GB2221642A - Regulating the wall thickness of an extruding parison - Google Patents

Abstract

A method of controlling the wall thickness of a parison for a blow-moulding operation, during the extrusion of the parison, employs a die 10 having a movable plug 15 controllable to adjust the die gap. The volume rate at which the thermoplastic material 12 is extruded is controlled, and the rate of advancement of the free end of the parison is monitored by a linear array of photocells 28 arranged close to the advancing parison. The rate of advancement is compared with empirical values determined for a satisfactory parison, and the die gap is adjusted to give an advancement rate approximating to the empirically determined rate. <IMAGE>

Description

PARISON PRODUCTION This invention relates to a method for controlling the extrusion of a parison, suitable for use in a blowmoulding operation, as well as to apparatus for the control of parison extrusion processes.

The first step of a blow-moulding process is the preparation of a parison of a suitable shape and size, having regard to the configuration of the mould cavity into which the parison is to be blow-moulded by a wellknown inflation technique. Such a parison conventionally is prepared by supplying an appropriate grade of thermoplastics material in granular form to an extrusion apparatus, where the thermoplastics material is heated to form a homogenous plastic mass which is then extruded out of an annular extrusion die, so as to produce a tube-like parison ready for inflation into the mould cavity during the inflation step of the process.

During the inflation of the parison so as to engage the mould walls defining the mould cavity, different parts of the parison are subjected to varying degrees of stretch. Also, certain parts of the finished article may be required to have greater strengths than other parts. For these purposes, it is known to provide a parison the wall thickness of which varies along the length of the parison, this being achieved by controlling the size of the annular die gap as the extrusion of the parison proceeds. In this way, a parison having a ?redefined wall thickness profile may be obtained - and the more complex the shape of the finished article, the more important becomes the need accurately to control the wall thickness profile of a parison.

Control of the wall thickness profile has in the past been achieved largely empirically, by determining through experimentation a suitable wall thickness profile for the parison having regard to the configuration of the mould cavity, and noting the adjustments made to the die gap as the extrusion of the parison proceeds. Thereafter, the die gap is adjusted in precisely the same manner on a time basis, for each parison which subsequently is produced, thereby allowing the production of a plurality of parisons all having the required wall thickness profile.

Unfortunately, experience shows that the known method as described above for controlling the wall thickness profile of a parison is not particularly reliable, and is subject to significant variations over a production batch. Generally, there is a tendency for the wall thickness profile of actual samples to drift away from the desired profile, due to slowly varying factors such as temperature, raw material variations, and so on. This results in the need for an operator to make alterations to the predetermined die gap adjustment regime whenever it is found that parisons are being produced outside the permitted tolerance range. Inevitably, losses will be incurred because a production run must then be interrupted, and those products falling outside the permitted range will be wasted.It is therefore a principal aim of the present invention to overcome the difficulties of the described known forms of parison wall thickness profile control, so permitting the production of parisons which have consistent and repeatable wall thickness profiles, over an entire production run.

According to this invention, there is provided a method for controlling the wall thickness profile of a parison during the extrusion thereof from a die having a variable die gap, in which method the volume extrusion rate of the parison is controlled to follow a predetermined regime, the velocity of the free end of the parison during extrusion is monitored, and the die gap is adjusted depending upon the result of the velocity monitoring.

In the method of this invention, a form of feedback is employed, in that the extrusion of the parison is monitored as the extrusion proceeds, and the die gap is adjusted depending upon the monitored progress of the extrusion process. However, rather than attempting to monitor the wall thickness of the parison as the extrusion proceeds, in the method of this invention the velocity of the free end of the parison is monitored and the die gap is adjusted accordingly so as to yield a parison having the required wall thickness profile.

In order to permit this method to operate satisfactorily, it will be appreciated that the outflow from the die of the thermoplastics material forming the parison must be controlled to follow a pre-determined regime. Most preferably, a constant volume extrusion rate may be employed, but in certain circumstances there may be advantages in employing a volume flow rate which varies as the extrusion proceeds - for example, to minimise stretch effects a higher flow rate may be employed in the later stages of the extrusion. The method must however be performed in a repeatable manner, from one parison to the next.

As an alternative to controlling the volume flow rate of the plastics material from the die, the outflow may be controlled in a repeatable manner by monitoring the pressure of the plastics melt in the die as the extrusion proceeds, and causing that pressure to follow some predetermined regime. For example, the pressure may be controlled to build up to some pre-set value at the start of the extrusion, then remain constant until the parison reaches some specified length, and then fall away to zero at the completion of the extrusion.

