GB2275441A

GB2275441A - A method of blow-moulding

Info

Publication number: GB2275441A
Application number: GB9403360A
Authority: GB
Inventors: Nicholas Flood; Patrick Beirne
Original assignee: PLASTECH DEV Ltd
Current assignee: PLASTECH DEV Ltd
Priority date: 1993-02-22
Filing date: 1994-02-22
Publication date: 1994-08-31
Anticipated expiration: 2014-02-22
Also published as: IES930126A2; IES58331B2; GB2275441B; GB9403360D0

Abstract

Thermoplastics material is blow moulded by a method which includes the steps of forming a tubular parison and as mould parts 40 are closing around the parison moving a blow pin 30 forward. Air at a pressure of 10 bar is blown through the pin 30 to deform an outer skin 41 of the thermoplastics material as the mould parts are closing. The pin 30 is then inserted through the skin 41 and an article such as a case is blow moulded. The pin 30 has a blunt tip 31 and a tubular sleeve 41c forms around the pin 30 as the pin 30 is operated to sleeve the pin 30 This prevents leak-back of air and allows a greater pressure blow moulding at higher cycle times and with improved surface finish. <IMAGE>

Description

"A Nethod of Blow-Mouldingw The invention relates to a method of blow-moulding thermoplastics material to form a blow moulded article such as a case, for example, a tool case comprising two case halves having an integrally moulded hinge therebetween.

It is known to form such blow-moulded articles by firstly extruding a tube or parison of hot thermoplastics material, bringing two halves of a mould together on opposite sides of the parison and inserting a blow pin through the mould into the parison. Air is then blown into the interior of the parison through the blow pin at high pressure to inflate the thermoplastics material against the walls of the mould. After a preset time when the moulding has set, the two halves of the mould are separated and the moulded article is released.

In conventional arrangements, the blow pin which penetrates the wall of the mould comprises a sharpened tip which is normally chamfered. On penetration, the sharpened tip essentially rips through the wall of the parison resulting in a ragged edge through which air may back-leak out from the mould. Because of this, relatively low moulding pressure of between 3 and 5 bar is generally only achievable using conventional technology. This results in an unsatisfactory surface finish as the air pressure is insufficient to avoid flow wrinkles being visible on the surface of the moulding where distribution of thermoplastics material has been uneven. Further, the small apertures left in the moulding after withdrawal of the blow pins are frequently poorly formed which again detracts from the surface finish of the finished product.

This invention is directed towards providing an improved method of blow-moulding which will overcome these problems.

According to the invention there is provided a method of blow-moulding thermoplastics material comprising the steps of: extruding a parison of hot thermoplastics material; closing a pair of mould halves around the extruded parison; deforming the wall of the paris on prior to penetration; penetrating the parison with a blow pin which defines a blunt tip; blowing pressure air through the blow pin to inflate the parison against the walls of the mould; withdrawing the blow pin; and releasing a formed article from the mould after a preset time.

In a particularly preferred embodiment of the invention, pressure air is blown through the blow pin prior to penetration to deform the wall of the parison ahead of the blow pin.

In an especially preferred embodiment of the invention, the wall of the parison is deformed by the blow pin and pressure air is blown through the blow pin to form a tubular support extending inwardly from the blow pin insertion hole and substantially surrounding the blow pin when inserted in the mould.

Advantageously, an end outlet opening of the blow pin defines a circular orifice. Preferably the external edge of the circular orifice is rounded.

In a preferred embodiment of the invention, air is blown through the blow pin at a substantially constant pressure greater that 5 bar. Nost preferably, the air is blown through the blow pin at a pressure of approximately 10 bar.

In a further embodiment of the invention, the method includes the steps of: blending preset quantities of virgin thermoplastics material, regrind material and masterbatch in a blender dedicated to a particular moulding station, extruding the mixture thus formed to form a parison; trimming excess material from the moulded article; grinding the excess material; delivering the ground excess material to a storage hopper; and drawing the recycled excess material from the hopper on demand for blending with the virgin and masterbatch material in the blender dedicated to the moulding station at which the excess material is generated.

In this case advantageously prior to blending, the recycled material is passed through a first metal detector. Preferably the recycled material passed through the first metal detector is passed through a second metal detector.

In a preferred embodiment of the invention, the method includes the step, after trimming, of cutting a slot in an inner skin of the moulded article. Advantageously, the material removed on slot forming is reground and recycled.

Typically there are at least two moulding stations.

The invention also provides a blow-moulded article such as a blow-moulded case whenever formed by a 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 accompanying drawings in which Fig. 1 is a flow chart illustrating the method of the invention; Fig. 2 is a side elevational view of moulding apparatus used in performing the method of the invention; Fig. 3 is a plan view of the moulding apparatus of Fig.

2; Figs. 4 to 6 are cross-sectional views illustrating steps in the method of the invention; and Figs. 7 and 8 are further views illustrating steps in the method of the invention.

Referring to the drawings, there is illustrated a method of blow-moulding thermoplastics material which comprises the steps of blending in preset quantities virgin thermoplastics material 3, masterbatch material 4 and regrind material 5 in a blender 2 dedicated to a particular moulding station. The blend of thermoplastics material is then extruded in step 7. to form a tube or parison 8. In step 9 a pair of mould halves are moved towards a closed position around the extruded parison 8.

As the mould halves are closing a blow pin is moved forward towards the thermoplastics material in step 10.

Air is blown through the pin in step 11. The mould is fully closed in step lla. The pin is inserted through the parison in step 12 and air is blown into the parison in step 13 to inflate the parison against the walls of the mould. After a preset time allowed for the moulded article thus formed to set, the article is released in step 14 from the mould.

