Mold cooling, part deux

Readers with long memories might recall that I started a very preliminary look at mold cooling in the February 2010 issue of Canadian Plastics.

The subject is phonebook-thick if you want to really understand it, but fortunately, the most important cost aspects of cooling the mold don’t require an engineering degree to understand.

Cooling is by far the longest portion of a typical mold cycle for commodity resins, so it’s natural that it’s also the best place to shave seconds off when looking to gain productivity. The temptation is to oversize the chiller and crank up the flow rate for the lowest possible outlet temperature. Colder is better, right? In theory, yes; in the real world, it’s a little more complicated.

The actual flow of heat from the molten resin goes through the metal of the mold cavity and then into the cooling water flow…but the proper way to think of it is as a one-step process, from the part to the mold wall. The water’s job is to keep the mold from heating up, which may seem to be the same thing, except for one important fact: Unlike a direct heat flux from part to coolant, the effect of lowering the water outlet temperature doesn’t automatically translate into noticeably faster cooling in the cavities.

Cranking up the flow rate can actually decrease cooling efficiency if it upsets the turbulent flow needed for efficient heat transfer at the walls of the cooling channel. Then there’s the shrinkage issue. Part cooling actually starts the instant the shot hits the cavities, and for common resins like PP and PE it will shrink noticeably. The hold time serves not only to fill the mold, but also to keep the part pressurized to counteract this shrinkage. Shrinkage control is often the key quality issue, and it isn’t a given that colder is better.

The blow molding community, for example, commonly uses oil–and hot oil, at that–as a cooling medium, so there’s more to optimizing cooling than lowering the set temperatures. And then there’s my favourite: ejection temperature. In a perfect world, you’d open the mold at the instant the parts are solid enough to hold their shape, which is generally somewhere between Tim Hortons’ double-double hot and blister-your-fingers hot. Why? Post-mold cooling is essentially free, and, in any case, they’re going to sit in those Gaylords for some time in even a captive molding/ assembly operation.

But if you optimize this part of the cooling cycle, keep in mind a couple of factors. First, any press-side dimensional checks won’t be at the comfortable air-conditioned temperature of the QA lab or office; this means there will be a temperature gradient between the parts at the top of the bin and at the bottom, so make sure your SPC police dig down into the pile to grab a true sample.

Second, don’t forget the operator. After-mold cooling can make the press side environment damn hot, and few plant engineers I’ve met consider this when laying out the production floor. I once built a “cool cab” for a miserable operator, basically an open-ended phone booth with a small residential A/C unit blowing cool air onto her as she worked the mold. Wasteful, yes, but check out how many bathroom breaks your crew takes when working in hot environments and you might find that stopgaps like that can pay. Sometimes you need to cool the people as much as the parts!