Plastics machinery doesn't have to fly, or float. It has to withstand a lot of force without breaking, and without bending much, and survive the occasional caress of a forklift.
Occasional detailed analysis might give you a better handle on where the 'rules of thumb' come from, but you really have no incentive to reduce the machinery's mass. In fact, other things being equal, astute buyers will prefer heavier machinery, just because that usually correlates to trouble free service.
You could go looking for work, right where you are. Do you have chronic unexplained field failures in any part of your product? Unexpectedly short bearing life would be a symptom. You might not know about it if it happens outside the warranty. Look for aftermarket traffic in replacement parts that shouldn't need replacing. Dig deep, analyze everything, find the real root cause, improve the product.
Or design something on the side. Like model airplanes or engines, or anything that strikes your fancy, just to keep your brain working. If you can get money for it, that keeps you in beer. If you can't, you have a hobby.
I would _not_ advise giving up a good job, or even a mediocre job, just because it doesn't challenge you in every possible way. Make lemonade.
Oh, yeah. I find this scary, but in some places, the calculations and FEA are done by designers, who happened to be sitting in front of the fast computers when the Pro/E salesman slithered through. The engineers were downsized to pay for the Pro/E. They'd be happy to take designer jobs. They'd be ecstatic to take your job.
Mike Halloran
Pembroke Pines, FL, USA