GB2308088A - A blow moulding process - Google Patents

Description

"A Blow Moulding Process" The invention relates to a blow moulding process.

The invention particularly relates to the blow moulding of relatively large plastics drums, for example of a size in the order of 120-220 litres capacity. With the blow moulding of such drums, relatively large amounts of plastics material have to be handled within the mould and this presents problems in trying to achieve an even distribution of plastics material within the mould and in forming drum side walls of a desired thickness profile.

Further, relatively long cooling times are required once the drum has been formed due to the large volume of plastics material in the drum. This limits the production throughput of the moulding machine. Also, improper cooling can cause distortion of the drum as it cools.

The present invention is directed towards providing a blow moulding process for manufacturing a plastics drum in an efficient manner which overcomes these problems.

According to the invention, there is provided a blow moulding process for manufacturing a plastics drum, comprising the steps:forming a tubular parison by extruding plastics material from an extruder accumulator head downwardly through a die, the die having an annular outlet opening; adjusting the width of the outlet opening for controlling the thickness of the parison side wall; locating a blow pin within a lower end of the parison; clamping the lower end of the parison about the blow pin; closing a split mould about the paris on in a controlled manner by locating an associated pair of mould halves at opposite sides of the parison, bringing the mould halves together at a desired mould closing speed in a number of steps, incrementally reducing the mould closing speed for each successive closing step; injecting air at a preset pressure through the blow pin into an interior of the parison, commencing air injection during closing of the mould parts together, inflating the parison against an inner wall of the mould to form the drum; opening the mould and discharging the drum from the mould.

The stepped closure of the mould advantageously assists in controlling expansion of the parison within the mould.

In a particularly preferred embodiment, the process further includes the step of cooling the drum when it is discharged from the mould by delivering the drum to a drum cooler, at the drum cooler clamping the drum between a pair of cooling plates, engaging each end of the drum with the cooling plates, circulating a cooling fluid through the cooling plates, sealing the drum, and circulating a cooling fluid through an interior of the drum, maintaining a preset desirable pressure within the drum while circulating the cooling fluid through the drum.

Advantageously, the cooling of the drum downstream of the moulding machine allows early removal of the drum from the moulding machine, the drum only being partially cooled on the moulding machine sufficient to allow handling. Thus, the throughput of the moulding machine can be increased.

Further, the drum cooler supports the drum during cooling thus ensuring proper cooling of the drum to a desired drum shape, thus improving the quality of the drum produced.

Maintaining a preset pressure within the drum prevents the tendency of the drum to buckle upon cooling.

In a particularly preferred embodiment, the process includes the steps of standing the drum on a fixed bottom cooling plate, engaging a top of the drum with a top cooling plate, moving the top cooling plate from a retracted disengaged position to a drum engaging position for clamping the drum between the cooling plates, circulating cooling water through the cooling plates, sealing one or more openings at a top of the drum for sealing the drum, passing cooling air through said openings for circulating cooling air through an interior of the drum whilst maintaining a preset air pressure within the drum. The circulation of cooling water through the cooling plates facilitates efficient cooling of the top and bottom of the drum which contain the largest amounts of material while at the same time, the cooling air readily easily and efficiently cools the side wall of the drum.

In a further embodiment, the process includes maintaining the blow pin at a temperature in the range of 50 C-80 C.

Preferably, the process includes maintaining the blow pin at a temperature of 70do. The heating of the blow pin has been found to improve the formation of a neck around the blow pin. More usually, such blow pins are cooled, however, such cooling can in some cases, particularly when handling the relatively large volumes of plastics material as in the present invention cause imperfections and cracking in the neck portion which can prove difficult or impossible to seal when a cap or cover is mounted on the neck portion in use.

In another embodiment, the mould has a core for forming an outwardly extending circumferential ring on the drum, the core defining a shaped slot extending outwardly of an inner side wall of the mould, the core having an open position forming an enlarged slot and a closed position of reduced slot size corresponding to a desired ring shape, the process including holding the core in an open position while closing the mould, blowing plastics material into the slot of the core, on attaining a preset desirable pressure within the mould closing the core for forming the captured plastics material into a desired ring shape, and moving the core into the open position after completion of blowing within the mould.

In a particularly preferred embodiment, a pair of cores are provided, namely, a top core and a bottom core spacedapart on the mould, the cores being closed at different speeds for forming a pair of spaced-apart circumferential rings on the drum. Ideally, the process includes commencing closure of the cores at the same time and closing the bottom core more slowly than the top core.

