Temperature Controllers forInjection and Compression Molds

18 T e m p e r a t u r e Injection 18.1 a n d Controllers for C o m p r e s s i o n M o l d s Function, Method, Classification Temperature controllers have the function to bring up molds connected to them to processing temperature by circulating a liquid medium and keep the temperature automatically constant by heating and cooling The basics of temperature control are shown in Figure 18.1 The heat-exchange medium in the tank (1) with built-in cooler (3) and heater (2) is delivered to the mold (10) by a pump (4) and returned to the tank The sensor (9) measures the temperature of the medium and passes it on to the maincontrol input (7) The controller adjusts the temperature of the heat-exchange medium, and, thus, indirectly the mold temperature If the mold temperature rises above the set value, then the magnetic valve (5) is actuated and cooling initiated Cooling takes place until the temperature of the medium, and with it that of the mold, have reached the set value again If the mold temperature is too low, heating (2) is activated in analogy to cooling Temperature controllers can be classified as follows: devices for operating with water and heat-transfer oil Devices for operating with water generally have an initial temperature of 90 0C or of roughly 160 0C if is pressurized with water Those for heattransfer oil without pressurization have an initial temperature of 350 C Figures 18.2 and 18.3 illustrate a temperature controller for water and oil operation up to 90 and 150 C Table 18.1 presents a summary of properties of additional equipment employed for temperature control Figure 18.1 Method of temperature control Tank, Heater, Cooler, Pump, Magnetic valve for cooler, Level control, Main control, Inlet, Temperature sensor, 10 Mold Figure 18.2 Temperature controller for water or oil operation Table 18.1 Figure 18.3 Temperature controller for water or oil operation (housing removed) Equipment for temperature control (summary) System Characteristics Cooling tank For cooling only (water) Control depending on skill of operator Mold temperature depending on temperature and pressure of line water Processing conditions (cycle time, coolant temperature, breaks) are not considered Cooling-water thermostat Automatic control considers processing conditions For cooling only (water) Control independent of skill of operator Use is limited to temperatures above temperature of line water For cooling only (except special design) Use independent of water supply No water consumption For cooling and heating Control independent of skill of operator because of automatic control Mold temperature independent of water temperature and pressure Processing conditions are considered Heating up to processing temperature possible Use is limited to temperature above temperature of line water Refrigerator Temperature control devices 18.2 Control 18.2.1 Control M e t h o d s There are three methods to control the mold temperature: Control of the heat-exchange medium (Figures 18.4 and 18.7) The sensor measures the temperature of the medium in the equipment Figure 18.6 presents the response of this control type in various stages The characteristics are as follows: - The temperature set at the controller and required for reproducible production is not measured Therefore, depending on the disturbance variables, different temperatures can occur in the mold, even though the same value is selected - Variations of the temperature in the mold may be relatively great because the disturbance variables (9) affecting the mold are not directly considered and controlled (such as cycle time, injection, melt temperature, etc.) Direct control of the mold temperature (Figures 18.5 and 18.8) The temperature sensor is in the mold This results, in most cases, in a much better stability of the mold temperature than with the control of the medium temperature Figure 18.8 shows the response of the temperature control of the mold in various stages The primary characteristics of this control are: Figure 18.4 medium Control of heat-exchange Figure 18.5 Control of mold temperature Temperature controller, Controller, main controller PI, master controller, Heating/cooling system, Set value, Actual value, Actual mold temperature, Temperature sensor for heat transfer medium, Temperature sensor for mold, Auxiliary controller (PfD), 10 Correcting set value, 11 Mold (consumer) Figure 18.6 Cascade control A Figure 18.7 C Control of coolant temperature with PID controller A Figure 18.8 B B C B Control of mold temperature with PID controller A B C Figure 18.9 Control of mold temperature with PID/PD cascade control Set value for heating, Set value for cooling, Temperature of coolant, Mold temperature, Limit temperature for coolant, ATW Difference in mold temperature A Heating, B Injection (cooling), C Shut down (heating) B - The temperature set with the controller corresponds with the mold temperature -Disturbance variables (9) affecting the mold are considered and leveled out This results in very small temperature variations of the mold - Processing data are reproducible Cascade control (Figures 18.7 and 18.9) This is a combination of the two different types of control presented so far By means of two coupled controllers (2) and (9), both the medium and the mold temperature are controlled Mold temperature constancy is further improved 18.2.2 Preconditions for G o o d Control Results The following preconditions have to be met to obtain good stability of the mold temperature: Properly dimensioned channels in the consumer (arrangement, heat-exchange surface, diameter, circuits) (see also Chapter 8) Use of temperature-controlled devices with properly dimensioned heating, cooling, and pumping capacity Correct adaptation of the controller to the controlled system (mold), i.e., actuation by means of control measurement Correct positioning of the temperature sensor in the mold in the case of regulation of mold temperature and cascade control (see Section 18.2.2.4) The heat carrier should have good heat-transfer properties to carry appropriate quantities of heat in a short time These preconditions are discussed in more detail in the following sections 18.2.2.1 Controllers The controllers employed in temperature-control equipment are three-point controllers with the positions "heating - neutral - cooling" (quasi-steady controllers) For special applications steady controllers are used Heating or cooling are not controlled in an in/out mode but in a "more/less" mode depending on the capacity demand An interface (e.g., RS 232, 485) enables software dialog with the processor of the injection molding machine The machine provides the data to be set, calls the parameters for the controller, orders the mold to empty, asks for the status of equipment (e.g., breakdown), etc 18.2.2.2 Heating, Cooling, and Pump Capacity Insufficient heating capacity prolongs the heating-up phase and levels out disturbances imperfectly or too slowly Oversizeing can make the control circuit oscillate During start-up, a temperature overshooting may occur The capacity of the pump determines the temperature gradient of the mold or the heatexchange circuit The greater the capacity, the smaller is the temperature difference of the heat carrier in the mold between inlet and outlet On the other hand, a large discharge causes a large pressure drop in the mold so that pumps with high discharge pressure are Figure 18.10 Microprocessor controller needed The consequence is that the temperature difference should be kept as small as necessary, but not as small as possible (Section 18.2.2.5) (< 0C) 18.2.2.3 Temperature Sensors One distinguishes two kind of temperature sensors: - Resistance sensors The resistor consists of a platinum wire, which has a well defined resistance over a wide temperature range This principle is based on the temperature dependence of the electric resistance of metals With increasing temperature the resistance increases and vice versa Main features: Very good long-term stability, simple connections (no special connectors needed) Measurements are absolute values, independent of ambient influences - Thermocouples (e.g., Type J/Fe-CuNi) If two wires of dissimilar metals are brought into intimate contact, a voltage is generated which depends on the temperature of the junction and the kind of metals used The voltage is proportional to the difference between the temperature of the control point (mold) and the outside, ambient temperature Main features: Special connection lines and connectors necessary; punctiform measurement, inexpensive, service life dependent on production quality 18.2.2.4 Installation of Temperature Sensors in the Mold For the installation of sensors in the mold one has to pay attention to the following criteria: - The most suitable position of the sensor in the mold depends, among others, on the configuration and the design of the mold as well as on the location of the cooling channels - The sensor should be located in a place where temperature plays a decisive role, e.g., dimensions with close tolerances, regions with a tendency to warpage, or high demands on mechanical properties - The sensor should be placed in a defined distance from the cavity wall because the largest temperature variations during a cycle occur there (Figure 18.11), and this could interfere with the response of the controller The temperature variations are caused by physical conditions (criteria: mold material, processed material, temperature) and cannot be influenced by the temperature controller Immediately before injection the cavity wall has the controlled temperature T Wmin When the hot plastic melt comes into contact with the cooler cavity wall, a contact temperature is generated, which is between the temperature of the cavity wall and that of the melt It decreases continuously during the cycle The contact temperature T Wmax depends on the heat penetrability of the mold and the plastic The amplitude of the temperature variation ATW decreases with increasing distance from the cavity-wall surface Calculation programs for determining the sensor distance from the mold cavity surface are obtainable from institutes