The method of this invention is preferably performed by empirically determining a suitable parison free-end velocity profile which gives rise to a satisfactory parison having the required wall thickness profile, paying due regard to the volume extrusion rate, and then to compare the instantaneous velocity of the free end of a parison during the extrusion thereof with the empirically determined values, the die gap then being adjusted as a result of that comparison. In this way, the actual velocity profile of the free end of a parison may be made to follow closely that of the empirically-determined values, so leading to a parison having a satisfactory wall thickness profile.

Though it would be possible when performing this invention to monitor the velocity of the free end of the parison on a substantially continuous basis, for example by obtaining an analogue electrical signal indicative of the velocity of the free end of the parison during extrusion and continuously comparing that signal in real time with another analogue signal indicative of the required pre-determined velocity value so as to obtain an error signal which is used to control the die gap, it has been found in general that the free end velocity, and thus the wall thickness of a parison tends not to deviate suddenly from the desired value: such deviations tend to be gradual, and cumulative over a number of extrusion operations. As a consequence, when performing the method of this invention, it has in general been found to be sufficient to determine the average velocity of the free end of the parison over a number of successive steps, the obtained average values being compared with the average values required to obtain a satisfactory parison. In turn, this allows relatively simple apparatus to be employed for determining the actual velocity of the parison free end. For example, it is convenient to determine the velocity of the free end of the parison by having a linear array of photosensitive elements arranged close to the line of advancement of a parison during extrusion, the parison itself interrupting light falling on the photosensitive elements from a suitably positioned light source, whereby the outputs from those photosensitive elements indicates the advancement of the parison.Then, by timing the interval required for the end of the parison to advance the known distance from one photosensitive element to the next adjacent element, by monitoring the photosensitive element outputs, a stepwise function of the average velocity of the free end of the parison may be obtained. In turn, this stepwise function may be compared with a predetermined velocity function for an "ideal" parison, that latter function typically being determined empirically during a setting-up operation for the parison extruder.

This invention extends to apparatus for controlling the wall thickness of a parison during the extrusion thereof, which apparatus comprises a parison extruder including a die having an annular gap, means to adjust the annular gap of the die, means to monitor the velocity of the free end of a parison during the extrusion thereof from the die, and control means acting upon the output from the velocity monitoring means to control operation of the gap adjustment means dependent thereupon. Most preferably, the control means is arranged to compare the instantaneous output of the velocity monitoring means with pre-determined velocity values stored in the control means, and to cause operation of the gap adjustment means dependent upon the result of the comparison.

By way of example only, one specific method of parison extrusion of this invention and of apparatus arranged in accordance therewith will now be described in detail, reference being made to the accompanying drawing which diagramatically shows extrusion apparatus for producing a parison suitable for inflation into a blow-moulding cavity to product a finished article.

Illustrated in the drawing is a parison extrusion die 10 having a main chamber 11 from which heated thermoplastic material 12 is extruded, the die proper being defined by an outer nozzle portion 13, a fixed die core 14 and a movable die plug 15. The nozzle 13, die core 14 and plug 15 are configured such that axial adjusting movement of the die plug 15 causes the annular die gap through which the plastics material is extruded to be varied, so altering the wall thickness of the parison 16 obtained by the extrusion operation. The adjustment of the die plug 15 is performed by a double-acting hydraulic piston-and-cylinder combination 17, the piston 18 being connected to the die plug 15 by means of a link rod 19.

The heated thermoplastics material 12 is supplied to the main chamber 11 by a plasticising screw and barrel (not shown) and is then extruded from the main chamber 11 by means of a plunger 20, connected by rods 21 to a piston 22 positioned within an hydraulic cylinder 23 mounted on the die 10 by means of columns 24. Hydraulic fluid under pressure is supplied both to the hydraulic cylinder 23 and to the piston-and-cylinder combination 17, under the control of respective servovalves 25 and 26.

A partly extruded parison 16 is illustrated in the drawing. The rate of movement of the free end 27 of that parison is monitored by means of a linear array 28 of photocells disposed to lie parallel to the axis of the die 10, closely adjacent the parison itself. A light source (not shown) is positioned to direct light on to a reflector surface 29, from where the light is reflected towards the array 28, across the path of advancement of the free end 27 of the parison. In this way, the parison obscures each photocell, in turn as the extrusion process proceeds.

A linear electrical position transducer 30 (such as an LVDT) is appropriately connected between the piston 22 and the hydraulic cylinder 23, to provide a signal indicative of both the position and the rate of movement of the piston 22. The output from this transducer 30 is supplied to a computer 31, as are the outputs from the individual photocells of the linear array 28 thereof. Operating in accordance with a predetermined programme, the computer controls the servovalves 25 and 26, to ensure the parison extrusion apparatus operates in accordance with the required regimes.

