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Taking time for temperature control

January 1, 2006

4 Min Read
Taking time for temperature control

Proper mold temperature control affects everything from part quality to cycle time, yet selecting a temperature control unit (TCU) is often given little consideration.

TCUs in fact can quickly impact the bottom line if they limit mold performance because the pump is sized incorrectly or they are awaiting seal replacement in the maintenance department. TCUs typically cost between $1200 and $5000, but the cost of improper selection is much more when you include increased cycle times, downtime, and maintenance.

TCUs provide mold heating and cooling via either direct or indirect operation. Both offer advantages and disadvantages based on the application. Direct-cooled units can cool to lower temperatures and offer a lower temperature, but performance is sacrificed-when the cooling valve opens to allow hot water to escape and cold water to enter, a major pressure and flow drop is experienced through the mold. Indirect-cooled units, however, use a closed system that offers many more features such as leak detection, mold purge (without compressed air), more consistent pressure and flow, tighter temperature control, and more.

The first consideration in selecting a TCU is to determine the heating requirements of the mold.

Use the formula:

P = m x cp x T

P is power in kW, m is mass in kg, cp is the specific heat of the mold material, and T is the difference between the mold starting and operating temperature.

For example, consider a mold that weighs 3000 lb, the specific heat of steel is .117, and we want to heat the mold from 75°F to 150°F (T=75 deg F).

P = 3000 x .117 x 75 = 26,325 BTU, or, to convert to kilowatts, divide by 3415 to get 7.7 kW.

Specifying the pump type and size is often where pressure is overlooked or sacrificed for flow. A quick calculation shows you can physically only pass so much water through a specific pipe diameter, which is why pressure is often much more important than flow. Centrifugal pumps produce higher flow rates but with lower pressure, while turbine pumps produce higher pressures with lower flow rates.

Comparing pump curves for the two pump types of identical horsepower, the centrifugal pump is shown to offer 30 gpm under no restriction, while the turbine pump offers only 16 gpm. The turbine pump can provide more pressure when a restriction is applied and hence, higher flow rates for better cooling than the centrifugal pump. If, for example, the restriction was a .25-inch cooling line, we see from the chart that the turbine pump produces approximately 35% better flow than the centrifugal pump.

The added benefits of sealless turbine pumps cannot be understated when you consider the annual cost of maintenance to replace pump seals, not to mention the costs of downtime from lost productivity. Consider a plant with 10 TCUs; if each one requires seal replacement every three months and it takes three hours each to perform, then the annual cost could easily exceed $16,000 ($100/seal, $150 labor, $150 downtime = $400 every three months x 4 quarters x 10 TCUs = $16,000) before adding the time to disconnect/connect the TCU and lost production. The savings would be like getting several free TCUs annually.

You also want to maximize the operation of the TCU through control options. In addition, there are many added features that are available for TCUs to maximize their performance, such as leak stop and negative pressure, which may be standard or options to consider. Something as simple as having mold purge can be a huge advantage if you perform frequent mold changes.

To avoid water quality and heat-transfer problems, you want to ensure all wetted surfaces are of nonferrous construction. Then there are items like insulated tanks for better temperature control or auto fill and leakage monitoring. A safety temperature limiter that does not rely on the control circuit or electrical power can be extremely beneficial. Consider maintenance friendliness with a rugged frame construction for easy or toolless access and longer life.

For heating requirements up to 320ºF, water units are available, but above that you need to consider oil. Or, you may require a second unit to meet the heating and cooling requirements, introducing the possibility of using a dual unit with reduced footprint and cost vs. two independent units.

To properly specify your TCU and to maximize the benefits, involve the OEM or your supplier when performing the heating and cooling calculations to review your specific requirements. Correctly specifying your TCU can result in substantial savings.

Wes Moffitt, water products manager, Wittmann Inc. www.wittmann-ct.com

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