Learn more about the main cost drivers in injection molding and actionable design tips that will help you reduce the costs of your project.
The biggest costs in injection molding are:
Tooling costs determined by the total cost of designing and machining the mold
Material costs determined by the volume of the material used and its price per kilogram
Production costs determined by the total time the Injection molding machine is used
Tooling costs are constant (starting at $3,000 and up to $5,000). This cost is independent of the total number of manufactured parts, while the material and production costs are dependent on the production volume.
For smaller productions (1,000 to 10,000 units), the cost of tooling has the greatest impact on the overall cost (approximately 50-70%). So, it’s worthwhile altering your design accordingly to simplify the process of manufacturing of the mold (and its cost).
For larger volumes to full-scale production (10,000 to 100,000+ units), the contribution of the tooling costs to the overall cost is overshadowed by the material and production costs. So, your main design efforts should focus on minimizing both the volume part and the time of the molding cycle.
Tip 1: Stick to the straight-pull mold
Side-action cores and the other in-mold mechanisms can increase the cost of tooling by 15% to 30%. This translates to a minimum additional cost for tooling of approximately $1,000 to $1,500.
In a previous section, we examined ways to deal with undercuts. To keep your production on-budget, avoid using side-action cores and other mechanisms unless absolutely necessary.
Tip 2: Redesign the injection molded part to avoid undercuts
Undercuts always add cost and complexity, as well as maintenance to the mold. A clever redesign can often eliminate undercuts.
Tip 3: Make the injection molded part smaller
Smaller parts can be molded faster resulting in a higher production output, making the cost per part lower. Smaller parts also result in lower material costs and reduce the price of the mold.
Tip 4: Fit multiple parts in one mold
As we saw in a previous section, fitting multiple parts in the same mold is common practice. Usually, 6 to 8 small identical parts can fit in the same mold, essentially reducing the total production time by about 80%.
Parts with different geometries can also fit in the same mold (remember, the model airplane example). This is a great solution for reducing the overall cost of assembly.
Here's an advanced technique:
In some cases, the main body of 2 parts of an assembly is the same. With some creative design, you can create interlocks points or hinges at symmetrical locations, essentially mirroring the part. This way the same mold can be used to manufacture both halves, cutting the tooling costs in half.
Tip 5: Avoid small details
To manufacture a mold with small details require longer machining and finishing times. Text is an example of this and might even require specialized machining techniques such as electrical discharge machining (EDM) resulting in higher costs.
Tip 6: Use lower grade finishes
Finishes are usually applied to the mold by hand, which can be an expensive process, especially for high-grade finishes. If your part is not for cosmetic use, don’t apply a costly high-grade finish.
Tip 7: Minimize the part volume by reducing wall thickness
Reducing the wall thickness of your part is the best way to minimize the part volume. Not only does it mean less material is used, but also the injection molding cycle is greatly accelerated.
For example, reducing the wall thickness from 3 mm to 2 mm can reduce the cycle time by 50% to 75%.
Thinner walls mean that the mold can be filled quicker. More importantly, parts thinner parts cool and solidify much faster. Remember that about half the injection molding cycle is spent on the solidification of the part while the machine is kept idle.
Care must be taken through to not overly reduce the stiffness of the part which would downgrade its mechanical performance. Ribs in key locations can be used to increase stiffness.
Tip 8: Consider secondary operations
For lower volume productions (less than 1000 parts), it may be more cost effective to use a secondary operation to complete your injection molded parts. For example, you could drill a hole after molding rather than using an expensive mold with side-action cores.
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