GB2087786A

GB2087786A - Vacuum baths

Info

Publication number: GB2087786A
Application number: GB8128856A
Authority: GB
Original assignee: FLOATAIRE Ltd
Current assignee: FLOATAIRE Ltd
Priority date: 1980-10-07
Filing date: 1981-09-24
Publication date: 1982-06-03
Also published as: GB2087786B

Abstract

A vacuum bath for calibrating and cooling hollow profile sections of extruded thermoplastic material comprises a bath (2) for liquid coolant through which the material (1) to be calibrated and cooled is passed, means for controlling the level of coolant in said bath and a closed loop coolant recirculation system by means of which coolant is extracted from the bath and reintroduced into the bath. The coolant level is controlled by a stand pipe (9) connected to a venturi which produces the bath vacuum and is operated by coolant pumped from a reservoir (16) by a pump (20). The closed loop coolant system comprises a pump (14), which circulates coolant from an outlet (7) to spray jets (5). <IMAGE>

Description

SPECIFICATION Vacuum baths This invention relates to vacuum baths and more particularly concerns vacuum baths having a coolant fluid level control system.

Vacuum baths are used to calibrate and cool under vacuum, hollow profile sections of extruded thermoplastic material, the extruded material being passed through the bath where it is cooled by means of a coolant which is normally water. Calibration is achieved by passing the section through a shaping orifice such as a perforated bronze bush. The coolant level control system is employed to maintain a predetermined maximum level of coolant in the bath.

According to the present invention, a vacuum bath for calibrating and cooling hollow profile sections of extruded thermoplastic material comprising a bath for liquid coolant through which the material to be calibrated and cooled is passed, means for controlling the level of coolant in said bath and a closed loop coolant recirculation system by means of which coolant is extracted from the bath and reintroduced into the bath.

Preferably the coolant is reintroduced into the bath by means of one or more spray heads. The spray head may be rotatable in order to obtain a better cooling effect on the extruded material.

In a preferred construction a single pump is employed to extract and to reintroduce coolant from and into the bath.

The coolant recirculation system may incorporated an inlet for coolant at a lowertemperaturethan that of the coolant being recirculated.

The means for controlling level of coolant in the bath may comprise an overflow device and the device is desirably adjustable to provide for different levels of coolant in the bath.

Preferably the overflow device is a stand pipe associated with a venturi jet which serves to provide the vacuum in the bath and which may be operated by coolant pumped from a coolant reservoir through the venturi jet system and back to the reservoir.

The coolant removed from the bath via the overflow device is usually cooled and then returned to the coolant recirculation system.

The vacuum baths of the present invention have many advantages over the prior art including the following :a) When operated in a spray only mode, that is with, for example, only a few millimetres of cooling fluid in the top tank, buoyancy problems which are experienced when large extrusions are calibrated in prior art calibrating baths are overcome; and b) Because the minimum amount of cooling fluid is used in the top tank, the danger of cooling fluid flooding out of the vacuum chamber in the event of extrusion breakage is eliminated; c) Variations in the amount of any additional relatively cold cooling fluid introduced into the cooling spray jet system have no effect on the cooling fluid level in the top tank of the calibrating bath; d) The rate at which cooling fluid is passed through the spray jet system has no effect on the cooling fluid level in thetop tank; and e) Because there is only a relatively small amount of cooling fluid flowing it is easy to regulate the temperature thereof. Some plastics require extremely close control during the calibration stage and the use of comparatively warm cooling fluid may be necessary. Large difficult to manage quantities of cooling fluid are thus avoided.

The present invention will be further illustrated with reference to the sole accompanying drawig which is a schematic representation of a preferred vacuum calibrating bath according to the present invention.

Referring to the drawing, the platics extrusion to be calibrated 1 is passed through the evacuated top tank 2 of the calibrating bath. The top tank 2 has a lid 3 which may be opened. The lid 3 carries a vacuum relief valve 4. A number of jet sprays 5 are attached to a tube (or tubes) arranged around the extrusion centre line and are capable of being swivelled for optimum cooling effect on the extrusion. The bottom of the tank 2 has a cooling fluid sump 6 having a fluid outlet 7 and a venturi 8 with a stand pipe 9. The tank contains an amount of cooling fluid (chain shading) whose level is the same as that of the height of the stand pipe 9. The outlet7 is in fluid communication by means of flow line 10 with a filter 11 which is itself in fluid communication, by means of flow lines 12 and 13 with a pump 14.The pump 14 is in fluid communication with each of the spray jets 5 by means of the flow line 15 which splits into three lines 15', each of which is connected to a respective spray jet 5. The venturi 8 is in fluid communication with a reservoir tank 16, which has a filter 17 by means of flow line 18. The reservoir tank 16 has an overflow flow line 23. The tank 16 and the filter 17 are in fluid communication with a pump 20 by means of a flow line 19, and the pump 20 is in fluid communication with the venturi 8 by means of flow line 21.

