GB2204265A - Forming hollow structures from powdered thermoplastic materials - Google Patents

Description

----------------- - A 1. 2204265 _METHOD. -AND APPARATUS FOR.FORMING

HOLLOW STRUCTURE9 FROM POWDERED THERMOPLASTIC MATERIALS POOR O.UALITY The present invention concerns a method fO r fabricating hollow structures by depositing a thermoplastic material in powdered form on an exterior surface of a mold form and an,apparatus for carrying out this process.

Hollow structures such as noncorroding fluid container liners have previously been formed from a (1,1Z -rj 'k) variety of thermoplastic materials, such as polyethylene, polypropylene, nylon, Celcon, and Hytre 11 One method of fabrication, called the "roto-casting" process, involves placing a predetermined quantity of thermoplastic material in powdered form inside a split mold, inserting the mold and powdered thermoplastic material combination into an oven and rotating the combination about one or more axes at a temperature approximately the same as the melting temperature of the powdered thermoplastic matexial. The powdered thermoplastic material splashes freely within the mold and adhers to the inner surface of the mold as the surface temperature approachs the melting point of the thermoplastic material. The mold is removed from the oven after the thermoplastic material has been deposited over the entire inner surface-of the mold and the resulting structure is removed from. the interior of the mold when cooled.

This proess, however, suffers from a number of disadvantages. Chief among these is an inability to control the wall thickness of the resulting thermoplastic material structure. Pinholes often occur in portions of the resulting structure due to poor distribution of the thermoplastic material within the mold during the -heated rotation-step. Alternatively, 2.

exce. ssively--thick and-hepyy structures -result fr-om attempting to achieve a minimum wall thickness--over--the e ntire surface of the structure. In addition, the interior surface of the resulting structure commonly has a rough or uneven surface. This rough surface can cause problems with fluid purging procedures when the resulting thermoplastic structure is used as a fluid container. It is also commonly difficult to adhesively bond most of the thermoplastic materials best suited for use as fluid container liners due to their noncorrosive characteristics. This bonding problem can be further exacerbated by an often necessary practice of coating the inside of a roto-cast mold with a release agent before inserting the powdered thermoplastic material. Because of this bonding problem, difficulties are frequently encountered in fabricating a fluid container having an exterior shell bonded to an inner liner formed from thermoplastic materials by the roto-casting process.

Attempts have been made to avoid the problems encountered in practicing the roto-casting process.

One process, described in U.S. patent No. 3,009,209, involves spraying a liquid reactive m,,xture capable of forming a solid polyurethane plastic onto a rotating mandrel form. While permitting a more careful monitoring of wall thickness, this process is limited to liquid reactive materials which can be spray deposited through an aerosol nozzle onto a mandrel form while still in a liquid state. Similarly, a process described in U.S. patent No. 3,379,591 involves forming tubular articles by spray depositing r'esin-glasS ' compositions and incorporating cut lengths of filament roving onto a rotating mandrel form. This process is also limited to those resin-glass compositions which can-be deposited from an aerosol nozzle while still in a liquid state.

3 1 Thus, there exists a nedd for a process for forming hollow structures from a powdered thermoplastic material permitting careful monitoring of the wall thickness during fabrication. There further exists a need folr a process for forming a hollow structure from powdered thermoplastic plastic materials to which external container shells and the like can be more easily bonded and which have a relatively smooth inner wall. In addition, there exists a need for an apparatus by which this process can be practiced.

According to one aspect of this invention, there is provided a method for forming a hollow structure from a thermoplastic material, comprising the steps of:

a. heating a mold form to at least the melting point of said thermoplastic materials; b. rotating said mold form; c. sprinkling said thermoplastic material onto the exterior of said mold form as it rotates while heated to form a semi-molten layer of said material on said mold; d. cooling said mold form to effect hardening of said layer; and e. removi ng the resulting formed hollow structure from said mold form.

According to a second aspect of this invention, there is provided an apparatus for forming a hollow structure from a thermoplastic material comprising:

support structure; mold form, rotatingly supported on said support structure; heating means for heating said mold form to a temperature at least as high as the melting point of said - thermoplastic material and maintaining said mold form at said elevated temperature; rotating means, coupled to said mold form, for rotating said mold form; and dispenser means, disposed exteriorly adjacent to 1 said mold form, for sprinkling powdered-thermoplastic material onto an exterior surface of said mold form.

