12566_18.pdf

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1 8 . 1 F u n c t i o n M e t h o d C l a s s i f i c a t i o n

Temperature controllers have the function to bring up molds connected to them toprocessing temperature by circulating a liquid medium and keep the temperatureautomatically 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 main-

control 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 initialtemperature of 90 0C or of roughly 160 0C if is pressurized with water. Those for heat-transfer oil without pressurization have an initial temperature of 350 0C .

Figures 18.2 and 18.3 illustrate a temperature controller for water and oil operation upto 90 and 150 0 C .

Table 18.1 presents a summary of properties of additional equipment employed fortemperature control.

Figure 18.1 Method of temperaturecontrol1 Tank, 2 Heater, 3 Cooler, 4 Pump,5 Magnetic valve for cooler, 6 Levelcontrol, 7 Main control, 8 Inlet,9 Temperature sensor, 10 Mold

1 8 T e m p e r a t u r e C o n t r o l l e r s f o r

I n j e c t i o n a n d C o m p r e s s i o n M o l d s

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Table 18 1 Equipm ent for temperature control (summ ary)

System

Cooling tank

Cooling-waterthermostat

Refrigerator

Temperaturecontrol devices

Characteristics

For cooling only (water)Control depending on skill of operatorMold temperature depending on temperature and pressure of line waterProcessing conditions (cycle time, coolant temperature, breaks) are notconsidered

Automatic control considers processing conditionsFor cooling only (water)Control independent of skill of operator

Use is limited to temperatures above temperature of line waterFor cooling only (except special design)Use independent of water supplyNo water consumptionFor cooling and heatingControl independent of skill of operator because of automatic controlMold temperature independent of water temperature and pressureProcessing conditions are consideredHeating up to processing temperature possibleUse is limited to temperature above temperature of line water

Figure 18 2 Temperature controllerfor water or oil operation

Figure 18 3 Temperature controller for wateror oil operation (housing removed)

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1 8 . 2 C o n t r o l

1 8 . 2 . 1 C o n t r o l M e t h o d s

There are three methods to control the mold temperature:

Control of th heat-exchange medium Figures 18.4 and 18.7).The sensor measures the temperature of the medium in the equipment. Figure 18.6presents the response of this control type in various stages. The characteristics are asfollows:- 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 thedisturbance variables (9) affecting the mold are not directly considered and controlled(such as cycle time, injection, melt temperature, etc.).

Direct control of th 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 Control of heat-exchang emedium

Figure 18 5 Control of mold temperature

1 Temperature controller, 2 Controller, maincontroller PI, master controller,3 Heating/cooling system, 4 Set value,5 Actual value, 6 Actual mold temperature,7 Temperature sensor for heat transfermedium, 8 Temperature sensor for mold,9 Auxiliary c ontroller PfD), 10 Correctingset value, 11 Mold (consumer)

Figure 18 6 Cascade control

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Figure 18 7 Control of coolant temperature with PID controller

A B C 8

Figure 18 8 Control of mold temperature with PID controller

A B C B

Figure 18 9 Control of mold temperature with PID/P D cascade control1 Set value for heating, 2 Set value for cooling, 3 Temperature of coolant,4 Mold temperature, 5 Limit temperature for coolant,ATW Difference in mold temperatureA Heating, B Injection (cooling), C Shut down (heating)

A B C B

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- The temperature set with the controller corresponds w ith 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 presen ted so far. By m eans oftwo coupled controllers (2) and (9), both the medium and the mold temperature arecontrolled. Mold temperature constancy is further improved.

1 8 . 2 .2 P r e c o n d i t i o n s f o r G o o d C o n t r o l R e s u l t s

The following preconditions have to be met to obtain good stability of the moldtemperature:

1. Properly dimensioned channels in the consumer (arrangement, heat-exchange sur-face, diameter, circuits) (see also Chapter 8).

2. Use of temperature-controlled devices with properly dimensioned heating, cooling,and pumping capacity.

3. Correct adaptation of the controller to the controlled system (m old), i.e., actuation bymeans of control measurement.

4. Correct positioning of the temperature sensor in the mold in the case of regulation ofmold temperature and cascade control (see Section 18.2.2.4).