The closing of the cores at different speeds advantageously facilitates an improved distribution of plastics material within the mould for forming the desired rings on an exterior of the drum.

The invention will be more clearly understood by the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic flowchart illustrating a blow moulding process according to the invention; Fig. 2 is a perspective view of a drum formed by the process; Fig. 3 is a perspective view of another drum type formed by the process; Fig. 4 is a diagrammatic illustration of the formation of a parison in the process; Fig. 5 is a diagrammatic illustration of a mould closing about the parison according to the process; Fig. 6 is a diagrammatic illustration showing the mould in a fully closed position and the parison inflated to form a drum within the mould; Fig. 7 is a detail sectional elevational view of portion of a mould used in the process shown in an open position; ; Fig. 8 is a detail view similar to Fig. 7 showing the mould portion in a closed position for forming a circumferential ring about the drum; Fig. 9 is a detail view similar to Fig. 7 showing the mould portion in an open position; Fig. 10 is a detail sectional elevational view of portion of a drum formed according to the process showing a circumferential ring on an exterior of the drum; Fig. 11 is a perspective view of drum cooling apparatus used in the process; Fig. 12 is a perspective view showing the drum cooling apparatus in use cooling a drum according to the process; Fig. 13 is a partially sectioned elevational view of a drum being cooled in the apparatus of Fig. 11 according to the process; and Fig. 14 is a perspective view of a cooling carousel used in the process.

Referring to the drawings, a process and apparatus for blow moulding a cylindrical plastics drum will be described. Fig. 2 shows a drum 1 formed by the process.

The drum 1 has a circular bottom wall 2 with an upstanding cylindrical side wall 3 and a top wall 4. A pair of spaced-apart outwardly extending circumferential rings 5 are provided at an upper end and at a lower end of the side wall 3. These rings 5 are generally L-shaped in section as can be seen in Fig. 10. A pair of openings are provided in the top wall 4 defined by spaced-apart neck portions 6 upstanding on the top wall 4. Each neck portion 6 is threaded for reception of a cap (not shown) to seal the opening.

Fig. 3 shows another drum 7 formed according to the process of the invention. The drum 7 has a bottom wall and an upstanding side wall 8. An outwardly extending circumferential ring 9 is provided at an upper end of the side wall 8. A lid (not shown) is engageable with the upper end of the side wall 8 to seal the drum 7.

Ingredient materials for forming a plastics drum are delivered from supply hoppers 10, 11 containing new plastics material and dye material respectively to a blender 12. Additionally, re-ground plastic material which is a waste by-product of the process is also delivered to the blender 12 from a regrind material hopper 13. The ingredient materials are mixed in the blender 12 typically in the ratio of about 70% new plastics material, about 30% regrind plastics material and about 0.4% dye material.

The ingredient materials are delivered from the blender 12 to an extruder 14 of a blow moulding machine 16.

A tubular parison 18 is formed at the moulding machine 16 by extruding the plastics material from an accumulator head of the extruder 14 downwardly through a die having an annular outlet opening. The width of the annular outlet opening can be adjusted during extrusion of the plastics material for controlling the thickness of the parison side wall to provide a desired thickness profile between an upper end 19 and a lower end 20 of the parison 18.

When fully extruded, the lower end 20 of the parison drops over a pair of blow pins (not shown). The blow pins are moved apart to spread the lower end 20 of the parison 18 and then the lower end 20 of the parison 18 is clamped about the blow pins.

Referring to Figs. 4 to 6, a split mould 22 of the moulding machine 16 has a pair of mould halves 23, 24.

The mould halves 23, 24 are closed from an initially open position as shown in Fig. 4 to a closed position as shown in Fig. 6. The split mould 22 is closed in a controlled manner in a number of steps incrementally reducing the mould closing speed for each successive closing step. For example, in the manufacture of a 210 litre drum, the mould halves are initially aligned with the parison 18, the mould halves being spaced 190 mm apart. In a first closing step, the mould halves are closed to 70 mm at 100% closing speed. Then in a second closing step, at 35% mould closing speed, the gap between the mould halves is reduced to 35 mm. Subsequently, in a third mould closing step, the mould halves are closed to 16 mm at 12% mould closing speed. Finally, in a fourth mould closing step, the mould halves are brought together at 1% mould closing speed.

