Oven for heating and stacking thermoplastic fiber reinforced resin sheets

A method and apparatus for heating fibers reinforced thermoplastic sheets is disclosed. The apparatus involves use of gas heating ovens adapted to allow several layers of material to be heated continuously, with the conveyors stacked are above the other. Stacking of the heated product can be provided at the oven exit. Provisions for cleaning and diffusing the gases over the work piece are also described.

The present invention relates to the heating of fiber reinforced 
thermoplastic sheets for subsequent molding. More particularly, the 
present invention relates to a method and apparatus suitable for use in 
preparing fiber reinforced thermoplastic sheets for molding or stamping. 
Still more particularly, the present invention relates to a method and 
apparatus suitable for use in preparing fiber reinforced thermoplastic 
sheets for subsequent molding in which several heated sheets are stacked 
after heating, one on top of the other prior to molding. 
BACKGROUND OF THE INVENTION 
In U.S. Pat. Nos. 3,626,053 and 3,621,092, processes are described for 
molding fiberglass reinforced thermoplastic sheets utilizing presses. In 
the processes described in both patents, a reinforcing mat, typically 
formed of glass fibers, is utilized to reinforce thermoplastic resin in 
sheet form. The mat reinforced sheets are then stamped into shaped parts 
utilizing a press. Prior to placing the sheets in the press for stamping 
into shaped parts, however, the sheets must be heated to a temperature 
sufficient to render the resin of the sheet molten or flowable while 
maintaining the temperature of the sheet below the decomposition 
temperatures of the thermoplastic resin used to prepare the sheet. The 
heating systems described in both of these patents involve infrared ovens. 
Various other patents have issued which relate to fiber reinforced 
thermoplastic sheet products such as those described in the aforementioned 
patents. Exemplary of some of these other patents are U.S. Pat. Nos. 
3,664,909, 3,684,645 and 4,335,176. In all of these patents the product 
described is suitable for use in stamping or compression molding 
operation. In utilizing any of the fiber reinforced thermoplastic products 
described in these patents, the fiber reinforced thermoplastic resin sheet 
product is first heated to bring it to a temperature sufficient to render 
the resin component of the sheet flowable or molten. The heated sheet, 
while the resin is still in the flowable state, is then placed on a mold 
in a suitable press such as a hydraulic or mechanical press and pressure 
is applied to stamp or mold the sheet into a shaped part. As described in 
the aforementioned patents, rendering the resin molten prior to molding 
the fiber reinforced sheet, permits the resin to flow during molding and 
the reinforcement flows with the resin. This provides a shaped part which 
has reinforcement uniformly distributed throughout. 
In preparing the reinforced thermoplastic resin sheets for the compression 
molding processes utilized in the art, recourse has been had to the 
utilization of infrared ovens for the heating of the sheets. It has also 
been common practice to employ an oven containing a standard cable 
conveyor system which oven is provided with opposing infrared heaters on 
the top and the bottom of the oven facing each other. The sheets are 
placed on the cable conveyor and moved through the oven or held there in a 
stationary position while heat is applied from the infrared heaters to 
render the resin contained in the reinforced resin sheets molten. Once the 
resin is molten the sheets containing the molten resin can then be placed 
on a cold mold in a hydraulic press. The press is closed quickly to 
provide for the reinforcement and resin to flow and fill the mold. The 
resin cools and solidifies as the part is stamped or molded resulting in a 
shaped product. 
While infrared heating of fiber reinforced thermoplastic blanks or sheets 
has been the rule in industry to date, such heating does present certain 
disadvantages with respect to providing an efficient process. Infrared 
ovens are by nature, in the environment utilized to heat thermoplastic 
resin sheets, inefficient. In the heating of fiber reinforced 
thermoplastic sheets it has been found in practice that the reflectors or 
heaters become quite dirty due to the release of dust into the atmosphere 
usually from the edges of the reinforced thermoplastic resin sheets passed 
through the ovens. This dust which settles on the reflectors contaminating 
their surfaces require substantial percentage increases in the power fed 
to the heaters over time to compensate for losses in efficiency caused by 
this surface contamination. Increased heating of the infrared heaters also 
has the disadvantage of requiring excessive amounts of power to be 
utilized in heating a given sized sheet thereby increasing the cost of the 
production process. Further, by utilizing the infrared heaters at high 
energy levels continuously, the life of the heaters is substantially 
reduced. 
