GB2218030A

GB2218030A - Controlling the vacuum of a tube vacuum calibrating tank

Info

Publication number: GB2218030A
Application number: GB8810618A
Authority: GB
Inventors: Walter Marshall
Original assignee: SPEEDEX ENGINEERING Ltd
Current assignee: SPEEDEX ENGINEERING Ltd
Priority date: 1988-05-05
Filing date: 1988-05-05
Publication date: 1989-11-08
Also published as: GB8810618D0

Abstract

The calibrating tank (2) has inlet and outlet apertures (5, 6) through which a hollow extruded body (8) can passes to undergo differential pressure calibration during cooling by water immersion, or sprays (10). A calibrator head (21) at the tank inlet end defines the size of the body (8). A port (17) communicates with a vacuum pump (20) which reduces the tank pressure to below atmospheric. A pressure tansducer (22) produces a signal proportional to the tank gauge pressure to which a control means (24) is responsive to effect remedial action whereby the pressure in the tank can be maintained substantially constant; either by altering the speed of pump (20); by throttling the air flow between the tank and the pump; by bleeding air into the pipe (17) or the pump (20); or by throttling air flow into the tank via a variable slot inlet (Fig 5), or via opening (51) controlled by oscillating a magnetic valve body (50) by a solenoid (54) located around a non ferrous (brass) housing (53). <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO THE CALIBRATION OF EXTRUDED PLASTIC PIPE The present invention relates to an improved apparatus and method for the calibration of extruded plastic bodies of elongate hollow form, such as pipe or tubing, and particularly to a certain type of calibration known as vacuum calibration or differential pressure calibration.

When a plastic pipe or tube leaves an extruding head it is still relatively soft and must first of all be cooled under controlled conditions in such a manner as to maintain the pipe at the desired size. This is achieved by passing the extruded pipe through a calibrating unit, and, in a differential pressure calibrating unit the pipe is maintained at the desired size by maintaining the pressure outside the pipe lower than the pressure inside the pipe, the latter being atmospheric pressure since the pipe is open to the atmosphere at the end furthest away from the extruder.

A A typical differential pressure calibrating unit comprises an enclosed elongate tank having hinged lids allowing access to the interior of the tank. An extruded pipe enters through an aperture in the first end wall of the calibrating unit and leaves through a further aperture in a second end wall of the calibrating unit. The aperture in the first end wall is fitted with a calibrator head which is typically in the form of a cylindrical metal sleeve, although many different shapes and sizes of head are available for use with different extrusion crosssections.

The calibrator head has a number of slots in it, the purpose of which will be described later.

The aperture in the second end of the calibrating unit is fitted with a seal having an aperture therein through which the extruded pipe passes. The hinge lids also have seals around their edges, so that once the extruded pipe has been threaded through the calibrating unit, the tank can be evacuated by a vacuum pump to create a pressure outside the pipe which is less than the atmospheric pressure, that is to say less than the pressure inside the pipe. The slots in the calibrator head sserve to allow the difference in pressure between the inside and the outside of the pipe to cause the pipe to be pressed outwardly against the calibrator head, thus defining the desired size of pipe.

The cooling of the extruded pipe in the calibrator can be achieved either by flooding the tank with water so that the pipe is completely submerged, or by spraying water onto the pipe from a number of nozzles spaced along the length of the tank. In the latter case a small amount of water collects in the bottom of the tank, so that the water level is maintained below the level of the lowest set of spray nozzles.

In either case, the water gradually becomes hotter and must be drained away and replaced by cold water, and this means that the level of water in the tank will temporarily rise and fall to a certain extent, thus causing fluctuations in the volume and hence the pressure of the air above the water in the tank.

These pressure fluctuations cause problems in that they result in irregularities in the size of the pipe which is still quite soft at this stage. Also, pressure fluctuations can result from changes in the leakage rates of the lid seals and the seal around the pipe at the exit end of the calibrating unit. The pressure fluctuations due to changes in the level of the water are more pronounced in the case where the extruded pipe is completely submerged, since these represent greater percentage volume changes than in the other case in which the pipe is not submerged.

