Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting
Patent
1992-06-04
1994-06-21
Aftergut, Karen
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
264 406, 264310, 425143, 425144, 425150, 425170, 425439, B29C 3302, B29C 3340, B29C 4104
Patent
active
053226542
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to apparatus for and a method of rotational moulding.
Rotational moulding is a processing method for producing hollow articles made of synthetic plastics material. It consists of three basic stages. Initially, cold plastics powder (or granules) is placed in a sheet metal (or cast alloy) mould. The mould is closed and is rotated simultaneously about two perpendicular axes. Whilst rotating the mould moves into an oven and as the metal surface becomes hot, the plastic in the mould starts to melt and coat the inside surface of the mould. When all the plastics powder has melted and a uniform coating has formed on the mould, it is taken out of the oven and, still rotating, it is cooled by an air and/or water spray. When the plastics has solidified the mould is opened and the moulding is removed.
The rotational moulding technique has been used commercially for about 30 years but there are a number of practical problems. Firstly, it is very difficult to know when the powder has melted. This is usually estabished by trial and error and results in a considerable amount of wasted material. This is because the heat transfer characteristics can vary depending on the type of oven used to heat the mould, the ambient conditions, the nature of the powder, the type of plastics, the wall thickness of the moulding, the material and thickness of the mould etc. Also, even when the powder has melted it is difficult to know how long the mould should be left in the oven to get optimum properties from the moulded article.
According to one aspect of the present invention, there is provided a rotational moulding apparatus comprising a mould, means for rotating the mould and means for measuring the temperature of the medium inside the mould but outside the material being moulded in order to provide a means for controlling the moulding process.
According to another aspect of the present invention, there is provided a method of rotational moulding comprising the steps of introducing moulding materials into the mould, heating and rotating the mould and monitoring the temperature of the medium inside the mould but outside the material being moulded and controlling the operation of the moulding machine in dependence upon the temperature monitored.
In a preferred embodiment, variation of temperature with time is plotted and three unique points on that plot are determined respectively representing when al the plastics material is melted, when the moulded article has achieved its optimum properties and when demoulding can take place. The first point is located at and identified by means of a discontinuity in the heating up part of the temperature time characteristic. The second point is determined by testing the properties of moulded articles at temperatures above the fusion temperatures for different moulding materials. In this way, the second point on the characteristic at which these properties are achieved can be accurately predicted, and by monitoring the temperature, articles having these properties can be produced. The third point is located at and identified by means of a discontinuity in the cooling down part of the temperature - time characteristic.
Temperature is monitored inside the mould by any appropriate temperature monitoring device. The device can be connected to a processor which can in turn be used to control the moulding process. If desired, the device may emit a continuous signal so that an instant temperature readout is obtained. Alternatively, a data logger for logging the temperature may be used in place of the monitoring device and processor, but this would not provide an instantaneous read out.
In order that the invention may be more clearly understood, one embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 shows a diagrammatic view of rotational moulding apparatus according to the invention, and
FIG. 2 is a graph of the variation of temperature with time inside the mould of the apparatus of FIG. 1.
Referring to FIG.
REFERENCES:
patent: 3796533 (1974-03-01), Vox
patent: 4217325 (1980-08-01), Colby
patent: 4235577 (1980-11-01), Groleau
patent: 4371483 (1983-02-01), Mattson
patent: 4422987 (1983-12-01), Arimatsu
patent: 4455268 (1984-06-01), Hinrichs et al.
patent: 4589072 (1986-05-01), Arimatsu
patent: 4725389 (1988-02-01), Hahn et al.
patent: 4740337 (1988-04-01), Gale et al.
patent: 4800050 (1989-01-01), Hahn et al.
patent: 4810438 (1989-03-01), Webster et al.
patent: 4933126 (1990-06-01), Shuman
patent: 5038709 (1991-08-01), Yamada et al.
patent: 5055245 (1991-10-01), Hisatomo et al.
patent: 5194197 (1993-03-01), Munk
Crawford Robert J.
Nugent Paul J.
Aftergut Karen
The Queen's University of Belfast
LandOfFree
Rotational moulding apparatus and process 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 apparatus and process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rotational moulding apparatus and process will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2219243