In systems using black faced (infrared) heating elements positioned in 
ceramics, the same problem exists due to environmental contamination 
occurring on the ceramic surfaces. Again, increased power is required to 
overcome the loss of efficiency created by the deposition of a foreign 
material on the surface of the ceramic. Another disadvantage of the 
infrared system is the operational preferences attributable to individual 
operators in staging the heating of sheets passing through such ovens. 
Thus, it is common practice in the heating of thermoplastic resin blanks 
reinforced with fibers to pass them through the infrared oven in stages. 
The first stage causes considerable swelling of the sheets since most of 
them are reinforced with mats which have been compressed during the 
formation of the sheet itself. These mats have a tendency to swell the 
sheet once the resin reaches a flowable state during the heating 
operation, because the compressive forces in the mat are released once the 
sheet resin is no longer a rigid solid. 
Thus, the sheets generally accept large quantities of heat at the inception 
of their passage through the ovens in the presence of infrared heaters 
operating at temperatures sufficient to cause the resin to melt. The sheet 
then, after swelling, is subjected to less severe temperature regimes but 
must soak in some heat in order to insure that the center of the blank 
receives sufficient heat to cause all of the resin in the blank to become 
flowable. It is in these latter stages of the heating, as the sheet passes 
through the oven and is subjected to stops that operators make the 
ultimate decision on how much power to input to that section of the oven. 
If too much heating occurs in these latter stages, the resin itself can 
deteriorate. 
Finally, it is a further disadvantage of infrared ovens that the ovens with 
opposing infrared heaters (top and bottom) transferring heat from their 
surfaces to the upper and lower surfaces of a sheet moving on cable 
conveyors through heating ovens, are incapable of heating several layers 
of sheets passing through on multiple conveyors positioned one above the 
other, in the same oven. In any such arrangement, the bottom heaters would 
be prevented from heating the bottom surface of an upper row of sheets 
passing through a conveyor located immediately below them. This seriously 
limits oven capacity to two banks of heaters, one on the top and one on 
the bottom and a single conveyor passing through the oven so that only the 
sheets on one conveyor can be treated at one time. 
Thus a need exists to improve the heating cycle for fiber reinforced 
thermoplastic sheet materials that are to be utilized in compression 
molding systems where heated sheets are placed in a cold mold and pressure 
is applied to shape those sheets into a formed part. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to provide a process which 
overcomes the deficiencies of the prior art infrared heating ovens used to 
treat fiber reinforced thermoplastic sheets. 
It is a further object of the present invention to provide a heating oven 
for heating fiber reinforced thermoplastic resin sheets utilizing hot 
circulating gas. 
It is still a further object of the present invention to provide a heating 
oven for fiber reinforced thermoplastic resin sheets which will lend 
itself to the utilization of several moving conveyors within a single 
heating oven. 
It is still a further object of the present invention to provide a method 
for preparing fiber reinforced thermoplastic resin sheets for subsequent 
molding in a hot air oven in which the sheets can be automatically stacked 
as they exit the oven. 
It is still a further object of the invention to provide a method for 
preparing sheets of fiber reinforced thermoplastic resin in a heating oven 
in such a manner that all sheets contained within the oven are of uniform 
temperature. 
These and other objects of the invention will be apparent from the ensuing 
descriptions of the preferred embodiments of the ovens and methods 
utilized to produce the results. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a method is provided for preparing sheets 
of fiber reinforced thermoplastic resins in which the sheets are passed 
through an oven on a continuous basis on a conveyor system which 
constantly circulates through the oven. Hot gases, preferably air, 
although any gas inert to the resin sheets treated may be used, are passed 
around the sheets contained on the conveyor on all surfaces thereof at 
temperatures in excess of the melting temperature of the resin utilized in 
the sheet. The circulation rate of the hot gas is maintained preferably at 
below 1000 cubic feet per minute and a residence time is provided for the 
sheet in the oven sufficient to permit the resin to become molten or 
flowable throughout the sheet. 
