GB2178361A

GB2178361A - Blow moulding

Info

Publication number: GB2178361A
Application number: GB08618580A
Authority: GB
Inventors: Akihiro Nomura; Hisahiko Fukase
Original assignee: IHI Corp
Current assignee: IHI Corp
Priority date: 1985-08-01
Filing date: 1986-07-30
Publication date: 1987-02-11
Also published as: IT8620626A0; SE8602466L; GB2178361B; IT1204887B; JPH035294B2; KR910000498B1; DE3619113C2; JPS6230020A; DE3619113A1; CN86104270A; SE8602466D0; GB8618580D0; KR870001920A

Abstract

Parisons (1), formed by extruding plastics material through an annular gap partially defined by a mandrel (4) and blocks (17), have portion(s) of constant wall thickness controlled by longitudinal movement of the mandrel and portion(s) of increased wall thickness controlled by transverse movement of the movable blocks. Thickness is controlled by storing signals indicative of the gap width in dependence on desired thickness values of the moulded articles in a control unit (24), arranged to control the positions of the mandrel and the blocks. A first parison is extruded whilst making marks (12) at predetermined positions on at least one portion of constant wall thickness and on at least one portion of increased wall thickness. The parison is then blow moulded into a test article (15) whose wall thickness is measured at the marks and compared with desired values. The values stored in the control unit are adjusted to eliminate or reduce the difference between the measured and desired thickness values. Subsequent articles are then moulded from further parisons after monitoring the parison length and comparing it with a desired length and adjusting all the values stored in the control unit such that the length of the next parison is substantially equal to the desired length. <IMAGE>

Description

SPECIFICATION Blow moulding The present invention relates to a method of blow moulding plastics articles and is concerned with such a method in which a parison is extruded through an annular nozzle and is then blow moulded into an article. The invention is concerned with controlling the thickness of such parisons prior to the blow moulding step.

In such a method, a generally cylindrical parison is extruded and its open ends are trapped between and closed by the cooperating edges of a two-part mould with an air injection tube extending into the parison.

The parison is at this stage of necessity of oval shape, at least at its top and bottom, due to its engagement by the mould halves. Certain portions of the parison thus contact the mould relatively rapidly when air is injected and thereafter their thickness remains constant. Certain further portions are stretched to a greater extent before they contact the mould and thus tend to be of reduced thickness. Similarly, very different degrees of stretching of the plastics material are required to permit it to conform to the shape of the corners of the mould. Thus, if the parison were of constant wall thickness the thickness of the moulded article would vary considerably in both the longitudinal and the circumferential directions.

Figure 1 is a schematic perspective view of part of a conventional plastics blow moulding machine including a double-cone-shaped head 2 attached to a die (not shown) for moulding a parison 1, that is to say a cylindrical plastics body extruded from a nozzle. A mandrel 4 within the head 2 defines with it an annular nozzle gap 5 and may be moved vertically by a servo cylinder 3 to vary the gap between the lower end of the mandrel 4 and the inner lower end of the head 2. A hollow injection plunger 7 for forcing material 6 into the space defined between the head 2 and the mandrel 4 may be vertically moved by an injection cylinder 8. A sensor 9 arranged to detect the injection stroke of the injection plunger 7 is connected to a setting device 10 including a setting means, such as a conventional pinboard or a slide switch, for setting the pattern of the width of the nozzle slit 5.Also connected to the setting device 10 are a servo valve 11 connected to the servo cylinder 3 and a marker 13 adapted to make marks 12 on the parison regularly spaced apart by a predetermined distance in the longitudinal direction of the parison 1, i.e. the direction in which the parison is extruded. A thickness gauge 14, e.g. of ultrasonic type, for measuring the section or wall thickness of a moulded article 15 at each of the marks 12 is also connected to the setting device 10.

With a blow moulding machine of this type, the wall thickness of the parison is set as follows. A parameter which varies as the parison is extruded, such as the injection stroke, i.e. the position, of the injection plunger 7, is selected and the width Sj of the nozzle slit is estimated for each movement of the injection plunger 7 by a predetermined distance to produce the desired thickness of the moulded article. The estimated nozzle slit width Si at each of the predetermined positions of the injection plunger 7 is fed into the setting device 10 which sets the pattern of the nozzle slit width and then the extrusion moulding process is started. A signal representative of the position of the injection plunger 7 is continuously fed to the setting device 10 by the sensor 9.At each of the predetermined positions of the injection cylinder 7 the servo cylinder 3 is actuated to move the mandrel 4 vertically so that the nozzle slit width Sj has the estimated value.

