Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Differential temperature conditioning
Patent
1989-01-17
1991-02-19
Heitbrink, Jill L.
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Differential temperature conditioning
2642105, 264550, 425325, 4253261, 425384, B29C 5142
Patent
active
049942298
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to the general art of extruding thermoplastic material in a sheet form, then directly feeding and forming this hot plastic sheet material into hollow objects, such as food containers.
Such processes in general require a device for extruding the thermoplastic in sheet form, a set of temperature controlled tempering rolls to control the thickness of the sheet web and to reduce its overall temperature to the desired forming temperature, a device to transport the molten, often sagging web into a forming machine and a device to trim out the finished parts from the web, after these are formed and stabilized.
The range of materials which can be extruded in sheet form and used in the method of invention generally includes almost the whole known range of thermoplastics, and combinations thereof, either blended or simultaneously co-extruded in discrete layers by multiple extruders feeding into a single die.
In the application of this invention we are particularly, but not exclusively, interested in those materials which have melts which behave more as viscous fluids than as rubbery membranes. Typically, crystalline polyolefines such as High Density Polyethylene and Polypropylene have a sharply defined melting point and a melt rheology resembling a highly viscous fluid. Such materials sustain stress primarily by viscous resistance, and therefore sag or creep, when suspended as a sheet without full support. The cohesive elasticity which materials such as PVC and Polystyrene exhibit in their molten state, make them relatively easy to feed into thermoforming equipment and these have traditionally been the preferred materials for thermoforming.
Recently however, the advent of the high oxygen barrier polymers such as Ethylene Vinyl Alcohol (EVOH) Polyvinylidene Chloride (PVDC), has given rise to a new class of food packaging, wherein food is packed and sealed into a plastic package and sterilized in steam retorts, in much the same way as metal cans, at retorting temperatures up to 140.degree. C. Polypropylene is one of the few readily available resins with the relatively high temperature resistance necessary to withstand steam serialization. It is often combined, usually by co-extrusion, with a layer of the aforementioned high barrier plastic, to produce the base material for high barrier plastic packaging.
Other forms of fabrication, such as injection molding, are not suited to economic production of multilayer hollow containers and the relatively mature art of thermoforming is therefore currently undergoing a developmental transformation aimed to achieving economic means of thermoforming retortable polypropylene-based high-barrier containers.
We have found that it is difficult to reheat and then thermoform pre-extruded polypropylene sheet. The melt sag which occurs immediately after the material passes through its crystalline melting point makes it very difficult to heat a suitably sized area of suspended sheet by known means, such as infrared radiation, and then to feed such a sheet of sagging molten material into a thermoforming machine of normal commercial size and output. Many of the current thermoforming process operators have found that they can often achieve reasonable results by forming polypropylene just below its crystalline melting point. This so-called solid phase forming usually leaves residual stress in the walls of a finished container and results in unsightly distortion when this stress is released during sterilization. A further problem of solid-phase forming is that the melting points of the commonly used EVOH and PVDC resins are higher than that of polypropylene, and a thin barrier layer of this material can be relatively easily damaged during forming at the solid-phase forming temperature of polypropylene.
Hence, we have developed a process and method by which polypropylene may be transported and formed in the so-called melt phase. Extruding the melt directly into a forming process overcomes many of the difficulties of reheating premade polypropylen
REFERENCES:
patent: 4105386 (1978-08-01), Thiel et al.
patent: 4150930 (1979-04-01), Asano et al.
patent: 4235579 (1980-11-01), Kurz et al.
patent: 4459093 (1984-07-01), Asano
patent: 4722820 (1988-02-01), Flecknoe-Brown
Heitbrink Jill L.
Hitek Limited
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