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What to Consider When Converting Parts from Machined to Molded

You’ve developed a product using machined parts and things seem to be going well, but the part price is bogging you down. You know you could improve your bottom line and grow your market share if you could get some cost out of those components. Switching to molded parts could make a difference, but when is the right time? Numerous factors must be considered when deciding to go molded.

Consider why you developed the product using machined parts in the first place. Machining parts from solid stock was a quick way to develop and test your design. Although the per-unit cost was high, the outlay was minimal because quantities were low. The commitment was low, too. If a design change was needed, a change in the machining program would do the trick. This was a fast and easy approach with a low total cost.

Now the design has settled down. Changes are fewer and further between; the product is ready for (or is gaining momentum in) the marketplace and it would be nice to start making a profit. Now is the time to switch to molding.

Making the decision to use molded parts can be intimidating. What are the real savings and how can you be sure that the timing is right? The answer is the standard economist’s answer: it depends.

All other things being equal, the simple financial decision is how quickly the part cost savings will pay for the mold. The three most important factors in this analysis are part complexity, material selection and the quantity of parts needed.

The cost of the mold is greatly influenced by part design complexity, which is also a key machining cost driver. Undercuts, side-actions (features not in the mold’s line of draw), threads and fine details increase mold cost. However, because these features also increase the cost of machining, the molded unit cost becomes more attractive as part complexity increases. The decision now depends upon material selection and part quantity.

Material waste plays a significant role in cost considerations, especially with more expensive grades like implantable PEEK. With some materials costing upwards of $4 per gram, your part cost could double due to the amount of material that is machined away. The molding process generates some waste, too, but is usually a much more efficient use of material.

Justifying the up-front mold cost with molded part price savings and material savings then becomes a question of quantity. Since the cost of each molded part is significantly less than a machined part, you must determine how many parts it will take to pay off the mold. You may be able to justify molded parts with as few as 100 parts per year if the machining costs are high or if machining generates significant material waste. Often, the expense of building a mold can be recovered by part cost savings within the first year of production—sometimes in six months or less.

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