THERMOFORMING MACHINES

MACHINES DE THERMOFORMAGE

English 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.

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.

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.
,.. ~;,~ ~F

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-- 2 --
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.

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-- 3 --
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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