5. The heat carrier should have good heat-transfer properties to carry appropriatequantities 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 controllerswith the positions heating - neutral - co oling (quasi-steady con trollers). For specialapplications steady controllers are used. Heating or cooling are not controlled in anin/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 theinjection molding machine. The machine provides the data to be set, calls the parametersfor 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 disturbancesimperfectly or too slowly. Oversizeing can make the control circuit oscillate. Duringstart-up, a temperature overshooting may occur.

The capacity of the pump determines the temperature gradient of the mold or the heat-exchange circuit. The greater the capacity, the smaller is the temperature difference ofthe heat carrier in the mold betw een inlet and outlet. On the other hand, a large d ischargecauses a large pressure drop in the mold so that pumps with high discharge pressure are

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needed. The consequence is that the temperature difference should be kept as small asnecessary, but not as small as possible (Section 18.2.2.5) (< 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 definedresistance over a wide temperature range. This principle is based on the temperaturedependence of the electric resistance of metals. With increasing temperature theresistance increases and vice versa.

Main features: Very good long-term stability, simple connections (no special connectorsneeded). Measurements are absolute values, independent of amb ient influences.

- Thermocouples (e.g., Type J/Fe-CuNi). If two wires of dissimilar metals are broughtinto intimate contact, a voltage is generated which depends on the temperature of thejunction and the kind of metals used. The voltage is proportional to the differencebetween the temperature of the control point (mold) and the outside, ambienttemperature.

Main features: Special connection lines and connectors necessary; punctiformmeasurement, 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 followingcriteria:

- The most suitable position of the sensor in the mold depends, among others, on theconfiguration and the design of the mold as well as on the location of the coolingchannels.

Figure 18 10 Microprocessor controller

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- 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 highdemands 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 couldinterfere 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.Imm ediately before injection the cavity wall has the controlled temperature TW m i n. Whenthe hot plastic melt comes into contact with the cooler cavity wall, a contact temperatureis generated, which is between the temperature of the cavity wall and that of the melt. Itdecreases continuously during the cycle. The contact temperature TW m a x depends on theheat penetrability of the mold and the plastic. The amplitude of the temperature variationATW decreases with increasing distance from the cavity-wall surface.

Calculation programs for determining the sensor distance from the mold cavity sur-face are obtainable from institutes and software companies. As a rule of thumb, adistance of 0.5 to 0.7 d is recommended (d = diameter of sensor).

18.2.2.5 Heat-Excha nge System in the M old see also Chapter 8)

The surface of channels of the heat-exchange system in the mold has to be dimensionedso that the generated heat can be carried away from the heat carrier of the temperaturecontroller or the necessary heat can be supplied. The larger the channel surface, thesmaller is the temperature difference between medium and mold and the faster thetemperature variations are leveled out.

Compared with water as a heat carrier, molds operated with a heat-transfer oil neededa 2 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 forequipment w ith expensive pum ps (high discharge pressure) or a dividing of the channelsystem into several parallel circuits to reduce the pressure drop.

Figure 18 11 Temperature ofcavity wall versus time1 Injection2 Ejection:T2 min M inim, temperature of

cavity wall,T2 max Maxim, temperature of

cavity wall,T2 Average temperature of

cavity wall (important forcooling),

T4 Temperature of demo lding,t c Cooling time,tc Cycle time.

J2

iS

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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 heatcarrier 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 maybecome an impermissible great temperature difference AT between the first and the lastcircuit, depending on the application.

Because of the great pressure drop, a higher discharge pressure is needed to conveythe required amount of heat carrier through the channel system and a relatively highpump capacity may be needed.

The main problem of the parallel arrangement is the difference in flow rate betweenthe individual circuits even from only slightly different flow conditions (e.g., unequalcross sections, lengths, number of bends of the channels). The branch with the smallestresistance to flow experiences the best heat exchange (smallest Ap). A correction ofunequal conditions with a hand-valve control cannot be recomm ended. An improvem entmay 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 thecirculating 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 anonuniform heat dissipation in the mold. Besides this, a reduction of the flow rate resultsin a poorer coefficient of heat transfer. This m eans a reduced heat exchange in the mold.

Figure 18 12 Control in series andparallel control

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1 8 . 3 S e l e c t i o n o f E q u i p m e n t

The selection of temperature-control equipment depends primarily on processedmaterial, 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, aswell as pressure and flow-rate conditions in the mold (Table 18.2).

The discharge pressure of the pum p can only be determined w ith a diagram pressuredrop as a function of flow rate . If there is no such diagram, the pressure drop can beestimated from experience with similar molds.