and software companies As a rule of thumb, a distance of 0.5 to 0.7 d is recommended (d = diameter of sensor) 18.2.2.5 Heat-Exchange System in the Mold (see also Chapter 8) The surface of channels of the heat-exchange system in the mold has to be dimensioned so that the generated heat can be carried away from the heat carrier of the temperature controller or the necessary heat can be supplied The larger the channel surface, the smaller is the temperature difference between medium and mold and the faster the temperature variations are leveled out Compared with water as a heat carrier, molds operated with a heat-transfer oil needed a to 3-times larger channel surface because of the smaller coefficient of heat transfer Small channel diameters cause a large pressure drop in the mold This calls for equipment with expensive pumps (high discharge pressure) or a dividing of the channel system into several parallel circuits to reduce the pressure drop J2 Figure 18.11 Temperature of cavity wall versus time Injection Ejection: T2 Minim, temperature of cavity wall, T2 max Maxim, temperature of cavity wall, T2 Average temperature of cavity wall (important for cooling), T4 Temperature of demolding, tc Cooling time, tc Cycle time i S Below, the characteristics of heat-exchange systems in series or parallel are explained (Figure 18.12) With an arrangement in series the individual circuits are passed through by the heat carrier one after the other With a parallel arrangement they are passed through after flowing through a distributor With an arrangement in series the main problem may become an impermissible great temperature difference AT between the first and the last circuit, depending on the application Because of the great pressure drop, a higher discharge pressure is needed to convey the required amount of heat carrier through the channel system and a relatively high pump capacity may be needed The main problem of the parallel arrangement is the difference in flow rate between the individual circuits even from only slightly different flow conditions (e.g., unequal cross sections, lengths, number of bends of the channels) The branch with the smallest resistance to flow experiences the best heat exchange (smallest Ap) A correction of unequal conditions with a hand-valve control cannot be recommended An improvement may result from a parallel layout in series 18.2.2.6 Keeping the Temperature as Stable as Possible It is usually better to maintain a stable temperature by controlling the temperature of the circulating heat carrier, but not by changing the flow rate The reason for this is the increase in the temperature gradient from a reduction of the flow rate This can lead to a nonuniform heat dissipation in the mold Besides this, a reduction of the flow rate results in a poorer coefficient of heat transfer This means a reduced heat exchange in the mold Figure 18.12 Control in series and parallel control 18.3 Selection of E q u i p m e n t The selection of temperature-control equipment depends primarily on processed material, the weight of the mold, the desired heat-up time, the amount of material to be processed per unit time (kg/h), the permissible temperature differential in the mold, as well as pressure and flow-rate conditions in the mold (Table 18.2) The discharge pressure of the pump can only be determined with a diagram "pressure drop as a function of flow rate" If there is no such diagram, the pressure drop can be estimated from experience with similar molds As already mentioned in Section 18.2.2.5, water should be preferred as the heat carrier if possible because of its better heat-transfer properties It should be used for mold temperatures up to 90 C Employing equipment for pressurized water allows its use up to temperatures of about 140 C Beyond 90 or 140 0C respectively, heat-transfer oil has to be employed as heat carrier Table 18.2 Criteria for selection of equipment Specification Objective Processed material Mold temperature Mold weight, heating time Amount of processed material per unit time Temperature gradient of mold Pressure drop versus flow rate Mold temperature Heat exchange medium (water/oil) Heating capacity Cooling capacity Discharge of pump Discharge pressure of pump 18.4 C o n n e c t i o n of M o l d a n d E q u i p m e n t Safety - Measures For reasons of safety and reliability of operation, maintenance, and avoidance of leaks the following items should be observed: - Only plugs with tapered pipe threads should be employed in heat-exchange circuits - Only pressure- and heat-resistant hoses should be used Twisting, too small bending radii, compression, etc have to be prevented - Cooling should always be connected for safety reasons - Heat-insulated lines should be employed for greater distances between equipment and mold - Periodic examinations of the circuit (controller, connections, mold) for leaks and proper function - For a change-over from water to oil in suitable equipment, one has to proceed cautiously: hazard of an accident during heating from excessive vapor pressure of water remnants in the circuit - Periodic