Patent Application
20080191394
This invention relates to a method and apparatus for blow moulding bottles and similar hollow articles.
The manufacture of bottles, containers and similar blow moulded articles is an enormous business, with millions of articles being produced per day. Even a small energy reduction per article can result in a considerable overall cost saving.
To achieve satisfactory expansion of many blow moulded articles it is necessary to achieve blowing pressures of around 40 bar, and achieving the required amount of air compression consumes large amounts of energy. When blow moulding large articles partial expansion is sometimes carried out at a relatively low pressure in order to achieve a suitable distribution of material within the finished article, but the main part of the expansion process is normally carried out at high pressure, resulting in considerable energy consumption.
U.S. Pat. No. 4,830,810 describes a method of blow moulding which involves fluorinating plastic containers. During the expansion phase a first blow moulding stage is carried out with nitrogen up to 1.5 bar followed by a second, more rapid, expansion stage up to 4 bar. There is then a short pause to check for leaks, which is followed by a third expansion stage in which pressure is increased progressively by introduction of fluorine up to 10 bar. Apart from stage 2, the three expansion stages generally involve a steady increase in pressure.
U.S. Pat. No. 5,543,107 describes a process for blow moulding a closed plastic drum. Again, the blowing operation is carried out in three stages, but the first primary expansion stage is performed at 30-60 psi, the second stage is performed at a lower pressure of 15-40 psi to stabilise the article, and the third stage is carried out at a higher pressure of 90-110 psi.
U.S. Pat. No. 5,622,735 relates to blow moulding of bottles. Although this involves three stepwise expansion stages there is an apparent pressure drop between the first and second stages.
The present invention seeks to provide a new and inventive method of blow moulding articles, and apparatus for carrying out the method, which can produce a significant reduction in energy useage.
The present invention proposes apparatus for blow moulding articles from thermoplastic material, which includes a mould defining a mould cavity for a heated thermoplastic parison, and control means arranged to operate such that, in an expansion phase, air is sequentially admitted to the mould cavity in a first expansion stage at a low moulding pressure, at least one intermediate expansion stage at an intermediate moulding pressure, and a final expansion stage at a high moulding pressure, in turn, to expand the parison and thereby produce a moulded article conforming to the internal shape of the mould cavity,
In this context it should be noted that “stepwise manner” means that a relatively rapid change in pressure is followed by a period during which there is a relatively slow or insignificant change in pressure.
The invention also provides a process for blow moulding articles from thermoplastic material, which includes:
The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings:
The duct 9 leads to four air accumulators 10, 20, 30 and 40. The internal storage volume of each accumulator is about one order of magnitude higher than that of the mould cavity 3. Thus, a mould which is designed to produce a 1 litre container will have accumulators of approximately 10 litres capacity. Although four accumulators are described in this example the number of accumulators may vary. For example, where the nature of the thermoplastic and the shape of the article being moulded allow lower blowing pressures to be used only three accumulators may be used, whereas higher blowing pressures, or a requirement for maximum energy efficiency, may make it economical to provide five or more accumulators. Each accumulator is connected to the air duct 9 via a respective control valve, 11, 21, 31 and 41. In addition, the accumulators are supplied with compressed air via a respective compressor 12, 22, 32 and 42. A vent valve 50 allows the mould cavity to be vented to atmosphere when required.
The valves, compressors and the ram 7 are all operated by a programmable electronic controller 52 which has overall control of the moulding process. The control device can also monitor the pressures within the four accumulators by means of respective pressure sensors, 13, 23, 33 and 43.
The moulding process is illustrated in
In stage 56 the parison 4 is introduced into the mould and the vent valve 50 closed at 57. In the blowing phase 58 the first valve 11 is opened to admit air pressure from the first accumulator 10 to the mould cavity via the duct 9. The curve 70 of
When the shape of the parison has stabilised within the confines of the mould cavity the recovery phase begins (stage 59,
Over several moulding cycles the pressures within the accumulators 10 to 30 is maintained at the approximate starting figures. The pressure in the final accumulator will fall slowly, but this is replenished in due course by the compressor 42 (stage 60). The efficient recovery of moulding pressure which is thus achieved results in a significant energy saving.
The power required to initially fill the accumulators is significantly less than the power required to blow a pre-expanded parison in a single expansion stage. This is demonstrated by the following rough calculation of the energy used:
10 litres@10 bar=100 units
10 litres@20 bar=200 units
10 litres@30 bar=300 units
10 litres@40 bar=400 units
In contrast, providing 40 litres of air @ 40 bar requires 1,600 units of energy.
The use of separate compressors is desirable in order that the efficiency of each compressor can be optimised for the target pressure, but it would also be possible to use a single compressor in combination with a suitable valve system arranged to permit the compressor to fill each of the accumulators to the required pressure.
The pressures and volumes stated in the above description are purely exemplary.
It will be appreciated that the features disclosed herein may be present in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.