As already mentioned in Section 18.2.2.5, wa ter should be preferred as the heat carrierif possible because of its better heat-transfer properties. It should be used for mold tem-peratures up to 90 0C. Employing equipment for pressurized water allows its use up totemperatures of about 140 0C . Beyond 90 or 140 0C respectively, heat-transfer oil has tobe employed as heat carrier.

Table 18 2 Criteria for selection of equipment

Specification Objective

Processed material Mold temperatureMold temperature Heat exchange medium (water/oil)Mold weight, heating time Heating capacityAmount of processed material per unit time Cooling capacityTemperature gradient of mold Discharge of pumpPressure drop versus flow rate Discharge pressure of pump

1 8 . 4 C o n n e c t i o n o f M o l d a n d E q u i p m e n t -S a f e t y M e a s u r e s

For reasons of safety and reliability of operation, maintenance, and avoidance of leaksthe 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 ofwater remnants in the circuit.

- Periodic change of the heat carrier.- Use of synthetic heat-transfer oil because of little tendency to form deposits.

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- Heat exchanger or expansion tank have to be placed somewhat higher than the mold ifthe latter is very large and contains a proportional quantity of heat carrier. Thisprevents a slow return of the content of the mold into the expansion tank and over-flowing during a shutdown. A return is almost inevitable because of tiny leaks atconnections in the circuit which permit air to enter. If a higher placement is notfeasible, then there are the following options:

- Size of the expansion tank has to be adequate to accommodate all returning fluidduring the shutdown.

- Mounting shut-off valves at inlet and outlet at the mold, which can be closed duringshutdown.

- 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.

1 8 . 5 H e a t C a r r i e r

In the following, the characteristics of water and heat-transfer oil are compared. Wateroffers the following advantages over heat-transfer oil:

- Cheaper, cleaner, and ecologically safe. Water from leaks in the circuit can be dis-charged into the sewer system without further precautions.

- Very good thermal properties, such as heat transfer (high heat transfer coefficienta) , heat capacity (high specific heat capacity c), and thermal conductivity (highthermal 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 togreater pressure drop and a deterioration of the heat exchange.

- Detrimental substances dissolved in water (n itrites, chlorides, iron, etc.) areincreasingly 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 temperature-control circuit (device and attached consumers), the water used must meet the followingrequirements:

Hardness 10 to 45 ppm equivalent calcium carbonate,pH value at 20 0C 6.5 to 8.5,

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Cl-ions max. 150 mg/1,Sum of chlorides and sulfates max. 250 mg/1.

Potable w ater often meets these requirem ents. 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 do not exhibit the disadvantages of water. Because of their higherboiling point they can be used up to more than 350 0C, depending on the type.

Disadvantages of heat-transfer oils are:

- poorer heat-transfer characteristics. Optimum heat transfer can only be obtained underdefined flow conditions. The flow rate (1/min) has to be precisely adjusted to thespecific 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 ofundesirable deposits considerably. Of course one has to pay a higher price than for amineral oil.

1 8 . 6 M a i n t e n a n c e a n d C l e a n i n g

Maintenance and cleaning of the heat-exchange system (device, mold, connecting lines)must be performed regularly as otherw ise creep ing deterioration (slow reduction ofcross-sections of channels by scaling) may occur.

To avoid production downtimes on expensive production equipment, preventaccidents, and maintain the temperature-control device at a high level of operationalsafety, the periodic controls and maintenance work prescribed in the manufacturer'smanual must be performed.

Deposits of rust and scale adversely affect the heat exchange between mold andcirculating water, if the latter is the heat carrier.

Contamination and deposits can constrict the flow-through cross-section and result ina higher pressure drop in the mold so that, after some time, the pump capacity does notsuffice for a troublefree operation anymore. Periodic examination and cleaning istherefore indispensable. Adding a corrosion inhibitor is a commendable means ofprevention.

Deposits in the mold from heat-transfer oil can also impair heat transfer and result inhigher pressure drop. Adding a detergent before changing the oil or preventively duringthe whole time between changes is likewise suggested. The oil should be examinedperiodically. This is best done by the supplier w ith an analys is. Dark color is by itself noquality criterion.

R e f e r e n c e s[18.1] Handbuch der Temperierung mittels fliissiger M edien. Regloplas AG , 5th Ed., St. Gallen,

Huthig Verlag, Heidelberg, 1997.

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