Apparatus for rotational molding

A method and apparatus for the rotational molding of thin-walled articles. The method is comprised of the steps of introducing a predetermined amount of plastic material into a mold, rotating the mold about a first axis at a predetermined speed, while simultaneously rocking the mold through a selective arc about a second axis which is normal to the first axis, heating the mold for curing the plastic material during the rotation and rocking, and cooling the mold and plastic material whereby the plastic material is molded to form a thin-walled object in conformance with the interior surface of the mold. The rotational molding apparatus is comprised of a heat insulating housing comprising a bottom half and a top half which may be disassociated for access to the interior of the housing. The interior of the housing contains a first frame which may be rocked about a first axis and a second mold-carrying frame which is attached to the first frame and rotates about a second axis normal to the first axis. Motors mounted exterior of the housing and operably connected to each of the frames cause the rotation and rocking of the frames. A heat source introduces heated air to the interior of the housing for purposes of curing plastic material held in a mold mounted in the apparatus.

BACKGROUND OF THE INVENTION 
This invention pertains to the art of rotational molding or casting and 
more particularly to a method and apparatus for the rotational molding or 
casting of thin walled items. 
The invention is particularly applicable to a rotational molding machine 
for the manufacture of thin walled or hollow items shaped in conformance 
with a mold. However, it will be appreciated to those skilled in the art 
that the invention could be readily adapted for use in other environments 
as, for example, where similar methods or apparatus are employed to 
manufacture items. 
Numerous types of apparatus and methods for molding thin walled articles 
are known in the art. A common rotational molding machine is one which 
fixedly mounts a plurality of mold carrying arms on a turret which rotates 
to move the arms from one operating station to another. At the operating 
stations the molds on the arms are loaded with a thermoplastic or like 
material, rotated and heated in an oven chamber, and cooled at a cooling 
station. During rotation and heating, the mold is typically rotated about 
two axes normal to each other. 
The deficiencies found in these types of rotational molding machines are 
such that the machines themselves are of limited economic and practical 
value due to a number of inherent problems. A principal problem is cost 
and size. The complexity of a machine which must carry a plurality of arms 
on a turret through different operating stations has produced an apparatus 
which is excessively costly to manufacture and maintain. In addition, the 
physical size of the apparatus requires a great amount of factory space. 
An additional problem in prior rotational molding machines has been the 
extremely short life span of bearings which support the mold carrying 
arms. In prior art machines when a mold is being rotated about two normal 
axes and heated, the bearings have been in the oven chamber or heating 
station. Likewise, when the mold is cooled. Such temperature changes have 
had an extremely debilitating effect on the bearings. 
A further problem in prior rotational molding machines has been the 
difficulties with mold loading, mounting and removal. A mold must be 
loaded with plastic material and then mounted on a turret arm. Where a 
plurality of turret arms are involved, loading, mounting and removal of 
molds has been particularly time consuming and inefficient. 
The present invention contemplates a new and improved apparatus and method 
which overcomes all of the above referred to problems and others to 
provide a new apparatus and method for rotational molding which is simple 
in design, economical to manufacture, readily adaptable to a plurality of 
uses with molds having a variety of dimensional characteristics, easy to 
install, easy to maintain and which provides improved molded articles. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided an apparatus 
and method for rotational molding or casting of materials comprising an 
outer housing including a first portion and second portion is cooperation 
with each other to define a selectively disassociable arrangement. A first 
frame is mounted in the housing for rocking about a first axis. A second 
frame is also mounted in the housing for rotation about a second axis 
transverse to the first axis. Both frames are driven by variable speed 
motors. A heat source is connected with the housing whereby material in a 
mold mounted in the rotational molding apparatus is rotationally molded to 
form a thin walled product. 
The method of the invention includes the steps of rotating a mold 
containing a predetermined amount of plastic material about a first axis 
while simultaneously rocking the mold through a selective arc about a 
second axis which is normal to the first axis. 
In accordance with another aspect of the present invention, motors which 
rotate and rock the respective frames are mounted exteriorly of the 
housing. 
In accordance with a further aspect of the present invention, the 
requirements for a plurality of stations for loading, rotating and 
heating, and cooling have been obviated. 
In accordance with a more limited aspect of the present invention, the 
rocking frame selectively rocks through a predetermined arc ranging 
between zero and ninety degrees. 
