IES930126A2

IES930126A2 - A blow-moulding process

Info

Publication number: IES930126A2
Application number: IE012693A
Authority: IE
Inventors: Nicholas Flood
Original assignee: Plastech Developments Ltd
Priority date: 1993-02-22
Filing date: 1993-02-22
Publication date: 1993-08-25
Also published as: GB9403360D0; GB2275441A; GB2275441B; IES58331B2

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.

Description

A BLOW-MOULDING PROCESS" The present invention relates to an improved blow-moulding process, in particular to a process for moulding objects including a hollow cavity section or sections.

Hitherto, it has been the practice to mould an object such as a speciality tool case, e.g. for holding an electric drill and drill bits, as follows. Firstly, a section of a tube or parison of hot thermoplastics material is extruded in a manner known per se, the two halves of the mould are brought together on opposite sides of the parison, and as this is happening a pin or pins penetrates through the mould into the parison, through which air is blown at high pressure to inflate the thermoplastics material against the walls of the mould, whereupon a period of time is allowed to elapse for the moulding to set prior to separation of the two halves of the mould. In this example, the moulding may be a tool case comprising an integral hinge dividing two hollow cavity sections which form the two sides of the case, which may be formed internally with appropriate hollows or depressions to accommodate a tool or tools to prevent their movement when the case is closed, and also to provide cushioning and protection if the case were to be dropped. Many speciality tools such as electric drills are presently sold complete with a custom-made tool case. ~ ~ ~~ ™ OPEN TO PUBLIC INSPECTION UNDER SECTION 28 AND RULE 23 IE 930126 IE 930126 - 2 In a conventional blow-moulding process, the blow pin or pins which are used are of approximately 3mm diameter and comprise a sharpened tip which is normally chamfered. However, with the conventional blow pin, the parison wall is pierced by the sharpened tip and the aperture so formed may be of greater diameter than the pin or be of ragged configuration, since the pin effectively rips through the parison wall. Pressure air must be introduced through the pin instantaneously after penetration of the parison wall so as to inflate the parison against the walls of the mould. However, in practice air leaks out through the aperture made by the pin and no effective seal is provided. A pressure of 3 to 5 bar may be achieved with the conventional moulding process, but this is insufficient to avoid flow wrinkles being visible on the surface of the moulding where distribution of thermoplastics material has been uneven, and the small apertures left in the moulding after withdrawal of the blow pins are frequently poorly formed lowering the quality and appearance of the finished product. These drawbacks result in higher reject rates, and reduced pressures due to air leakages may result in poor surface definition where surface ornament is to be applied to the moulding.

The present invention seeks to overcome these disadvantages by providing a blow-moulding process which uses an improved blow pin arrangement.

According to the present invention, there is provided a blow-moulding process comprising the steps of: - extruding a parison of hot thermoplastics material, bringing together two halves of a mould around the parison, penetrating the parison with at least one blow pin and blowing pressure air through the pin to inflate the parison against the walls of the mould, and - allowing a period of time to elapse before releasing the mould, characterised in that the blow pin defines a blunt tip adapted to deform the wall of the parison before penetration.

IE 930126 - 3 Preferably, the opening of the blow pin defines a circular orifice.

Most preferably, the external edge of the circular orifice is rounded.

Advantageously, pressure air is blown through the pin at a pressure which is greater than 5 bar, most preferably about 10 bar.

The invention also provides a thermoplastics moulding including a 10 hollow cavity made by the process of the present invention in which the blow pin aperture or apertures is/are defined by thickened aperture wall sections formed by a sealing and accumulation of material against the blow pin(s) during the inflation step.

The invention will be further described in detail with reference to the accompanying drawings, in which Figures 1 to 4 show the penetration of a conventional blow pin with a sharpened tip during a prior art moulding process, Figures 5 to 8 show the corresponding steps during the process of the present invention, Figure 9 shows a detail view in part cross-section of a blow pin for 25 use in the present invention, Figure 10 shows, in cross section viewed on line A-A in Figure 11, an example of a tool case in an open position moulded by the process of the present invention, and Figure 11 shows a plan view from above of the tool case of Figure 10.

