Tension frame for releasable clamping of a film

In the production of shaped plastic elements using deep-drawable thermoplastic film, a tension frame is required to clamp the film. The tension frame is formed by lengthwise and transverse frame parts, on the outside edges of which is located one angle clamp on each. The angle clamps can be actuated by means of closure levers into a releasable clamping position, in which the clamp arms of the angle clamp press with force against bottom fixing members of the frame parts.

The invention relates to a tension frame for the releasable clamping of a 
film as disclosed in the disclosure part of claim 1, especially tension 
frames for implementation of the process of German Patent Application No. 
P 35 05 828.5. 
Tension frames of this sort are required for the deformation of 
thermoplastic films, in order to hold the film tightly at the edges during 
the treatment process. Known processes for vacuum deformation of 
thermoplastic films are already described in detail in "Modern Plastics 
Encyclopedia", 1969-1970, pages 534-563 and in DE-OS No. 3 130 584. 
As has been shown in practice, one notable drawback of the known process 
for vacuum deformation of thermoplastic films resides in that during 
actual use the properties which are required cannot always be produced 
with the low deformation temperatures which are used. 
One decisive improvement is disclosed in the process of the German Patent 
Application No. 35 05 828.5, in which the film held by a tension frame is 
heated to the heat molding temperature of the plastic, for the production 
of a shaped plastic element, especially an expandable truck interior 
paneling made of a deep-drawable thermoplastic film, and is made arcuate 
by preforming corresponding to the general shape of a shaping tool, 
whereupon the film is subjected to final deformation after introduction of 
the shaping tool into the arc by placing the reverse side of the film on 
the surface of the shaping tool using a pressure differential and is 
removed therefrom after cooling. 
The decisive advantage of this process resides in that for the final 
deformation a thick area of the film is heated on its reverse side by 
means of high radiation intensity in the plastic temperature range of the 
plastic. A certain portion of the film is then deformed in the temperature 
range in which the plastic does not return to its original shape. 
Therefore the employment of for instance instrument panels of a power 
vehicle is possible even under extreme climatic conditions. 
The tension frame plays an important role in all processes for the 
deformation of thermoplastic films, in which it is to clamp the film at 
the edges and thus to facilitate the desired deformation. A rigid frame 
construction under great stress forces is required so as to avoid tearing 
of the film while it is under stress. This leads to costly frame 
structures. Great stress force conditions (identical to or greater than 20 
kp/m lengths between clamps) are particularly to be dealt with during use 
of the aforementioned advantageous process according to German Patent 
Application No. P 35 05 828.5. 
Basically of course the production of a rigid frame construction for great 
stresses raises no particular difficulties, disregarding the outlay and 
costs. However there is still another important aspect. The frame 
construction which is employed at this time requires a relatively sizable 
floor space. In practice, in devices for implementation of the process for 
the deformation of films, cramped worksite dimensions are present between 
the shaping tool and the molding box or suction box. With the advantageous 
process cited above the film held by the tension frame is laid out on 
these molding boxes, in order then to execute the desired deformation. 
The limited floor space dimensions complicate a rigid frame construction 
for high stresses, since only a small space is available for any locking 
or clamping members. 
Furthermore it must be considered that pieces of film of different 
dimensions can be used as desired for the production of shaped plastic 
elements. Therefore different tension frames with different frame contact 
surfaces must also be available. Since one single tension frame for each 
piece of film is already very costly on account of the different 
requirements and conditions, the provision of a plurality of tension 
frames represents a further drawback in terms both of storage and of cost. 
It is also to be considered that the changing of the tension frame is 
connected with not inconsiderable work outlay and causes down time. 
The object of the invention is to provide a tension frame which as a result 
of simple construction has a easily operable locking mechanism for 
clamping the film when great stress forces are present, and actually while 
using only a limited space, and which with simple means facilitates the 
clamping of films of different sizes. 
The invention attains this goal with the tension frame disclosed in the 
disclosure part of claim 1, characterized in that a mounting base composed 
of frame parts is provided with a bottom fixing member, that a pivotal 
clamp element with a clamp arm is arranged extending over the length of 
the relevant frame part on the outside of each frame, and that the clamp 
arms can be moved by means of associated closure levers into a releasable 
clamp setting, in which the clamp arms press with force against the fixing 
member of the frame parts. 
