Method and apparatus for controlling the temperature in thermoforming machines

The temperature of upper and lower parts of a forming tool making deep-drawn thermoplastic articles is controlled with a system which performs the steps of passing a coolant of predetermined total volume through the upper part and the lower part of the forming tool; measuring the temperature of the upper and lower parts; determining the actual temperature difference between the upper and lower parts; comparing the actual temperature difference with a predetermined desired temperature difference; generating a signal representing the magnitude of deviation between the actual and desired temperature differences; and, as a function of the signal, increasing the volume of coolant passing through one of the forming tool parts and decreasing the volume of coolant passing through the other of the forming tool parts for reducing the magnitude of deviation while maintaining the total volume unchanged.

BACKGROUND OF THE INVENTION 
This invention relates to a method and an apparatus for controlling the 
temperature of the upper and lower portions of forming and stamping tools 
for the manufacture of thermoplastic containers. The apparatus is 
installed in the forming station of a thermoforming machine which draws a 
film web from a supply roll, heats the film web and thereafter 
sequentially deforms lengths of the film web in the forming station by 
means of the tool and by means of a differential pressure. Immediately 
following the deformation the same tool stamps out the container. After 
the tool is opened, the containers are conveyed away by means of systems 
known by themselves. The scrap web is wound or comminuted. 
In thermoforming machines a shortening of the operational cycles (that is, 
an increase of the operational frequency) is sought for the purpose of 
increasing the output rate. The operational frequency is significantly 
affected by the cooling period necessary for the article made by the 
thermoforming machine. Such cooling period depends from several factors 
such as the type of the film, the film thickness, the film temperature 
during deformation and the shape of the formed article (container), 
particularly at its upper edge. 
Since the film web is deep-drawn at temperatures in the order of magnitude 
of approximately 150.degree.-200.degree. C., the forming tool heats up 
during operation unless specifically cooled. During such heat-up the upper 
and lower portions of the tool will have different temperatures: normally 
a greater amount of heat is taken up by that tool part (usually the lower 
tool portion) which carries the mold proper. In practice it is therefore 
conventional to cool the lower tool part in order to achieve higher 
operational frequencies. 
The critical zone during cooling of the article shaped in the thermoforming 
machine is mostly at the upper article edge which is thicker than the 
article wall and therefore needs a longer period for cooling. If the 
article (such as a container) is removed from the mold prematurely, its 
edge is distorted which is unacceptable if such an edge should, in use, 
perform a sealing function. 
Because of the different temperature conditions in the upper and lower tool 
parts, conventionally two coolant circuits have to be controlled. As a 
rule, the thermoforming machine is associated with a refrigerating 
apparatus which delivers a coolant having the required low temperatures. 
Such a refrigerating apparatus operates usually with water as the coolant 
which is cooled to approximately 3.degree. C. and is then introduced into 
the tool halves. Dependent upon operational conditions, with the aid of 
such a system the tool temperature may be maintained between 20.degree. 
and 40.degree. C. 
The requirements for ever-increasing output in thermoforming machines have 
led to arrangements in which the forming and stamping tools can, without 
difficulty, work on film widths of 800 mm and the containers can be made 
in groups, in which the containers are distributed in several rows. In 
general, 8 to 10 containers per row are feasible, dependent upon the 
diameter of the container. 
Since the containers are stamped in the tool subsequent to the forming 
operation, the tools have to be high-precision parts. Cutting dies and 
matrix bores have to be in an accurate alignment during stamping; the 
clearance between the two (that is, the cutting play) is in the order of 
magnitude of 0.04 mm. 
If the forming tool is of substantial width, the temperature difference 
between the upper and lower tool parts may become excessive, whereupon, 
due to the unlike expansion of the parts, the cutting play may disappear. 
As a result, the outer cutting dies in the row run onto the matrix edges 
which leads to damages resulting in high costs and interruption of 
production. 
By throttling the flow of the individual coolant streams to the upper and 
lower portions of the tool, it would be, to be sure, feasible to obtain 
for the two tool portions predetermined separate temperatures set at 
respective temperature regulators. Such an arrangement requires throttle 
mechanisms in the supply conduit for the coolant leading to the upper and 
lower tool portions and a system which controls the throttles. It is a 
disadvantage of this type of coolant control that not the entire coolant 
volume capable to be supplied by the refrigerating apparatus is utilized. 
This then means that the tool is not brought to the lowest possible 
temperature. Such a lowest possible temperature, however, is desirable 
because it would result in an optimal cooling of the formed container, 
thus leading to a maximum output rate. It is noted that the value of such 
a temperature is usually not known and depends from many factors. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved method and 
apparatus for controlling the temperature of an upper and lower part of a 
molding tool in a thermoforming machine in such a manner that the entire 
coolant volume which the refrigerating apparatus is capable of supplying 
is admitted to the tool to thus achieve the lowest possible temperatures 
for the tool halves. The temperatures of the upper and lower parts may be 
identical or may differ from one another in a predetermined manner. 