In accordance with yet another more limited aspect of the present 
invention, the upper portion of the housing of the rotational molding 
apparatus includes a flexible vent adapted to allow the egress of heated 
air. 
In accordance with another aspect of the present invention, the molds are 
carried in a mold support apparatus which is easily installed and removed 
from the rotational molding machine and is adapted to carry a plurality of 
molds of a variety of dimensions. 
One benefit obtained by use of the present invention is a method and 
apparatus for rotational molding which has the capability of manufacturing 
thin walled articles which have a variable wall thickness. 
Another benefit obtained from the present invention is a rotational molding 
apparatus which substantially reduces maintenance costs and the space 
requirements of prior art rotational molding machines. 
A further benefit of the present invention is improved efficiency in the 
manufacture of thin walled articles due to the ease of loading, 
installation and removal of molds and the mold carrying apparatus during 
the steps comprising the manufacture of a rotationally molded article. 
Other benefits and advantages for the subject new method and apparatus for 
rotational molding will become apparent to those skilled in the art upon a 
reading and understanding of this specification.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings wherein the showings are for purposes of 
illustrating the preferred and alternative embodiments of the invention 
only, and not for purposes of limiting same, FIG. 1 shows an alternate 
embodiment of a rotational molding apparatus A comprising an outer housing 
10 mounted on a fixed support 12 and a control unit 14. The housing 10 is 
comprised of an upper portion 16 and a lower portion 18. Upper portion 16 
is selectively disassociable from lower portion 18 for purposes of ready 
access to the inner areas of housing 10 for the loading and unloading of 
molds and mold-carrying apparatus. Housing 10 selectively rocks about an 
axis at bearing 20 with an arc range of approximately 90 degrees. Dash 
lines 22 and 24 of FIG. 1 show the housing 10 rocked at opposing 45 degree 
angles from the horizontal. Drive means are contained in support 12 for 
selectively rocking housing 10. Molds are carried in the apparatus in the 
inner portions of housing 10 on a mold supporting apparatus connected to 
variable speed rotating motor drive 26. Heat is introduced into the inner 
areas of housing 10 for curing plastic material contained in the molds by 
gas heater 28. The pitch and speed of the rocking movement, the speed of 
rotation of the mold turning apparatus, and the rate of heating of the 
inner areas of the housing 10 are all controlled by control unit 14. 
With reference to FIG. 2, the preferred embodiment of the rotational 
molding apparatus embodying the invention is shown. The rotational molding 
apparatus B includes an outer casing 30 constructed of heat insulating 
material and is comprised of an upper portion 32 and a lower portion 34. 
Control unit 36 controls the operation of rotational molding apparatus B. 
The lower portion 34 of casing 30 is fixed to support base 38 while upper 
portion 32 is selectively arcuately disassociable from lower portion 34 
through rotation about pivot connector 37. Drive motor 39 and gears 40, 
41, which are rigidly affixed to lower portion 34, operate the opening and 
closing of upper portion 32 and are controlled by control unit 36. 
With reference to FIGS. 2, 3 and 5, casing 30 contains a first frame 42 
generally disposed horizontal at rest and pivotally attached to support 
members 43, 45 at opposing bearing members 44, 46 for rocking frame 42 
about an axis 48. Frame 42 is connected to bearing members 44, 46 by 
shafts 50, 52 respectively. Shaft 50 is fixedly attached to quadrant gear 
54 which is driven by worm gear 56 and variable speed motor 58. The extent 
of the pitch in the rocking action of frame 42 is selectively controlled 
by adjustment of the positions of limit switches 60 and limit switch 
actuator members 62. The speed of rocking action is selectively controlled 
by varying the speed of motor 58 through associated controls in control 
unit 36 and typically has a speed ranging from two to twenty cycles per 
minute. 
With particular reference to FIG. 5 it can be seen that bearing members 44, 
46, motor 58, and gears 54, 56, are all mounted exteriorly of casing 30 
and are fixedly attached to support members 43, 45. 
The action of the first frame 42 is to selectively rock about a first axis 
48 at a variable speed in a predetermined arc ranging from zero to ninety 
degrees; although if desired the rocking action may be completely 
eliminated during rotational molding. Numerous types of conventional 
controls for producing such a rocking action are known in the art. The 
controls disclosed in the preferred embodiment are only one means of 
obtaining such action. 