The described features of the new tension frame facilitate the provision of 
two lengthwise frame parts and two transverse frame parts for each, which 
together define the frame surface, and both transverse frame parts are 
releasably mounted between the lengthwise frame parts. The advantage is 
thus obtained of having a variability in the size of the tension frame to 
enable grasping of different shaped of films. 
A desired change of shape requires only the shifting of the transverse 
frame parts and with that a change of the relevant spacing of the facing 
transverse frame parts, if a different length is desired with the same 
width of the film section. In case a different width of the film is also 
required, the releasable transverse frame parts can be replaced by 
suitable larger or smaller transverse frame parts. Different film sections 
then do not require the use of different and separate tension frames. This 
is an important advantage provided by the invention, since the tension 
frame can be suitably readjusted or changed over for different shaped with 
few hand grips. 
The pivotal clamp arm arranged on the outside of each frame part and 
extending over the length of the frame part allows the possibility of 
clamping the film with simple means such as closure levers. This 
construction advantageously facilitates the arrangement of the closure 
lever horizontally to the frame plane and outside the frame, so as to 
tolerate the conditions of cramped space. Also the horizontal arrangement 
of the closure lever can be effected in a simple manner from above by use 
of a compressed air cylinder, in order either to hold or to release the 
clamp setting. 
The effect of the compressed air cylinder applied from above has the 
advantage that it does not prevent horizontal movement of the tension 
frame after operation of the closure lever, in other words, the tension 
frame can immediately be moved to the side. This condition has an 
especially favorable effect on automation of the entire process for the 
production of the shaped plastic elements which is generally sought, and 
the tension frame is moved by means of a conveyor system from the 
deformation setting over the aforementioned molding box sideways into the 
mold opening site (and vice versa). 
In another configuration of the invention the frame parts are configured as 
steel frame parts which are U-shaped and the bottom horizontal arm of the 
"U" forms the fixing member. Furthermore the clamp element is formed by an 
angle clamp of which the bottom is the clamp arm. This structure in 
connection with the closure lever allows production of the desired stress 
forces without any further adjustment, even despite the aforementioned 
cramped floor space between the shaping tool and the molding box. 
According to another advantageous further development of the invention the 
tension frame has an inside lining with highly polished aluminum sheets. 
These cause a reflection of the outward-directed infrared radiation of the 
heating element provided for heating the film into the marginal areas. 
This means that the marginal areas of the film as well as the middle of 
the film are heated uniformly. 
Another advantageous feature of the invention resides in that both the 
clamp arm and the fixing member and also the top arm of the base mount 
have joint faces. In working position the tension frame can therefore also 
function as a packing element between the shaping tool and the molding 
box, so that it is possible to perform the deformation in a vacuum. 
Other appropriate configurations of the invention are provided in the 
dependent claims and are to be assumed from the drawing.

For the general explanation of the use of a tension frame the production of 
a shaped plastic element will be explained in some detail relative to the 
device shown diagrammatically in FIG. 1. The device includes a shaping 
tool 10 with a plurality of suction openings 12 and 14. Shaping tool 10 is 
mounted on a tool holder 16, which can be moved vertically over a crank 
drive which is not shown in detail. 
Tool holder 16 which holds shaping tool 10 is configured as a reinforced 
box girder, of which the hollow space 18 can be connected through a 
connection 20 and through a flexible line to a vacuum unit which is not 
shown. Hollow space 18 also opens into another connection 22 with a not 
shown solenoid valve, which allows for pressure compensation with the air 
pressure of the environment following the deformation process. 
Beneath shaping tool 10 is arranged an infrared radiator 24 with a side 
shield 26 to serve as heat source. Double arrow B indicates that infrared 
radiator 24 can be moved horizontally. 
One essential component in FIG. 1 is the tension frame 32, in which a film 
28 is held by clamping on all sides. Tension frame 32 is located in its 
position shown in FIG. 1 over a molding box (suction box) 34 which is open 
on top, and tension frame 32 can likewise be moved side to side and 
horizontally. 
The molding box which is open on the top has connections 36 and 38 for 
application of the thrust air and also another connection 40, through 
which a vacuum can be produced within molding box 34. 
