METHOD AND APPARATUS FOR MAKING PLASTIC PIPE WITHOUT MECHANICAL PRESSURE ON INNER PIPE WALL

Abstract: A method of making double wall plastic pipe (8) having inner (8) and outer (5) pipe walls adhered to one another without mechanically holding the inner pipe wall against the outer pipe wall comprises extruding separate streams of molten plastic into a mold tunnel (3) to form the inner and outer pipe walls and producing a positive gas pressure which acts internally of and forces the inner pipe wall (8) outwardly against the outer pipe wall (5) while the pipe is being formed in the mold tunnel (3).

Description: Title: METHOD AND APPARATUS FOR MAKING PLASTIC PIPE WITHOUT MECHANICAL PRESSURE ON INNER PIPE WALL FIELD OF THE INVENTION The present invention relates to a method and apparatus for making double wall plastic pipe. BACKGROUND OF THE INVENTION According to conventional methods of manufacturing double wall plastic pipe, the inner and outer pipe walls are formed separately from one another within a mold tunnel. A mechanical assist and principally a cooling plug is used to force the inner pipe wall against the outer pipe wall to secure the two pipe walls to one another. This slows down production of the pipe through the drag created by the contact between the plug and the inner pipe wall and also produces distortions on the inner surface of the pipe wall. When the pipe is in use, these distortions are extremely detrimental as they create undesirable build up or collection areas for product passing through the pipe. SUMMARY OF THE INVENTION The present invention provides a method of making double wall plastic pipe where the inner and outer pipe walls are adhered to one another without using mechanical pressure to hold the inner pipe wall against the outer pipe wall. In accordance with the present invention, separate streams of molten plastic are extruded into a mold tunnel to form the inner and outer pipe walls and the inner pipe wall is forced outwardly against the outer pipe wall by gas pressure produced internally of the inner pipe wall and acting on the inner pipe wall while the pipe is in the mold tunnel. The use of gas pressure internally of the inner pipe wall does not produce scratching or other type of marking on the inside surface of the inner pipe wall and further substantially produces production speed of the pipe. BRIEF DESCRIPTION OF THE DRAWINGS The above as well as other advantages and features of the present invention will be described in greater detail according to the preferred embodiments of the present invention in which: Figure 1 is a schematic view of a pipe manufacturing apparatus; Figure 2 is a sectional view showing preferred features of the pipe forming apparatus of Figure 1. DETAILED DESCRIPTION ACCORDING TO THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION Figure 1 shows the general outlines of a pipe manufacturing apparatus which includes an extruder die 1 at the upstream end of the apparatus and a pipe forming mold tunnel down stream of the extruder die. Double wall pipe P emerges from the downstream end of the mold tunnel. This pipe, as better seen in Figure 2 of the drawings, comprises a profiled and in this case, corrugated outer wall 6 with a flat inner wall 8 secured or adhered to the troughs of the outer wall. The adhesion occurs because the two walls meet with one another in a molten state and once cooled remain secured to one another as is known in the art. However, unlike prior art methods, the method of the present invention does not use a mechanical assist to physically hold the inner wall against the outer wall while the pipe is in the mold tunnel. In accordance with the present invention, gas pressure is produced to force the inner wall outwardly against the outer wall. More specifically, Figure 2 shows the extruder die as having a first extrusion orifice 5 through which a stream of molten plastic is fed to the rib forming profile on the inner surface of the mold tunnel 3 to form the outer pipe wall 6. Also provided is a second extrusion orifice 7 downstream of the first extrusion orifice 5 which feeds a second stream of molten plastic to form the interior pipe wall 8. In the preferred embodiment as shown, a small lip 4 is provided at the second extrusion orifice to guide the inner wall forming stream of plastic outwardly towards the outer wall forming stream of plastic as the two streams leave the die head. However, it should be noted that this lip or guide is not required and although the lip applies a slight momentary pressure on the plastic, it is no way maintains pressure to hold the inner wall against the outer wall while the two walls are fusing together. Pressure is however applied to force the inner wall 8 outwardly against the troughs of the outer wall 6 as the plastic starts to cure and harden in the mold tunnel. This pressure is created by gas or air pressure within the mold tunnel. In the particular arrangement shown, the die head has a passage 18 which opens to the outside atmosphere and a fan 17 is provided to push air into the mold tunnel through passage 18. A baffle 15 is provided interiorly of the inner pipe wall 8. Although there is sufficient clearance to avoid contact between the inner pipe wall and baffle 15, the baffle does act as a block to maintain much of this pressure on the upstream side of the baffle. This pressure is sufficient to hold the inner wall against the outer wall while the two walls cure with one another. The pressure provided inside the inner pipe wall must be sufficient to hold the inner wall against outer wall overcoming the pressure within the pockets 9 between the inner pipe wall and the crests of the outer pipe wall. In order to produce a positive securing of the two walls, the pressure inside the inner wall will generally be slightly greater than the pressure within the pockets. After the pipe emerges from the mold tunnel where it is sufficiently hardened to maintain its shape, a cap 19 can be applied to the end of the pipe and this cap again acts as a block to prevent escape of the gas or air pressure built up inside the pipe. It should be noted that the positive air pressure can also be produced from the downstream end and forced towards the upstream end of the system in which case the end cap, the baffle and the path through the die head would not be used. Figure 2 also shows the use of a cooling coil 13 which cools the air internally of the inner pipe wall. This decreases curing or hardening time for the inner pipe wall but in no way provides anything in the way of mechanical pressure against the inner pipe wall. Although various preferred embodiments of the present invention have been described in detail, it will be appreciated by those skilled in the art, that variations may be made without departing from the spirit of the invention or the scope of the appended claims.

