What will your part cost?

Estimating your part cost can be frustrating, because a full quote requires a final part design and decisions on how big of a mold to make. But that can take a lot of effort, and determining whether that effort is worth it is made easier with…a part cost.

So what affects part costs and how can you estimate them?

Below are seven major factors that influence what your part will cost per unit.

1. Material selection

Commodity grade resins are lower cost than engineering grade resins. Black parts are lower cost than brightly colored parts.

If it performs satisfactorily for your part, we recommend choosing polypropylene or high density polyethylene for most injection molded parts. Selecting high-end engineering grade resins is an option when certain rigidity or temperature requirements call for it, but typically come at significant cost.

2. Cavitation

‘Cavitation’ refers to the number of parts produced every cycle. A single cavity mold produces one part each cycle. By making a mold that has several cavities, such as a 2x2 grid of cavities, it’s possible to produce four parts per cycle. While that makes for a more expensive mold, increasing the number of part produced per cycle means that the cost of running the machine is amortized over more parts, and that reduces your part cost.

3. Run length

As you can imagine, you’ll get lower part costs from your supplier as you order more parts. But why? Is it because they figure you’ll shop around more?

Injection molding has high returns to run length. That means that it takes work to put a mold in a machine and get it dialed in, but once it’s running, it takes much less supervision. So longer runs become much more economic than short runs, because the cost of that setup time is averaged out over many days of production.

So a major determinant of your part cost is not just how many you’ll order in a year, but how many you’ll order at one time. There’s a big difference between ordering 20,000 parts one time per year, and the same quantity but spread out over ten runs: ten runs means ten setups, and much more idle time for the molding machine.

For that reason, it’s often more economic to order fewer times per year, but with bigger jobs, to drive your cost down. Weigh that against storage costs to find an ideal run length for your business.

4. Wall thickness

The thickness of the walls matters because of the way the part cools in the mold. Plastic is a good insulator, so thick walls allow the outer edges of the plastic - the plastic adjacent to the mold’s cooled steel - to insulate the plastic in the middle of the wall. The thicker the wall, the more insulation there is - and the longer the part takes to cool off the center.

When a part takes longer to cool, then fewer parts are produced per hour, and the cost of operating the injection molding machine is amortized over lower output. Costs go up.

That’s why it’s often ideal to find ways to strengthen your part that do not involve increasing wall thickness, by using ribs or part curvature to create strength.

5. Weight

The more a part weighs, the more material cost it requires. In part design, it’s best to make the part as thin as possible while still meeting performance requirements. Using features such as ribs can help provide extra strength while adding only minimal weight.

6. Tonnage

Injection machines come in different sizes, typically rated by “tonnage,” which is the amount of force that is required to keep the two halves of the mold together during injection.

Small presses are typically under 200 tons, while medium sizes presses range from 300 to 600 tons, and large presses can range up to 2,000 tons.

Large presses cost much more to run per hour than small presses, so you don’t want to run your product in a bigger press than needed.

The size of press needed can be calculated through fluid dynamics simulation, but a simpler proxy is the ‘projected area’ of a part. Take your part, and pretend you smash it down into a two-dimensional surface, in the direction the part would come out of the mold. The surface area of that two-dimensional surface is your projected area.

Now, take the projected area (in square inches) and multiply by 3-5. That’s the typical range of tonnage required to process your part.

For example, if you have a circular part of 6” diameter, the projected area is (pi * 3^2), or about 28 square inches. The tonnage required would likely be in the range of about 100 - 150 tons. If you chose to upgrade to a two-cavity tool, then the tonnage is approximately twice that: 200 - 300 tons.

7. Post Processing

Any post-processing adds to part cost. For example, some long, flat parts must be cooled in special fixtures after molding to prevent warping. This adds cost.