Infrared radiator 24 is inserted from the side into the position shown in 
the device in order to heat film 28 which is clamped in tension frame 32. 
Thrust air is simultaneously blown through connections 36 into molding box 
34, and the thrust air can flow out through opening 38 into the 
environment. The velocity of the air being discharged is thus a measure of 
the pressure level being produced in molding box 34, which supports film 
28 during the heating up process. 
Film 28 is clamped within tension frame 32 so that the patterned side is 
turned away from the infrared radiator 24. Thus the reverse side of the 
film is heated by infrared radiator 24. When the desired deformation 
temperature has been reached connections 36 and 38 are closed by valves 
which are not shown in the drawing, and vacuum is applied simultaneously 
to connection 40. As a result, film 28 is pre-deformed and takes 
approximately the position shown by a broken line with reference number 
30. 
Only at this time is infrared radiator 24 moved to the side into a hold 
position outside the deep-drawing device, and tool holder 16 with shaping 
tool 10 is introduced obliquely into molding box 34. At this moment a high 
vacuum is applied to shaping tool 10 through connection 20, and the other 
connection 22 remains closed. 
Connection 40 is also closed simultaneously and connection 38 is opened, so 
that the air pressure of the surrounding air is adjusted in molding box 
34, which presses film 28 precisely to the outline of shaping tool 10. 
After a cooling time of approximately 30 seconds connections 20 is closed 
and connection 22 is opened, so that the surrounding air pressure is reset 
in shaping tool 10. Now shaping tool 10 is again moved up into the 
position shown in FIG. 1 over molding box 34. Tension frame 32 is likewise 
moved to the side to a site for removal from the mold, and now the 
completely shaped deep-drawn film can be completely removed from tension 
frame 32. 
The details of construction of the tension frame which is shown only 
diagrammatically in FIG. 1 are shown in FIGS. 2-8. The plan view of FIG. 2 
shows that tension frame 32 consists of two lengthwise frame parts 42, 
between which extend two transverse frame parts 44. A pivotal and angular 
clamp element 52 with a bottom arm forming a clamp arm 54 and a top 
portion 56 (see FIG. 5) extends on the outside of each lengthwise frame 
part 42 and transverse frame part 44. As is clear from FIG. 6, the 
lengthwise and transverse frame parts are configured as U-shaped in 
profile with a top arm 46, a bottom arm 48 serving as fixing members as 
well as a perpendicular arm 50. 
Angle clamps 52 are mounted or suspended by means of mountings 58 which are 
configured as types of rocker bearings on lengthwise frame parts 42 and/or 
transverse frame parts 44. Each mounting 58 as shown in FIGS. 3 and 4 
includes an axis 60 supported by angle members 62, to which angle clamp 52 
is pivotally suspended. Angle members 62 are fastened by means of screw 
connections 64. 
Closure levers 66, of which the structure is shown in FIGS. 5-8, are also 
provided on lengthwise frame parts 42 and transverse frame parts 44 of 
tension frame 32 at some distance from mountings 58. Closure lever 66 is 
arranged to be pivotally mounted around a bolt 70 between two support arms 
72, which are connected with clamp arms 52 and support bolts 70. 
Supports arms 72 supporting both closure levers 66 over bolts 70 on top arm 
56 of angle clamp 52 are attached by means of screw connections 68. The 
plan view of FIG. 2 shows that two closure levers 66 are provided in 
lengthwise frame parts 52, while only one closure lever is provided in the 
case of the shorter transverse frame parts 44. 
FIG. 5 shows angle clamp 52 in open setting, while FIGS. 6-8 show angle 
clamp 52 in closed clamping setting. Proceeding from FIG. 5, film 28 is 
brought up into contact with a joint face 88 on bottom arm 48 (fixing 
profile). Another joint face 88 is found on the inside of bottom arm 54. 
After film 28 engages on top joint face 88, the four angle clamps 52 are 
pressed by not shown compressed air cylinders on lengthwise frame parts 42 
and/or transverse frame parts 44, in order to close tension frame 32. The 
effect of the compressed air cylinder is indicated by arrow A in FIG. 5. 
After acting on bottom clamp arm 54 in the direction of arrow A and as a 
result of a movement to the left in FIG. 5, the closure lever 66 which is 
connected with angle clamp 52 takes a position in which it engages with 
its bevel 82 at its front end on the front of a tempered catch ledge 86, 
which is arranged on top arms 46 of lengthwise frame parts 42 and 
transverse frame parts 44. 