With reference to FIGS. 2, 3 and 5, casing 30 contains a second frame 68 
for carrying molds. Frame 68 is fixedly attached to first frame 42 by 
opposed attachment means 70 and 71. Attachment means 70 and 71 may 
comprise any conventional attachment devices which provide for quick and 
easy attachment and disattachment of the frame. 
With particular reference to FIGS. 5 and 6 it may be seen that frame 68 
comprises two opposed end members 72, 74 with four support members 76, 78, 
80 and 82 extending between end members 72, 74 at spaced intervals. Frame 
68 is disclosed as having two removable and adjustable segregating members 
82, 84 to provide for three mold carrying compartments in frame 68. Any 
number of segregating members could be used to provide for a plurality of 
mold carrying compartments, or none may be used to provide for a single 
large compartment. 
With particular reference to FIG. 6, it may be seen that support member 80 
is selectively and arcuately disassociable from frame 68 through rotation 
about pivot connection 86. Support member 80 may be rigidly affixed to 
frame 68 by locking means 88 to provide for fixed containment of molds in 
frame 68. Support member 80 contains a plurality of recesses 90 adapted to 
receive a portion of the edges of the molds in the frame 68 to further 
fixedly contain the molds. 
With particular reference to FIGS. 2 and 3, attachment means 70 is fixedly 
attached to shaft 92. Shaft 92 is fixedly attached to gear member 94 and 
variable speed motor drive 96. Variable speed motor drive 96 operates to 
rotate frame 68 on an axis normal to axis 48 and at predetermined speed 
typically ranging between four and twenty revolutions per minute and is 
operated by means of control unit 36. Variable speed motor drive 96 and 
gear member 94 are mounted exteriorly of casing 30. 
With reference to FIG. 3 it may be seen that the rotational molding 
apparatus B operates to rock frame 42 between the arcuate positions 
designated by dashed lines 98 and 100, while simultaneously rotating 
second frame 68 about an axis normal to the rocking axis 48. 
With reference to FIG. 2, an apparatus 102 for introducing heat into the 
casing for heating and curing plastic material contained in a mold during 
rotational molding comprises a safety solenoid 104 which is connected to 
limit switch 106 to provide for automatic shutoff of gas to burner 107 
when the upper portion 32 of the casing 30 is open, a conventional 
pressure controlled diaphragm 108 and an air inlet 110. The egress of 
heated air from the casing 30 is provided by means of flexible vent 112. 
With reference to FIG. 4, shafts 114 and 116 are fixedly attached to 
support plate 118 for motor drive 96. Shafts 114 and 116 are support 
members which are fixedly attached to frame 42 to provide a fixed support 
against the torque force applied to shaft 92 by means of gear member 94 
and motor drive 96. Longitudinal slots 120, 122 and 124 of casing 30 allow 
movement of shafts 114, 92 and 116 through their rocking arc. 
With particular reference to FIG. 4A which is taken along line 4a--4a 
depicted in FIG. 3, it may be seen how the longitudinal slots 120, 122, 
124 are sealed around the shafts 114, 92, 116, respectively to prevent the 
undesirable egress of heated air. Opposing flexible asbestos type 
skirtings 126 and 128 operate to allow the easy movement of the shaft 
along the slot by giving away as the shaft moves along the slot, yet 
returning to an abutting sealing association after the shaft has passed. 
With reference to FIGS. 6 and 7, a mold 130 with a spherical inner portion 
132 is shown. Such a mold could be used for the production of hollow balls 
or the like, such as basketballs. The mold 130 is typically constructed of 
aluminum and is generally comprised of two distinct parts which are 
affixed together and sealed by conventional attachment means such as a 
plurality of bolts and nuts or camming members through associated 
cylindrical openings 134. One edge of the mold 130, such as at edge 136, 
is adapted to be closely received in associated recess 90 in support 
member 80 of frame 68. 
FIGS. 8 and 9 show a carousel type arrangement of a plurality of rotational 
molding machines embodying the invention. In such an arrangement the 
carousel 140 rotates about an axis 142 by means of drive motor 144. 
Overhead hoist 146 is utilized to load and unload molds or mold carrying 
apparatus. 
