A temperature control unit or “TCU” is exactly what the name implies: a device that is used to regulate the temperature of an injection molding, blow molding, extrusion or other plastics process within a narrow range in order to ensure the quality and efficiency of that process.
Unlike a chiller that mechanically removes the heat, a TCU can raise the temperature with internal heaters and cool by directly exchanging with lower temperature water or through a heat exchanger. In high temperature applications (above 300°F), some may consider a thermal transfer fluid.
Go into any plastics processing plant and you’ll see them: little metal boxes, usually on casters, plumbed to an injection molding, extrusion tooling or an extrusion cooling tank. Many of them are labeled Thermolator®.
Thermolator® is a well-known brand name for temperature control units first introduced to the plastics industry in the 1950s. Conair acquired the brand in 1977. Through the years, Thermolator TCUs have been so reliable and have become so widely used that plastics processors often use the term “Thermolator” when they are talking about any TCU.
These compact units incorporate a pump, a heater, and a cooling system, together with the electronics to ensure the proper temperature is achieved and maintained in the injection mold or whatever system is being used.
Why is Temperature Control Important?
Regardless of the process (molding or extrusion), temperature has a critical influence on how the polymer reacts, how it flows, how rapidly and consistently it sets or ‘shrinks’ into a form that can be removed from a mold or otherwise handled. Cooling of the plastic influences many different attributes of the finished product, from surface finish to dimensional stability to its physical and mechanical properties.
In injection molding, precise temperature control is required to allow cooling to take place rapidly enough to minimize cycle time, but not so fast that heat-transfer-related quality problems arise, such as improper polymer flow resulting from over-cooling or under-cooling. The latter condition would apply when parts are not completely solidified and, therefore, stick in the mold or deform after being ejected from the mold. Over-cooling may be suspected when some or all of the mold cavities do not fill properly. Surface detail may not be perfectly replicated or, worse, parts may be incomplete (short shots). In general, high mold temperature will cause the polymer to cool at a slower rate, this increases part shrinkage and causes the part to stick in the mold. A low mold temperature may cool the part too quickly and cause excessive in-mold stress.
The situation in extrusion is not that much different except for the fact that, until recently, the popular assumption was that colder water removed heat faster and would allow for higher throughput rates. In some cases that may be true, but extrusion processors still run up against many of the same problems with surface finish and dimensional stability if they are not careful. In addition, when heavy-walled extrusions are produced, rapid chilling of the surface structure can actually insulate the interior and prevent proper cooling. In many cases, extruders have begun to use TCUs to raise the temperature of cooling water to provide more controlled heat transfer for better results and even higher productivity.
How does a TCU work?
There are several different basic designs used in temperature control units but, as noted, they all have several common components:
- Electrical heater
- Precision controller
- Cooling valve to control water flow
The pump, of course, is responsible for circulating the cooling fluid through the process and back to the TCU. To achieve the specified temperature for the process – commodity plastics like polyolefins for instance, are usually cooled in a range between 70 and 80°F (21 – 27°C) and engineering materials like nylon or polycarbonate, may be “cooled” at 100 – 200°F (38 – 93°C) – the electric heater may be required. As the fluid circulates through the process, it inevitably picks up heat from the polymer before it returns to the TCU. The digital controller compares the from-process temperature to the specified to-process temperature and may initiate cooling by one of several different means. When a cold mold is being started up, the process fluid will almost certainly need to be heated, but once it has reached the proper temperature and hot material is repeatedly injected, heat removal becomes the more critical task. Being able to both cool and heat allows the TCU to maintain consistently ideal temperatures.
TCU Fluid Circuit Configurations
Direct Injection: This is the simplest configuration. It uses the same source (a chiller, cooling tower or other source) to fill the circuit and pump the fluid to the process. Fluid returning from the process enters a mixing tank. If the fluid temperature needs to be raised, the heater element is engaged to add heat to the fluid. If the process fluid needs to be cooled, a solenoid valve opens to “inject” cool water from a chiller, tower, or other water source until the correct temperature is reached. Excess warm water flows out of the circuit and back to the tower or chiller. These systems typically operate up to 250°F (121°C).
Closed Circuit: This type of circuit also uses the same source for process water and cooling water, but only to initially fill the circuit or to make up for system loss. So, instead injecting cool water to reduce the temperature of from-process water, a closed circuit system uses either a brazed-plate heat exchanger to cool the process water. This design best suits applications where the cooling process is critical, since its heat exchanger offers more heat-transfer capacity than the simple cooling solenoid valve used in Direct Injection. Closed-circuit systems also can tolerate potentially contaminated cooling water and can operate at temperatures up to 300°F (148°C).
