CA1090902A - Mold temperature control system - Google Patents

Abstract

TITLE
MOLD TEMPERATURE CONTROL SYSTEM

ABSTRACT OF THE DISCLOSURE
A system for automatically sensing and controlling the temperature of a mold within a desired temperature range.
Steam or chilled water proportionately supplied is used to control the operating temperature in response to continuous sensing of the actual temperature of the mold.

Description

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TITLE
MOLD TEMPERATURE CONTROL SYSTEM
BACKGROUND OF THE INVENTION
(1) Field of the Invention _ The present invention relates to controlling the temperature of molds and more particularly to a continuous temperature control system capable of maintaining the internal mold temperature within a narrow temperature range.

(2) Description of the Prior Art Systems for automatically sensing and controlling the temperature of molds are well known. Representative systems are disclosed in U.S. Patents 3,071,967 issued January 8, 1963 to R.J. Mouly and U.S. Patent 3,566,439 issued March 2, 1971 to R.J. Mouly and R.L. Thomas. In the area of injection molding, commercial systems are available from such manufacturers as Sterlco and Mokon. These commercial mold temperature control systems typically employ a closed loop feedback system consisting of a pump, reservoir, heating elements, mixing valve controls and thermocouples.
In a representative commercial system, assuming it is desired to operate a mold at a temperature of 150F, that temperature is set on the mold temperature controls.
When the system is turned on 55F, water fills the mold, control unit and water lines. This action then lowers the mold temperature from room temperature to 55F, until in-ternal heaters bring the water temperature up to 150F.
During this heating operation, a pump is utilized to circu-late the water throughout the mold. Cycle time for an average size mold in a 450 ton molding machine operating in this mode is approximately 35 minutes.

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When the molding cycle is started the temperature of the mold is increased by the thermal input of the injected hot plastic material. The water being pumped through the mold increases in temperature and returns to the controller.
This increase in temperature is sensed and 55 water is then added to the system through a mixing valve, the resulting action lowering the water temperature to 150F. In the commercial system outlined the volume of water in the system is ten gallons and the temperature of the water would rise to 170F. To lower the 170F temperature to 150F, 3.63 gallons of 55F water are required.
Several disadvantages exist in such commercial systems. These include a high chilled water requirement, relatively long start-up time and a high scrap rate on manu-factured molded parts, during the start-up period and any time the cycle is interrupted. Also since the mold tempera-ture is always hiyher than the water temperature, this temperature differential is directly proportional to the distance and volume between the temperature sensors in the molded cavity. Such commercial systems have traditionally exhibited a high maintenance record.
Accordingly, it is the object of the present invention to provide a new mold temperature control system which precisely controls the operating temperature of any molds within a narrow temperature range and reduces mold cycle times, return water temperature, start-up time and start-up scrap.
SUMMARY OF THE INVENTION
In the present invention a thermocouple for sensing mold temperature, a steam valve for applying steam as a U~3~Z

heating agent, a cold water valve for applying cold water as a cooling agent, check valves to prevent reverse flow and digital controls consisting of a digital potentiometer digital readout, thermocouple amplifier, proportional control amplifier, two signal controllers and two servo valves are employed. Like the available commercial systems the present invention is a closed loop feedback system, except that the thermocouple or heat sensing device is located in the mold instead of in the water reservoir.
In accordance with the present invention DC signals from the input thermocouple are amplified with the resultant amplifying signal read out digitally to indicate the mold temperature. The same DC signal is applied to a proportional control amplifier whose output is determined by settings under control of a digital set point device or digital potentiometer. When the input signal indicates a tempera-ture in the mold in excess of that established by the digi-tal set point device, the proportional control amplifier generates a negative DC output signal and when the detected signal indicates a temperature below the established set point the proportional control amplifier generates a positive DC output signal. These signals are applied to respective signal controllers which provide operating potentials to connected servo valves.
Each of the servo valves are in turn connected to a compressed air supply and respond to operation to pro-vide air to connected air-operated steam or chilled water valves. In response to a positive signal the connected controller operates its associated servo valve to control the associated steam valve and in response to a negative