In preferred practice of this invention the hollow mold form is,in a preliminary step, coated with a commercially available release agent before being heated.

The mold form is then heated to approximately the melting point of the thermoplastic material to be deposited. The mold form is then continuously rotated as a predetermined quantity of powdered thermoplastic material is sprinkled over the exterior surface of the mold form. The deposition rate, temperature, and rate of rotation are coordinated to achieve the deposition of a semi-molten layer of thermoplastic material having a predetermined thickness. The deposition step continues until a plurality of semi-molten layers have been built up on the mold form having a composite thickness approximately the, same as the desired resulting structure.

In a preferred embodiment of the present inventive process, an adhesively bindable material is then sprinkled over the semi-molten outer layer. The mold form is subsequently cooled to a lower temperature and an edge of the resulting thermoplastic material structure may be trimmed to form a uniform edge. The mold form may then be further cooled and the resulting structure may be removed from the mold form by introducing pressurized air between the resulting structure and the mold form.

The apparatus of the present invention preferably includes a hollow mold form open at one end and rotatingly coupled to the support structure, a means for heating the mold form, and an apparatus, removably disposed above the mold form, for dispensing powdered thermoplastic material.

A preferred dispensing apparatus comprises a support frame; a reservoir for powdered thermoplastic material attached to said support frame and having a bot-tom defining an aperture; a gate at the bottom of said reservoir for selectively opening and closing said aperture; a dispensing cylinder rotatably supported by said support frame and disposed below said shutter;-and means for rotating said dispensing cylinder. The rotating dispensing cylinder is preferably driven by a suitabli variable speed drive. Both elements are attached to a support frame independent of the mold form support structure. The dispensing apparatus support frame may be vibrated to assist in the even distribution of powdered thermoplastic material over the mold form.

The support structure for the mold form preferably includes a hollow cylinder partially disposed within the mold form and a suitable structure for physically supporting this hollow cylinder. The cylinder is open at an exposed end outside the mold form and sealed off at the opposing end disposed within the mold form. A fluid-tight bushing is disposed about and sealed to the hollow cylinder and a bulkhead similarly disposed about and sealed to this bushing. The bulkhead is attached to the mold form at the open end of the cavity therein thus enclosing the space within the mold form.

The mold form may be heated by heating a fluid, such as oil, to a temperature approximately twice the m,lting terr.rature (in OF.) of t1ya thermtPlastic material to be used. This fluid is introduced into the enclosed volume within the mold form through pipes disposed within the hollow supporting cylinder. Internal heating elements disposed adjacent and coupled to the hollow supporting cylinder maintain the temperature of the heated fluid.

External radiant energy heaters disposed about the mold form may provide supplemental heating. Thermocouples may be disposed within the mold form, attached to the hollow supporting cylinder, to assist in temperature management.

After the thermoplastic material has been satisfactorily deposited on the exterior of the mold form, the internal and external heating elements are turned off, the heated fluid removed from the mold form, and a quantity-of the - same or a similar fluid at,say, approximately room 1 temperature is - intro - duced into the mold forM-to begin cooling the mold form.

After cooling, the resultant thermoplastic structure may be removed from the mold form by introducing air between,the mold form and the resulting structure through a tube disposed within the mold form which extends through both the bulkhead and the tip of the mold form.

For a better understanding of the invention and to show how the same can be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, wherein:

FIGURE 1 is a perspective view of an illustrative embodiment of the apparatus for carrying out the inventive process for forming hollow structures from powdered thermoplastic materials; FIGURE 2 is a cutaway side view of the mold form, support structure, heating and cooling means, and dispensing apparatus shown in Figure 1; FIGURE 3 is a cutaway end view of the mold form and dispensing apparatus also shown in Figures 1 and 2.

FIGURE 4 is a cutaway side view of the mold form used in the apparatus of Figure 1.

7 Referring now to the drawings, and more par,ticularly FIG. 1 thereof, there is shown a preferred embodiment (if the inventive apparatus for forming hollow structures from powdered thermosetting materials. The major elements of this apparatus include a hollow mold form 10 rotatably coupled to a support structure 13. A dispensing system, generally designated as 16, is removably disposed above and to one side of the mold form 10. Auxiliary external heating devices 19, typically radiant heaters, are disposed on the opposing side of the mold form 10.

