Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Composite article making
Reexamination Certificate
1998-09-15
2001-07-17
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Composite article making
C264S045400, C264S054000, C264S255000
Reexamination Certificate
active
06261490
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to rotational moulding and other casting processes. More specifically, but not exclusively, the invention relates to improved methods of producing a lightweight rigid, foamed or cellular thermoplastic article by the process known as rotational moulding, roto-moulding or rotational casting, using one-shot rotational moulding methods.
BACKGROUND OF THE INVENTION
Using known rotational moulding techniques, moulded articles can be produced having a solid or unfoamed “skin” or envelope and a cellular interior, the latter having a density in the region of 0.15 g/cm
3
. The composite structure is rigid and of lower overall density than many common woods, and can be used for the manufacture of chemically resistant pallets, boxes, containers and similar goods.
Typical examples of plastics materials used in the production of hollow articles having cellular interiors produced by a rotational moulding technique are polyethylene as the wall, envelope or skin material of the article, and polyurethane as the foam. Polyethylene has useful chemical resistance and toughness and polyurethane foam is rigid and provides the desired degree of stiffness.
There is disclosed in CA 983226 (Du Pont) a method of rotational moulding of polyolefin articles having a foamed inner layer and a substantially solid skin. Molded articles are made in a one-step rotational moulding process using a mixture of powdered non-foamable ethylene polymer, which forms the outer skin, together with a foamable ethylene polymer in pellet form, which forms the foamed inner layer.
In the process of the Canadian patent, the particle size difference as between the foamable pellets and the powdered non-foamable polymer enables a separation of these materials to be achieved during the rotational moulding process so that the moulded product has the necessary solid outer envelope and associated foamed lining.
We have conducted tests of the process described in the Canadian patent and these show clearly that in practice only very thin and uneven skins or envelopes are achieved when using the conditions described in the Canadian patent. These very thin skins have occasional thick spots, but the inadequacy of the skin thickness is such that the process is not really practical, and this is maybe the reason why this one-shot process has never been commercially exploited. Further, the foamed material does not completely fill the moulded product and instead only forms a relatively thin lining on the interior surface of the skin, which limits the effectiveness of the material. Furthermore, we have found that if more of the foamable polymer is added in an attempt to fill the mould, the powder and the pellets do not separate properly, resulting in even poorer skin formation.
We have found that in a one-shot rotational moulding process it is not sufficient merely to provide the two main components of the mould charge with differing particle sizes. While such difference in particle size can achieve a degree of separation of the components to enable formation of the impermeable outer envelope and the foamed material within it, in the absence of other contributory factors as defined below, the process as described in the Canadian patent does not achieve an acceptable wall or envelope thickness.
GENERAL DESCRIPTION OF THE INVENTION
To achieve such an acceptable wall or envelope thickness, we have ascertained that it is necessary to provide two distinct temperature stages in the process, for the materials within the mould. First, there needs to be an initial phase in which the in-mould temperature is high enough to melt the powder of the plastics material which forms the skin or envelope, but is not sufficient to melt the large particles or pellets. Then, there needs to be a second phase in which the in-mould temperature is increased to melt and cause foaming of the large particles or pellets of foamable polymer.
A further contributory factor, which assists in achieving good results, is to increase the rate of rotation of the mould during the initial phase to a higher rate than normal (for example to 15 rpm) so that the large particles or pellets do not stay in one position in the mould long enough to adhere to the melting powder. During the second phase, the rate of rotation may be reduced to a more usual rate of, for example, 5 rpm.
When the skin or envelope has formed in the above-described initial phase, the mould temperature can be increased to that needed to cause the pellets or particles of foamable material to melt and expand so as to fill or partially fill the envelope within the mould.
Our experiments have shown that without the initial lower-temperature envelope-formation phase in the moulding operation, an envelope could not be obtained which had a consistent thickness greater than 1 mm. For most applications of rotational moulding, such as the production of pallets, floats, tote boxes, fish bins etc., a minimum envelope thickness of 3 mm is required and thicknesses up to 6mm are usually needed for the more mechanically demanding applications.
By providing a process wherein the initial envelope formation stage is conducted at a lower temperature than the subsequent foaming stage, the embodiments are able to provide a moulded product having a uniform envelope or wall thickness.
Our test work has shown that the temperature required in the initial envelope or skin formation stage is important and should be in the range defined by the melting point of the plastics material which forms the skin or envelope and a temperature of 10° C. above that melting point. For example, in the case of polyethylenes, the melting points are around 120°C. Our test show that if the temperature rises significantly beyond this range, there will be a reduction in skin thickness.
At this lower temperature, the skin or envelope forms typically at a rate of approximately 1 mm of thickness per 10 minutes of moulding time. It appears that for a particular particle size and material, the rate of skin thickness formation cannot be readily increased as it is dependent upon the melting point of the polyethylene and the thermal conductivity thereof.
The temperatures selected in any given case need to take account of the following additional factors in relation to the mechanism of skin formation. Thus, the temperature in the initial envelope-formation phase should be such as to avoid as far as possible the likelihood of the particles of the envelope-forming plastics material melting right through from one side to the other on contacting the hot mould surface, and thereby forming sites at which the larger particles of the foamable second plastics material can adhere thereto and thus be trapped in or on the envelope or skin so that when the subsequent higher temperature foam-forming phase commences, there is produced a blowhole in the envelope of the moulded article, which will render it commercially unacceptable.
Our test work has shown that although the difference in particle size between the skin-forming component and the foam core-forming component is not sufficient on its own to ensure good separation of the two components and therefore good skin formation, it is important. We have found that for the skin-forming component a ground non-foamable polymer powder of maximum particle size 500 to 600 microns is suitable, whereas for the foam core-forming component foamable polymer granules having maximum dimensions in the range 3mm to 5mm are most suitable.
By employing comparatively large granules, preferably pellets, of the foam-producing material the result is achieved that these pellets are able, by virtue of their comparatively large size, to roll around within the hollow interior of the partly-moulded article, and indeed to pass through relatively narrow restrictions inherent in the shape thereof, so as to achieve a substantially uniform distribution. This uniform distribution is due, not least, to the fact that the larger particle size has the result that a slower melting and slower corresponding increase in tackiness of the
Kuhns Allan R.
Merchant & Gould P.C.
Rotec Chemicals Limited
LandOfFree
Rotational moulding does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Rotational moulding, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rotational moulding will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2458352