Claims: THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS: 1. A method of making double wall plastic pipe having inner and outer pipe walls adhered to one another without mechanically holding the inner pipe wall against the outer pipe wall, said method comprising extruding separate streams of molten plastic into a mold tunnel to form the inner and outer walls and producing a positive gas pressure which acts internally of and forces the inner wall outwardly against the outer wall while the pipe is in the mold tunnel. 2. A method of making double wall plastic pipe having inner and outer pipe walls adhered to one another without mechanically holding the inner pipe wall against the outer pipe wall, said method comprising extruding separate streams of molten plastic into a mold tunnel to form the inner and outer pipe walls and forcing the inner pipe wall outwardly against the outer pipe wall by gas pressure acting internally of the inner pipe wall while the pipe is in the mold tunnel. 3. A method as claimed in Claim 2, wherein the gas pressure is produced by pressure producing means located outside of the mold tunnel. 4. A method as claimed in Claim 3 wherein said die head is located at an upstream end of the mold tunnel and is provided with an outside air passage to the mold tunnel and wherein said inner pipe wall is moved past without touching a baffle provided interiorly of said inner pipe wall downstream end of the die head, the baffle blocking the escape of the gas pressure from the mold tunnel. 5. A method as claimed in Claim 2 comprising capping the pipe emerging from the mold tunnel to maintain the gas pressure internally of the inner pipe wall. AMENDED CLAIMS [received by the International Bureau on 26 August 1998 (26.08.98); original claims 1-5 replaced by amended claims 1-3 (1 page)] 1. A method of making double wall plastic pipe having inner and outer pipe walls adhered by curing to one another without mechanically holding the pipe wall against the outer pipe wall, said method comprising extruding separate streams of molten plastic into a mold tunnel to form the inner and outer walls and producing a positive pressure by a gas internally of and forcing the inner wall outwardly against the outer wall while the pipe is in the mold tunnel, the gas being cooled to decrease curing time of the pipe walls . 2. The method of Claim 1, wherein the gas is cooled by a cooling device located interiorly of the inner pipe wall within the mold tunnel, said method including moving the inner pipe wall past the cooling device without any contact between the inner pipe wall and the cooling device. 3. The method of Claim 1, including forming the outer pipe wall with a profiled surface and forming the inner pipe wall with a flat profile and wherein the positive pressure internally of the inner pipe wall is greater than any pressure produced as a result of air trapped between the inner and outer pipe walls .

Inventors: LUPKE MANFRED A A, LUPKE STEFAN A, LUPKE, MANFRED, A., A., LUPKE, STEFAN, A.
Applicants: LUPKE MANFRED ARNO ALFRED, LUPKE STEFAN A, LUPKE, MANFRED, A., A., LUPKE, STEFAN, A.
Application year: 1997
Main classification: B29D23
IPC codes: B29C47, B29C49, B29D23