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signal the associated controller operates its associated servo valve to cause operation of the associated air operated chilled water valve. Thus, steam or chilled water from respective sources and through the respective operated valves is directed to the mold to which the system is con-nected to increase or decrease the mold temperature. Check valves in both the steam and chilled water lines prevent backflow.
The present arrangement effectively senses the actual temperature of the mold and then meters steam or chilled water to control the operating temperature in the manner outlined.
DESCRIPTION OF THE DRAWINGS
The single figure of accompanying drawings is a schematic diagram of a mold temperature control system in accordance with the present invention.
PREFERRED EMBODIMENT OF THE INVENTION
Referring now to the accompanying drawing the mold temperature control system of the present invention consists of a number of portions all of which individually are well known and commercially available. Located in the mold itself is a temperature sensing device ll consisting of an input thermocouple. Such units as the Watlow Type J
have been found effective for this use. The output of the thermocouple 11 is connected to the input of a DC amplifier whose output (a DC signal in the range from 0 to 5 vqlts DC) is connected to a proportional control amplifier 14.
In the present embodiment an Action Pak Model #4100-170 amplifier has been utilized to amplify the thermocouple 30 output while an Action Pak Model 2102-2109 amplifier has been used for proportional control.

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A digital read out device 13 such as the Fairchild Model 53 unit effectively reads out the mold temperature as detected by the input thermocouple 11. The proportional control amplifier has its threshold established by means of a digital set point unit (a digital potentiometer or similar device). For this application a Spectrol Model 534 digital potentiometer has been found satisfactory.
The positive output from the proportionate control amplifier 14 is applied to a signal controller 16 while the negative output is applied to signal controller 17.
In the present system both signal controllers are also manufactured by Action Pak with a Model 4010-1201 employed for signal controller 16 and a Model 4010-1200 utilized for signal controller 17. Electrical output signals from the signal controllers 16 and 17 are applied to servo valves 18 and 19 respectively. These servo valves are electrically controlled valves for an air supply system connected to each valve. Robertshaw Model 443 control units are utilized in the present system for this purpose.
Servo valve 18 has its output air line applied to the control input of steam valve 22 which in the present system is a Robertshaw 4401-Al-Bl-A2 valve. The air line output from servo valve 19 is applied to the control input of chilled water valve 23 which in the present system is a Robertshaw DC-210-BLR valve. Output lines from valves 22 and 23 are connected through check valves 24 and 25 respectively to a common fluid line extending to the asso-ciated mold. The check valves are both conventional in nature and may be any of many commercialiy available units.

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The best understanding of the present invention will be had by reference to the following description of operation:
It is assumed first that it is desirable to operate the associated mold at a temperature of 150F. The 150F
temperature will then be set on the digital set point device 15. When the system is turned on the control unit through the input thermocouple 11 senses that the mold is at room temperature and that it is necessary that it be heated to 150F. The mold temperature is shown by digital read out 13.
Since the mold temperature is below the desired value a positive signal will be generated by control ampli-fier 14 and applied to signal controller 16. Operation of signal controller 16 in turn will operate servo valve 18 which will permit air from the associated air supply to be applied to the control portion of valve 22 after which the steam valve 22 will open and apply 350F steam to the mold. As the mold temperature approaches 150F the thermo-couple will generate a smaller signal and proportionatelythe afore outlined control circuitry will cause the steam valve to begin to close on a proportional basis to prevent temperature overrun. When the mold temperature reaches 150F, controls will then shift to a dead-band position (usually a 2-4 temperature range) and remain in this posi-tion until a temperature change is sensed. The total time required with an average size mold in a 450 ton molding machine for this operation is approximately 12 minutes instead of the 35 minutes required for existing commercial systems.