Simultaneously, a signal is transmitted from the setting device 10 to the marker 13 to make the marks 12 on the parison at each of the predetermined positions of the injection plunger 7. The marks 12 are spaced apart in the longitudinal direction of the parison 1 since they are made in response to the injection stroke of the injection plunger 7.

Upon completion of the moulding of the parison 1, the latter is blow moulded in a two-part mould and the wall thickness of the article at each of the marks 12 is measured by the thickness gauge 14. An operator compares the desired thickness with the actual or measured thickness and adjusts the setting device 10 such that when the desired thickness is less than the actual or measured thickness, the nozzle slit width Sj is decreased while when the desired thickness is greater than the actual or measured thickness, the nozzle slit width Sj is increased. The moulding operation is then repeated in the same manner and the wall thickness of the moulded article at each mark 12 is measured and compared with the preset thickness.When there is substantial coincidence between the actual section or wall thicknesses and the preset thickness the setting up or adjustment procedure is completed and continuous production is started.

However, the moulding method described above has various problems: It presupposes that the nozzle slit width Sj is uniform in the circumferential direction. If the wall thickness of the parison 1 is to be increased in the circumferential direction to provide, for instance, an opening or a reinforced part, it is impossible to estimate the gap profile in the circumferential direction. Furthermore, the nozzle slit width is set by the operator which is a complex and inefficient operation.

In the steady state, the length of the parisons may vary due to changes in temperature or the like which results in variations in their thickness.

According to the present invention there is provided a method of controlling the thickness of parisons for blow moulding into articles in which plastics material is extruded through an annular gap partially defined by a mandrel, the parisons having one or more portions of constant wall thickness controlled by longitudinal movement of the mandrel and one or more portions of differing wall thickness controlled by transverse movement of movable members partially defining the annular gap, the method including storing signals which are arbitrarily selected or indicative of magnitudes of the gap in accordance with desired thickness values of the moulded articles in a control unit arranged to control the positions of the mandrel and the movable members and extruding a first parison on the basis of the stored signals, making marks during extrusion of the parison at predetermined positions in the longitudinal direction on the parison on at least one portion of constant wall thickness and on at least one portion of differing wall thickness spaced therefrom in the circumferential direction, blow moulding the parison into a test article measuring the wall thickness of the test article at the marks and comparing with the desired values thereof and adjusting the values stored in the control unit to eliminate or reduce the difference between the measured and the desired thickness values, and subsequently moulding articles from further parisons after monitoring the length of each parison and comparing it with a desired length and adjusting all the values stored in the control unit such that the length of the next parison is substantially equal to the desired length.Preferably the widths of the annular gap corresponding to the portions of constant wall thickness and of differing wall thickness are displayed by the control device as a function of the length of the parisons.

The invention also embraces a method of blow moulding parisons into articles in which the thickness of the parisons is controlled as set forth above.

Further features, details and advantages of the present invention will be apparent from the following description of a preferred embodiment thereof which is given with reference to Figure 2 of the accompanying drawings which is a schematic view of a machine for carrying out a method in accordance with the invention. The same reference numerals as in Figure 1 are used to designate similar components.

Disposed at the lower end of the head 2 are movable blocks 17 which are spaced apart in the circumferential direction of the head 2 and are movable in the radial direction by means of respective actuators 16 to define portions of the annular gap of width g1 and g2 corresponding to thick portions of the wall of the parison 1. A marker 13 for making marks 12 on the parison at predetermined intervals in the longitudinal direction of the parison 1 is disposed below each movable block 17. Another marker 13 (not shown) is also provided for making marks at a constant wall thickness portion of the parison 1 which correspond to a width G of a portion of the annular gap. The actuators 16, the markers 13 and the servo cylinder 3 for vertically moving the mandrel 4 are actuated in response to control signals from a control device 24.

A length detector 18, such as a phototube, is arranged to detect the length of the parison 1.

The length signal 19 from the detector 18, the plunger-position signal 20 from the sensor 9 and the wall-thickness signal from the thickness gauge 14 are applied to the control device 24 and the instantaneous local widths G, g, and g2 of the annular gap are displayed as functions of the length of the parison 1.