As shown in FIGS. 2 and 4, the mold form 10 has a hollow interior 11 communicating with an aperture 21 at one end. A forward support member 22 is disposed within the mold form 10 towards the closed portion of the mold form 10. An attachment ring 23 is connected to the inside surface of the mold form 10 adjacent the aperture 21.

The particular shape of the mold form 10 is derived from the desired shape of the structure being manufactured. The shape of the resulting structure manufactured using the mold form 10 1-116strated in FIGS. 2 and 4 is intended for use as an external fuel tank for an aircraft or helicopter. The mold form 10 illustrated in FIGS. 2 and 4 therefore has a generally aerodynamic shape approximately corresponding to the desired shape of an external aviation fuel tank.

Structures having other shapes could also be formed by the process of the present invention. The particular shape qf the-mold form employed in the prefer. red embodiment is, therefore, illustrative only and should not be interpreted as a limitation.

As shown in FIG. 3, the support structure 13 of the i3resent inventi on includes a nonrotating hollow support beam 24 connected to a support frame 27. When the mold form 10 is rotatably mounted to the- support ------------- structure -1-3 (as -shown in FIG. 2), a portion of the hollow support ' beam-74 As- disposed within the moldr6im- 10. The end poxtion 24a of the hollow support; beam 24 disposed within the mold form 10 fig Sealed off by a cap 28. The opposing end portion 24b of the hollow support beam 24 has an aperture opening to the interior 25 of the hollow support beam 24. Thus, the hollow support beam 24 defines a stationary conduit into the interior cavity 11 of the mold form 10.

The mold form 10 is rotatably mounted to the support structure 13 by being connected to a bulkhead which is disposed about and sealingly attached to a fluid-tight bushing 33, which is in turn disposed about and similarly sealingly connected to the hollow support beam 24. A high temperature gasket 34 is disposed between the mold form 10 and the bulkhead 30 to provide a fluid-tight seal. The mold form 10 may be connected to the bulkhead 30 by b6lts passing through the bulkhead 30 and engaging the attachment ring 23, or by any other convenient fastening means.

To facilitate subsequent removal of a thermoplastic structure from the exterior surface of mold form 10 an air tube 35 is disposed along the interior surface of the mold form 10. One end of the air tube 35 penetrates through the enclosed tip 10a of the mold form 10 while an opposite end of the air tube is connected to and sealingly passes through the bulkhead 30.

Rotation of-the mold form 10 is effectuated by a variable-speed motor 36'connected to the support frame 27 and engaging a sprocket wheel 39 by means of a chain 41. The sprocket wheel 39 is attached to te bulkhead 30.

The dispensing system 16 (FIGS. 2 and 3) includes a thermoplastic material reservoir, or hopper 44 connected to- a support frame 45. The bottom of the hopper 44 defines an aperature-to which a gate 48 is- 9 attached-. -This gate 48 permits a controlled quantity of the rmop la.5ft4-c_-_mater ia I to-fall througb the-bot.tom of the hopper 44 'thereby controlling the rate of diposition of thermoplastic material onto the surface of. the mold form 10.

A dispensing cylinder 51 is rotatingly mounted to the support frame 45 and disposed directly below the hopper 44. The dispensing cylinder 51 extends along almost the entire length of the mold form 10. In the presently preferred embodiment of the present invention the dispensing cylinder 51 has a s mooth surface. A knurled or otherwise roughened surface should, however, assist in providing a more uniform deposition of thermoplastic material onto the mold form 10.

Alternatively the dispensing cylinder 51 may have a surface made of some form of rubberized material to further effectuate a more uniform deposition of powdered thermoplastic material.

Rotation of the dispensing cylinder 51 is effectuated by a second variable-speed motor 56, connected to the support frame 45, engaging a standard "v-belt" drive belt and gear connected to the dispensing cylinder 51. A vibrating mechanism 60 for vibrating the hopper 44 and dispensing cylinder 51 is also attached to the frame 45. As shown in FIG. 3, the support frame 45 has an offset geometry to permit the hopper 44 and dispensing cylinder 51 to be placed directly over the mold form 10.

For material dispensing the vibrating mechanism 60, and variable-drive motor 56 are activated and a limited quantity of thermoplastic material sifted through the gate 48 to fall onto the now rotating dispensing cylinder 51. As the dispensing cylinder 51 rotates, a uniform stream of thermoplastic material is deposited onto the-top of the mold form 10. The density of the thermoplastic material stream and consequently the rate of deposition of theremosetting material onto the -mold form 10 is governed by the size of the opening a_t the ------- - - ---- ----------- 10- b6ttOM of the- hopper 44--zkp -regtklated- by the- gate 48.

