US 4,902,455 AGranted

Method and extrusion apparatus for degassing thermoplastic plastics material melts

AssigneeHermann Berstorff Maschinenbau GmbH

InventorHans Wobbe

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Abstract

A method of degassing thermoplastic plastic melts over a wide range of viscosities in a degassing extruder including a plurality of sequential degassing sections. Each degassing section includes at least one screw section having flights formed thereon. Each section includes a degassing region and a metering section downstream thereof. The height and length with which the flights are filled in the degassing regions in the individual degassing sections is controlled by the comparison of a pressure gradient measured in the metering region of that section with a desired pressure gradient and on the basis of this, the melt is conveyed with substantially the same throughput capacity in each degassing section. As a result of these measures, the degassing is effected in all of the degassing sections with optimally partially filled screw threads and with an optimum filling length of the worm threads, which is independent of the viscosity of the melt. The number of flights, the depth and width of the flights and the pitch of the flights may be identical in all degassing sections thus simplifying the manufacture of the screw.

Claims8

Dependent claims

A method of degassing a thermoplastic plastic material melt in a degassing extruder comprising a plurality of degassing extruder sections connected in series with one another, each said degassing extruder section including housing means, screw means rotatable within said housing means, and means for rotating said screw means, said screw means including core means having a periphery and flight means of a predetermined depth helically disposed around the periphery of said core means, said screw means defining, within said housing means, a degassing region and a metering region disposed downstream of said degassing region in each said section, said degassing region including degassing means, said degassing means comprising degassing port means formed in said housing, means for producing a reduced pressure at said degassing port means and means for conveying said melt within said housing past said degassing port means at a minimal extrusion pressure;

said metering region including means for producing an elevated pressure to be applied to said degassed melt and means for determining a pressure gradient in said metering region, comprising the steps of feeding said melt successively into and through each said section wherein said melt is degassed and subsequently subjected to said elevated pressure, determining said pressure gradient in each said metering region of each said section, comparing said measured pressure gradient, and controlling the height and length with which said flight means are filled with melt in each said degassing section in dependence upon said measured pressure gradient so as to cause said desired pressure gradient to be attained, the melt being conveyed at substantially the same throughput through each said degassing section.

A degassing screw extruder for degassing a thermoplastic plastic material melt comprising a plurality of longitudinally spaced degassing extruder sections, each section comprising housing means and screw means rotatable within said housing means, each said screw means comprising a core portion, having a periphery and flight means helically disposed around the periphery of said core portion, said screw means defining, within said housing, a degassing region and a metering region disposed downstream of said degassing region in each said section, said degassing region including degassing means, said degassing means comprising degassing port means formed in said housing and means for producing a reduced pressure at said port means and means for conveying said melt past said degassing port means at a minimal extrusion pressure, each said metering region including means for producing an elevated extrusion pressure to be applied to said degassed melt and at least two pressure measurement devices longitudinally spaced apart from one another for measuring the pressure of said melt in said metering region such that the pressure gradient between said measuring devices is determined, drive means for rotating said screw means in at least the upstream extruder section, means for adjusting the rate of rotation of said screw means, flow restrictor means disposed between two adjacent said extruder sections for controlling throughflow;

means associated with said flow restrictor means for adjusting the throughflow cross-section of said restrictor means, melt pump means disposed at the downstream end of said extruder, said melt pump means including a melt pump, drive means for rotating said melt pump and means for adjusting the rate of rotation of said melt pump means;
control means connected to said screw rotation rate adjustment means, to said throughflow cross-section adjustment means and to said melt pump rotation rate adjustment means;
said control means also being connected to each said pressure measurement device whereby said pressure measurements from each said metering section are transmitted to said control device and are compared therein with a desired pressure gradient whereby disparity between said measured and said desired pressure gradient in any said metering section causes said control device to regulate at least one of said screw rotational rate adjustment means, said free throughflow cross-section adjustment means and said melt pump rotational speed adjustment means, said screw means having a substantially equal number of flights, depth of flight and flight pitch in each said degassing extruder section.

Dependencies are inferred from the claim text.

Facts

Application: 07/290,488Filed 27 Dec 1988
Priority: DE 374419324 Dec 1987
Granted: 20 Feb 1990
Term ends: Not estimatedThe source records don’t support an estimate.

Classification25 codes

B29C 48/387
Plasticisers, homogenisers or feeders comprising two or more stages: using a screw extruder and a gear pump
B29B 7/421
With screw or helix: with screw and additionally other mixing elements on the same shaft, e.g. paddles, discs, bearings, rotor blades of the…
B29B 7/429
Parts or accessories, e.g. casings, feeding or discharging means: screws
B29B 7/726
Measuring, controlling or regulating: measuring properties of mixture, e.g. temperature or density