Canadian Plastics

Lightweighting? I’m skeptical

Lightweighting is all the rage these days, with volatile resin pricing and razor-thin margins driving processors and their customers to win back a few pennies per part any way they can.

November 1, 2006   By Jim Anderton, technical editor



Lightweighting is all the rage these days, with volatile resin pricing and razor-thin margins driving processors and their customers to win back a few pennies per part any way they can.

That’s understandable, and laudable, but whenever I hear about huge savings to be had by shaving the part mass to featherweight proportions, a little red flag pops up in the back of my mind. There are a hundred ways to get into serious trouble with a lightweighting program and most of them, as usual, will involve the molder. That’s ironic, since part design and material selection are two parameters usually outside the molder’s control.

Why would a redesigned, lighter part be trouble in the making? There are many reasons. Adding void spaces in non-structural regions of the part is as old as engineering itself (look at those old iron lattice bridges for example) but to achieve it without expensive core pulls, the part is often reoriented relative to the mold parting line, where the “void” is formed. For I/M processors, the result can be the same number of cavities, a slightly smaller shot weight, but a larger mold.

Another factor that might conflict with the new orientation is the need (for many resins) to put the part’s heaviest wall sections closest to the gates because of the pressure gradient as the resin flows deeper into the mold. If gates have to be radically relocated, it may be necessary to go to a totally new hot runner setup, with the learning curve that that inevitably brings.

Then consider cooling. A smaller, lighter part should cool more quickly, for faster cycle times, right? Ideally, yes, but thick section parts cool at differential rates at their surface and interior, creating residual stresses that the mold engineer considers and may exploit in the mold design. Thicker sections generally shrink more than thin sections, so if shrinkage has the effect of helping the part release, bigger draft angles may be necessary.

Then consider mold filling. A part with lower resin mass will fill more quickly, right? Not necessarily! Herbert Rees’s excellent “Understanding Product Design for Injection Molding” (Hanser, 1996) expresses it algebraically:

Q = PxH3

———-

12

Where Q is the volume of resin passing through gap H under a pressure drop of Px with a resin melt viscosity of .

What’s the most important part of this equation for our purposes? It’s the “H cubed” term. If the H represents the height of one dimension of a cavity, then halving that dimension reduces the flow by eight times! The equation is a huge simplification for complex part shapes, but the point is clear: Make the part a little thinner and there can be major implications for mold filling.

The worst part of it all for job shops is that none of this is under the molder’s control, yet the molder is expected to mold the new lightweighted part at a lower unit cost than on the previous tool. There are many more examples, too.

What can you do? The main thing is to regard a new mold for a lightweighted part as an entirely new job, and quote it to include the usual downtime, learning curve and loss rates that you’d expect from any new job at start-up. The customer will insist that it’s just like the old part, but with faster cycle times. That may be true eventually, but for a while you can bleed pounds of dollars for every ounce of saved resin.


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