It has been found-that the combination of a hopper spilling a limited quantity of thermoplastic material onto a rot:ating dispensing cylinder provides a more uniform distribution of thermoplastic material onto the mold form 10 than a hopper alone.

- The mold form 10 of the present invention is heated to about twice the melting temperature ("F) of a thermoplastic material by introducing a heated fluid into the cavity 11 of the mold form 10. Typically oil having a flash point of 2880C (550"F) or higher is used. Any fluid having a boiling temperature and/or ignition temperature greater than twice the melting temperature of the thermoplastic plastic could alternatively be used for heating. The same type of oil is used for both the heating and cooling.

As shown in FIG. 2, the heating system of the present invention includes a heating fluid reservoir 63 and a separate cooling fluid reservoir 66 connected by appropriate plumbing to a high temperature reversible fluid pump 71. An exterior heat source 74 is used to heat the oil within the heating reservoir 63. This heat source 74 may be incorporated into the rese,rvoir if desired. For example, insulated electrical heating elements may be disposed within the reservoir 63 Alternatively, devices such as commercial oil boilers could be employed.

In the presently preferred embodiment of the- invention, the heated oil reservior 63 and cooling fluid reservior 66 are respectively connected to separate two-position "on-off" type valves 67 and 68.

Pipe segments 69 and 70 extend respectively from the off-on valves 67 and 68 to alternative inputs of an alternatively selective "T-valve" 72 the output of _whidh is i n turn connected to the reversible pump 71 via pipe segment 76- - From the _pump 7l, heated or cool oil is communicated to the interior cavity:11 of- the -mold form 10---via by a pipe segment 71-connected-to a pipe segment-80 disposed within the the Anterior25-.o.f the hollow -support beam 24 and passing through the ol3ening at the hollow support beam end 24b. The pipe segment &0 includes a lateral section 80a which passess through the bottom of the hollow support beam 24 within the cavity of the mold form 10. A fluid-tight seal is forined between the pipe section 80a and the hollow support beam 24. The pipe section 80a descends towards but does not contact the bottom inner surface of the mold form 10.

A second pipe segment 83 is also disposed within the interior 25;f the hollow support beam 24 similarly passing through the opening at the hollow support beam end 24b. Pipe segment 83 also includes a lateral section 83a which passes through the bottom of the hollow support beam 24 decending towards but not contacting the bottom inner surface of the mold form 10. Pipe segments 80a and 83a are generally disposed near opposing ends of the enclosed portion of the hollow support beam 24. In the presently preferred embodiment of the invention heated oil is heated exteriorally and then inserted into the interior 11 of the mold form 10... Internal heating elements 86 are then employed to maintain the elevated temperature of the heated oil. In this presently preferred embodiment pipe segment 83 is capped off at an end adjacent the hollow support beam end 24b. In an alternative embodiment, however, the heated oil could be circulated through the interior 11 of the mold form 10 with the 3_0 elevaed temperature of-the heated oil further augmented by an externally located commercially I available heating source. In this alternaive embodiment, pipe segments 80 and 83 would serve as input and output means for the circulating heated oil.

As shown in FIGS. 2' and 3, several internal heating elements 86 are-disposed w-ithin the cavity of 12 the mold form 10 directly-below the hollow support beam 7A. -The internal Weat-ing elements 86 are connected to two interface boxes 89 and 92 which are sealingly connected to the hollow support beam 24. These interface boxes 89 and-92 provide fluid-tight seals around the internal heating elements 86 and electrical cables passing through the hollow support beam 24 to energi.ze the internal heating units 86.

As mentioned above, in the preferred embodiment internal heating elements 86 are used to maintain the temperature of a heated oil disposed within the cavity 11 of the mold form 10. Several external heating units 19 are disposed along one side of the mold form 10 to supplement the internal heating of the mold form 10 and to maintain the exterior surface of the mold form 10 at a uniform temperature. Heating units 19 are typically radiant heaters.

Internal thermocouples 95 are used to monitor the interior temperature of the mold form 10. As shown in FIG. 2, these thermocouples descend from the bottom portion of the hollow support beam 24 towards the inner surface of the mold form 10. The thermocouples 95 penetrate through the hollow support beam 24 via fluid tight seals.