When the apparatus of the present invention is in operation, the plastics extrusion to be calibrated 1 is continuously passed through the evacuated top tank 2 of the vacuum calibrating bath. As the extrusion 1 passes through the tank 2 it is sprayed with cooling fluid issuing from the sprays 5 whereby it is cooled.

Cooling fluid is caused to issue forth from the spray jets 5 by means of the pump 14 which pumps cooling fluid along the flow lines 15 and 15' and thereby into the jets 5. The cooling fluid issuing from the jets 5 sprays on to the extrusion 1 and then falls therefrom to the bottom of the tank 2. Cooling fluid is withdrawn from the sump 6, in the closed loop recirculation system, by means of the aforementioned pump 14. The pump 14 pumps the fluid from sump 6 via the outlet 7 thereof, along the flow line 10, through the filter 11 where any particular matter is removed, and into the pump 14 itself via flow lines 12 and 13. Having reached the pump 14 the fluid is pumped along the flow line 15 whereby the cycle is repeated. Since the descibed loop is filled with cooling fluid it may be seen that a recirculation system is formed.If it is desired to introduce comparatively cold cooling fluid into this recirculation system for the reasons discussed above, it is introduced into the closed loop from a source generally designated A via a valve 22, and into the pump 14 by means of flow line 13. When the pump 14 is operating to cause the above discussed flow cycle, the pump 20 is simultaneously operating. The pump 20 pumps cooling fluid from the reservoir 16 via the filter 17, which removes any particulate matter therefrom, and by means of flow line 19 into the pump 20 itself. The pump 20 also pumps cooling fluid into the venturi chamber 8 by means of flow line 21.Cooling fluid passes through the venturi chamber 8 whereby a venturi effect occurs, and the suction thus produced at the end of stand pipe 9 exhausts any excess cooling fluid (by which is meant fluid above the level of the end of the stand pipe 9) from the bottom of the tank 2. The fluid thus exhausted (if any) and the fluid causing the venturi effect pass out of the venturi chamber 8 and into the reservoir 16 by means of flow line 18 whereby the cycle is repeated, said cycle, of course, being continuous. Any cooling fluid which is in excess of the capacity of the venturi loop can pass out of the system by means of the overflow line 23 which is in fluid communication with the reservoir 16.

It will be appreciated from the above that the amount of cooling fluid passing out of the apparatus of the present invention shown in the drawing from the overflow line 23 will be equal to the amount of any relatively cold water introduced thereinto from the source A. Typically, in an apparatus of the present invention this amount of cooling fluid, when water is used as the fluid, will be of the order of 31 litres per minute per venturi incorporated in the apparatus.

Claims

1. A vacuum bath for calibrating and cooling hollow profile sections of extruded thermoplastic material comprising a bath for liquid coolant through which the material to be calibrated and cooled is passed, means for controlling the level of coolant in said bath and a closed loop coolant recirculation system by means of which coolant is extracted from the bath and reintroduced into the bath.

2. A vacuum bath according to Claim 1 in which the coolant is reintroduced into the bath by means of one or more spray heads.

3. A vacuum bath according to Claim 2 in which the spray heads are rotatable.

4. A vacuum bath according to any of Claims 1 to 3 in which a single pump is employed to extract and to reintroduce coolant from and into the bath.

5. A vacuum bath according to any of Claims 1 to 4 in which the coolant recirculation system incorporates an inlet for coolant at a lower temperature than that of the coolant being recirculated.

6. A vacuum bath according to any of Claims 1 to Sin which the means for controlling the level of coolant in said bath comprises an overflow device.

7. A vacuum bath according to Claim 6 in which the overflow device is adjustable to provide for different levels of coolant in the bath.

8. A vacuum bath according to Claim 6 or 7 in which the overflow device is a stand pipe associated with a venturi jet which serves to provide the vacuum in the bath.

9. A vacuum bath according to Claim 8 in which the venturi jet is operated by coolant pumped from a coolant reservoir through the venturi jet and back to the reservoir.

10. A vacuum bath according to any of Claims 6 to 9 in which coolant removed from the bath via the overflow device is cooled and then returned to the coolant recirculation system.

11. A vacuum bath according to any of Claims 1 to 10 in which the coolant is water.

12. A vacuum bath for calibrating and cooling hollow profile sections of extruded thermoplastic material substantially as herein described with reference to and as shown in the accompanying drawing.