Various attempts have been made to limit these pressure fluctuations, for example by incorporating an adjustable valve which may be of the spring controlled type, somewhat similar to a safety valve, in the top of the tank which can be pre-set to allow control leakage of atmospheric air into the tank. However, this is not entirely satisfactory as it is a manually pre-set valve and does not give the degree of control necessary to maintain perfect size of the pipe as it cools. Also, the characteristics of a spring loaded valve are dependent on the rate of air flow through the valve so that accurate control is not possible.

It is therefore the aim of the present invention to provide an improved calibrator and method of calibrating which overcomes the above-mentioned disadvantages.

According to a first aspect of the present invention there is provided a differential pressure calibrating unit for cooling and maintaining the size of extruded plastic bodies in elongate hollow form, the calibrating unit comprising an elongate enclosed tank having a first end with an aperture formed therein through which the extruded body enters the calibrating unit, a second end opposite said first end through which the extruded body leaves the calibrating unit, a calibrator head located at said first end for defining the desired size of the body, a port provided in the tank which communicates with a pump, said pump being operable to reduce the pressure in the tank to below atmospheric pressure, in which a pressure transducer is located in the tank or in association therewith, said pressure tranducer producing a signal proportional to the gauge pressure in the tank, and control means responsive to changes in said signal to effect remedial action whereby the pressure in the tank is maintained substantially constant.

Preferably, the arrangement is such that the pressure difference between the pressure in the tank and atmospheric pressure can be maintained substantially constant at a preselected value.

Preferably, the control means is coupled to the motor driving the pump, and changes the speed of the motor in response to a change in the signal from the transducer.

The control means conveniently comprises a variable speed drive which acts to vary the pump speed. Since the cost of a variable speed drive increases in proportion to the pump size, a second smaller pump may be included which works in parallel with the main pump, and the smaller pump is provided with a variable speed drive.

Another alternative is to use the signal from the pressure transducer to throttle the airflow from the tank during its passage to the vacuum pump.

A A further alternative is to use the signal from the pressure transducer to bleed atmospheric air into the pipe leading to the vacuum pump. The same effect can be achieved by utilising the transducer signal to bleed atmospheric air into the vacuum pump itself.

However, the same advantageous effect can equally be achieved by coupling the control means to a valve or proportional valve, such that the control means will open or close the valve or change the state of the proportional valve in response to changes in the transducer signal to admit the required amount of atmospheric air into the tank.

Thus, by using the signal from the transducer to control the speed of the pump motor, or to operate a valve, the changes in pressure in the tank are automatically compensated for, thus keeping pressure fluctuations to a very small acceptable level.

Conveniently, the pressure transducer is located in an upper portion of the tank.

Preferably, the pressure transducer incorporates a strain gauge.

According to a second aspect of the present invention there is provided a method of operation of a differential pressure calibrating unit for cooling and maintaining the size of extruded plastic bodies in elongate hollow form, the method comprising introducing the extruded body into the calibrating unit through an aperture in a first end of an elongate tank forming the calibrating unit, and causing it to leave the calibrating unit through a second end of the tank, defining the desired size of the extruded body by passing it through a calibrator head located at said first end, maintaining the pressure in the tank at below atmospheric pressure, sensing any changes in the pressure in the tank, and utilizing said pressure changes to effect remedial action whereby the pressure in the tank is maintained substantially constant.

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a differential pressure calibrating unit in accordance with the present invention, in use during the plastic extrusion process; Figure 2 is a cross-section through the calibrating unit shown in Figure 1; Figure 3 is a cross-section through a flooded calibrating unit; Figure 4 shows an alternative method of operation of the calibrating unit shown in Figure 1; Figure 5 shows a proportional valve for use in accordance with the present invention.

Referring to the drawings, a differential pressure calibrating unit 1 in accordance with the present invention comprises an enclosed elongate tank 2 having hinge lids 3,4 at the top, the lids including lid seals 3a,4a.

The tank has an inlet aperture 5 at a first end thereof and an outlet aperture 6 at a second end thereof, the latter outlet being covered by a rubber seal 7 having an aperture therein through which the extruded plastic pipe 8 passes to leave the calibrating unit. The pipe enters the aperture 5 directly from the plastic extrusion head 9.

