GB2302520A

GB2302520A - Control of air pressure in blow moulding

Info

Publication number: GB2302520A
Application number: GB9512749A
Authority: GB
Inventors: Nicholas Flood
Original assignee: Menza Ltd
Current assignee: Menza Ltd
Priority date: 1995-06-19
Filing date: 1995-06-22
Publication date: 1997-01-22
Also published as: IES950446A2; IES66135B2; GB9512749D0

Abstract

A blow moulding apparatus includes a blowpin 1 and a dedicated air control system for optimising the blow moulding operation comprising an air supply vessel 2, for holding typically 10 litres of air at a pressure of at least 8 bar, from which pressurised air is delivered through a control valve 6 to the blowpin 1. A pressure sensing valve 8 senses the air pressure in the vessel 2 and if the pressure falls below 10 bar a signal is outputted to a controller 9 which disables operation of the blow moulding station. The temperature of the air is also controlled to be about 8{C by a temperature sensor 10 which also provides a signal to the controller 9. Air may be exhausted into an exhaust vessel 15 maintained at a negative pressure before exhaust to atmosphere. Cycle time is reduced and product quality is improved.

Description

"Blow Moulding Method and Apparatus The invention relates to an apparatus and method for blow moulding.

Extrusion blow moulding is a technique that is used for moulding hollow plastics articles such as plastics cases and the like. A thermally plasticised thermoplastic tubular parison is first extruded. A two part moulding die is then closed around the parison. Air is injected into the parison enclosed by the die to blow mould the parison into the shape of the cavity of the die. The mould is then opened and the blow moulded article is removed.

There are several problems in blow moulding to a high standard of quality in a cost efficient manner. One of the problems is to minimise the cycle time to produce a moulded article.

This invention is directed towards providing an improved apparatus and method for blow moulding to a high quality standard while minimising cycle time.

According to the invention there is provided a blow moulding apparatus comprising: a blow moulding station having a blowpin through which pressurised air is blown during blow moulding; and an air control system dedicated to the blow moulding station; the air control system comprising: a vessel for holding air at a pressure of at least 8 bar; the vessel having an inlet for air supply and an outlet through which pressurised air is delivered; a delivery line from the vessel outlet to the blowpin of the dedicated blow moulding station; a control valve on the delivery line for delivery of air on demand from the vessel to the blowpin; an exhaust unit for exhaust of air from the delivery line; a pressure sensing means for sensing the pressure of air in the vessel; and pressure control means for preventing operation of the dedicated blow moulding station if the pressure in the vessel is below a preset pressure.

In one embodiment of the invention the volume of the vessel for holding air is at least 10 litres. The volume may be typically 10 or 25 litres.

Preferably the preset pressure is approximately 10 bar.

In a particularly preferred embodiment of the invention the air control system includes temperature sensing means for sensing the temperature of air for delivery to the blowpin and temperature control means for preventing operation of the dedicated blow moulding station if the temperature of the air is not within a preset temperature range. In this case preferably the temperature sensing means is located to sense the temperature of the air in the air vessel.

Preferably the preset temperature range is from 8"C to 10 C, especially approximately 8"C.

In another preferred embodiment of the invention the air control system includes water level sensing means for sensing the level of condensate in the vessel and a drain system for removing condensate from the vessel.

Preferably the control valve on the delivery line for delivery of air to the blowpin is a fast acting directional control valve having ports for maximum flow of air to the blowpin with minimum pressure drop across the control valve.

In a preferred arrangement the inlet to the air vessel is supplied with air at a pressure of at least 8 bar through an inlet pipe having a diameter of at least 15 mm preferably at least 25 mm.

In one embodiment of the invention the apparatus includes an exhaust vessel maintained at a negative pressure into which exhaust air is delivered before exhausting to atmosphere. Preferably the exhaust vessel is maintained at a negative pressure of from 0.5 to 0.5 bar by a vacuum pump. Usually the air is exhausted from the exhaust vessel through a non-return valve. Preferably the exhaust vessel has a capacity larger than that of the air exhausted in the blow moulding cycle.

The invention also provides a method of blow moulding including the steps of: delivering pressurised air to a vessel dedicated to a blow moulding station, the vessel having an air inlet and an air outlet; delivering air from the vessel outlet through a delivery line to a blowpin of a blow moulding station; delivering the air through a control valve on the delivery line; sensing the pressure of air in the vessel; and preventing operation of the dedicated blow moulding station if the pressure in the vessel is below a preset pressure.