Isolated Circuit: As the name implies, this TCU design completely isolates the process fluid from the water used to cool it. The two fluids never mix, so different fluids can be used in the process and cooling circuits. If transfer fluid or and ethylene-glycol mix is used as the process fluid, this kind of system must be used. An isolated-circuit system still uses heat exchanger (shell-in-tube or brazed-plate) but the cooling water from a chiller or tower and any contamination cannot affect the process circuit. Because the cooling circuit is open to atmosphere, this configuration is limited to up to 180°F (82°C).
To ensure that proper process temperatures are maintained, TCUs must not only deliver heat-transfer fluid at the proper temperature, but must also provide ‘turbulent flow.’ This is important because as the process fluid flows through the passages of a mold, only the fluid in contact with the cooling channel surface will readily transfer heat. If the fluid moves too slowly, the result is what’s called ‘laminar flow,’ which is characterized by smooth, constant layers of fluid, traveling in a neat, straight, uninterrupted stream. Under laminate flow conditions, outer fluid layers insulate the inner fluid layers and limit heat-transfer capacity. Increasing the speed of the fluid through cooling channels achieves “turbulent flow,” which provides the random eddies, vortices and other flow instabilities needed to break up the fluid layers. Thus, the entire volume of fluid comes into contact with the walls of the cooling passages and heat transfer is maximized, resulting in more even temperatures and more efficient processing.
Note: Different types of fluid flow are characterized by a dimensionless number called a Reynolds number, which is calculated based on fluid velocity, internal channel diameter and fluid viscosity. Laminar flow occurs at low Reynolds numbers while turbulent flow occurs at Reynolds numbers above 4000. As will become clear, Reynolds numbers are an important when it comes to calculating the amount of fluid flow required to cool the mold under process conditions.
Selecting and Sizing a TCU
In general, it is always best to seek the assistance of an expert from your equipment supplier when selecting and sizing a temperature control unit. However, you can expect to look at design considerations such as pump size, heater capacity, cooling capacity and control features. And there is certain information that is critical to the final decision. The following description assumes an injection-molding application, but the basic principles can apply to any process.
- Calculate resin loads
Based on their properties, different polymer materials give off their heat differently, and this factor must be considered when determining the capacity of a temperature control unit. If you run a variety of materials, base the required capacity of the TCU on the material that is most difficult to cool. If your mold uses hot runners, add 0.15 ton of cooling capacity per kW of heating load.
- Calculate Flow
As noted, it is important to ensure that fluid is moving through the heating/cooling channels fast enough to achieve turbulent flow. Generally, that is a Reynolds number between 4000 and 8000. Knowing the resin-cooling load and the difference between the to-process temperature and the from-process temperature (also known as the ‘approach temperature’) makes it possible to calculate the flow rate (gal/min or l/min) required. Then, pressure losses due to cooling hose diameter, number and size of couplings, etc., must be considered. Charts are readily available from TCU suppliers to help you use this information to determine the size of the pump required.
- Pump Selection
Using a pump performance chart, you can compare the performance curves on the pumps available for your selected TCU model to the performance (pressure/flow) that you need. Select a pump that provides your required pressure/flow in the “middle” of its performance curve, so that the pump motor and seals are not overstressed and the pump provides a long operating life.
- Cooling Valve
The cooling valve is selected based on the total tons of cooling required and the approach temperature. It is important to remember that the pump itself contributes to the overall heat load. The total cooling requirement is the sum of the resin load plus the heat load contributed by the pump. Again, charts are available to help determine the ideal size of the cooling valve based on the approach temperature and the total tons of cooling required. Modulating valves, as distinct from simple on/off solenoid valves, help eliminate thermal shock in the process circuit.
- Heater Size
As noted above, the heater is required when starting up a cold mold so that it can be brought up to processing temperatures. The heater size (kW) is determined based on the size of the mold, what material it is made out of and how long you want to take to heat up the mold.
Take Advantage of Your Supplier’s Expertise
The basic principles used to size and select a temperature control unit are relatively simple and have been well-established over many years. You can find a more detailed explanation in the Conair webinar, “The Benefits of Temperature Controllers and How to Choose a System.” However, there is no substitute for experience when it comes to understanding and adjusting a TCU to meet the subtleties of specific processing applications.
TCU technology is constantly changing, especially in the area of controls, which today offer a wide variety of convenience features, diagnostic capabilities and communications protocols. There are different types of heater controls and alternatives in heat-exchanger design.