This invention is concerned with a blowhead for use in the manufacture of glass containers for blowing air through a neck opening of a glass parison positioned in a cavity of a mould to cause the parison to expand to the shape of the cavity.
In the manufacture of glass containers, a gob of molten glass is first formed into a parison either by a blowing or by a pressing operation and the parison is then transferred to a cavity of a mould having the shape which it is desired that the container should have and air is introduced into the parison to cause it to expand to the shape of the cavity. The air is supplied by means of a blowhead which comprises a body defining an air supply passage through which, when the blowhead is positioned on the mould, air can enter the parison, and an exhaust passage through which air can exhaust from the parison.
In some types of conventional blowheads, the exhaust passage remains unobstructed throughout the blowing operation so that air can enter the parison and pass out through the exhaust passage throughout the blowing operation. This enables heat to be extracted from the interior of the parison by the air flowing therethrough and, therefore, reduces the time during which the glass must remain in the mould cavity. However, the ratio of the cross-sectional area, transversely to the direction of air flow, of the air supply passage to the exhaust passage is critical to an efficient blowing operation. This ratio has to take account of the increase in the volume of the air in the parison due to the temperature rise experienced by the air and also of the pressure that it is necessary to create in the parison to enable the blowing to take place. In conventional blowheads, the ratio between the aforementioned areas is fixed and cannot be adjusted to suit varying operational conditions. In another type of known blowhead, a valve is used to keep the exhaust passage closed throughout the blowing operation and the valve is then opened to allow a separate cooling operation during which air flows through the air supply passage around the inside of the blown parison and out through the exhaust passage. However, this type of blowhead has the disadvantages that a longer period is required in the mould as the blowing operation is followed by a cooling operation and also that the blowhead is more complex because of the need to provide a valve.
It is an object of the present invention to provide a blowhead in which the ratio between the areas transversely to the direction of air flow, of the air supply passage to the exhaust passage can be adjusted.