Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Including application of internal fluid pressure to hollow...
Reexamination Certificate
2000-06-22
2004-04-27
Davis, Robert B. (Department: 1722)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Including application of internal fluid pressure to hollow...
C264S550000, C264S554000, C425S326100, C425S387100, C425S526000, C425S529000, C425S533000
Reexamination Certificate
active
06726873
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of forming an article via injection of plastics material into a mould.
BACKGROUND OF THE INVENTION
Various methods of forming thermoplastics materials into articles are known:
1. Injection moulding involves injection of the material into a mould, which is filled. The material solidifies in the mould and takes up its shape. The general rule is that the wall thickness of the article, that is the gap between opposite parts of the mould should be uniform. There is a practical limitation on the wall thickness, in that it is difficult to mould walls thinner than the order of 0.3 mm. With thinner wall thickness, the mould becomes very difficult to fill completely and reliably. This is because a long thin passage for the material exists from the injection port into the mould to the opposite end of the article. The so called flow length: thickness ratio is too high. The result is that injection moulding is impractical and/or uneconomic for certain classes of product, such as vending machine cups.
2. Injection/blow moulding is a process whereby the wall thickness of an article initially formed by injection is reduced by blowing the article to a large diameter—or other cross-sectional dimension. In an injection/blow moulding process, the initially formed article—the preform—is indexed from the original cavity to another cavity into whose shape it is to be blown. The process of indexing, coupled with adequate cooling of the preform before opening of the initial cavity away from the core results in the preform being likely to be too cool for blowing immediately on introduction into the second cavity. Further, cooling of the preform gives it stability for the indexing. After indexing, a delay is necessary before blowing with gas pressure at the core side, to allow for heat soak from the core to warm the preform again to a plastic state. Consequently, injection/blow moulding is a relatively slow process, but nevertheless widely used for bottle production.
3. An alternative, especially for wide mouth containers is for the material to be thermoformed. In this process, an extruded sheet is peripherally gripped and blown into a cavity. The process stretches the material and a wall thickness less than that practical with injection moulding can be achieved. Usually the result is an uneven wall thickness, with a thicker wall in a base of the article and a thinner wall in its sides. Further there is liable to be wastage at the periphery and in gaps between successive articles formed from the extrusion.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an improved method of forming a plastics article.
According to a first aspect of the invention there is provided a method of forming a plastics material article, consisting in the steps of:
forming an injection moulded preform between a pair of complementary mould parts, which define an initial mould cavity for injection of the preform,
separating the one of the complementary mould parts (hereinafter “the Substitutable Mould Part”) from the preform,
replacing the Substitutable Mould Part with one or more replacement mould parts (hereinafter “the Replacement Mould Part(s)”) to enlarge the mould cavity so as to allow stretching of the preform,
stretching at least part of the preform away from the other of the complementary mould parts (hereinafter “the Base Mould Part”) for the forming of it to a finished article shape against the Replacement Mould Part and
removing the Replacement Mould Part(s) for release of the finish formed article.
Normally, the Base Mould Part will be a core part and the Substitutable Mould Part will be a cavity part. However, it can be envisaged that these two parts could be two identical parts to which the core and cavity terminology would not be appropriate.
The method of the invention differs from the conventional injection/blow moulding process in that the mould parts are not indexed as such. In an indexing mould tool, the preform is withdrawn from the initial cavity and moved to the second cavity, whilst simultaneously another core part is introduced into the first cavity part. Injection and blowing occurs simultaneously at the two cavities. Whilst this may seem to have advantage in producing a finished article for each indexing step, the cycle time is determined by the time required for cooling the preform, indexing and reheating the plastics material to plastic state after cooling for indexing. In the method of the invention, the first cavity mould part will normally be separated early, the second cavity assembled without delay and the preform stretched quickly. It is anticipated that the cycle time will be of the order of 2.5 seconds, in comparison to a typical 5.0 seconds cycle for conventional injection blow moulding.
As explained in more detail below, the Replacement Mould Part(s) may be a plurality of individual pans assembled to form the enlarged cavity or a single Replacement Mould Part for the or each enlarged cavity, the part being moved bodily into position. It is anticipated that the former arrangement will be faster in terms of bringing the second cavity around the preform, but the overall cycle time with the latter arrangement is likely to be quicker, especially where final cooling and/or temperature stabilisation for crystallisation occurs in the enlarged cavity after removal of this from the core.
Whilst the degree of stretching may be greater or less, the stretched portion of the preform will normally be stretched by between a factor of 2:1 and 4:1.
A significant advantage of the invention is that it results in controlled bi-axial orientation of the article or at least its blown portion. The stretching involves axial strain and orients molecules of the plastics material in the direction of stretching. The blowing, involving as it does radial expansion hence circumferential strain, strains the material orthogonally to the axial strain. Hence the bi-axial orientation.
In the method of the invention, the preform is unlikely to move transversely of the machine, although due to the construction of the tool, it is likely to move axially, particularly where injection has been via an injection gate in the Substitutable Mould Part and where the mould tool is a multi-impression stack tool. The preform remains in contact with the Base Mould Part at a temperature suitable for plastic extension until after separation. Further, because there is axial movement only of the preform, the Substitutable Mould Part can be removed early, as soon as the plastics material has been cooled, preferably mostly by this part, to a sufficient extent to hold its shape. The material skins against the Substitutable Mould Part at least prior to withdrawal of the latter. However, the material is unlikely to be solid throughout its thickness (which, it should be remembered is about to be reduced) with the central region of the wall remaining hotter and more flexible. Thus heat soak is able to occur into the skin rendering the entire wall sufficiently flexible to stretch into the Replacement Mould Part.
Normally the method will include injection of gas between it and the Base Mould Part to separate at least part of the preform from the Base Mould Part. The stretching of the preform, or at least part of it, can also be effected by injection of gas between the preform and the Base Mould Part.
Preferably, the stretching of the preform is effected by lifting a portion of the preform from at least part of a main piece of the Base Mould Part by a movable piece of the Base Mould Part. Normally the lifted portion of the preform will be moved by the movable piece of the Base Mould Part as far as a corresponding piece of the Replacement Mould Part(s), and the lifted portion of the preform will be captivated between the movable piece of the Base Mould Part and the corresponding piece of the Replacement Mould Part(s) at the end of the stroke of the movable piece. This has the advantage of enabling moulded features to be reproduced in the base—or equivalent part—of the articl
Coraltech Limited
Davis Robert B.
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