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[Antoine Lefèvre]

Hi! Deciding between low-volume production options for a small elastomeric over-molded grip on a handheld power tool. The part has undercuts and thin ribs, and we need decent tear resistance, but only ~200–300 pcs/year. Considering tooling cost and functional performance, has anyone compared urethane casting to injection molding for this kind of soft, detailed grip geometry, and what trade-offs in surface finish and material behavior you ran into?

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[Benjamin Marchand]

In this instance, it will certainly be the more cost-effective manufacturing route for that low production volume. Generally, injection molding is only used for volumes of 10,000 or more, because the cost of producing the steel molds is so high, so injection molding is off the table for you here.

However, there are some limitations with urethane molding. For one, you’re limited to using a polyurethane (I am assuming that’s the material you’re already planning to use). Is it the right elastomer for your grip? A second limitation is that only smaller sizes are possible to manufacture, but I think a grip for a handheld power tool will be small enough to be managed. Also, you’ll need to make a few copies of your silicon molds; they’re generally only good for about 30 units or so, so you’ll never do your annual production from a single mold.

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[Sébastien Colin]

Urethane casting can be a bit tricky, especially since you want a detailed geometry. You need to be careful to get the air bubbles out before the urethane cures, and features like your planned thin ribs are a key place where bubbles can be trapped. You may need to consider a prepolymer casting approach (premix for a more stable pre-polymer) or use a vacuum chamber. Lots of ribs will also cause your silicon mold to degrade faster as you demold.

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