In the conventional process, shown in Figures 1 to 4, a blow pin 1 with a sharpened tip 2, of about 3mm diameter, approaches a parison 3 of hot thermoplastics material (see Figure 1). The tip 2 pierces the parison IE 930126 - 4 3 (see Figure 2), and an instant later, air is introduced through the pin into a space within the parison enclosed by the mould (see Figure 3). However, due to the fact that the wall of the parison is pierced, no effective seal is formed around the blow pin and pressure air tends to leak out of the enclosed space past the blow pin, through a minute gap 4. (see Figure 4).

In accordance with the present invention, a blunt-tipped blow pin 5 10 approaches the parison (see Figure 5). Before penetrating the wall of the parison, the pin deforms or puckers the wall of the parison (see Figure 6). As pressure air is introduced, an portion of thermoplastics covering the blow pin orifice is ejected, thus forming a clean aperture in which the parison seals around the blow pin (see Figure 7).

Pressure air acting against this seal further spreads material around the aperture formed by the blow pin to thicken the aperture walls 6 (see Figure 8). Once set, the blow pin is withdrawn leaving a clean, well formed aperture.

Figure 9 is a detail of the blow pin 5 showing the rounded edge 7 and essentially circular orifice 8.

Figure 10 shows a moulded tool case 10 having two hollow cavities 11, defining the sides and handles of the case. Depressions 13 are formed on the inside of the case to receive a tool or tools such as an electric drill and bits therefor. Two blow pin apertures 14, 15 mark the positions at which the cavities 11, 12 were inflated during moulding. However, it will be appreciated that the blow-moulding process of the present invention may be applied to many different articles which include a hollow cavity or cavities requiring to be inflated during the moulding process, such as hollow car bumper sections, for example.

IE 930126 - 5 Much higher air pressures can be used with the blow pin of the present invention, for example, up to about 10 bar, which gives the following significant benefits: 1. Deeper penetration of the mould by the thermoplastics gives a better surface definition of fine surface ornamentation, for example, a leather or grain effect, or fine raised surface lettering. A lower air pressure of 3 to 5 bar may give poor or incomplete surface definition, especially when there is air leakage. 2. Improved contact of the thermoplastics with the mould surface leads to a better heat transfer and faster cooling and setting rate, which in turn leads to higher production throughput. 3. Better material distribution caused by faster inflation of the mould cavities means that less material may be used, i.e. a lower part weight may be achieved giving higher production. 4. A larger diameter blow pin may be used. For example, a blow pin of 5mm diameter as compared to one of 3mm diameter gives a fourfold increase in orifice cross-sectional area resulting in faster exhaust of pressure air which in turn leads to higher production rates.

. The blow pin design requires no sharpening while running due to the rounded blunt tip of the penetrating end. 6. The better surface definition results in a higher quality product and permits designs to be embossed onto the surface of the moulding with improved clarity which have not hitherto been possible. 7. Because the quality of the moulded product is improved, there is a lower reject rate which again contributes to higher overall production.

Claims

1. A blow-moulding process comprising the steps of: extruding a parison of hot thermoplastics material, bringing together two halves of a mould around the parison, penetrating the parison with at least one blow pin and blowing pressure air through the pin to inflate the parison against the 10 walls of the mould, and allowing a period of time to elapse before releasing the mould, characterised in that the blow pin defines a blunt tip adapted to deform the wall of the parison before penetration.

2. A process according to claim 1, in which the opening of the blow pin defines a circular orifice, preferably wherein the external edge of the circular orifice is rounded. 20

3. A process according to claim 2, in which pressure air is blown through the pin at a pressure which is greater than 5 bar, most preferably about 10 bar.

4. A thermoplastics moulding made by the process according to any of 25 claims 1 to 3 in which the blow pin aperture or apertures is/are defined by thickened aperture wall sections formed by a sealing and accumulation of material against the blow pin(s) during the inflation step. 30

5. A blow-moulding process, or product of such a process substantially as described herein with reference to any of Figures 5 to 11 of the accompanying drawings.