Normally, identical temperatures for the upper and the lower tool parts are 
desired. There are, however, instances where a temperature difference is 
advantageous. Some of the reasons which warrant such temperature 
differences are as follows: 
(a) Significant components of the upper and lower tool part which undergo 
heat expansion during operation are of different materials having 
different coefficients of heat expansion. 
(b) The small inner caliber differences of the stamping dies and matrix 
bores obtained in the mechanical manufacturing process may be adjusted by 
means of different temperatures. 
(c) Usually, difficulties are involved as to how the temperature sensors 
should be mounted on the upper and lower tool portions in order to measure 
a characteristic temperature which is determinative for the heat expansion 
of the tool half. If in one tool half the temperature sensor is too close 
to a "cold" zone, whereas in the other tool half the temperature sensor 
has been mounted too close to a "hot" zone, such an "error" can be 
corrected by setting a predetermined temperature difference between the 
upper and lower tool halves. 
These objects and others to become apparent as the specification 
progresses, are accomplished by the invention, according to which, briefly 
stated, the temperatures of the upper tool part and the lower tool part 
are measured by temperature sensors which apply corresponding signals to a 
temperature difference regulator. The latter controls a distributor valve 
such that the entire volume of the coolant is so distributed between the 
upper tool part and the lower tool part that a predetermined temperature 
difference is obtained between these tool components. 
It is noted that as concerns the invention, the absolute temperatures 
assumed by the upper and lower tool parts are of no significance. What is 
of essence is to set a temperature equality or a temperature difference 
between the upper and the lower tool parts. The absolute temperature 
depends from many factors (air temperature, coolant temperature, film web 
temperature, type of film, thickness of film and operational frequency) 
and fluctuates during operation. At the beginning of the operation it has 
room temperature and thereafter oscillates to assume a predetermined value 
for the operation. 
According to an additional feature of the invention, several temperature 
sensors for each tool half are used for deriving a mean temperature to 
enhance the accuracy of the temperature sensing. 
According to a further feature of the invention, in case the temperature 
difference is excessive as compared to a predetermined value, the 
temperature difference regulator emits a signal which triggers either an 
optical or an acoustic signal or turns off the thermoforming machine. This 
ensures that damage to the tool is reliably prevented.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning first to FIG. 1, a thermoforming machine adapted to incorporate the 
invention includes a frame 1, a support 2 for carrying a reel of film 
supply, a film web conveying mechanism 3, a heating station 4, a forming 
and stamping station 5 and a drive 6 for vertically reciprocating a table 
7 which supports a lower portion 8 of a forming tool. An upper portion 9 
of the forming tool is mounted on a stationary traverse 10. During 
operation, a film web 11 is drawn off the supply roll 12 and is 
subsequently heated in the station 4 and then deep-drawn in the forming 
and stamping station 5 by means of a plunger-assisted differential 
pressure. The finished articles (containers) 13 are conveyed out of the 
station 5, whereas the scrap band 11a is wound or comminuted. 
Turning now to FIG. 2, there is illustrated in axial section a single-row 
forming tool for making containers. The lower portion 8 of the tool has a 
base plate 14, a cooling block 15 receiving, for example, eight molds 16 
(only one shown) and a bottom 17 for each form 16. The bottoms 17 are 
mounted on a rod 18. In each mold form 16 there is provided a sharp 
peripheral inner top edge 22 which serves as a cutting edge. The mold 16 
may be surrounded by a stripper 23. For cooling the forms 16, bores 19 and 
20 are provided in the cooling block 15. Inner walls of the cooling block 
15 and outer walls of the forms 16 define annular chambers (only one 
shown) 21 which communicate with the bores 19 and 20 by means of 
respective ports 24 and which surround the respective mold 16. The bores 
19 and 20 may be, respectively, coolant inlets and coolant outlets. It is, 
however, feasible to so connect the chambers 21 that the coolant flows 
therethrough in sequence. 
The upper tool portion 9 is formed of a head plate 25, an intermediate 
plate 26, a matrix 27, a stripper or hold-down part 28 and a plunger 29 
which assists in the drawing of the film 11 and which is mounted on a rod 
30. A coolant inlet bore 31 and a coolant outlet bore 32 extend in the 
matrix 27 and, by ports 33 communicate with an annular chamber 34 defined 
by a sleeve 35 and the matrix 27. In this manner a particularly thorough 
cooling of the hold-down component 28 is achieved. Temperature sensors 36 
and 37 (such as thermo-elements) are provided in the upper tool part 9 
(preferably in the matrix 27) and the lower tool part 8 (preferably in the 
cooling block 15). 