Operating on the output from the linear array 28 of photocells, the computer is able stepwise to calculate the velocity of the free end of the parison, by timing the interval for the parison to advance the known distance from a position where one photocell is obscured to a position where the next adjacent photocell in the vertical array is obscured. That velocity may then be compared by the computer with a velocity profile held in the computer memory, which profile will have been pre-determined to give a parison having a required wall thickness profile. Such a pre-determined velocity profile typically will have been obtained empirically, the computer allowing minor variations to be stored to permit adjustments to be made to the wall-thickness profile. Equally, the computer may have stored in its memory various different profiles, so permitting rapid change-over from one parison form to another, on changing the blow-moulding die to produce a different finished article.

The computer controls the operation of the servovalve 26, dependent upon the result of the velocity comparison, so adjusting the annular die gap to give the required parison free end velocity, and hence the required wall thickness profile. Simultaneously, the computer also controls operation of the servo-valve 25, to ensure that the parison push-out rate also follows a pre-determined regime - which typically is a constant rate, as determined by the output from the transducer 30.

Claims (15)

1. A method for controlling the wall thickness profile of a parison during the extrusion thereof from a die having a variable die gap, in which method the volume extrusion rate of the parison is controlled to follow a predetermined regime, the velocity of the free end of the parison during extrusion is monitored, and the die gap is adjusted depending upon the result of the velocity monitoring.

2. A method according to claim 1, in which the outflow from the die of the thermoplastics material forming the parison is controlled to be at a constant volume extrusion rate.

3. A method according to claim 1, in which the outflow from the die of the thermoplastics material forming the parison is controlled by monitoring the pressure of the plastics melt in the die as the extrusion proceeds, and causing that pressure to follow some predetermined regime.

4. A method according to claim 3, in which the pressure is controlled to build up to some pre-set value at the start of the extrusion process, is then controlled to remain constant until the parison reaches a pre-set length, and is then controlled to reduce to zero at the completion of the extrusion.

5. A method according to any of the preceding claims, in which an empirical determination is performed to find a suitable parison free-end velocity profile which gives rise to a satisfactory parison having the required wall thickness profile, and then the instantaneous velocity of the free end of the parison during the extrusion thereof is compared with the empirically-determined values, the die gap then being adjusted as a result of that comparison.

6. A method according to any of the preceding claims, in which the velocity of the free end of the parison is monitored on a substantially continuous basis to obtain an analogue electrical signal indicative of the velocity of the free end of the parison during extrusion, and that signal is compared in real time with another analogue signal indicative of the required predetermined velocity value so as to obtain an error signal which is used to control the die gap.

7. A method according to claims 1 to 5, in which the average velocity of the free end of the parison is determined over a number of successive steps, the obtained average values being compared with the average values required to obtain a satisfactory parison, and the die gap being controlled dependent thereon.

8. A method according to claim 7, in which the velocity of the free end of the parison is determined by a linear array of photosensitive elements arranged close to the line of advancement of a parison during extrusion, the parison itself interrupting light falling on the photosensitive elements from a suitably positioned light source, whereby the outputs from those photosensitive elements indicate the advancement of the parison.

9. A method according to claim 8, in which the interval required for the end of the parison to advance the known distance from one photosensitive element to the next adjacent element is timed by monitoring the photosensitive element outputs, to yield a stepwise function of the average velocity of the free end of the parison.

10. A method for controlling the wall thickness profile of a parison during the extrusion thereof and substantially as hereinbefore described, with reference to the accompanying drawings.

11. A parison whenever extruded by a method according to any of claims 1 to 10.

12. Apparatus for controlling the wall thickness of a parison during the extrusion thereof, which apparatus comprises a parison extruder including a die having an annular gap, means to adjust the annular gap of the die, means to monitor the velocity of the free end of a parison during the extrusion thereof from the die, and control means acting upon the output from the velocity monitoring means to control operation of the gap adjustment means dependent thereupon.

13. Apparatus according to claim 12, in which the control means is arranged to compare the instantaneous output of the velocity monitoring means with pre determined velocity values stored in the control means, and to cause operation of the gap adjustment means dependent upon the result of the comparison.

14. Apparatus according to claim 12 or claim 13, in which the velocity measuring means comprises a linear array of photosensitive elements together with a light source arranged so that the parison may during extrusion interrupt the light falling on to the linear array of photosensitive elements.

15. Apparatus for controlling the wall thickness of a parison during the extrusion thereof and substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.