CA1122767A

CA1122767A - Thermoforming machines

Info

Publication number: CA1122767A
Application number: CA324,284A
Authority: CA
Inventors: Raimondo Arippol
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-03-27
Filing date: 1979-03-27
Publication date: 1982-05-04

Abstract

ABSTRACT OF THE DISCLOSURE

A pre-heating apparatus for a thermoforming machine for uniformly heating a thermoplastics sheet or plate throughout comprises a first heating section, a cooling section and a further heating section. Such arrangement ensures that both the outer and inner layers of the thermo-plastics sheet or plate are all at the same temperature when the sheet or plate is subjected to a thermoforming process in a mould.

Description

~2Z767 The present invention relates to improvements in thermoforming machines, particularly in such machines which work with thermoplastics sheet material, such as poly-propylene.
Known thermoforming machines are characterized by a final product in which, after a number of operations, the thermoplastics sheet attains a high temperature and is ready for the thermoforming process. I~ is, however, very common that the heated sheets do not receive a uniform temperature throughout, due to the fact that only the outer layers of the sheet reach an adequate temperature, the inner layers not reaching a sufficient temperature.
Reference mav be had in this context, for instance, to U.S. patent 3,476,840 issued November 4, 1969 (Glassford).
Generally it is desired that the thermoplastics sheet has a uniform temperature throughout both for a better sheet utilization as well as for an improved final product.
The uniform temperature is known to be generally achieved either by extending the heating station to a length sufficient for a thorough heating of the entire cross-section of the sheet or by intermittent feeding of the sheet into the heating station and maintaining the sheet within the heating station for a predetermined period until the thorough heating has been achieved. The latter embodiment is exemplified in the aforesaid U.S.
patent. Neither of the above two solutions is totally satisfactory; the long heating station is space and energy consuming, while the intermittent feeding means a slowdown in production. The overall problem is ~urther agg~avated by the fact that there is considerable limitation in the temperature that can be applied to the surfaces of the thermoplastic sheet.
It is an object of the present invention to provide a sheet pre-heating apparatus for a thermoforming machine which at least minimizes the above-mentioned disadvantage regarding attaining a uniform temperature throughout the thermoplastics sheet.

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l~Z2767 Another object of the present invention is to increase the capacity of thermoplastic forming machines by producing a uniform temperature throughout the thermoplastic sheet while avoiding the drawbacks as referred to above.
According to the present invention, there is provided a pre-heating apparatus for a thermoforming machine for uniforml~ heating a thermoplastics sheet or plate throughout, such apparatus comprising a heating section, followed by a cooling section and then a further heating section.
Preferably, the pre-heating apparatus of the invention comprises a vertical heating section, followed by a vertical cooling section and then a further horizontal heating section.
Also in accordance with the present invention there is provided a process for pre-heating a thermoplastics sheet or plate prior to subjecting such sheet or plate to a thermoforming process, comprising firstly passing the sheet or plate through a heating section, thereafter passing the sheet or plate through a cooling section and fina-lly passing the sheet or plate through a further heating section.
Preferably, such process comprises firstly passing the sheet or plate vertically upwards through a heating section, therafter passing the sheet or plate vertically downwardly through a cooling section and fin ally passing the sheet or plate horizontally through a further he~ting section.
The invention will be further illustrated, by way of example, with reference to the accompanying drawings, in which:
Fig. 1 is a schematic view of passage of a thermoplastics sheet through the pre-heating arrangement of the invention, prior to thermoforming; and Fig. 2 is a section through a thermoplastics sheet indicating the changes which take place as the sheet passes through the various heating and cooling sections.