After reaching this setting each closure lever 66 is now pressed downward 
by other not shown compressed air cylinders in the direction of arrow D, 
so that closure levers 66 engage and lock, and film 28 is clamped 
accurately for its particular size and tightly in tension frame 32. The 
two-step closing process is ended. The selected compressed air cylinders 
are carried back into their original position, and tension frame 32 is 
moved into work position by a chain conveyor (see FIG. 1) with guide 
rollers. Before shaping tool 10 is moved thereover, first film 28 is 
heated by means of infrared radiator 24 over molding box 34. 
The highly polished aluminum sheets 92 mounted on perpendicular arms 50 on 
the inside of tension frame 32 are provided to prevent marginal radiation, 
so that the marginal areas of film 28 are heated uniformly as well as the 
middle of the film. 
The engagement of closure lever 66 causing the final closing of angle clamp 
52 is facilitated by a cotter pin 74 located within closure lever 66, of 
which the heat 78 is provided with a recess 80 adapted to the shape of the 
edge of catch ledge 86. With the engagement, cotter pin 74 is pressed 
counter to the force of a spring 84 which is likewise located within 
closure lever 66, which presses cotter pin 74 with heat 78 against catch 
ledge 86 and thus secures the closed setting of closure lever 66. The 
horizontal movement of cotter pin 74 is limited by a pin 76 which is 
guided in a lengthwise aperture. 
After the tension frame 32 which is thus closed is moved with film 28 into 
the aforementioned work position, and after film 28 has been heated by 
infrared radiator 24, vacuum is applied to molding box 34 for the 
preforming. Then shaping tool 10 is moved into molding box 34. At this 
instant vacuum is applied to the shaping tool for the final deformation 
and a high pressure is produced in molding box 34. Tension frame 32 then 
works for all practical purposes as a packing element between shaping tool 
10 and molding box 34. Foam rubber packings 90 as shown in FIG. 6 are 
provided to produce an especially tight closing. 
After the final deformation shaping tool 10 is again moved upward, and 
tension frame 32 is moved to the side into the site for removal from the 
mold, after molding box 34 has been opened. Tension frame 32 is reopened 
in the site for removal from the mold. 
Closure lever 66 of FIG. 5 is pressed downward at the outside end by 
compressed air to open tension frame 32, which is indicated by arrow C. 
The catch connection between head 78 and catch ledge 86 is thus released, 
so that angle clamps 55 are opened and thus take the position shown in 
FIG. 5. The part of the mold drops downward and can then be reprocessed. 
The new tension frame 32 can be adapted or changed over in a simple manner 
for pieces of film of different shapes. But when sections of film are 
identical width with different lengths only transverse frame parts 44 need 
by shifted to the front in a suitable manner. If on the other hand the 
width of the section changes, the transverse frame parts (with suitable 
angle clamps) are to be replaced by others. 
Tension frame 32 with bottom clamp arms 54 advantageously allows the 
possibility of cooling the heat sensitive patterned side of film 28, lying 
downward, to be cooled by the thrust or support air. The film section on 
account of the U-shaped profile of the tension frame and on account of the 
bottom clamp arm 54 lies quite low in the frame, so that the cooling can 
be effected uniformly over the surface. Dead zones are thus avoided. 
A conveyor system for moving tension frame 32 is shown in FIG. 9, provided 
with a chain conveyor 94 (the separate individual conveyor chains are 
already shown in FIG. 2, where it is clear that chain conveyors 94 engage 
outside the actual tension frame 32 on the ends of lengthwise frame parts 
42). 
Chain conveyors 94 are mounted on lengthwise frame parts 42 by screw 
connections 96, and frame parts 42 are also connected through angles 98 
with guide rollers 100. 
Guide rails 102 are provided for the shifting of tension frame 32 with the 
use of conveyor chains 94, and guide rollers 100 which slide on rails 102. 
When the deformation position is reached, the guide rollers go into 
indentations 104 provided at suitable points in guide rails 102, so that 
tension frame 32 is lowered and the joint faces come essentially 
automatically into position because of the specific weight of tension 
frame 32.