As the mould 22 is being closed, air injection commences for inflating the parison 18 within the mould 22. Air injection takes place in a number of steps. A first preblow of air at 4.5 bar pressure for 2.5 seconds is applied to the interior of the parison within the mould. After stopping the first pre-blow, a 0.1 second dwell time is allowed. Then a second pre-blow of air at 2.5 bar pressure for 4.8 seconds is applied to the parison interior within the mould. After stopping the second preblow, a 5 second dwell time is allowed. Then a main blow is carried out for 90 seconds. During the main blow, the air pressure within the parison is cycled at intervals between 9 bar and 4 bar pressure. This advantageously allows the air within the mould to be changed at intervals to provide cooling of the drum formed within the mould.

After the main blow, air is vented from the mould. The mould is opened and a drum 1 formed within the mould is discharged to a trimming station 26 at which excess plastics material is cut away from the drum 1. The waste plastics material is passed through a grinder 27 and returned to the regrind hopper 13 for re-use.

Downstream of the trimming station, the drum 1 is delivered to a drum cooling apparatus 50 which is described later.

During moulding, a pair of spaced-apart L-shaped support rings 5 are preferably formed around a circumference of the drum 1 projecting outwardly from an exterior surface of the drum. Figures 7 to 9 show the sequence for forming a pair of spaced-apart support rings 5 on an exterior of the drum 1. The mould 22 has a pair of spaced-apart cores, namely a top core 30 and a bottom core 31 spacedapart on the mould 22. Each core 30, 31 defines an Lshaped slot 32, 33 extending outwardly of an inner side wall 34 of the mould 22. Each core 30, 31 has an open position (as shown in Fig. 7) forming an enlarged slot 32, 33 and a closed position of reduced slot size corresponding to a desired ring shape as shown in Fig. 8.

Initially the cores 30, 31 are held in an open position while closing the mould. After the mould 22 has been fully closed at a preset internal pressure, the top core 30 and bottom core 31 close. It will be noted that both cores 30, 31 commence closure at the same time, however, the bottom core 31 closes more slowly than the top core 30. The top core 30 has a closing time of approximately 2.75 seconds, while the bottom core has a closing time of approximately 4.1 seconds. Thus, the closing time for the bottom core 31 is approximately 1.5 times that of the top core 30. This closure differential advantageously assists in the even distribution of plastics material into the slots 32, 33. In the closed position shown in Fig. 8, the slots 32, 33 shape the plastics material into a desired ring shape. Upon subsequent opening of the cores as shown in Fig. 9, an L-shaped ring 5 thus formed can be released from the core 30, 31.

Fig. 10 shows the desired L-shaped profile of the ring 5 which is achieved according to the method.

Downstream of the moulding machine 16, the drum 1 is delivered to a cooling apparatus 50 (Fig. 11). The cooling apparatus 50 has a support frame 51 with a fixed bottom cooling plate 52 and an associated top cooling plate 53 spaced-apart above the bottom cooling plate 52 and movably mounted on the frame for vertical movement by means of a ram 54. An upstanding arcuate drum locating band 55 is mounted at a rear of the bottom cooling plate 52. Each of the cooling plates 52, 53 are connected to a cooling water supply (not shown) for circulating cooling water through the plates 52, 53 for cooling a top wall 2 and a bottom wall 3 of a drum 1 placed in the cooling apparatus 50 as shown in Fig. 10.Openings in a top of the drum 1 are closed by sealing discs 60, 61, each carried by an associated ram 62, 63 on the frame 51 for movement between a retracted disengaged position and a drum engaging position as shown in Fig. 12. Air pipes 65, 66 are connected to each disc 60, 61 for circulating cooling air through the drum 1 whilst maintaining a preset air pressure within the drum 1. It will be noted that in the cooling apparatus 50, the drum 25 is supported between the cooling plates 52, 53 which support the drum in a desired alignment during cooling of the drum to form high quality drums of a uniform shape. By pressurising an interior of the drum during cooling, this prevents the tendency of the top wall 2 of the drum 1 to collapse or buckle during cooling which weakens the drum.

Advantageously, the cooling apparatus 50 allows the drum to be removed earlier from the moulding machine 16 without the need for fully cooling the drum on the moulding machine 16. Thus, advantageously a greater throughput of drums can be achieved with the moulding machine.