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When hot plastic material is inserted in~o the mold and the molding cycle begins, the temperature of the mold will increase due to the thermal input of the hot plastic material. As the mold starts to increase above 150F, the cold water valve 23 begins to operate propor-tionally in a manner similar to that outlined for the steam valve 22 above. This action will then cause the correct amount of cold water to enter the mold to maintain the temperature between 149F and 151F. Obviously any other outside variables entering into the molding process will also cause operation of the associated controlled circuitry and the mold temperature will be maintained within a narrow range of the temperature established by the digital set point apparatus 15.
In the system according to the present invention the volume of water employed is two gallons. With the same thermal input as discussed in connection with available commercial systems above, an attempt is made to drive the system toward 170F but since the system compensates imme-diately for temperature shifts, the temperature will never exceed 151 or 152F. The same ratios remain in effect as outlined above so that the amount of 55 chilled water required to maintain the 150F temperature will be .72 gallons. A portion of the effectiveness of the present system is due to the return water head which maintains water in the system and prevents hot or cold spots in the mold.
The rapid response time and compensation for temperature changes in the present system also permits faster molding cycle times.

Other forms of temperature sensing and control other than that described in the present embodiment may also be employed in the present system as well as the use of a combination steam and cold water thermal mixing valve, however the principle of operation will remain the same.
It will be obvious to those skilled in the art that numerous modifications of the present invention can be made without departing from the spirit of the invention which shall be limited only by the scope of the claims appended hereto.

Claims (10)

WHAT IS CLAIMED IS:

1. A system for controlling the temperature of an associated forming mold comprising: temperature sensing means located in said mold, operated to generate electrical output signals proportionate to the temperature of said mold; control means connected to said temperature sensing means, operated in response to signals below a predetermined value to generate proportionate control signals of a first characteristic, and operated in response to signals above said predetermined value to generate proportionate control signals of a second characteristic; first and second signal controllers, each connected to said control means, said first signal controller operated in response to control signals of said first characteristic and said second signal controller operated in reponse to control signals of said second characteristic; first servo means connected to said first signal controller and second servo means connected to said second signal controller, said servo means each operated in response to proportionate control signals from its connected signal controller; first control valve means connected between a source of heating fluid and said mold and including an operating input connected to said first servo means; second control valve means connected between a source of cooling fluid and said mold and including an operating input connected to said second servo means; said first control valve means operated in response to said first servo means to conduct proportionate amounts of heating fluid from said heating fluid source to said mold, to in-crease the temperature of said mold, and said second control valve means operated in response to said second servo means to conduct proportionate amounts of cooling fluid from said cooling fluid source to said mold to decrease the tempera-ture of said mold.

2. A mold temperature contolling system as claimed in claim 1 wherein: said temperature sensing means comprise a thermocouple.

3. A mold temperature controlling system as claimed in claim 1 wherein: there is further included amplification means connected between said temperature sensing means and said control means.

4. A mold temperature controlling system as claimed in claim 2 wherein: there is further included digital read out means connected to said amplification means.

5. A mold temperature controlling system as claimed in claim 1 wherein: said control means conprise a proportionate control amplifier and digital set point means connected thereto.

6. A mold temperature controlling system as claimed in claim 4 wherein: said digital set point means comprise a digital potentiometer.

7. A mold temperature contol system as claimed in claim 1 wherein: said servo means each comprise a valve connected to an air supply, electrically operated to control proportionate amounts of air at the respective outputs.

8. A mold control system as claimed in claim 6 wherein: said first and second control valves each are operated in responsive to proportionate amounts of air from said respective connected servo valves.

9. A mold control system as claimed in claim 1 wherein: there is further included first check valve means connected between said first control valve means and said mold; and second check valve means connected between said second control valve means in said mold; said first check valve means operable to inhibit conduction of cooling fluid to said heating fluid source and said second check valve means operable to inhibit flow of heating liquid to said cooling liquid source.

10. A mold control system as claimed in claim 1 wherein: said control means are inoperable in response to signals within a substantially narrow range of said predetermined value.