METHOD AND OPERATION 
With particular attention to FIGS. 3 and 6, the improved rotational molding 
characteristics of the new rotational molding apparatus will be 
specifically discussed. 
The invention can be used to mold anything that will set under heat, 
generally thermoplastics, plastisols, or polyethylene type materials. The 
invention can also be used to mold common molding resins such as epoxies, 
acrylics, or polyesters, where catalysts may be used instead of heat. A 
predetermined amount of plastic material in either liquid or powder form 
is placed in a mold. A mold is generally constructed of aluminum, 
comprised of two halves, and sealed by screws or cams. A silicon type 
spray is generally used to coat the inner surfaces of the molds in order 
to inhibit adhesion of the plastic materials to the surface during 
heating. 
After the insertion of the plastic materials, the mold is then loaded in 
the mold carrying frame 68 and tightly secured therein. The entire loading 
operation will require approximately one minute per mold. 
The mold carrying frame 68 may or may not be attached to the mold carrying 
apparatus B at the time of the loading of the molds. After the loaded 
molds and mold carrying frame 68 have been securely attached in the 
rotational molding apparatus casing 30, and the casing 30 has been heat 
sealed, the molding and curing of the plastic material to form a thin 
walled object in conformance with the interior of the mold is ready to 
begin. Molding entails rocking about a first axis 48 in an arcuate pitch 
ranging between zero and ninety degrees and a rocking speed ranging 
between two and twenty cycles per minute. Simultaneous rotation about a 
second axis normal to the first axis 48 also occurs. The rotation speed 
has a range between four and twenty revolutions per minute. While the 
simultaneous rocking and rotating action is occurring, heat is introduced 
into the mold carrying apparatus casing by heating means 102 (FIG. 2) for 
purposes of curing the plastic material. Although the necessary 
temperature is dependent upon the plastic materials to be molded, the air 
temperature of the heated air introduced into the casing 30 is 
approximately 1,000 degrees Fahrenheit and a common mold temperature for 
curing is approximately 300 degrees Fahrenheit. The curing time may 
typically range from two to ten minutes with typically five to six minutes 
curing time necessary to obtain 1/8 of an inch wall thickness in the 
molded article. 
If desired, the rocking action may be eliminated during molding entirely, 
resulting in only a rotating action of the mold carrying frame 68. Mere 
rotating action is desired in casting of hollow articles such as pipe or 
tubing. 
A particular advantage of the invention includes the attainment of variable 
wall thicknesses in a molded article due to the independently controlled 
rotation and rocking action. For example, where a canoe is to be 
rotationally molded which requires greater structural strength in central 
portions of the canoe as opposed to the end portions, dwelling the mold 
carrying apparatus in a substantially horizontal position for a longer 
time during the rocking action will result in a thicker wall thickness in 
the central portions of the canoe. 
After the curing time has elapsed, the casing 30 is opened and either the 
molds or the mold carrying frame 68 is removed from the rotational molding 
apparatus B. The next step comprises quenching of the molds, typically in 
a liquid bath, in order to cool the plastic materials in the molds. 
Another particular advantage of the present invention includes increased 
efficiency in rotational molding since the quenching operation is 
performed apart from the heating operation. For example, a mold carrying 
frame 68 containing cured molds may be removed by means of a hoist from 
the mold carrying apparatus B and loaded into a quenching bath, while 
another mold carrying frame may be immediately thereafter loaded into the 
mold carrying apparatus for curing of the molds, which in turn may then be 
unloaded and quenched and a third frame loaded, and so on. The speed of 
the operation will be limited by the curing time and not by the loading of 
the plastic materials into the molds nor the quenching of the molds. 
With reference to FIGS. 8 and 9, this assembly line type of manufacture may 
be most advantageously realized in a carousel operation where a plurality 
of rotational molding machines may be loaded, heated and cured, and 
unloaded for quenching, all on a sequential basis. That is to say a 
machine may be loaded and thereafter heated and cured while other machines 
are loaded. 
The invention has been described with reference to the preferred 
embodiment. Obviously, modifications and alterations will occur to others 
upon the reading and understanding of the specification. It is my 
intention to include all such modifications and alterations insofar as 
they come within the scope of the appended claims or the equivalents 
thereof.