Plastic and nonmetallic article shaping or treating: processes – Vacuum treatment of work
Patent
1990-03-21
1992-08-18
Silbaugh, Jan H.
Plastic and nonmetallic article shaping or treating: processes
Vacuum treatment of work
2643289, 26432815, 26432816, 264537, 425533, 425548, 425549, 425552, B29C 4503, B29C 4520, B29C 4572
Patent
active
051397242
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for injection moulding blanks, especially preliminary blanks suitable for inflation moulding. In particular, the invention relates to a method in which a crystallizable synthetic moulding compound, which is first heated to a temperature T.sub.H in the vicinity of the crystalline melting temperature T.sub.M, is pressed from a heated injection moulding nozzle through a gate opening into a cold mould. The method is especially useful for moulding preliminary blanks suitable for inflation stretching from chilled crystallizable synthetic moulding compounds in association with a coolable moulding apparatus having a heatable nozzle consisting of a body and a tip, the tip comprising a nozzle mouth, the coolable mould forming a mould cavity consisting of an outer mould and an inner mould, the outer mould comprising a gate opening, and the nozzle mouth and gate opening being substantially in alignment with each other.
PET (polyethylene terephthalate) material is very sensitive to processing, especially to processing temperatures. If the processing temperature is too low, crystalline zones occur, giving rise to streaks in injection moulded blanks, especially when the blanks are inflation stretched. On the other hand, if the temperature is too high, an undesirable amount of acetaldehyde is produced, and if PET inflation stretched containers are used for delicately flavoured food products, the acetaldehyde impairs the flavour of such products.
The special properties of crystallizable synthetic moulding compounds known to the expert will be dealt with hereinafter, using PET as an example, as far as they are essential to the present invention, in order to clarify problems arising during the processing of this material.
PET is a thermoplastically processable polymer which is very hard and rigid at room temperature. PET is, therefore, a preferred material for thin walled bottles. There are essentially three basic types of polymers: a) amorphous polymers with a completely random arrangement of molecule, such as PVC or glass, characterized in that they become softer and softer with increasing temperature; b) so called "crystalline" polymers whose molecules are carefully aligned and which, therefore, have a relatively sharp melting point, such as polybutylene terephthalate, linear polyethylene, or ice; and c) so called "crystallizable" polymers which may be either amorphous or crystalline at room temperature and in which the degree of crystallinity can be controlled. PET pertains to this last group.
It is typical of PET that it exhibits "crystallinity" only at a specific temperature above which leads to milkiness. This temperature range is defined at its lower limit by the minimal temperature for crystallization by heating (T.sub.CH), at its upper limit by the maximal temperature for crystallization by cooling (T.sub.CC), and at its between glass transition temperature (T.sub.G) and melting temperature (T.sub.M). A typical glass transition temperature is about 80.degree. C., and a typical melting temperature is between 250.degree. and 255.degree. C. The rate at which crystallinity forms is temperature dependent and is very slow, both in the vicinity of the minimal temperature T.sub.CH and of the maximal temperature T.sub.CC.
A detailed description of the behaviour of PET may be found in "A Layman's Guide to PET Chemistry and Processing", EDWARD E. DENISON; 4th Annual International Conference on Oriented Plastic Containers; Mar. 25, 1981.
In casting PET blanks, and especially preliminary blanks for subsequent inflation stretching, it is usual to inject the PET material, which has been rendered pressure-conveyable by heating to a suitable temperature, through a nozzle and the gate opening in the outer mould into a mould. As soon as the mould is filled, the nozzle, or the passage between the nozzle and the mould, is usually closed by a mechanically operated element, such as a nozzle needle. Thereafter, the blank, usually with the countermould, is lifted from the outer mould and gate openi
REFERENCES:
patent: 4213751 (1980-07-01), Fernandez
patent: 4378963 (1983-04-01), Schouenberg
Injection Molding Handbook, Rosato, editor, 1988, pp. 292-297.
Fernandez Luis
Hofstetter Otto
Otto Hofstetter AG
Silbaugh Jan H.
Timm Catherine
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