25- The oil within the heating fluid reservoir 63 is heated to a temperature about twice the melting temperature of the thermoplastic material to be used.

The "off-on" type valve 67 is opened and alternatively selective "T-valve" 72 turned to place the reversible pump 71 in communication with the heating fluid reservior 63. A quantity of heated oil is then transferred by the pump 71 to the interior cavity of the mold form 10 through pipe segment 80. Internal heating elements 86 and auxiliary heating devices 19 are -then activated.

When the mold form 10 is to be cooled, the internal heating elements 86 and auxiliary heating 13 -devices 19 ariF deactivated and the pump 71 activated in a reverse mode to remove the heated_oil from the interior of the mold form 10. Valve 67 is then closed, valve 68 opened and the alternatively selective OT-valve" 72 turned to place the zeversible pump 71 in communication with the cooling fluid reservior 66.

Cool oil, typically at room temperature, is then pumped into the cavity 11 of the mold form 10. This process reduces the temperature of the mold form 10 to approximately the solidifying temperature of the thermoplastic material to be used. Thereafter, the mold form 10 is allowed to cool to room temperature without introducing additional cool oil.

As discussed above the mold form 10 could alternatively be heated and maintained at an elevated temperature by circulating the heated oil through the cavity of the mold form 10 and an exterior heat source.

Cool oil could be similarly circulated to facilitate cooling of the mold form 10. If more rapid cooling were desired, the cool oil could be circulated through an external refrigeration unit or a waste heat exchange unit.

Having described the inventive appdratus used to practice the'process of the present invention, the process will now be detailed.

Thermoplastic materials are normally produced for industrial pu-rposes as pellets, - typically having a 3mTi (1/811) diarretex arxi a 3mn (1/8") length. Powdered thermoplastic materials are formed by grinding or milling these pellets -into a powder having a consistency generally similar to course flour. Numerous thermoplastic materials in powder form can be used with the process of the present -invention. Useful materials include NNYLON 6/6" (a trademark) and "HYTREL" (a trademark).

- A hollow structure is form(d from a powdered thermoplastic material by first-.coating the exterior surface of the mold form 10 with a conventional release 14 agent such-as "RELEASE. ALL 30_"___(a_-tr_ademark) -or "'RELEASE ALL _50'w -(a trademark) After c6a'ti-ng_tb-e mold form 10, the oiJL residing within the heating f luid reservoir 63 is heat ed, typically, to a temperature about twice the melting temperature of the thermoplastic plastic to be used. Th is process has been performed by heating the oil within the heatinq fluid reservoir to a temperature of approximately 2600C ( 5500 F) when w5ed with thermoplastic materials melting at about 121"C (250"F)._ After heating the-oil in.th.e hea.ting fluid reservior 63 to approximately 260C (5500F), valve 67 is opened and alternatively selective "T-valve" 72 turned to communicate heating fluid reservior 63 with the pump 71. The pump 71 is then activated in a forward mode and heated oil is transferred into the interior of the mold form 10. Typically 189.3 to 283.9 drr (50 to 75 US gallons) of heated oil is used to heat the mold form 10 of the preferred embodiment which has a diameter of about 76cm (2.5 feet) and a length of about 152 an (5 ffeet). The internal heating elements 86 are then activated to maintain the temperature of the heated oil. The auxiliary external heating devices 19 are also activated to maintain the exterior surface of the mold form 10 at a uniform temperature. Typically the internal temperature of the mold form is maintained at about 260"C (5500F) and the external temperature of the mold form maintained at about 218"C (425"F). - Continuous rotation of the mold form, typically at a rate of 1-3 revolutions per minute, is then initiated using the motor 36 and the powdered thermoplastic material dispensing system 16 is placed over the rotating mold form 10. The dispensing system 16 is then activated as described above and a predetermined quaritity of powdered thermoplastic material deposited on the exterior surface of the mold form 10 along the entire length thereof.

