DE2120601A1 - Process and cascade screw press with vacuum chamber for processing and degassing powdered thermoplastics - Google Patents

Description

Bag.

h ar in ag

Banner Maschinenfabrik Aktiengesellschaft

Wuppertal

Process and cascade screw press with vacuum chamber for processing and degassing powdered thermoplastics

The invention relates to a method and a screw press in a cascade arrangement for processing and degassing powdery thermoplastics, wherein the powdery feed material in a first cascade stage compressed, conveyed into a vacuum chamber, degassed here and then in a second cascade stage is plasticized, mixed and pressed.

It is known from DT-OS 1 ^ 54 920, powdery Process and degas thermoplastics in a screw press of the cascade design. The well-known Cascade screw press consists of a first screw press with a feed hopper as a conveying device and a second screw press as a snap-on press, a hermetically sealed vacuum chamber being provided between these presses. The vacuum chamber has a valve assembly at the outlet of the first screw press and also a Lvakuierii! <rs device on.

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212060Γ

According to the known process technology, the powdery feed material is first placed in the first cascade stage conveyed and compressed against the pressure force of the valve. As an essentially powdery material, that is compressed, it leaves the outlet of the first screw press and passes through the opened valve in the vacuum chamber. This is where air, moisture and other volatile components become available to it withdrawn. In the further cascade stage downstream of the vacuum chamber, the degassed, im essential powdery material melted, plasticized, mixed and pressed.

The first cascade stage of the known screw press is dimensioned relatively short and the Function according to a pure conveyor device that is not heated. As a result, only the Heat that is generated due to the friction between the worm and the worm housing, npnnoch it cannot be avoided that as a result of the compression process :. res, at which additional heating is effective the powdery material sometimes forms lumps in small amounts. This in turn interfere with the proper functioning of the valve. As a result, there are then differences in terms of the amount of material and pressure before. This influences the degassing effect and different particle sizes lead to uneven feed and pressure fluctuations in the screw press of the second Cascade level. In addition, powder residues and dust can settle on the valve, which are under the influence the temperature at the press outlet and the steam temperature decompose.

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To eliminate these disadvantages, in many cases one is complicated in structure and mode of operation pressure-dependent valve control device required. Since the degassed material is also essentially powdery is present, the second cascade stage is for the purpose of drawing, conveying and also melting des llmaterials preferably designed as a twin screw press. In particular, feeding and pulling in powdered plastics in a screw press of any type is always with considerable difficulties tied together.

Twin screw press types are known per se for processing powdered thermoplastics, and Although with or without a degassing device, these twin screw presses are disadvantageous in terms of operational safety and investment costs, whereby the products obtained are unsatisfactory in terms of quality, because air and moisture components are insufficiently extracted due to the chosen process technology Melt are to be removed.

The aim of the invention is to feed the feed material in the first cascade stage in the sense of a processing operation to treat, to the extent that it is below the melting temperature Temperature at the end of this cascade stage, due to which it is present in a form in the vacuum chamber, on the one hand a cheap and intensive degassing and on the other hand a cheap, uniform one Moving into the second cascade stage is guaranteed. Associated with this is a gentle one Processing with a view to the least possible damage to the melt and an improvement in the

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Throughput / time unit at the same time cheaper Drive power compared to the known.

This aim is achieved according to the invention in that the feed material is agglomerated in the screw press of the first cascade stage and in the gelled state at a point between the softening point and the melting point lying temperature at the end of the first cascade stage and thereby in approximately more evenly porous-granular, crumbly (crumbling) or flaky form transferred into the vacuum chamber, at a The temperature lying between the softening point and the melting point is degassed by evacuation and then degassed in the same form that is available from the cutting, pulled into the screw press of the second cascade stage, melted, plasticized, mixed, is homogenized and pressed.

Due to these proposed procedural steps continuously a grainy, crumbly, or flaky one Intermediate product produced in almost the same form, which is favorably degassed and from the press the second Cascade stage is drawn in favorably. This also gives a relatively small one Temperature-time product in the second cascade stage. V takes place against this temperature-time product The least possible damage to the finished product and thus, including the previous favorable one Degassing, high product quality.

The method according to the invention can be used advantageously when the feed material is PVC dry blend. For this thermoplastic material, the preferred temperature of the agglomerate is in the range between 30 ² SO ⁰ C, preferably 40 ° -6O ° C,

20 $ 8.48 / 0.8

- ε * -

below the melting temperature.