A number of spray nozzles 10 are located along the length of the tank, and are oriented towards the pipe 8 to spray water onto the pipe to cool it as it passes through the calibrating unit. Typically, there would be four lines of nozzles designated 10a, lOb, lOc and lOd in Figure 2.

Each line of nozzles is supplied with water by pipe line 12 which is connected at its other end with a drain 13 to remove water from the tank, so that water is recirculated to the nozzles.

Pipe line 12 includes a pump 14, the purpose of which will be described later, an outlet 15 and a cold water inlet 16, both controlled by solenoid operated outlet and inlet valves 15a,16a respectively. The water inlet 16 is preferably located in the base of the tank, but it may alternatively be located in a pipe line 12, upstream of pump 14 (this alternative position is shown in broken lines in Figure 1).

The tank 2 includes a port 17 which communicates with a pump 20 typically of the "water-ring" type, which is a motor driven rotary pump. The pump is operable to reduce the pressure in the area 18 inside the tank (exteriorly of the extruded pipe) to below atmospheric pressure, that is to say below the pressure in the interior of the extruded pipe 8.

The pipe 8 is open at its end furthest away from the extrusion head, so that the pressure inside the pipe 8 is atmospheric pressure.

A calibrator head 21 in the form of a cylindrical sleeve is located at the first end of the tank 2, adjacent the first aperture 5, and the extruded pipe is forced to pass through the interior of this sleeve to define the desired size of the finished pipe. Slots 21a cut into the sleeve 21 allow the higher pressure inside the pipe to press the pipe outwardly against the sleeve.

A pressure transducer 22 incorporating a strain gauge is located in a portion of the tank, either in the tank wall or in one of the lids 3,4, and is connected electrically to a controller 24 which is in turn connected to the motor 25 driving the pump 20.

During the extrusion process, the pressure in area 18 can fluctuate due to changes in the water level and/or changes in the leakage rates of seals 7,3a and 4a. The pressure in area 18 is continuously sensed by pressure transducer 22, which produces a signal proportional to the pressure in area 18. Any changes in the signal from the transducer will cause the actuator 24 to change the speed of the motor 25, thus restoring the pressure in area 18 at a desired constant pre-selected reading, which can be observed on pressure gauge 27.

Alternatively, the signal from the transducer may be electrically connected to an actuator 28 which operates a valve 29 to admit atmospheric air into the tank. The valve 29 may be a simple on-off valve, or may be a proportional valve such as that shown in Figure 5 which comprises a slot 30 covered by a profiled body 31 which is movable longitudinally of the slot to change the admission rate of the valve.

Figure 6 illustrates an alternative valve which is an oscillating valve. This comprises a magnetic valve body movable along axis X to open and close opening 51 in the tank wall 52. The magnetic valve body is surrounded by a non-ferrous housing 53, typically brass, and a solenoid 54 surrounds housing 53. By energising the solenoid with a suitable pulsed electric current, the valve body can be caused to oscillate between its open and closed positions By varying the proportion of the time during which the valve is closed relative to the proportion of time during which it is open, the rate of air flow through the valve can be varied. Suitable waveforms to achieve this effect are shown in Figure 7.

A quick release valve 35 is provided to allow the pressure in the tank to quickly be brought up to atmospheric pressure prior to opening the lids 3,4.

Figures 2 to 4 show the drain 13 in more detail.

Conveniently, this may take the form of an upright pipe 37 which may be quite tall as shown in Figure 3 which shows the tank being flooded so that water covers the extruded pipe 8 completely. The top of the pipe 37 is at the desired water level, so that any excess water will be drained away. In Figure 2, the cooling is achieved by spraying water onto the pipe, and therefore there is only a small amount of. water in the bottom of the tank, so that the pipe 38 need only be very short. In Figure 4, the tank is flooded just sufficiently to cover the extruded pipe 8, and the pipe 39 in this case is of intermediate length.

In each case, because the water is being drawn off at less than atmospheric pressure, it requires a pump 14 to draw it off and, in the case of the spray mode, to recirculate it to the spray nozzles.