Preferably the method includes the steps of: sensing the temperature of air for delivery to the blowpin; and preventing operation of the dedicated blow moulding station if the temperature of the air is not within a preset range.

In a preferred embodiment of the invention the method includes the step of exhausting air from the blow pin into an exhaust vessel maintained at a negative pressure prior to exhaust to atmosphere.

The invention also provides a blow moulded article whenever produced using the apparatus and/or method of the invention.

The invention will be more clearly understood from the following description thereof given by way of example only with reference to the accompany drawings, in which: Fig. 1 is a schematic view of part of a blow moulding apparatus according to the invention.

Referring to the drawings there is illustrated portion of a blow moulding apparatus according to the invention which comprises a blow moulding station including a blowpin 1 through which pressurised air is blown during blow moulding. An air control system dedicated to the blow moulding station comprises an air supply vessel 2 for holding typically 10 or 25 litres of air at a pressure of at least 8 bar. The vessel 2 has an inlet 3 for air supply and an outlet 4 through which pressurised air is delivered along a delivery line 5 to the blowpin 1. A large port, quick acting, large flow rate, directional control valve 6 on the delivery line 5 delivers pressurised air on demand to the blowpin 1. Air is exhausted through a quick exhaust unit 7.Air may be exhausted initially to an exhaust vessel 15 maintained at negative pressure of typically 0.5 to 0.8 bar by a vacuum pump 16 into which exhaust air is delivered before exhausting to atmosphere through a non return valve 17.

The exhaust vessel 15 has a capacity larger than that of the air exhausted in a typical blow moulding cycle. For example, the vessel may have a capacity of 10 to 25 litres. Because the air exhausts initially to a negative pressure vessel it exhausts more quickly than in conventional systems in which air is exhausted directly to atmosphere.

The control system also includes an air pressure sensing means provided by a senstronic pressure sensing valve 8 which provides an electronic signal along a control line to a machine programmable logic controller 9. In the event of the air pressure falling below a preset value, typically 10 to 15 bar, ideally about 10 bar, the controller 9 outputs an alarm signal and also a signal to disable operation of the blow moulding station. A proportional pneumatic value 19 may also be provided on the supply line to maintain a high flow rate of air.

A temperature sensor 10 is located to sense the temperature of the air in the air vessel 2 and if the temperature falls outside a preset range of typically 8 to 1 2 OC , preferably 8"C + 1"C, it provides a signal to the controller 9 which outputs an alarm signal and provides an output to disable operation of the blow moulding station.

Although not illustrated the control system may also include a moisture sensing means, typically provided by a water level sensing device humidity probe, for sensing the level of condensate and moisture control means provided by an automatic drain system which removes condensate from the supply vessel 2.

To minimise pressure drop an air line supply 12 to the vessel inlet 3 is through a relatively large diameter, preferably at least 1S mm, ideally at least 25 mm. Air is supplied at a minimum of 8 bar and preferably at 10 bar.

A pressure gauge 13 is also provided on the vessel 2. A non return valve 20 is provided on the supply line 12.

In use, each blow moulding station is provided with a dedicated air control system as described above.

Pressurised air is delivered into the vessel 2 and the pressure and temperature of the air is closely controlled to lie within preset limits. If any of the parameters fall outside the preset limits an alarm is given and the controller 9 disables the further operation of the moulding station. During normal operation air is supplied through the control valve 6 to the blowpin 1 at optimum operating condition. In this way the air which is delivered into a parison to blow the parison against a die cavity to form an article is consistently delivered at high pressure and at minimum temperature and humidity.

Adverse effects on the blow moulding process and blow moulded articles are thereby avoided. Dimensional changes and surface finish variations in the product are avoided while minimising cycle time. This is because the inflation of the parison is faster and improved contact between the plastic and the surfaces of the die cavity are achieved.

Example A two litre polyethylene container was blow moulded on a continuous extrusion blow moulding machine using conventional pressurised air supply direct from a ring main. This resulted in a production rate of 156 tanks per hour to an acceptable quality finish.

An air control system as described above was installed on the machine as follows: volume of air vessel 25 litres; set pressure range 10 bar + 0.5 bar; set temperature range 8"C to 12"C.

The equipment was set up and operated as described above and resulted in a significant increase in production with associated improvements in quality of the polyethylene containers.