-15 In using thermoplastic --matex ials such as "NYLON-- 6/6 (a tradema-rk) or "HYTRELO'-(a trademark) j- good results have been obtained when the quantity of.

ited on the exterior thermoplastic material deposi surface of the mold form 10F in conjunction with the rotation of the mold form 10 at a rate of 1-3 revolutions per minute, produces a layer approximately 0.05 to 0.13 mm (.00" -.005")thick during each revolution. The deposition of powdered thermoplastic material is continuously performed until the cumulatively deposited layers bu-ild up to a composite thickness of approximately 0.5 to 0.76 mm (.020" -.030"). Deposition of the powdered thermoplastic material is then stopped. While the dispensing system 16 provides a generally uniform deposition of thermoplastic material onto the exterior surface of the mold form 10, the resulting composite layers may be examired at this point and, if necessary, additional thermoplastic material may be selectively deposited onto any areas of apparent sub-minimal thickness.

To substantially assist the adhesive bonding characteristics of the resulting structure, the surface of the molten thermoplastic material structure is then covered with a material having better adhesive bonding characteristics. Typical of the materials that can be used are glass or graphite fibers, milled fibers, or microballoons. Milled fiberglass fibers have been successfully used.

After covering the surface of the molten structure with a more'adhesively bondable compound, the auxiliary external beating devices 19 and internal heating elements 86 are deactivated. iThe pump 71 is then reversed to remove virtually all of thd heated oil from -the interior of the mold form'10. The valve 67 is then closed, valve 68 op ened and-alternatively selective "T-valve' 72 turned to communicate the cold fluid reservoir 66 with the pump 71. The pump 71 is again 16 1 activated in the fory9fd--iode- and room temperature d6ol fluid, typically the same type of oil as used for heating, is pumped into the interior of the mold form 10.

Cool oil is pumped into the interior of the mold form 10 until the interior temperature has dropped to approximately 121 0 C (250 0 F). Thermosetting materials such as "NYLON 6/611 (a trademark) and "HYTREL" (a trademark) adhering to the exterior of the mold form usually solidify at approximately 121 0 C (250 OF). The edge of the solidified structure adjacent the bulkhead 30 is then trimmed to produce a straight edge. The mold form 10 and resulting structure are then allowed to cool further and the resulting structure subsequently removed from the mold form 10.

Pressurized air is introduced into the tube 35 communicating with the exterior tip of the mold form 10 to facilitate removal of the resulting structure.

Alternatively, pressurized air may be directed-along the trimmed edge of the resulting structure to effect removal.

Although the invention is described with respect to a particular shape ofthe mold form 10, the invention is adaptable for use with mold forms of other geometries and is therefore not limited in form to that of the preferred embodiment herein described.

17 1 Cl-aims:

1 -1.' A method for forming a hollow structure from a thermoplastic material, comprising the steps of:

a. heating a mold form to at least the melting 5_ 1 point of said thermoplastic materials; b. rotating said mold form; c. sprinkling said thermoplastic material onto the exterior of said mold form as it rotates while heated to form a semi-molten layer of said material on said mold; d. cooling said mold form to effect hardening of said layer; and e. removing the resulting formed hollow structure from said mold form.

2. The method of claim 1, wherein said mold form is set in rotation after heating of it has commenced.

3. The method of claim 1 or 2, wherein said sprinkling step c is continued until a plurality of layers having a predetermined composite thickness have been deposited on said rotating heated mold form.

4. The method of any preceding claim wherein an adhesively bindable powder is sprinkled onto the outer surface of said semi-molten layer before step d.

5. The method of any preceding claim wherein said mold form is coated with a release agent before step a.

18 6.- The method of-any prec(diffgj ulaim wherein one_ edge of sa id resulting structure is trimmed to form an even edge after step d and before step e.

7. The method of any preceding claim wherein step e is carried out by forcing air between said mold form and said resultingly formed hollow structure.

8. The method of any preceding claim wherein said mold form defines an interior cavity and has a plurality of heating elements disposed within said cavity and wherein step a is carried out by introducing into said mold form cavity a quantjty of heated'fluid having a boiling or flash temperature greater than the melting temperature of said thermoplastic material.

9. The method of claim 8 wherein the temperature of said mold form is maintained by activating said internally disposed heating elements.

10. The method of claim 8 or 9 NdV-rein step d is carried out by removing said quantity of heated fluid and introducing a quantity of fluid, having a boiling or flash temperature greater than the melting temperature of said thermoplastic material, at a cooling temperature.

11. The method of claim 8 wherein step a is further carried out by operating a plurality of radiant energy heating elements ext6xnally disposed. about and directed towards said mold.