change of the heat carrier - Use of synthetic heat-transfer oil because of little tendency to form deposits - Heat exchanger or expansion tank have to be placed somewhat higher than the mold if the latter is very large and contains a proportional quantity of heat carrier This prevents a slow return of the content of the mold into the expansion tank and overflowing during a shutdown A return is almost inevitable because of tiny leaks at connections in the circuit which permit air to enter If a higher placement is not feasible, then there are the following options: - Size of the expansion tank has to be adequate to accommodate all returning fluid during the shutdown - Mounting shut-off valves at inlet and outlet at the mold, which can be closed during shutdown - Use of magnetic valves, which close when the equipment is turned off - Reduction of cross-sections in the circuit, only if needed, and then close to the mold A measure for the size of the connections are the connectors of the control equipment This permits the full use of the pump capacity 18.5 Heat Carrier In the following, the characteristics of water and heat-transfer oil are compared Water offers the following advantages over heat-transfer oil: - Cheaper, cleaner, and ecologically safe Water from leaks in the circuit can be discharged into the sewer system without further precautions - Very good thermal properties, such as heat transfer (high heat transfer coefficient a), heat capacity (high specific heat capacity c), and thermal conductivity (high thermal conductance f) - Comparatively low channel surface area required for heat supply and dissipation Disadvantages are: - low boiling point In the case of tap water: - Danger of corrosion and scale development in the heat-exchange circuit leading to greater pressure drop and a deterioration of the heat exchange - Detrimental substances dissolved in water (nitrites, chlorides, iron, etc.) are increasingly precipitated at elevated temperatures Preventive measures depending on shop conditions are: - use closed-loop cooling circuit, removal of solids with strainers, periodic flushing of equipment and mold with a scale remover, treatment of the circuit with a corrosion inhibitor, correct water quality To avoid damage to the cooler of the temperature-control device and in the temperaturecontrol circuit (device and attached consumers), the water used must meet the following requirements: Hardness pH value at 20 0C 10 to 45 ppm equivalent calcium carbonate, 6.5 to 8.5, Cl-ions Sum of chlorides and sulfates max 150 mg/1, max 250 mg/1 Potable water often meets these requirements If the water is too soft (distilled water, rain water), it can cause corrosion Corrosion is also promoted by the prescuce of chlorides Water that is too hard promotes scale and sludge development Heat-transfer oils not exhibit the disadvantages of water Because of their higher boiling point they can be used up to more than 350 C, depending on the type Disadvantages of heat-transfer oils are: - poorer heat-transfer characteristics Optimum heat transfer can only be obtained under defined flow conditions The flow rate (1/min) has to be precisely adjusted to the specific heating capacity (W/cm2), - inflammable under certain conditions (there are not yet flame retardant oils), - aging (oxidation, increase in viscosity), - costs Synthetic heat-transfer oils exhibit better solubility of products from the aging process, which are primarily formed near the temperature limits This reduces the danger of undesirable deposits considerably Of course one has to pay a higher price than for a mineral oil 18.6 Maintenance and Cleaning Maintenance and cleaning of the heat-exchange system (device, mold, connecting lines) must be performed regularly as otherwise "creeping" deterioration (slow reduction of cross-sections of channels by scaling) may occur To avoid production downtimes on expensive production equipment, prevent accidents, and maintain the temperature-control device at a high level of operational safety, the periodic controls and maintenance work prescribed in the manufacturer's manual must be performed Deposits of rust and scale adversely affect the heat exchange between mold and circulating water, if the latter is the heat carrier Contamination and deposits can constrict the flow-through cross-section and result in a higher pressure drop in the mold so that, after some time, the pump capacity does not suffice for a troublefree operation anymore Periodic examination and cleaning is therefore indispensable Adding a corrosion inhibitor is a commendable means of prevention Deposits in the mold from heat-transfer oil can also impair heat transfer and result in higher pressure drop Adding a detergent before changing the oil or preventively during the whole time between changes is likewise suggested The oil should be examined periodically This is best done by the supplier with an analysis Dark color is by itself no quality criterion References [18.1] Handbuch der Temperierung mittels fliissiger Medien Regloplas AG, 5th Ed., St Gallen, Huthig Verlag, Heidelberg, 1997