Many variations on the specific embodiments of the invention described will be readily apparent and accordingly the invention is not limited to the embodiments hereinbefore described which may be varied in detail.

Claims

1. A blow moulding apparatus comprising: a blow moulding station having a blowpin through which pressurised air is blown during blow moulding; and an air control system dedicated to the blow moulding station; the air control system comprising: a vessel for holding air at a pressure of at least 8 bar; the vessel having an inlet for air supply and an outlet through which pressurised air is delivered; a delivery line from the vessel outlet to the blowpin of the dedicated blow moulding station; a control valve on the delivery line for delivery of air on demand from the vessel to the blowpin; an exhaust unit for exhaust of air from the delivery line; a pressure sensing means for sensing the pressure of air in the vessel; and pressure control means for preventing operation of the dedicated blow moulding station if the pressure in the vessel is below a preset pressure.

2. A blow moulding apparatus as claimed in claim 1 wherein the volume of the vessel for holding air is at least 10 litres.

3. A blow moulding apparatus as claimed in claim 1 or 2 wherein the volume of the vessel for holding air is approximately 10 litres.

4. A blow moulding apparatus as claimed in claim 1 or 2 wherein the volume of the vessel for holding air is approximately 25 litres.

5. A blow moulding apparatus as claimed in any preceding claim wherein the preset pressure is approximately 10 bar.

6. A blow moulding apparatus as claimed in any preceding claim wherein the air control system includes temperature sensing means for sensing the temperature of air for delivery to the blowpin and temperature control means for preventing operation of the dedicated blow moulding station if the temperature of the air is not within a preset temperature range.

7. A blow moulding apparatus as claimed in claim 6 wherein the temperature sensing means is located to sense the temperature of the air in the air vessel.

8. A blow moulding apparatus as claimed in claim 6 or 7 wherein the preset temperature range is from 8"C to 120C.

9. A blow moulding apparatus as claimed in any of claims 6 to 8 wherein the preset temperature is approximately 8"C.

10. A blow moulding apparatus as claimed in any preceding claim wherein the air control system includes water level sensing means for sensing the level of condensate in the vessel and a drain system for removing condensate from the vessel.

11. A blow moulding apparatus as claimed in any preceding claim wherein the control valve on the delivery line for delivery of air to the blowpin is a fast acting directional control valve having ports for maximum flow of air to the blowpin with minimum pressure drop across the control valve.

12. A blow moulding apparatus as claimed in any preceding claim wherein the inlet to the air vessel is supplied with air at a pressure of at least 8 bar through an inlet pipe having a diameter of at least 15 mm.

13. A blow moulding apparatus as claimed in claim 8 wherein the air inlet pipe has a diameter of at least 25 mm.

14. A blow moulding apparatus as claimed in any preceding claim wherein the apparatus includes an exhaust vessel maintained at a negative pressure into which exhaust air is delivered before exhausting to atmosphere.

15. A blow moulding apparatus as claimed in claim 13 wherein the exhaust vessel is maintained at a negative pressure of from 0.5 to 0.5 bar by a vacuum pump.

16. A blow moulding apparatus as claimed in claim 14 or 15 wherein the air is exhausted from the exhaust vessel through a non-return valve.

17. A blow moulding apparatus as claimed in any of claims 14 to 16 wherein the exhaust vessel has a capacity larger than that of the air exhausted in the blow moulding cycle.

18. A blow moulding apparatus substantially as hereinbefore described with reference to the accompanying drawings.

19. A method of blow moulding using the apparatus of any of claims 1 to 18 including the steps of: delivering pressurised air to a vessel dedicated to a blow moulding station, the vessel having an air inlet and an air outlet; delivering air from the vessel outlet through a delivery line to a blowpin of a blow moulding station; delivering the air through a control valve on the delivery line; sensing the pressure of air in the vessel; and preventing operation of the dedicated blow moulding station if the pressure in the vessel is below a preset pressure.

20. A method as claimed in claim 19 including the steps of: sensing the temperature of air for delivery to the blowpin; and preventing operation of the dedicated blow moulding station if the temperature of the air is not within a preset range.

21. A method as claimed in claim 19 or 20 including the step of exhausting air from the blow pin into an exhaust vessel maintained at a negative pressure prior to exhaust to atmosphere.

22. A method of blow moulding substantially as hereinbefore described with reference to the accompanying drawings.

23. A blow moulded article whenever blow moulded using the apparatus of any of claims 1 to 18 or the method of any of claims 19 to 22.