Referring to Fig. 14, there is shown a cooling carousel 70 used in the process. The cooling carousel 70 essentially has a plurality of the cooling apparatus 50 previously described with reference to Figs. 11 to 13. Parts similar to those described with reference to Figs. 11 to 13 are assigned the same reference numerals. The carousel cooler 70 has a central post 71 about which a support frame 72 is rotatably mounted. A plurality of cooling stations are provided by spaced-apart pairs of bottom cooling plates 52 and associated top cooling plates 53 arranged on the frame 72.

In use, drums 1 received from the moulding machine 16 are mounted between a pair of cooling plates 52, 53 and the cooling of the drum 1 is carried out as previously described with reference to Figs. 11 to 13. The frame 72 of the carousel cooler 70 is rotated through one revolution in the time taken to cool a drum 1. When the cooled drum 1 is returned to the loading station, it is unloaded and a fresh drum 1 from the moulding machine 16 is mounted on the carousel cooler 70.

The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail.

Claims (15)

CLAIS

1. A blow moulding process for manufacturing a plastics drum, comprising the steps: forming a tubular parison by extruding plastics material from an extruder accumulator head downwardly through a die, the die having an annular outlet opening; adjusting the width of the outlet opening for controlling the thickness of the parison side wall; locating a blow pin within a lower end of the parison; clamping the lower end of the parison about the blow pin; closing a split mould about the parison in a controlled manner by locating an associated pair of mould halves at opposite sides of the parison, bringing the mould halves together at a desired mould closing speed in a number of steps, incrementally reducing the mould closing speed for each successive closing step;; injecting air at a preset pressure through the blow pin into an interior of the parison, commencing air injection during closing of the mould parts together, inflating the parison against an inner wall of the mould to form the drum; opening the mould and discharging the drum from the mould.

2. A process as claimed in claim 1 which further includes the step of forced cooling of the drum when it is discharged from the mould.

3. A process as claimed in claim 2 which includes the step of cooling the drum when it is discharged from the mould by delivering the drum to a drum cooler, at the drum cooler clamping the drum between a pair of cooling plates, engaging each end of the drum with the cooling plates, circulating a cooling fluid through the cooling plates.

4. A process as claimed in claim 2 or 3 which includes sealing the drum, and circulating a cooling fluid through an interior of the drum, maintaining a preset desirable pressure within the drum while circulating the cooling fluid through the drum.

5. A process as claimed in any of claims 2 to 4 which includes the steps of standing the drum on a fixed bottom cooling plate, engaging a top of the drum with a top cooling plate, moving the top cooling plate from a retracted disengaged position to a drum engaging position for clamping the drum between the cooling plates, and circulating cooling water through the cooling plates.

6. A process as claimed in claims 4 or 5 which includes sealing one or more openings at a top of the drum for sealing the drum, passing cooling air through said openings for circulating cooling air through an interior of the drum whilst maintaining a preset air pressure within the drum.

7. A process as claimed in any preceding claim which includes maintaining the blow pin at a temperature in the range of 50"C-80"C during moulding of the drum.

8. A process as claimed in claim 7 which includes maintaining the blow pin at a temperature of 70"C.

9. A process as claimed in any preceding claim wherein the mould has a core for forming an outwardly extending circumferential ring on the drum, the core defining a shaped slot extending outwardly of an inner side wall of the mould, the core having an open position forming an enlarged slot and a closed position of reduced slot size corresponding to a desired ring shape.

10. A process as claimed in claim 9, the process including holding the core in an open position while closing the mould, blowing plastics material into the slot of the core, on attaining a preset desirable pressure within the mould closing the core for forming the captured plastics material into a desired ring shape, and moving the core into the open position after completion of blowing within the mould.

11. A process as claimed in claim 9 or 10 wherein a pair of cores are provided, namely, a top core and a bottom core spaced-apart on the mould, the cores being closed at different speeds for forming a pair of spaced-apart circumferential rings on the drum.

12. A process as claimed in claim 11 which includes commencing closure of the cores at the same time and closing the bottom core more slowly than the top core.

13. A blow moulding process substantially as hereinbefore described with reference to the accompanying drawings.

14. A plastics drum whenever formed by the process as claimed in any preceding claim.

15. Apparatus substantially as hereinbefore described for forming a plastics drum according to the process as claimed in any of claims 1 to 13.