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Purchasing Basics: Plant water cooling systems

May 23, 2001

7 Min Read
Purchasing Basics: Plant water cooling systems

Effectively and efficiently controlling the temperature of molds through water or other fluids is one of the most basic functions your molding operation needs to handle well. Poor cooling affects cycle time, which affects residence time, which influences part quality, and so on. Controlling mold temperature, whether by cooling or heating, is not guesswork, but based on tried-and-true engineering formulas. The function of controlling the temperature in the mold can be achieved by a portable chiller, a central chilling system, or by a cooling tower. The cooling may be by air or by water. Mold temperature control that requires heat can use either water or oil as a medium, depending on the need. 

Chiller capacity for common resins

Use this chart to calculate chiller capacity for various parts and molds. For example, if you are molding 300 lb/hr of ABS, divide 300 by 50. The result would be 6.0, the chiller capacity required for this job for the mold alone. 

HDPE

30

 

Urethane

40

LDPE

35

 

PET

45

Acrylic

35

 

ABS

50

PP

35

 

PS

50

Nylon

40

 

Acetal

50

PPE

40

 

PVC

75

Where to Start 
Is this purchase for a specific application requiring chilled water, a replacement chiller to add capacity, a supplement to a central system, or for a new or expanded facility? A central system makes sense if most of the applications are in a similar temperature range. Those that fall outside that range can be handled by a portable chiller. Many plants have a combination of solutions. 

If the plant is an existing one, vendors will want to know what you are doing now and how it is working. Is there additional capacity in your present system? What equipment are you now using? What materials are you running, and on what cycles? The plant's climate is important. Can excess capacity be used as heat to warm the plant in the winter? Are you looking at air cooling or water cooling? 

What kind of water is available? Is it city water or well water? What is the available power in the plant, in voltage and amperage? What is ambient air temperature? What's the temperature of the water coming from the cooling tower? Do you know the desired temperature range for the material you are running? (see box, bottom). In an existing application, what is the current starting and ending temperature of the item being cooled? What is the entering and leaving water temperature? How long does it take for the part to reach the proper level of cool? 

More than the Mold 
One important factor to note when gathering information to help size a chiller or a central system is that there are other things that need cooling besides just the mold. There are the machine hydraulics, for instance. Machine hydraulics need to be cooled only to 85F, however, so usually a cooling tower can handle this load. If you add up the total horsepower of all your molding machine hydraulics, then multiply by .1 ton/hp, you'll find the total chiller capacity for the machine hydraulics. For example, if your plant has two machines with 175 hp, four with 125 hp, seven with 100 hp, and four with 60, you have a total hydraulic cooling load of 1790 hp, or 179 tons. A 180-ton air-cooled chiller will operate using approximately 200 hp. 

A 144-ton tower, on the other hand, which has no compressor, will provide cooling for this same load using just 10 hp. With an operating cost of approximately $300/year/hp, the air-cooled chiller would cost roughly $60,000/year to operate, while the tower would cost $3000. Not a tough decision to make in favor of the tower for hydraulic system cooling. 

Rules of Thumb

  • If 80 percent of your materials are processed within a ±5 deg F temperature range, you should consider a central chilling system. If not, consider portables.

  • A chilling system requires 2 percent more capacity for every degree below the nominal rating of 50F.

  • Nominal design flow from a cooling tower is 3 gal/min/ton; from a chiller, it's 2.4 gal/min/ton. These are the flow rates necessary to achieve a 10 deg F change in the process. 

However, there are still other machine-related cooling needs. Hot runner systems have a heat load of .15 ton/kW. Cooling requirements here are often overlooked. If you don't know the kW usage, multiply the amps times the volts and divide by 1000; this equals the kW load. If you are running nylon, the mold will have to be heated using a temperature control unit; the heaters generate a heat load of .2 ton/hp. Machine feed throat cooling, while usually lumped with machine hydraulics, has a 1/2 ton heat load per machine. 

Here's an example of why cooling the mold by material formula isn't enough. A 300-ton molding machine processes 60 lb/hour of polyethylene in a hot runner tool with 27 kW of embedded heaters and uses two 3-hp mold temperature control (MTC) units. What is the mold load? Let's calculate it. 

Material load = 60/30 lb/hr/ton = 2 tons 

Hot runner load = 27 x .5/3.5 = 3.85 tons 

MTC load = 6 hp x .2 ton/hp = 1.2 tons 

Total load = 2 + 3.85 + 1.2 = 7.05 tons 

Beyond that, you may require cooling for air compressors or material dryer aftercoolers. All should be taken into consideration before talking with your vendor. 

Price Variations 
One of our contributing vendors explains why prices on any given size of chiller can vary by more than 200 percent. This is due to the variations in quality and components built into chillers. There are different types of compressors, each with its specific advantages. Semihermetic are more expensive, but can be torn apart and rebuilt. Hermetics are generally less expensive, but cannot be rebuilt. Scroll compressors, as hermetic, cannot be rebuilt, but operate more efficiently and at a reduced operating cost, though initially more expensive. Condensers vary from chiller to chiller; less expensive chillers combine condensers, fans, and compressors in a configuration that is relatively inexpensive to manufacture, but hard to service. 

Mold temperatures by resin

Most resin suppliers post a range of mold temperatures for their resin grades. These are merely guidelines for the most common levels. Consult your resin supplier or Pocket Specs for Injection Molding, a publication of Injection Molding Magazine and IDES, which lists mold temperatures for most of 15,000 individual grades of injection molding resins. For more information on Pocket Specs, see www.immbookclub.com

 

ABS

100-140

PET (high heat)

300

ABS, filled or reinforced

140-200

PETG

60-100

Acetal

170-200

Plastomer

50-85

Acrylic

100-175

Polyaryletherketone

300-400

Cellulose acetate

110-165

Polyetherimide

250-350

Ionomer

40-120

Polyethersulfone

200-350

LCP

85-200

Polyethylene, high density

40-100

LCP, filled

150-220

Polyethylene, low density

80-150

Nylon 6

150-200

Polyimide

300-400

Nylon 6/6

100-200

Polypropylene

70-140

Nylon 6/10

130-200

Polystyrene

100-180

Nylon 4/6

180-300

Polysulfone

200-300

Nylon 11

100-150

Polyurethane

60-140

Nylon 12

150-200

PPE

150-220

PBT

130-180

PPS

200-300

PBT, filled

140-250

PVC

50-150

PC

160-240

SAN

120-175

PET

180-250

Styrene butadiene

50-120


Materials also make a difference; some chillers use stainless steel, while others do not. Cast vs. welded tanks, types of heaters and gauges, types of controls, and many other factors influence how well the chiller is built and how long it will last. 

With our thanks . . .
. . . to all the suppliers who provided information for this article: Advantage Engineering, Capitol Temptrol, Conair, First Choice Chillers, Mokon, and Sterling. 

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