Desired wall-thickness signais 22 representative of the desired thickness of the constant wall thickness portion and the desired thickness of the thick wall portions and a signal 23 representative of the desired length of the parison are applied to the control device 24.

In response to signals 22 representative of the desired thickness values 22 and the actual thickness 21 of the test moulded article 1 5 at each of the marks, the control device 24 obtains a new slit width value at each of the marks in the circumferential direction and thereby obtains a circumferential gap profile at predetermined positions spaced in the longitudinal direction of the parison. In response to the gap profiles thus obtained, the servo valve 11 of the servo cylinder 3 and the actuators 16 are automatically actuated so that the constant thickness wall portion and the thick wall portions have the predetermined wall thicknesses.

In operation based on a controlled thickness distribution, the length of the parison may vary due to a number of disturbances, such as changes in temperature, which may cause distortion of the thickness distribution referred to above. The length of the parison 1 is constantly monitored by the sensor 18 and the length signal 19 produced by it is applied to the control device 24 so that the servo cylinder 3 and the actuators 16 are controlled to adjust the overall thickness to the desired thickness in the circumferential direction of the parison 1 in order that the parison 1 may have a preset length 23 (the length of the parison corresponding to the desired thickness distribution) .

In a modified embodiment the signals initially applied to the control device 24 do not correspond to the desired thickness of the moulded article but are arbitrarily selected and are corrected after the test article has been moulded. It is to be understood that the present invention is not limited to the features of the embodiment described above and that various modifications may be effected. For instance, the movable blocks 17 may be afforded by a component, e.g. the nozzle 2, defining the nozzle slit and have sufficient elasticity that movement of the blocks 17 takes the form of elastic deformation of the component. Furthermore, various methods of moulding the thick wall section, for making the marks and for detecting the length of the parison may be employed.

The effects, features and advantages of the present invention may be summarised as follows: In moulding a parison having portions of constant and enlarged wall thickness, the gap profile in the circumferential direction of the parison is detected and in response thereto the wall thickness in the circumferential direction of the parison can be automatically controlled. As a result, complex setting operations can be eliminated and controllability can be enhanced. The positions of the mandrel and movable blocks are adjusted in response to the detected length of the parison. As a result, the length of the parisons can be automatically controlled by adjusting the overall thickness in the circumferential direction. The precision with which the wall thickness of a moulded article is controlled is improved and furthermore the comparison between the distribution of the wall thickness of a moulded article and a preset nozzle-slit-width pattern can be easily made and the distribution of the wall thickness and the preset nozzle-slit-width pattern can be therefore changed in a simple manner. As a result, quality control can be enhanced. The measurement of the wall thickness of respective portions of a moulded article can be automated by the suitable design and construction of a jig upon which the moulded article is placed so that on-line production utilising a computer can be realised.

Claims

1. A method of controlling the thickness of parisons for blow moulding into articles in which plastics material is extruded through an annular gap partially defined by a mandrel, the parisons having one or more portions of constant wall thickness controlled by longitudinal movement of the mandrel and one or more portions of differing wall thickness controlled by transverse movement of movable members partially defining the annular gap, the method including storing signals which are arbitrarily selected or indicative of magnitudes of the gap in accordance with desired thickness values of the moulded articles in a control unit arranged to control the positions of the mandrel and the movable members and extruding a first parison on the basis of the stored signals, making marks during extrusion of the parison at predetermined positions in the longitudinal direction on the parison on at least one portion of constant wall thickness and on at least one portion of differing wall thickness spaced therefrom in the circumferential direction, blow moulding the parison into a test article measuring the wall thickness of the test article at the marks and comparing with the desired values thereof and adjusting the values stored in the control unit to eliminate or reduce the difference between the measured and the desired thickness values, and subsequently moulding articles from further parisons whilst monitoring the length of each parison and comparing it with a desired length and adjusting all the values stored in the control unit such that the length of the next parison is substantially equal to the desired length.

2. A method as claimed in claim 1 in which the widths of the annular gap corresponding to$ the portions of constant wall thickness and of differing wall thickness are displayed by the control device as a function of the length of the parisons.

3. A method of controlling the thickness of parisons for blow moulding into articles substantially as specifically herein described with reference to Figure 2 of the accompanying drawings.

4. A method of blow moulding parisons into articles in which the parisons are extruded through an annular gap and their thickness is controlled by a method as claimed in any one of the preceding claims.