In another aspect of the invention, an oven is provided for heating these 
fiber reinforced resin sheets which involves a heating chamber and at 
least two conveyors in the chamber positioned one above the other. The 
lower of the conveyors used traverses the oven from one end to the other. 
The conveyor above the lower conveyor, in the preferred embodiment, 
terminates short of one end of the chamber and is provided with a sloped 
plane at the end thereof to thereby urge sheets on its surface in a 
downward direction at the end of that conveyor to the surface of the lower 
conveyor. Means are provided to circulate gas to all surfaces of the 
conveyors and to heat all sheets carried by the multiple conveyors to a 
desired temperature. Means are also provided to recirculate and reheat the 
gases continuously while the sheets are conveyed throughout the oven. 
In another aspect of the invention, means are provided within the ovens 
contemplated to constantly clean the gas circulating therein to remove all 
foreign debris present in the atmosphere. The particulars of the apparatus 
and the methods herein provided will be made clearer in the ensuing 
description of the drawings and the preferred embodiments.

DETAILED DESCRIPTION OF THE DRAWINGS 
Turning now to the drawings and FIGS. 1 and 2 in particular, there is shown 
an oven generally indicated at 1 having at one end an entrance port 2 and 
at the other end an exit port 3. Traversing the oven is a conveyor 4 which 
is constructed of a series of cables shown in more detail in FIG. 2. 
Conveyor 4 is driven through motor 5 which is mounted on a mounting pad 40 
and is provided with a shaft 6 which rotates a pulley in the housing 7 
that rotates belt 8 off of pulley 9 which is a double track pulley having 
a track for the belt 8 and a separate track for belt 10 which passes 
around idler roll 11 contacting the under surface of the plurality of 
cables making up conveyor 4. The belt 4 is threaded around idler 11 and 
pulley 9, rollers 12, 13 and 14 and passes through the underside of roller 
15 which is biased in a downward direction by shaft 16 to which it is 
attached. The roller 15 in conjunction with shaft 16 applies pressure to 
the belt 4 to maintain tension at a desired level. The cable conveyors 4 
then pass around idler roll 17 over roller 18 and at that point re-enter 
the oven through port 2. The oven is provided with two stacks, 20 and 21, 
for the removal of hot gases at whatever rate is desired. Each stack is 
provided with an appropriate damper 22 and 23 for stacks 20 and 21, 
respectively. As seen more clearly in FIG. 2, the oven is also provided 
with an electrical heating element generally indicated at 24. The heating 
element is positioned behind a blower 25 and the blower introduces air in 
an upward direction from plenum 26 to an upper chamber or duct 27 and a 
lower chamber or duct 28. The chambers 27 and 28 are provided with grills 
so that air can be introduced above and below the conveyor 4 and in that 
manner provide gas to the upper and lower surfaces of sheets 60 which are 
transported by the conveyors 4 through the oven. 
Fiber reinforced thermoplastic sheets as used herein means thermoplastic 
sheets reinforced with inorganic or organic fibers in fibers, mat or 
fabric form. Fiber glass is the preferred fiber and continuous strand mat 
is the preferred form for the fiber glass embodiment. 
In the operation of the oven as shown in FIGS. 1 and 2, the fiber 
reinforced thermoplastic resin sheets to be heated are placed on the 
conveyor 4 and passed into the oven 1 through port 2. During their passage 
through the oven, hot gas is introduced from a blower 25 into plenum 26 
and passes in the bifurcated upper and lower chambers 27 and 28 into the 
oven and around the sheets 60 on all sides. The sheets 60 are thus 
uniformly heated on the top and bottom and the gas temperature in the oven 
passing through as indicated by the arrows 31 shown in FIG. 2 is 
maintained at a uniform temperature. The gas after heating the sheets is 
then passed downwardly through the filter 30 and across the heating 
element 24 to raise its temperature to the desired amount prior to 
introducing it into blower 25 for recirculation to the furnace. 
