Continuing on from where we left off, in this ongoing series of dissertations regarding the field of engineering design, I started to discuss how a simple feature on a plastic part, made by the “Injection Molding” (IM) process, might have tremendous cost ramifications. The case in point involves a plastic carrying case for a home sewing machine.
The four main components were the Cover, Base, Handle and two Side Locks that were all IM parts. The sewing machine was either attached to or just sat in the Base. The Cover, with a retractable Handle and two Side Locks was placed onto the base, covering and protecting the sewing machine. The Cover was the largest and heaviest part. To help you visualize it, consider an office waste basket measuring approximately 20” x 10” x 18” high. Now, turn it upside down and you have the Cover. It had a nominal wall thickness of 1/8” and weighed about 5 lbs. (For those more familiar with IM machines it was produced on a 700 ton, 200 cubic inch toggle-press machine.)
It was a really modern, smart looking carrying case. The industrial design department for the customer had certainly done their homework. That is, except for one small detail. Picture a house with a gable roof that extends over the side walls. They had come up with a design that had the front, top and back outside surfaces extending over both of the side walls. In order to mold the part, as designed, would require side-pulls that measured 10” x 18” on either side of the cavity. It certainly could be done and I said so.
However, doing so would result in a mold that was much weaker, a lot less reliable, would require a great deal of maintenance and would cost 3x the price, unless they made a very simple design change. The recommended change was simply to take the feature that caused the problem and recess it into the part instead of having it extend from the part. The “look” that the designers wanted would still be there. That, along with a tooling bill that was $100,000 less and a mold that could run 24 hrs/day, sealed the deal.
I’m not telling you that I hit home runs every time, but I definitely do get on base. Let me pinch hit for you by calling me for a no-cost, no-obligation appointment, so I can assist you with any of your product design or re-design issues. Tooling assistance is provided too.