Excess material is trimmed from the moulded article in step 15 and ground in step 16. The regrind material is delivered to a hopper 17 for feeding a holding bin 18 ready for delivery to the blender 2 on demand. The recycled regrind material is passed through a first metal detector 19 and a second metal detector 20 prior to delivery to the blender 2. Any material in which metal is detected by the detectors 19, 20 is rejected.

In one arrangement the moulded article is a blow-moulded case for speciality tools or the like and a slot is cut in step 21 in an internal skin of the case, material removed on cutting of the slot being also recycled along line 22 to the grinder 16. The cases thus formed are tested in step 23.

The usage of plastics material is optimised in the process of the invention as excess material trimmed or cut from the finished moulded article is reground and after checking is used in the same moulding machine in producing similar moulded articles with the same formulation of virgin, masterbatch and regrind materials.

In more detail and referring particularly to Figs. 4 to 8, in the method of the invention a blow pin 30 is used which has a blunt tip end outlet opening 31 which defines a circular orifice 31, the external edge of which is preferably rounded. The blow pin 30 extends from an enlarged head portion 35 which is fixed to a rod 36 of a pneumatic ram 37. Pressure air is delivered into the pin 30 by an air line 37 which is attached to a spigot 38 on the head portion 35 of the pin 30. In Figs. 4 to 8 one mould part 40 is illustrated onto which an outer skin 41 of thermoplastics material is blown.

In the method of the invention, air is blown through the blow pin 30 just before it meets the outer skin 41 of the parison and forms a slightly bulbous portion 41a in the skin ahead of the pin 30. As the pin 30 continues to travel through the mould 40, the bulbous portion 41a is elongated and deformed further to form a parabolic type shape 41b ahead of the pin 30. The pin 30 then penetrates through the skin 41 blowing air ahead of it at all times so that the deformed section of the skin is formed into the tubular shape 41c illustrated in Fig. 6 which substantially surrounds the blow pin 30. The tubular extension 41c strengthens the blow pin orifice in the skin 41 allowing air at relatively high pressure of approximately 10 bar to be blown into the parison to inflate the parison against the mould halves to form the moulded article. As the tubular section 41c substantially surrounds the blow pin 30 an effective air seal is formed so that all the air pressure applied through the blow pin 30 is used to inflate the mould without any leak-back through the blow pin hole.

The invention offers very considerable advantages.

Because of the higher air pressure and deeper penetration of the mould by the thermoplastics, a substantially improved surface finish is achieved. In addition, there is improved contact of the thermoplastics with the mould surface with consequent improved heat transfer and cooling efficiencies which in turn leads to lower cycle times and improved rates of production. As the material is inflated faster, less material may be required with a lower part weight giving higher production. Because of the improved method a larger diameter blow pin may be used resulting in faster exhaust of pressure air which in turn leads to lower cycle times and higher rates of production. As the blow pin has a blunt end no sharpening is required and a better surface finish in the area of the blow pin holes in the finished article is achieved. In view of the improved surface definition in the moulded articles designs may be embossed on the surface of the moulding with greater clarity than has been possible to date.

As the quality of the moulded article as produced using the method of the invention is improved there is a lower reject rate on testing, with consequent increase in production.

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

Claims

CLAIMS:

1. A method of blow-moulding thermoplastics material comprising the steps of: extruding a parison of hot thermoplastics material; closing a pair of mould halves around the extruded parison; deforming the wall of the parison prior to penetration; penetrating the parison with a blow pin which defines a blunt tip; blowing pressure air through the blow pin to inflate the parison against the walls of the mould; withdrawing the blow pin; and releasing a formed article from the mould after a preset time.

2. A method as claimed in claim 1 wherein pressure air is blown through the blow pin prior to penetration to deform the wall of the paris on ahead of the blow pin.

3. A method as claimed in claim 1 or 2 wherein the wall of the parison is deformed by the blow pin and pressure air is blown through the blow pin to form a tubular support extending inwardly from the blow pin insertion hole and substantially surrounding the blow pin when inserted in the mould.

4. A method as claimed in any preceding claim wherein an end outlet opening of the blow pin defines a circular orifice.

5. A method as claimed in claim 4 wherein the external edge of the circular orifice is rounded.

6. A method as claimed in any preceding claim wherein air is blown through the blow pin at a substantially constant pressure greater that 5 bar.

7. A method as claimed in claim 6 wherein the air is blown through the blow pin at a pressure of approximately 10 bar.

8. A method as claimed in any preceding claim including the steps of: blending preset quantities of virgin thermoplastics material, regrind material and masterbatch in a blender dedicated to a particular moulding station, extruding the mixture thus formed to form a parison; trimming excess material from the moulded article; grinding the excess material; delivering the ground excess material to a storage hopper; and drawing the recycled excess material from the hopper on demand for blending with the virgin and masterbatch material in the blender dedicated to the moulding station at which the excess material is generated.

9. A method as claimed in claim 8 wherein, prior to blending, the recycled material is passed through a first metal detector.

10. A method as claimed in 9 wherein the recycled material passed through the first metal detector is passed through a second metal detector.

11. A method as claimed in any of claims 8 to 10 wherein the method includes the step, after trimming, of cutting a slot in an inner skin of the moulded article.

12. A method as claimed in claim 11 wherein material removed on slot forming is reground and recycled.

13. A method as claimed in any of claims 8 to 12 wherein there are at least two moulding stations.

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

15. A blow-moulded article whenever produced by a method as claimed in any preceding claim.

16. A blow-moulded case comprising two case halves having an integrally moulded hinge therebetween whenever moulded by a method as claimed in any claims 1 to 14.