In the description which follows, the method of regulating the temperature 
of the upper part 9 and the lower part 8 will be set forth with reference 
to FIG. 3. 
A refrigerating apparatus 38 supplies a predetermined volume V of a coolant 
of predetermined temperature through a supply conduit 39 to a distributor 
valve 40. The distributor valve 40 has an outlet 40a which is connected by 
means of a conduit 41 with the bore 31 of the matrix 27 forming part of 
the upper tool part 9. The distributor valve further has an outlet 40b 
which is connected by means of a conduit 42 with the bore 19 of the 
cooling block 15 forming part of the lower part 8. A return conduit 43 
re-introduces the coolant into the refrigerating apparatus 38 from the 
outlet bore 32 of the matrix 27 and the outlet bore 20 of the cooling 
block 15. 
The temperature sensors 36 and 37 provided in the upper and lower tool 
parts, respectively, are connected with a temperature difference regulator 
45. By means of a setting knob 46 a desired temperature difference between 
the upper tool part 9 and the lower tool part 8 may be set. The 
temperature difference regulator 45 generates an output signal from a 
comparison between the predetermined desired deviation and the actually 
measured temperatures of the upper tool part 9 and the lower tool part 8. 
The output signal is applied to the distributor valve 40 by means of a 
conductor 47. The output signal may be a power signal for driving a motor 
44 which turns a setting spindle to displace a valve head 48 of the 
distributor valve 40. The position of the valve head 48 determines the 
distribution of the volume flow V through the valve outlets 40a and 40b 
and thus to the two tool halves 8 and 9. 
If, for example, the temperature difference regulator 45 determines, for a 
preset desired temperature difference of 0.degree. C., that the upper tool 
part 9 is 3.degree. C. warmer than the lower tool part 8, then the motor 
44 is so energized that to the upper tool portion 9 more coolant is 
supplied and, at the same time, the coolant supply to the lower tool part 
8 is accordingly reduced, leaving the total volume V unchanged. Thus, the 
motor 44, to achieve this purpose, moves the valve head 48 downwardly as 
viewed in FIG. 3. If, in such a set position of the valve head 48 there is 
achieved a temperature equality, the valve head 48 remains in this 
position until, for example, because of operational circumstances, a 
temperature difference between the two tool parts 8 and 9 appears, 
whereupon an appropriate change of the position of the valve head 48 is 
again effected. The actual temperatures of the upper tool part 9 and the 
lower tool part 8 are immaterial; they are as low as can be achieved by 
the refrigerating apparatus 38 in the available volume flow V. It is thus 
seen that a throttling (reduction) of the flow V is not effected: the 
coolant flows at all temperature differences and at all actual 
temperatures of the upper part 9 and the lower part 8 in a constant total 
flow rate out of and back into the refrigerating apparatus 38 without any 
hindrance. 
If, for example, because of an increase of the output rate of the 
thermoforming machine or an increase of the film temperature the tool 
temperature increases, the set temperature difference is maintained 
constant entirely automatically. Thus, the danger of a unilateral heating 
of one of the tool halves and the inherent temperature-caused expansion 
which would cause the cutting edge 22 to collide with the matrix 27 are 
securely eliminated. 
Since, with the temperature sensors 36 and 37 only a limited zone of the 
upper part 9 and the lower part 8 are covered, it is of advantage to 
provide a plurality of temperature sensors for each tool half 8 and 9 and 
to form a mean temperature value in the temperature difference regulator 
45. The latter then compares the difference between the mean temperature 
values determined for the upper tool part 9 and the lower tool part 8 
respectively, with the preset desired temperature difference. 
Since exceeding a predetermined temperature difference may cause serious 
damage, it is advantageous to connect the temperature difference regulator 
45 by a conductor 49 with an optical or acoustic signalling device 50 
which sounds an alarm if the predetermined temperature difference is 
significantly exceeded. Such an alarm signal may also be used for an 
automatic shut-off of the thermoforming machine. While normally, the 
temperature difference regulator 45 should prevent the occurrence of such 
an excess, a malfunction may nevertheless occur as a result of certain 
causes, such as clogging of one of the supply conduits 41 or 42. 
In principle, it is feasible to arrange the distributor valve 40 at the 
outlet side of the tool halves 8 and 9 and then regulate the proportion of 
the flow V exiting from the upper part 9 and the lower part 8, 
respectively. 
Using the method according to the invention solely in forming tools is in 
certain instances advisable in order to maintain, there too, a 
predetermined temperature difference in the upper and lower tool parts. In 
such an environment the purpose--rather than to control the heat 
expansion--is to ensure that the components which contact the film have 
approximately the same temperatures in order to prevent a unilateral 
significant quenching of the film. This is of importance particularly in 
forming foam films in a negative and positive matrix. 
An usual temperature difference regulator 45 is delivered by Metrawatt, D 
8500 Nurnberg, Germany under the number GTR 221. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.