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Clean cooling water clears up molding problems.

One automotive injection molder finds that raising the quality of its cooling water translates into higher quality molded parts. Closed-loop cooling is the key.

"Cooling water is one of the most important parameters that affect production of plastic parts, and the most overlooked," says Bill Plate, a 30-year veteran injection molder. As U.S. operations manager for the Automotive Components Div. of Windsor Mold, he has seen the benefits of rethinking the typical cooling system found in plastics plants. One benefit is more consistent temperature control. The other is cleaner cooling water, which results in better productivity, higher product quality, and longer mold life.

Plate first saw the advantages of a non-traditional, "closed-loop" cooling system at Windsor Mold's Autoplas molding operation in Bellevue, Ohio. That experience was confirmed at the new plant opened in Bellevue last year for a sister company, Precision Automotive Plastics, a Tier 1 automotive injection molder. The closed-loop cooling system for that plant was designed and installed by Engineered Process Cooling Systems (EPCS) of Huron, Ohio. "Our purpose is to remove cooling water from the list of variables that bedevil injection molders," says president Glen Ginesi.

Dirty facts of cooling

Dirty cooling water is a prescription for molding headaches. Biological fouling from algae deposits can clog up plumbing. Lime scale build-up inside mold cooling lines can steadily reduce cooling efficiency until the tool must be pulled out of production to have its cooling passages cleaned out.

Precision Automotive found that these contamination problems can be overcome, and cooling-system maintenance greatly reduced, by eliminating the open reservoir in the cooling system. Typical "open-loop" cooling systems commonly found in plastics plants are particularly susceptible to water contamination, according to Felix Theys, v.p. of engineering for EPCS. Open-loop cooling systems use a large water reservoir as a heat sink to minimize water-temperature variation. However, because of its exposure to open air, this reservoir is also a potential avenue for bio-contaminants such as algae to enter the system. Also, minerals tend to accumulate in the water supply as water evaporates from the open tank.

Thus, open-loop cooling systems require vigorous maintenance to keep the water lines open and functioning properly. Water must be bled off to keep algae from forming and chemically treated to prevent both scaling and bio-fouling. Water lines also need regular inspection for scale build-up, advises Theys.

Plate says he chose a closed-loop system to obtain more consistent temperature control. In his experience, including 23 years at Ford Motor Co., an open-loop system with an exposed water reservoir is influenced more by swings in ambient temperature. "We'd have to constantly monitor and manually adjust cooling-water temperature with an open system," he says.

Closing the loop

Precision Automotive Plastics opted for closed-loop cooling when it built its 75,000-sq-ft Bellevue plant last year. The plant required a cooling system big enough to handle 10 injection machines, each larger than 2000 tons clamp force.

The plant's closed-loop system has four separate but interacting cooling loops (see diagram). Two separate, totally enclosed piping systems serve the molds and the oil-cooling systems on the injection machines. This set-up prevents evaporation or contamination of the water in those two loops. The hydraulic-oil cooling circuit contains water and is kept at 85 F. It has 250 tens of cooling capacity. The mold-cooling circuit has 150 tons of capacity and operates at 50 F. Total cooling capacity for the hydraulics and the molds is 400 tons.

The closed loops dispose of collected process heat by transferring it to a third closed loop containing 30% glycol that goes to either a chiller or a cooling tower, depending on the season. The only open part of the system is the water reservoir at the base of the tower. Water from the reservoir is sprayed over a closed coil through which is pumped the water carrying heat transferred from the molds and hydraulic oil. But the water in the reservoir is never in direct contact with the process-cooling water.

Cooling water in the closed loops is chemically treated at the initial fill to dissolve the solids and keep them in suspension, explains Theys. Then an oxygen scavenger is added to remove dissolved oxygen, so that the water will neither support microbes nor cause rust. A water meter is attached to the incoming water line to keep track of new water added to the system to replace water lost from leakage and mold changes. New chemical treatment is added only in proportion to the replacement water and is thus kept to a minimum. "The system becomes much easier to maintain and keep clean," says Theys.

Dave Musick, maintenance supervisor at Precision Automotive, was leery about the unfamiliar closed-loop concept at first. "Now that I see all it does for us, I'd never go back to an open system," he says. "I figure we save close to 75% of the maintenance time we'd have to spend operating with an open-loop system. With open-loop, we'd be constantly cleaning the cooling tower. Indoor sumps fill up with sediment regardless of the filtration. We'd always have to watch for scale build-up in the water lines. It doesn't take much scale to ruin your day. With the closed-loop system, we clean the tower every quarter, and once a year we grease the pump motors and check the suction strainers and electrical connections. We don't even have to clean the heat exchanger two or three times a year."

The closed-loop system cost about 5% more to install. This cost was offset by savings in maintenance time, using 90% less chemical treatment, and eliminating replacement of water lost to evaporation. Payback was less than one year, according to Theys.

Another advantage of the closed-loop system is space savings. Because the large water reservoir is eliminated, the closed-loop system typically takes up 40-50% as much space as an open-loop system, Theys notes. "Every square foot we can save is like finding gold," says Plate.

'Free cooling' outdoors

Precision Automotive's cooling system has other features that cut operating costs and improve productivity. One is using outdoor temperature to provide "free cooling" when the season permits, thereby reducing usage of the central chiller. The plant runs in the "free-cooling" mode for about seven months of the year and uses the chiller for the remaining five.

Tracks loads closely

The Trane chiller selected by EPCS for the Bellevue plant has rotary-screw compressors instead of the usual reciprocating type. Typical chillers with reciprocating compressors operate at "stepped" loads between zero and 100%. Instead of being limited to discrete steps, rotary-screw compressors ramp up or down continuously to track the precise load imposed by the molding machines. Theys says this is more efficient and allows the system to maintain tighter control of process temperature. Plate estimates that the new cooling system at Precision Automotive has helped reduce the scrap rate there by up to 5% on some parts.

A balanced system

Cooling water is supplied to the molds and hydraulics using a "reverse-return" piping system, which maintains equal pressure to each molding press in the plant. Essentially, the length of the coolant piping is the same for every machine in the circuit. This allows the system to self-balance because the distance from the pump, through any of the loads, and back to the source is exactly the same. This is important because it provides the same conditions at every press and mold on the production floor, says Theys.
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Title Annotation:plastics molding
Author:De Gaspari, John
Publication:Plastics Technology
Geographic Code:1USA
Date:Nov 1, 1998
Words:1238
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