The conveyor 4 is regulated in its travel speed so that it maintains a 
residence time for the sheet in the oven sufficient to render the resin 
contained within the sheets 60 completely molten throughout the sheet. 
This can be determined by the thickness of the sheet, the extent and rate 
of travel of the sheet and the absorption capabilities of the particular 
resin sheet being fed. Experience will dictate the quantity of time 
required to take a sheet in a given high temperature atmosphere of heated 
gas to the requisite molten state. It is an important consideration in 
dealing with sheets of this character that the center of the sheet contain 
molten or flowable resin and for this reason, it is important to ensure 
that this state is reached. With the gas circulating at a uniform rate at 
the top, bottom and sides of the sheets as they pass through the oven, the 
sheets can be easily raised to the requisite temperature and maintained at 
that state for the necessary period of time to ensure the resin in the 
sheet is completely molten. 
The gas utilized in the chamber is circulated at a low rate, generally 
below 1,000 cubic feet per minute. Preferably a rate of 100 to 750 cubic 
feet per minute is employed but any circulation rate of gas coupled with 
temperature of the gas and residence time of sheets in the oven that 
produce a molten or flowable resin in the sheet will suffice. The gas 
leaving the chamber is preferably fed through the filter and recirculated 
so that it can be purged of any foreign material entering the atmosphere 
through the edges of the sheets which normally have been cut to given 
sizes in preparation for molding. Thus, the sheets utilized in oven are 
generally speaking, of various precise dimensions required by the customer 
for insertion into the cold mold of the stamping press that will be 
utilized to shape the final part. The edges of the sheets, therefore, 
where the sheets have been cut to provide these requisite sizes have a 
tendency to shed some fiber. For this reason, the atmosphere in the 
circulating oven can become contaminated as it has been in the past 
utilizing infrared heaters and thus, must be purified. In the Applicants' 
system, this involves positively circulating the air in the chamber 
through the filter 30 prior to reheating it for passage into the blower. 
Removing the debris at this point also provides an air entering the 
reheating system that is devoid of debris and therefore, allows the heater 
24 to operate at a more efficient level. 
In the embodiment of the oven 50 shown in FIGS. 3 and 4, a method is 
provided for stacking multiple sheets, one above the other, inside of an 
oven while still providing the necessary heating to raise the temperature 
of the sheets or blanks to 60 to a temperature sufficient to render the 
resin contained in the blank molten throughout. As shown therein, the 
conveyors 55, 56 and 57 are passed through the oven in the same horizontal 
mode, however, conveyors 56 and 55 terminate in the oven at a point in 
front of the exit port 51 of the oven while conveyor 57 passes entirely 
through the oven. Conveyor 57 is tracked by rollers 70, 71, 72, and 73. 
Conveyor 56 which is tracked by rollers 80, 81, 82, and 83 is provided 
with an inclined plane 87 at the end thereof. The conveyor 55, which is 
tracked by rollers 74, 75, 76, and 77 is provided with an inclined plane 
84 at the end thereof and located above conveyor 56. In operation, the 
sheets 60 are placed on the conveyors 55, 56 and 57 and through the 
operation of the drive rollers 74, 70 and 73 and idle rolls shown, convey 
the sheets 60 through the oven from the entrance ports 52, 53 and 54 to 
the exit port 51. 
The arrangement of the conveyors 55, 56 and 57 is such that sheets 60 are 
placed on the conveyor and the conveyor speeds are timed so that the 
sheets 60 contained on conveyor 57 pass under the inclined plane 87 at a 
point in time when a sheet 60 on conveyor 56 is riding down the inclined 
plane so that it is picked up by a sheet 60 conveyed on conveyor 57 as it 
passes under that inclined plane. Similarly, the sheets on conveyor 56 
pass under the inclined plane 84 of the conveyor 55 at a point in time 
when the sheets 60 on the conveyor 55 are sliding down the inclined plane 
84 so that those sheets are picked up by the sheets 60 on the conveyor 56. 