12. The method of any one of claims 1 to 7 wherein said mold form defines an interior cavity and step a is carried out by circulating a heated fluid, having a boiling or flash temperature greater than the melting temperature of said thermoplastic material, through said cavity, 19 and temperature being main fainedby externally -beating said heated fluid.

13. The method of claim l2wherein step d is carried out by removing said continuously heated circulating fluid and circulating a fluid, having a boiling or flash temperature greater than the melting temperature of said thermoplastic material, at a cooling temperature.

14. An apparatus for forming a hollow structure from a thermoplastic material comprising:

a support structure; a mold form, rotatingly supported on said support.-structure; heating means for heating said mold form to a temperature at least as high as the melting point of said thermoplastic material and maintaining said mold form at said elevated temperature; rotating means, coupled to said mold form, for rotating said mold form; and dispenser means, disF-4n-=jSA_-=xteriorly adjacent to said mold form, for sprinkling powdered thermoplastic material onto an exterior surface of said mold form.

15. The apparatus according to claim 14 wherein said mold form has a first end, a second end, and a continuously decreasing cross-sectional areadecreasing from a predetermined valu e at said first end to zero at said second end.

16. The apparatus according to claim 14 wherein said mold form defines an inner cavity with an aperture - at Aaid first end of said mold form communicating with said cavity- 17. 7he apparatus according to claim 16-wherein said beating means comprisesi:

a fluid reservoir in communication with the interior of said mold form; a fluid, disposed within said reservoir, having a boiling temperature greater than the melting temperature of said thermoplastic material; heating means, disposed adjacent said reservoir, for beating said fluid; pump means, in communication with said reservoir and the interior of said mold form, for circulating said fluid between said reservior and the interior of said mold form.

18. The apparatus according to claim.16 or 17 further comprising a plurality of internal heating elements disposed within said mold form and coupled to said hollow mold form support structure.

19. The apparatus according to claim 16, 17 or 18, further comprising a means, disposed within said mold form, for measuring the temperature within said mold form.

20. The apparatus according to any of clains 16 to 19 wherein said support structure comprises:

a hollow mold form support beam having an inner portion disposed within said mold from and an - outer portion disposed outside said mold form and having a sealed end adjacent.said inner portion and an open end adjacent said outer portion; a fluid-tight bushing coupled to said hollow form support beam; a bulkhead disposed about and coupled to said f luid-tight bushing, and detachably coupled to said mold form.

-21 21. The apparatus according to claim 20, additionally comprising a resulting thermoplastic material structure removing means which comprises a tube, disposed within said mold, connected to said bulkhead, and further connected to a forward interior tip of a second end of said mold form, said tube communicating with the exterior surface tip of said mold form.

22. The apparatus of-any one of.-claims 14 to 21, wherein said- dispenser mdAns comprises:

a support frame," a reservoir for powdered thermoplastic material attached to said support frame and having a bottom defining an aperture; a gate at the bottom of said reservoir for selectively opening and closing said aperture; a dispensing cylinder rotatably supported by- said support frame and disposed below said shutter; and means for rotating said dispensing cylinder.

23. The apparatus of claim22 wherein said gate 2n opens and closes said aperture to a selected flow rate controlling width.

24. The apparatus according to claim 22 or 23,further comprising a means for vibrating said support' frame.

25. An apparatus for sprinkling powdered material comprising:

a support frame; a reservoir for powdee_d material attached to said support frame and having 6L bottom defining an aperture; a gate at the bottom of said reservoir for selectively opening and closing said aperture; a dispensing cylinder rotatably supported by said support frame and disposed,below said shutter; and means for rotating -said dispensing cylinder.

22 26. The appar atus according to claim 25 wherein said shutter opens said aperture to a selected flow rate controlling width.

27. The apparatus according -to claim 25 or 26 further comprising a means for vibrating said support frame.

28. A method for forming a hollow structure from a thermoplastic material, substantially as described herein with reference to the accompanying drawings.

29. A hollow structure whenever formed from a thermoplastic material by the method claimed in any one of claims 1 to 13 and 28.

30. Apparatus for forming a hollow structure from a thermoplastic material, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

Publis ied 1988 at The Patent of:ftce, state House. 66"71 High Holborn, London WCIR 4TP.

Sales Further copies may be obtained from The Patent office, Branch, St MarY CraY. Orpington, Kent BRS 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1/87.