow down cycle times.
On a per part basis, cycle times can be greatly improved by adding additional cavities to the molds. By using multi-cavity molds one cycle can produce several parts instead of just one in the same amount of time.
Cost of Labor
Labor costs contributing to per part cost can be split into two categories: set-up and operator.
Set-up costs reflect the time it takes to mount the mold onto the molding machine, warm up and pre-cycle the machine and the material, and calibrate the run for maximal efficiency. Set-up costs become insignificant for long run parts since the small cost is averaged over a great number of parts. Short runs, however often include a set up charge since the cost remains significant on a per part basis. Epsilon charges a $350 set up charge on orders of less than $1500 and a $500 set up charge for clear material for orders of less than $2000.
Operator costs are added if the part needs to have an operator involved in the cycle. Operators are needed for hand-loaded inserts, special no-ejection instances, clean or cosmetic parts that cannot be dropped into a hopper, or a variety of other instances. Generally the presence of an operator is determined at the design phase of the part and the additional part cost is weighed against the tooling costs which be necessary to automate the operators' tasks.
Here is an example to demonstrate the above: Suppose we had a part that contained 10¢ of material and a $1.00 cycle. If we had a one cavity mold, the parts would cost $1.10 (material + cycle); however, if we had a four cavity mold, the cycle would produce four parts instead of just one resulting in a per part price of 35¢. (material + cycle/4)
|Cycle (per part)||1.00||0.25|
|Cost Per Part||1.10||.35|
Of course the other determining factor is how many parts will be run. If a lot of parts will be made, then the savings on the parts justified the extra expense of the additional cavities. However, if only limited quantities will be made, then it would be a better decision to pay more per part and avoid the extra tooling expenses.