The method according to the invention is also advantageous for PVC dry blend processing, if that Agglomerate in the form of porous grains, crumbs or flakes for a residence time of 5-60 s, is preferred 20-30 s, degassed in the vacuum chamber and then processed further.

The shape of the grains, crumbs or flakes can be achieved by squeezing one or more Agglomerate strands at the outlet of the first cascade stage be mechanically divided.

The method according to the invention is also advantageous because the material state of the agglomerate can be predetermined arbitrarily because the degassing phase is highly effective because the material surface of the present fragmented intermediate product is porous and large and it can therefore be degassed inexpensively and because the shape of this intermediate product is still favorable for even drawing into the Second cascade stage screw extruder. As a result of the intensive degassing, there is an improved To achieve quality compared to known process technologies. There is a gentle melting in Stages at relatively low temperatures and low back pressure, with a high throughput / unit of time is achieved with relatively low shear.

The screw press in a cascade arrangement with an interposed vacuum chamber for carrying out the process according to the invention consists essentially of a screw press forming the first cascade stage with single screw and 6-12 D effective screw housing length, with two systems in the press outlet

of projections are provided, which are arranged axially adjacent and are rotatable relative to one another » a vacuum chamber known per se between the cascade stages and one forming the second cascade stage Screw press with single screw and 1O-14 D effective screw housing length.

The one system of projections is preferably a component the screw, in particular the screw tip, and the further system of projections is fixed to the housing arranged. The projections can be present as cams, pins, webs or comparable means. One system has the task of forming passages for the agglomerate and the other system has that Task to break up this agglomerate. These two systems make one structurally and functionally simple and effective device formed, whereby the interaction of the systems in a simple way and form one or more agglomerate strands generated and these are divided in the desired form.

α Through the combination of the named construction features of the Cascade screw press gives relatively short residence times, and they are proportionate high speeds with low drive power possible, the degassing phase takes place without difficulty and favorable, since no dead spaces are formed, the degassing effect is also due to the vacuum exposed material surface of the intermediate product present in granular, crumbly or flaky form very intense and the condition of the material can also be arbitrary! ; be predetermined.

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In a preferred embodiment, the Screw tip of the screw of the first cascade stage as a head with 1 D diameter knife, and there are formed in it by projections distributed circumferentially arranged longitudinal grooves. In the area of the face of the screw head are fixed to the housing and radially arranged dividing means, especially distributed on the circumference arranged hessians or pens, provided.

In a modified embodiment, the Screw tip of the screw of the first cascade stage be the carrier of the dividing means, which are then fixed to the housing arranged means for forming the passages for the agglomerate are connected upstream. Such Means can be in the form of a perforated plate or a series of individual webs or projections.

Both according to one and the other embodiment, what training and arrangement of the dividing means and the passages for the agglomerate strands are relatively simple and advantageously porous agglomerate grains or agglomerate crumbs or agglomerate flakes almost comparable shape.

An embodiment of the cascade screw press according to the invention is shown in the drawing. It shows

Fig. 1 A cascade press with vacuum chamber in longitudinal section,

Fig. 2,

3 Detail of the dividing device at the outlet of the screw press forming the first cascade stage and snail tip of this snail im

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Longitudinal section and in front view,

Fig. 4,

5 Detail of a modified embodiment of the dividing device and screw tip in longitudinal section and in front view.

The cascade screw press according to Fig. 1 consists in essentially of a first heated screw press 1, a vacuum kettle 2 and a second heated one Screw press 3. The feed hopper 4 is filled with powdery thermoplastic or a powder mixture. The screw 5 is conventional Executed design and has (unspecified) feed, conversion and discharge zone. At the essentially open outlet 1 'of the screw press 1 is a relatively long, preferably vertically arranged vacuum chamber 2 connected. The evacuation line is numbered 8a and one The vacuum pump is denoted by 8b.

The chess-like vacuum chamber 2 is connected to a chess-like inlet 10 of the screw press 3. The screw 11 of this screw press is also of conventional design and has (unspecified) feeding, melting and ejection zones. The drives of the two screw presses are denoted by 12, 13. In the outlet I ^{1 of} the screw press 1, two systems of projections are provided. The free spaces between the projections 7 of the one system form passages for producing agglomerate strands. The projections 9 of the further system form a dividing device for the agglomerate strands. Axial arrangement and relative rotational movement of the two systems cause the agglomerate to be more uniform

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or approximately uniform shape as crumbs, flakes etc. is present, which enters the vacuum chamber immediately after the cutting process. Here it is intensively degassed and then drawn in cheaply by the screw press of the second cascade stage.

