Water mold temperature controllers are at the heart of a plastics manufacturing process. A temperature controller pumps water to and from the process. As water returns to the controller, it either adds or removes heat as directed by the system.
Water temperature controllers have progressed in technology over the past decade, with today’s units providing tighter control and more consistent deviations. Most technological advances have occurred in the area of programming while some have involved the mechanics. Solid-state relays for heating and improved designs for cooling valves allow the controllers to be switched more often to reach shorter regulation cycles.
Vast differences exist in the many types of controllers available. The choice largely depends on the material being molded, the mold itself and the process. Some processes can operate well with 5°F (3°C) swings in temperature, while others require tight tolerances of 1 to 2°F (0.6 to 1°C) or less. In many cases, process temperatures can be affected by outside interferences such as a door opening nearby. Although such situations cannot be entirely controlled, mold temperature controllers have come a long way in their ability to react to changing conditions.
So how do you know whether your controller is the best selection for your process? Every process is different, and a model that is ideal for one application might not be the best for another. Some key factors to consider when optimizing mold temperature control include the type of cooling (direct vs. indirect), the amount of flow through the process, the size of the heater and the capabilities of the microprocessor.
Direct vs. Indirect CoolingA direct-cooled temperature control unit (TCU) uses the cold water from the plant’s source, usually either a cooling tower or chiller (figure 1). When the process requires cooling, the temperature control unit dumps the hot water back into the plant’s water system and introduces the cold water directly into the process. This type of temperature control unit can be used in most applications up to 250°F (121°C).
Processes that have a high cooling demand or a low ΔT between the incoming plant water and the setpoint generally can benefit from direct cooling. However, water will travel the path of least resistance, and flow to the process will be sacrificed when the cooling valve is open. Additionally, the pump and cooling valve must be sized for the application to ensure efficient cooling. If the valve is too small, the unit will not be able to cool enough; by contrast, a valve that is too large will not control with much accuracy. The plant’s water supply pressure and temperature also affect the system’s cooling capacity and control: With high incoming pressure, more water can flow into the unit faster than with low pressure for increased cooling. Conversely, the lower the incoming water temperature, the more cooling is available.
An indirect-cooled temperature control unit uses a tank or vessel to hold the process water, and a heat exchanger or cooling coil inside the tank to cool the water - in other words, the cold water and process water are separate from each other (figure 2). This type of temperature control unit can control to a tighter tolerance because an open cooling valve does not allow the initial shock of cold water into the hot process water, and it also does not affect the pressure and flow going to the process. Instead of using the cooling valve as the controlling factor, an indirectly cooled temperature control unit relies on the heat exchanger; hence, the lower the ∆T, the lower the cooling capacity. However, this type of temperature control unit operates much better at higher temperatures (for example, above 100°F [38°C]).
In either case, proper sizing of the cooling valve is required. Consulting with a manufacturer can help you select the unit that best fits the specific application.
Proper FlowProper flow through a circuit is another key factor in ensuring tight temperature control. Worn pumps or calcified cooling channels can cause a reduction in the flow and can reduce heat transfer rates. Likewise, if the pump is undersized, it cannot produce the proper flow or pressure through the mold and will affect the ability of the temperature control unit to control the temperature within a tight tolerance. A lack of turbulent flow will cause an undesirable ΔT across the mold and might affect the part quality and cycle time. A slightly larger pump might need to be used with a large cooling application, depending on the design of the cooling circuits. However, be careful not to oversize the pump. Pumps that are too large add unneeded heat to the process and consume a significant amount of energy.
Opinions vary drastically regarding the number of pumps that should be used to achieve optimal flow and temperature control. Some manufacturers believe that using one large pump for a single mold or application is sufficient; others believe that using one for each half of the tool is better. However, a more important consideration is the number of temperature control units that are installed. In some instances on large molds, eight or more temperature control units can be used. Although this level of instrumentation can be expensive, it provides the best flow and temperature control.
On large molds, the temperature control unit typically pushes water to a manifold, and the manifold distributes it to many cooling circuits throughout the mold. These mold circuits have different restrictions in the form of fittings, internal diameters, baffles and radiuses, all of which play a role in restricting flow. When only one temperature control unit is used, the circuits with fewer restrictions take away from the flow required for the more restrictive circuits.
Multiple temperature control units also can be beneficial in place of several series connections on a mold with only one in and one out water connection. In this type of application, the pressure drop through the tool is high and the ΔT across the mold steel is substantial, resulting in a less uniform temperature. Connecting several temperature control units to the mold can help ensure that every circuit receives the proper flow for the optimum temperature control.
Right-Sized HeatingAlthough every mold and process is different, most plastic manufacturing applications require an initial heatup period to achieve the desired temperature before the molding process begins. A mold is a heat sink and will absorb the heat from the warm water being pushed through the circuits, so achieving the setpoint temperature is relatively easy.
Maintaining that temperature, however, is a bit more challenging. If the heater is not sized properly, it will affect the temperature control unit’s ability to keep the temperature consistent. The heaters are controlled by either triacs (solid-state relays) or contactors. Triacs are more expensive but offer faster reaction and longevity. Regardless of the method used to activate the heater, proper sizing is a key element.
Microprocessor CapabilitiesThe last piece of the puzzle, and probably the most crucial to temperature stability, is the temperature control unit’s microprocessor, or “brain.” The microprocessor is the central point for receiving information from the system’s temperature sensor or thermocouple and sending the signal to heat or cool. The logic for receiving and sending these outputs uses what is known as PID (proportional integral derivative) control. The algorithm for calculating cooling or heating power can be fine-tuned for special applications by adjusting these PID parameters.
A typical molder might need to use the same temperature control unit on several different applications, so having the ability to change the algorithm can be helpful. A low P-parameter (proportional value) means that it takes longer for the controller to reach the temperature setpoint, as only slow changes of regulation ratio occur. In that case, the I-parameter (integral value) has more effect. If the actual value is below the setpoint, the integral value is multiplied with ΔT, summed up and added to the proportional value.
The D-parameter (derivation value) is proportional to the temperature change during the last regulation cycle. If you know the derivation of a curve, you can predict your next action based on knowing the direction of the temperature curve within the next few seconds. This capability allows you to prevent a temperature change in the process, rather than reacting when the temperature is already beyond the setpoint.
The D-parameter is helpful but is difficult to manage. Small changes can cause large temperature swings. Setting the D-parameter to zero is safe but probably will not optimize the process. However, it is possible to make changes with the P- and I-parameters to compensate. Making small changes to these parameters and observing them over several cycles can change the reaction of the control.
Because there are no hard-and-fast rules about PID settings, it can be difficult to optimize control. Working closely with the instrument manufacturer can help the manufacturer identify the best settings for your process and program the unit accordingly.
In conclusion, as simple as it might appear to connect “just any” temperature control unit to the process, this idea cannot be farther from the truth. The more generic and universal the temperature control unit, the less likely it will control accurately. Many variables help determine the temperature control unit’s ability to perform.
It is important to understand the needs of your process. Selecting the proper type of cooling with a right-sized cooling valve, pump and heater is crucial. Finally, working closely with an instrument manufacturer to determine the best number of temperature control units for your process, as well as the correct PID parameters, can help you further optimize temperature control.