In the embodiment shown in Figure 2, two pumps are shown. Pump 14a recirculates the water back to the spray nozzles, and pump 14b removed excess water to the drain.

However, it is possible to draw excess water off by means of solenoid valve 15a, or alternatively excess water may be removed by the vacuum pump 20 provided it is the type which is capable of ejecting mixtures of air and water.

In the embodiment shown in Figure 1, a temperature sensor (not shown) will sense when the temperature of the cooling water exceeds a given maximum, and will cause solenoid operated valves 15a,16a to open, letting the warm water drain away and admitting cold water to the system.

Discs 40,41 located within the tank 2 along its length help to maintain the size of the pipe as it passes through the calibrator.

It should be appreciated that the present invention is not limited to the extrusion of circular pipes or tubing, but is equally applicable to the extrusion of any elongate hollow plastic body of any cross-sectional shape or size.

The differential pressure between the interior of the pipe and the exterior of the pipe inside the tank is chosen to suit the particular pipe being extruded. The pump is capable of producing a vacuum of up to 0.7 bar, and in practice, a differential pressure of 0.5 bar would be maintained between the interior and the exterior of the pipe within the tank, or perhaps as little as 0.1 bar for thin walled pipes.

Claims

1. A differential pressure calibrating unit for cooling and maintaining the size of extruded plastic bodies in elongate hollow form, the calibrating unit comprising an elongate enclosed tank having a first end with an aperture formed therein through which the extruded body enters the calibrating unit, a second end opposite said first end through which the extruded body leaves the calibrating unit, a calibrator head located at said first end for defining the desired size of the body, and a port provided in the tank which communicates with a pump, said pump being operable to reduce the pressure in the tank to below atmospheric pressure, in which a pressure transducer is located in the tank or in association therewith, said pressure transducer being operative to produce a signal proportional to the gauge pressure in the tank, and a control means is provided responsive to changes in said signal to effect remedial action whereby the pressure in the tank can be maintained subtantially constant.

2. A unit according to Claim 1, in which the arrangement is such that the pressure difference between the pressure in the tank and atmospheric pressure can be maintained substantially constant at a preselected value.

3. A unit according to Claim 1 or 2, in which the control means is coupled to a motor driving the pump, and is operative to change the speed of the motor in response to a change in the signal from the transducer.

4. A unit according to any one of Claims 1 to 3, in which the control means comprises a variable speed drive which is operable to vary the pump speed.

5. A unit according to Claim 4, in which a second smaller pump is included which works in parallel with the main pump, and the smaller pump is provided with a variable speed drive.

6. A unit according to any one of Claims 1 to 3, and arranged to utilise the signal from the pressure transducer to throttle the air flow from the tank during its passage to the pump.

7. A unit according to any one of Claims 1 to 3, and arranged to utilise the signal from the pressure transducer to bleed atmospheric air into a pipe leading to the pump. -

8. A unit according to any one of Claims 1 to 3, and arranged to utilise the signal from the pressure transducer to bleed atmospheric air into the pump itself.

9. A unit according to any one of Claims 1 to 3, in which the control means is coupled to a valve or proportional valve, and is arranged such that the control means can open or close the valve or change the state of the proportional valve in response to changes in the transducer signal to admit the required amount of atmospheric air into the tank.

10. A unit according to any one of the preceeding claims, in which the pressure transducer is located in an upper portion of the tank.

11. A unit according to any one of the preceeding claims, in which the pressure transducer incorporates a strain gauge.

12. A method of operation of a differential pressure calibrating unit for cooling and maintaining the size of extruded plastic bodies in elongate hollow form, the method comprising introducing the extruded body into the calibrating unit through an-aperture in a first end of an elongate tank forming the calibrating unit, and causing the body to leave the calibrating unit through a second end of the tank, defining the desired size of the extruded body by passing it through a calibrator head located at said first end, maintaining the pressure in the tank at below atmospheric pressure, sensing any changes in the pressure in the tank, and utilising said pressure changes to effect remedial action whereby the pressure in the tank is maintained substantially constant.

13. A unit according to Claim 1 and substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.

14. A method according to Claim 12, and substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.