GB2246567A - Moulding glass articles - Google Patents

Abstract

In a method of producing glass articles by the press-blow method in which a gob of molten glass is introduced into a chamber and a pressing tool presses upwards into the gob to form a parison after which the parison is blow molded into the final article, the pressing tool is cooled by means of a mixture of liquid nitrogen and air.

Description

PRODUCTION OF GLASS ARTICLES The present invention relates to the production of glass articles, and more particularly to mass producing glass containers by the well-known blow moulding process in which as a first step a parison of glass is made by the so-called press-blow method.

In the press-blow method, a gob of molten glass is introduced into a chamber and a pressing tool presses upwards into the gob to form an intermediate shape of glass known as the parison, the pressing tool is removed from contact with the parison, and the parison while gripped at its outer edge is turned over and introduced to a blow moulding station for blow moulding of the final shaped article.

As with any industrial process, developments are made to improve the speed of operation, consistency and quality of the product.

In regard to the pressing stage, a critical feature has been that of temperature control.

In any glass forming process for the most part, the heat required is carried in the glass itself and the glass looses heat and becomes firmer in form, and the various stages of shaping take place.

Thus at the parison stage the temperature will have fallen slightly from the fully molten to enable a part shaped article to be formed. However in a continuous production process where a succession of parisons are being formed, the pressing tool will be in continuous use and will absorb heat from each of the successive gobs.

If as a result the tool becomes too hot the glass will not loose sufficient heat and the parison will be too hot to retain its correct shape. It then becomes necessary to cool the tool to ensure that the gob is pressed at the correct temperature. The usual method of cooling the tool is to use a controlled flow of air through the interior of the tool and provided production speeds are not too great a balanced operation can be achieved where the heat gain and loss are balanced and the pressing tool remains at the correct operating temperature, neither too hot nor too cool.

However when greater production speeds are contemplated, the rate at which heat is imparted to the tool increases, and some other form of cooling becomes necessary. Previous attempts at improving the rate of cooling have involved the use of pumped water or of refrigerated air, but in each case this has resulted in incorrectly controlled cooling.

In the case of refrigerated air, the problem was to provide a sufficient quantity of cooling, whereas with water the cooling could not be controlled adequately.

When the temperature of the plunger is too high, there is a tendency for the glass to stick to the plunger and this then produces spikes on the parison, which in turn creates problems in the later stages of blowing, and when too low the parison will deform or even collapse.

The general conclusion therefore has been that the current method of air cooling the pressing tool has to be retained owing to the criticality of temperature control, and this in turn provides a barrier to improving production speeds.

The present invention solves this problem by cooling the interior of the pressing tool with a controlled mixture of liquid nitrogen and air. The great advantage of using the liquid nitrogen mixture is that the mixture proportions can be varied to vary the degree of cooling in a very controlled manner.

This overcomes the problems of the previously attempted methods where difficulties in control of cooling arose.

Prior U.S Specifications 4 553 999, 4 652 292 and 4 708 730 propose using liquid nitrogen for various cooling purposes during the production process, and one suggestion is that the mixture is applied to the mould cavity in the so-called 'roughing' stage. This is to cool the parison directly, but it does at the same time result in some cooling of the external surface of the pressing tool. This however does not solve the problem because it does not achieve control of the temperature of the pressing tool. Also, it can only be applied during the time the cavity is open.

An embodiment of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which Figures 1 to 7 show schematically in side elevation the normal known stages of blow moulding a glass container using the press-blow method.

Figure 8 is a sectional view of a plunger and Figure 9 is a schematic diagram showing how a mixture of liquid nitrogen and air is fed to the apparatus.

Referring to Figure 1, a gob 1 of molten glass is fed into a pressing chamber 2, and a plunger 3 is arranged for pressing a parison from the gob.

Figure 2 shows the second phase of this pressing process where the supply of glass forming the gob has been closed off and the plunger is being introduced into the gob. Figure 3 shows the final stage of pressing the gob and the internal cooling of the plunger is shown.

Figure 4 shows a rotary carrier 5 which enables the formed parison to be turned over and moved from the pressing chamber to a blow moulding chamber 6.

Figures 5, 6 and 7 show how injection of a blowing medium via an injector 7 produces the final shaped article 8. Up to now what is described is perfectly standard. What now follows relates to the present invention.

Figure 8 shows a detail of the plunger 3. This includes an inner tube 9 carried by a screw-in base unit 10 within a base section 11 of the plunger itself. The composite of base units 10 and 11 of the plunger are screwed into a multi-head inlet manifold 12 shown in the diagrammatic drawing of Figure 9.

Referring now more specifically to Figure 9, the inlet manifold 12 for supplying cooling to the plunger and a similar inlet manifold 13 for supplying blowing medium to the blowing chamber 6 (see Figures 4, 5 and 6) are each supplied with a controlled mixture of air and liquid nitrogen. Each manifold is vacuum insulated.

A liquid nitrogen supply source LN (generally an insulated high pressure container) supplies liquid nitrogen via mixer valves M1 and M2 to each of the manifolds 12 and 13.

In each case temperature probes within the manifolds feed back a temperature reading to a temperature processing unit TC1, TC2 respectively which in turn feeds back a control signal to the mixer valves M1, M2 respectively.

An air supply under high pressure is fed via a combined filter and air drier FD to control valves C1 and C2 respectively and thence to each of the mixer valves M1 and M2 respectively. The control valves are to control the pressure of air supply to the respective mixer valves by reference to pressure gauges Pl and P2 respectively. Thus, the two control systems for providing mixture of liquid nitrogen and air to the two separate manifolds are virtually the same, however the adjustments for each have to be different. In the case of supply to the interior of the plunger 3 via the manifold 12, a pressure of the region of 20 to 40 psi is required whereas a slightly lower pressure of between 5 and 30 psi is required at the blow head manifold 13. To supply these pressures therefore, the air supply AS should be at a pressure of at least 45 psi.

In regard to temperature, the control valve controls the ratio of liquid nitrogen to air to achieve the required temperature. In the case of maintaining the required temperature of the plunger, the liquid nitrogen mixture should be controlled to enter the plunger at a temperature of approximately -400C whereas the blowing manifold 13 should be maintained at a temperature of approximately -600C.

In each of these cases it will be appreciated that the ambient temperature, that is to say the temperature of the air as it originates at the pressurised air supply system will-affect the required ratio of liquid nitrogen to air and in each case the temperature control feed-back and the mixer valve will ensure that the required temperature is maintained.

Claims (4)

1. A method of producing glass articles in which a gob of molten glass is introduced into a chamber and a pressing tool presses upwards into the gob to form a parison of glass, the pressing tool is removed from contact with the parison, and the parison while gripped at its outer edges is turned over and introduced to a blow molding station for blow molding of the final shaped article, characterised in that the interior of the pressing tool is cooled by provision of a controlled mixture of liquid nitrogen and air.

2. An apparatus for producing glass articles in accordance with the method of claim 1, comprising a parison forming chamber including a pressing tool for pressing upwards into a gob of molten glass and means for transferring the parison to a blow molding station, the pressing tool having internal passages to permit cooling by a mixture of liquid nitrogen and air.

3. A method of producing glass article substantially as herein described with reference to the accompanying diagrammatic drawings.

4. An apparatus for forming glass article substantially as herein described with reference to the accompanying diagrammatic drawings.