Method for molding plastic pipe with enlarged portions formed therein

Claim 2

IN A METHOD OF PRODUCING A PLASTIC TUBE BY BLOW MOLDING WHEREIN ENLARGED SLEEVE PORTIONS ARE FORMED AT SPACED LOCATIONS THROUGHOUT THE LENGTH OF THE TUBE, AND WHEREIN THE METHOD INCLUDES CONTINUOUSLY EXTRUDING A TUBE OF HOT PLASTIC MATERIAL INTO A BLOW MOLDING ZONE FORMED BY A SERIES OF COOPERATING PAIRS OF FORWARDLY MOVING DIE BLOCKS HAVING CONCAVE INNER SURFACESDEFINING A FORWARDLY MOVING BODY-FORMING MOLD CAVITY AND AN ADJACENT FORWARDLY MOVING ENLARGED SLEEVE-FORMING MOLD CAVITY OF INTERNAL DIMENSION GREATER THAN THAT OF THE BODY-FORMING CAVITY, THE IMPROVEMENT WHICH COMPRISES EXPANDING AND MOLDING THE TUBE AGAINST THE BODYFORMING AND SLEEVE-FORMING MOLD CABITIES DURING FORWARD MOVEMENT OF THE ASAME THROUGH SAID BLOW MOLDING ZONE BY SUPPLYING INTERNAL FLUID PRESSURE IN THE HOT PLASTIC TUBE WHILE PERIODICALLY LOWERING THE INTERNAL FLUID PRESSURE IN THE HOT PLASTIC TUBE TO A REDUCED LEVEL DURING MOLDING OF THE TUBE IN THE ENLARGED SLEEVE-FORMING MOLDCAVITY SO AS TO AVOID BLOWING HOLES IN THE HOT PLASTIC TUBE AS THE HOT TUBE IS STRETCHED AND EXPANDED TO CONFORM WITH THE ENLARGED SLEEVE-FORMING MOLD CAVITY AND FORM ENLARGED SLEEVE PORTIONS IN THE TUBE.

Claim 3

In a method according to claim 1, wherein the step of periodically lowering the internal fluid pressure in the hot plastic tube during molding of the tube in the enlarged mold cavity comprises periodically lowering the fluid pressure at least during reception of the hot plastic tube within the enlarged mold cavity.

Claim 4

In a method according to claim 1, wherein the step of periodically lowering the internal fluid pressure in the hot plastic tube comprises reducing the pressure of the fluid being supplied to the interior of the tube.

Claim 5

In a method according to claim 1, wherein the step of periodically lowering the internal fluid pressure in the hot plastic tube comprises reducing the pressure of the fluid being supplied to the tube in response to the arrival of the enlarged sleeve-forming mold cavity at a predetermined position immediately prior to receiving the hot plastic tube therein.

Claim 6

In a method according to claim 4, including restoring the full application of the internal fluid pressure in the tube in response to the arrival of the enlarged sleeve-forming mold cavity at a predetermined downstream position in the blow molding zone after receiving the hot tube therein.

Claim 7

In a method of producing a corrugated plastic tube by blow molding wherein enlarged sleeve portions are formed at spaced locations throughout the length of the corrugated tube, said method including continuously extruding a tube of hot plastic material into a blow molding zone formed by a plurality of cooperating pairs of forwardly moving die blocks arranged in opposing endless series and having concave inner surfaces defininG a forwardly moving annularly corrugated body-forming mold cavity and an adjacent forwardly moving enlarged sleeve-forming mold cavity of internal diameter greater than that of the body-forming mold cavity, the improvement comprising supplying internal fluid pressure in the hot plastic tube during forward movement of the same through said blow molding zone to expand and mold the tube against the body-forming and sleeve-forming mold cavities and thereby form the same into successive corrugated body portions of a predetermined diameter and relatively short enlarged sleeve portions interconnecting adjacent body portions, while periodically lowering the internal fluid pressure in the hot plastic tube to a reduced level at least during reception of the hot plastic tube within the enlarged sleeve-forming mold cavity so as to avoid blowing holes in the hot plastic tube as the plastic tube is stretched and expanded to conform with the enlarged sleeve-forming mold cavity to form enlarged sleeve portions in the tube.

Claim 8

In a method of producing a corrugated plastic tube by blow molding wherein enlarged sleeve portions are formed at spaced locations throughout the length of the corrugated tube, said method including continuously extruding a tube of hot plastic material into a blow molding zone formed by a plurality of cooperating pairs of forwardly moving die blocks arranged in opposing endless series and having concave inner surfaces defininG a forwardly moving annularly corrugated body-forming mold cavity and an adjacent forwardly moving enlarged sleeve-forming mold cavity of internal diameter greater than that of the body-forming mold cavity, the improvement comprising supplying internal fluid pressure in the hot plastic tube during forward movement of the same through said blow molding zone to expand and mold the tube against the body-forming and sleeve-forming mold cavities and thereby form the same into successive corrugated body portions of a predetermined diameter and relatively short enlarged sleeve portions interconnecting adjacent body portions, while periodically lowering the internal fluid pressure in the hot plastic tube to a reduced level at least during reception of the hot plastic tube within the enlarged sleeve-forming mold cavity so as to avoid blowing holes in the hot plastic tube as the plastic tube is stretched and expanded to conform with the enlarged sleeve-forming mold cavity to form enlarged sleeve portions in the tube.

Claim 9

A method according to claim 6 including the step of sensing the passing of certain of the moving die blocks in one of the endless series past a predetermined location relative to the endless series, and wherein the step of lowering the internal fluid pressure in the hot plastic tube comprises reducing the pressure of the fluid being supplied to the tube in response to the sensing of said certain die blocks.

Claim 10

In a method according to claim 6, wherein the step of supplying internal fluid pressure in the tube includes supplying pressurized fluid from first and second relatively high pressure and relatively low pressure fluid supply lines, and wherein the step of lowering the internal fluid pressure in the tube includes interrupting the supply of fluid from said first relatively high pressure line in response to the arrival of said enlarged sleeve-forming mold cavity at a predetermined position immediately prior to receiving the hot plastic tube therein.