The screw tip 6 according to FIGS. 2 and 3 is formed by web-like projections 7 as a groove head with an outer diameter 1 D trained.

The passages for the agglomerate are in the form of longitudinal grooves 14 distributed over the circumference. Direct in front of the head face of the snail tip there is a splitting device 9, which consists of a number of Hesser fixed to the housing or distributed on the circumference Pins 15, which are arranged radially.

The screw tip according to FIGS. 4 and 5 is present as a pin 16, which has a number of circumferentially distributed and radially arranged knife or pins 17 carries. Between these dividing means and the screw 5 a perforated plate 18 with passages is arranged fixed to the housing. Instead of the perforated plate, a Row of circumferentially distributed, fixed cams, pins or the like can be provided, the free spaces between them Form passages for the agglomerate.

Example:

A PVC dry blend mixture or similar raw material as the feed is agglomerated in the first, heated cascade stage. At the outlet of this stage, the agglomerate strands formed are broken up into crumbs or flakes. The material temperature in the area of the cutting point is 140 c. This created flaky or crumbly Material is then with a dwell time of approx. 30 s

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degassed in the vacuum chamber, giving him undesirable volatile constituents, but not the higher-boiling lubricating, gelling and stabilizing additives as a result of the The present low fiaterial temperature of 14O C and the intended vacuum of about 50 Torr were withdrawn will. The degassed material is drawn in favorably: lanri by the screw press of the second cascade stage, plasticized and pressed. The diameters of the snails of the two cascade stages are GO nua, the effective screw length of the first stage is 10 D and that of the second 12 D. The volume output is 85 kg / h.

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Claims (7)

tentansi. rac } {Method 2να :: Processing and degassing powdered thermoplastics to melt plastic by feeding the powder or powder feeder to a first stage of a cascade screw press, compressing the feed material and conveying it into a vacuum chamber, degassing and feeding it to a second stage of the cascade screw press , Plasticizing, mixing and ejecting the melt, characterized in that the feed material agglomerates in the first cascade stage and in the gelled state at a temperature lying between the softening and melting point at the end of this stage and divided into an approximately more uniform, porous, granular, crumbly or flaky condition Form transferred into the vacuum chamber, here degassed by evacuation at a temperature between softening and melting point and drawn into the second cascade stage in the form that is present through the division, then melted, plasticized, mixed, homogenized and pressed out. 2) Method according to claim 1, characterized in that the agglomeration takes place at a temperature which is between 3O ° - OO C, preferably 40 ° - 6O ° C, below the melted! the nature of the hoof material lies. 209848/0897 3) Method according to claim 1 and 2, characterized in that the degassing takes place during a dwell time in the vacuum chamber from 5-60 seconds, preferably 20-30 see. he follows. 4) screw press for performing the method according to claim 1 to 3, consisting of a Screw press in a cascade arrangement with a vacuum chamber located between the cascade stages, marked by; ^{a) a screw press (1} ) forming the first cascade stage with a single screw (5) and 6 to 12 D effective screw housing length, in which two systems of projections (7, 9) are provided in the press outlet (I 1), which are arranged axially adjacent and relatively are rotatable relative to one another; b) a known vacuum chamber (2) with an attached evacuation device and c) a second cascade stage forming Screw press (3) with single screw (11) and 10 to 14 D effective screw housing length. 5) screw press according to claim 4, characterized in that the screw tip (6) of the screw (5) is present as a head with 1 D diameter, that in the head by projections (7) circumferentially distributed longitudinal grooves (14) are formed and that in the area in front of the End face of the head fixed to the housing and radially arranged dividing means (9), in particular knives or pins (15) arranged distributed around the circumference, are provided (Fig. 2, 3) 2 098 48/08 97 6) screw press according to claim 4, characterized in that that the screw tip (16) of the walking screw (5) is present as a pin and at the same time the carrier of the dividing means (7.17) and that axially between the dividing means and the screw a housing-fixed, passages (13) forming means (18), for example Perforated disc, is provided. 7) screw press according to claim 4 to 6, characterized in that the second cascade stage forming Screw press has a twin screw. 209848 / U897