llZZ767 As illustrated in Fig. 1, a thermoplastics sheet or plate 1, is initially passed vertically upwards through section (A) and between heaters 2, the said sheet or plate thereafter descending vertically through a cooling section (B) and then travelling horizontally through section (C) and between heaters 3. Only after such heat treatment is the sheet passed into the countermould 4.
As can be seen in Fig. 2, in the section (A) the outer layers 1' of the sheet are heated to a temperature of, for example, approximately 120C. In the section (B), with a normal or forced cooling, the temperature of the outer layers 1' falls to approximately 70C, whilst the inner layer 1", on cooling more slowly, falls to approximately 90C. As the sheet passes between the heaters of section (C), and as the outer layers 1' are subjected to a more rapid heating than the inner layer, the temperature of all layers reaches approximately 140C, thereby creating ideal thermoforming conditions, when the sheet is passed into countermould 4.
Those skilled in the art will readily appreciate that a substantial number of modifications of the present invention can be effected without departing from the scope of the present invention as defined in the accompanying claims.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. For use with a thermoforming apparatus:
a pre-heating apparatus for uniformly heating a thermoplastics sheet or plate throughout, said apparatus comprising a first heating section, followed by a cooling section which is followed by a second heating section, conveying means for passing a sheet or plate through said sections in a sequence as set forth, said conveying means being arranged to enable the transfer of the respective sheet or plate processed in said second heating section to a respective associated thermoforming machine, said apparatus being devoid of means for subjecting the sheet or plate to mechanical stress during the passage thereof through and between said heating and said cooling sections.

2. A pre-heating apparatus as claimed in claim 1, comprising a vertical heating section, followed by a vertical cooling section and then a further horizontal heating section.

3. Apparatus for molding a thermoplastic sheet, including conveying means for passing said sheet through thermal treat-ment means of the type applying a selected thermal impact to both faces of said sheet, said apparatus further comprising a molding section disposed downstream of said thermal treatment means and adapted to mold said sheet following the exit thereof from said thermal treatment means, said thermal treatment means comprising:
a) a first heating section disposed at an upstream portion of said conveying means and adapted to subject both faces of said sheet to heat radiation to increase the temp-erature of said sheet at both faces thereof;
b) a cooling section disposed downstream of said first heating section and adapted to subject both faces of said sheet to a cooling effect to decrease the temperature of said sheet at both faces thereof;

c) a second heating section disposed downstream of said cooling section and adapted to subject both faces of said sheet to heat radiation to again increase the temperature of said sheet at both faces thereof; and d) a molding section disposed downstream of said second heating section and adapted to mold said sheet following the exit thereof from said second heating section;
whereby the uniformity of the sheet temperature throughout its cross-sectionat said molding station is improved.

4. Apparatus as claimed in claim 3, wherein that portion of said conveying means which coincides with said first heating section is adapted for a generally vertical travel of said sheet through the first heating section.

5. Apparatus as claimed in claim 3, wherein that portion of said conveying means which coincides with said cooling section is adapted for a generally vertical travel of said sheet through the first heating section.

6. Apparatus as claimed in any of claims 3, 4, or 5, wherein that portion of said conveying means which coincides with said second heating section is adapted for a generally horizontal travel of said sheet through said second heating section.

7. Apparatus as claimed in claim 3, wherein said first heating section and said second heating section are so arranged and disposed that the surface temperature of said sheet leaving said first heating section is lower than that of the sheet leaving said second heating section.

8. Apparatus as claimed in claim 7, wherein the surface temperature of said sheet leaving said first heating section is approximately 120° C and the temperature of the sheet leaving said second heating section is approximately 140°C.

9. A process for pre-heating a thermoplastics sheet or plate prior to subjecting such sheet or plate to a thermoforming process, comprising firstly passing the sheet or plate through a first heating section, there-after passing the sheet or plate through a cooling section, in said cooling section cooling the sheet or plate such that the inner part of the sheet is hotter than its outer surfaces, and finally passing the sheet or plate through a second heating section and reheating the sheet or plate in said second heating section to the extent that the temperatures throughout the thickness thereof is generally uniform.

10. A process for pre-heating a thermoplastics sheet or plate as claimed in claim 9, comprising firstly passing the sheet or plate vertically upwards through a heating section, thereafter passing the sheet or plate vertically downwardly through a cooling section, and finally passing the sheet or plate horizontally through a further heating section.