In this manner, three stacked sheets then are conveyed by conveyor 57 
through port 51 to the outside of the oven 50. In this manner, it is 
possible to stack sheets for subsequent molding where stacked sheets are 
required for fill of a given molded part. 
THE PREFERRED EMBODIMENT 
Turning now to FIGS. 5 and 6 which depict the preferred embodiment of the 
instant invention, there is shown therein an oven generally indicated at 
60. The oven is provided with a loading zone on one side thereof, 
generally indicated at 37 and the loading zone is comprised of a plurality 
of cables 98 shown in section in FIG. 6 and on which the product rests in 
its transport through the oven 60. On the opposite side of the oven 60, is 
an unloading zone generally indicated at 41 which again, is composed of 
the same cables 98 from which the material treated by the oven 60 is 
removed after transport through the oven. In the area of loading zone 37 
there is positioned an idler roller 38 over which the cables 98 ride as 
they pass into the oven 60. A similar idler roll 39 is provided at the 
unloading station 41. The cables 98 are driven by a drive roller 40 shown 
on the right hand side of FIG. 5 and drive roll 40 draws the cables 98 
through the oven 60 and passes them over pulleys 92 and then downwardly 
and through the bottom portion of the oven to the return area or loading 
area 37 as a series of continuous belts. The oven 60 is provided with a 
blower 43 driven by blower motor 42 shown most clearly in FIG. 5. The 
return air is passed through a return air ports 36 which are associated 
with filters to filter out dust and debris picked up by the hot gases as 
they pass through the oven 60 and heat the materials being treated by the 
oven 60. Blower 43 takes gases coming from the return ports 36 and passes 
them across the heating elements 45 shown in both FIGS. 5 and 6. The 
heated gases are passed from the heaters 45 across the dampers 34 and 33 
and then strike the diffuser 91 for the upper oven and the diffuser 94 
which diffuses the gases to the lower oven. The gases from the upper oven 
pass through the upper diffuser plate 90 which is a metal plate having a 
plurality of holes, not shown, therethrough. The gases passing to the 
lower oven pass through a similar diffuser plate 95 located at the bottom 
of the oven so that the hot gases and diffused as they enter the work area 
and contact the work pieces carried on the cables 98 uniformly as shown 
more clearly in FIG. 6. An exhaust blower 32 is located in the oven and is 
utilized to vent gases from the oven when it is desired to relieve the 
circulating gases from the oven when desired. The box 46 shown in FIG. 5 
represents a power supply for the oven which is utilized to activate drive 
rolls 40, blower 42 and other equipment associated with the oven. As can 
be seen in FIG. 6, the oven is provided with an access door 48 and a 
window 49 so that the operator can observe the workings of the oven as the 
work pieces proceed on the cables 98 through the oven. In the embodiment 
shown in FIG. 6, the oven is mounted on rollers 47 so that it can be moved 
from one location to another and is provided with a control panel 61 which 
can be utilized to control the gas feeds, blower operations, oven 
temperatures and the like in a manner conventional to convection oven 
operation. 
Obvious modification to the invention may be made without departing from 
the spirit of the invention. Thus, for example, the conveyers of FIG. 4 
can be arranged so that they terminate outside of the oven rather than 
inside as shown. While cable conveyors are preferred, the conveyor can 
have a foraminous surface as long as the sheets on the surface can be 
heated by the hot gases from below and above the conveyor surface. 
Further, in the embodiment shown in FIGS. 5 and 6, the oven can be 
modified to provide for a more than a one layer conveyor cable system such 
as shown in FIG. 6. Thus, a second row of cables 98 can be provided above 
or below the one shown in FIG. 6 by modifying the oven to accomplish this 
as shown in FIG. 4. In utilizing multiple cable systems, it will of 
course, be understood by those skilled in the art that the contents of the 
materials conveyed by the cables will be contacted by all of the gases 
circulating through the diffuser plates 90 and 95 respectively of FIG. 6 
in the preferred embodiment. 
Thus, while the invention has been described with respect to certain 
specific illustrated embodiments and examples, it is not intended that the 
invention be limited thereby except insofar as appears in the accompanying 
claims.