GB2422327A

GB2422327A - Thermoplastic laboratory mixer

Info

Publication number: GB2422327A
Application number: GB0501499A
Authority: GB
Inventors: George Martin Gale
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-01-25
Filing date: 2005-01-25
Publication date: 2006-07-26
Also published as: GB0501499D0

Abstract

The invention relates to a laboratory batch mixer shown in figure 1 for producing test samples of compounded thermoplastics compositions from small quantities of materials using mixing actions similar to those of intermeshing twin screw compounding extruders. The mixer also discharges mixed materials in an extruded form convenient for subsequent testing. The mixer has a pair of intermeshing rotating sets of mixing elements (5) of the type and arrangement commonly used in co-rotating twin screw compounding extruders, combined within a barrel with a rotating screw (7) mounted parallel and peripherally in-line to the mixing elements; the screw combining with the elements to achieve the distributive mixing achieved during forwarding in a compounding extruder. The screw also discharges after completion of mixing by extrusion of a strand, strip, or other form suitable for such evaluations as rheological testing, colour matching, fire testing or as may be required.

Description

THERMOPLASTICS LABORATORY MIXER

REFERENCES CITED

I Nelson W K US 1,868,671(1932) 2 Erdmenger R US 2,670,188 (1954) 3 Erdmenger R US 2,814,472 (1957) 4 Manas-Zloezower I and Tadmor Z. "Mixing and compounding of po1ymers Theory and practice" Eise K "Recent developments in extrusion and processing machmery" in Advances in Polymer Technology vol3 No2 1983 I I 3 Wang Y. Compounding in co-rotating twin screw extruders" Rapra Review Report 116 Vol 10 No8 2000 7 Brabender Ol-IG, Kulturstrasse 5 1-55, D-47055 Duisburg Germany 8.DSM ResearchBV, Geleen, The Netherlands.

9 Murray B "The role of laboratory compounders" in "Enhancing Polymers using Additives and Modifiers" Rapra Technology Ltd 6 Oct 1993.

Bartel U "Comparison of discontinuous and continuous hot mcli production".

European adhesives and sealants 904 1992 p24 II List AG EP 6,329,092 A 12. Erdmenger and Oetke. German patent 1,1 11,154 1961

IN VENT WE STEP

This invention relates to a laboratory batch mixing apparatus capable of mixing additives into molten plastics and other viscous fluids using small quantities of materials and which replicates the mixing of production size continuous twin screw compounding extruders.

The mixing apparatus achieves both good dispersive and distributive mixing by incorporating two sets of co-rotating mixing elements of the types and arrangements commonly used in co-rotating twin screw compounding extruders but combined wIth a separate rotating screw mounted parallel and peripherally to the mixing elements, which aids both distributive mixing and also discharges the mixed material, hence obviating the need for manual removal.

ADVANTAGES

The apparatus mixes small amowits of material, typically 10 to lOOg which enables samples to be prepared using conditions which replicate production mixing such that economic savings in time and materials in the course of such activities as colour matching, quality control, and research, can be attained.

A further advantage is that the mixed matrial can be discharged as a strand for chopping into pellets suitable for testing in a rheometer or as a strip for colour measurements, or for physical and mechanical testing.

Current laboratory practice for mixing additives into thermoplastics, liqwd plastics, doughs and other viscous fluids is to use torque rheorneters, very small conical twin screw extruders with recirculation channel, or small versions of production compotinding twiii screw extruders The torque rheorneter, such as that described by Brabender OHG (7), is intended primarily for the comparitive measurement of viscosity during mixing and is particularly useful in evaluating the fusion and melt properties of PVC However such instruments have a disadvantage for other polymers of having non-intermeshing rotors As a result, the limited dispersive mixing fbrces of the non-intermeshing rotors may be inadequate to fully mix many additives into a polymer matrix For example, to achieve full colour development of a pigment in a thermoplastic A further disadvantage is that the mixed material must be removed by hand and is consequently produced in irregularly shaped lumps which are difficult to convert into samples in a form suitable for testing Small conical screw contra-rotating twin screw extruders with a recirculation channel combine twin screw extrusion mixing usmg intermeshmg screws with batch mixing (8) However they do not provide the accepted desirable dispersive mixing action of the intermeshing co-rotating twin screw compounding extruders. Furthermore, in the recirculating channel, viscous fluids such as molten thermoplastics will experience veiy low velocity at the channel surface so that this material may experience very little exposure to the screw mixing and may even stagnate and thermally degrade The material remaining in the channel when mixmg has been completed will reqwre manual removal Very small co-rotating intermeshmg twin screw compounding extruders such as that described by Murray (9) which are miniature versions of production machines will produce samples which are similar to production materials and in a form suitable for testing However, they need significant amounts of material to fill the barrel, and between mixes they either need purging which uses more material, or cleaning which is time consuming These machines are also relatively expensive.

Open batch mixers with interrneshing contra-rotating Z blades can have a tangential axial screw to aid mixing with reversal for discharge, but these machines are limited to mixing dough and paste materials such as for bread, adhesives and mnastics (10) List AG have described a kneading machine consisting of two intersecting cylinders with a figure-of-eight cross section and twin axial shafts with mixing elements (I I) 1-lowever these kneading elements consist of kneading bars on one shaft and scraper bars on the other or intersecting T shaped blades, but are not the cam type arrangements used for thermoplastic pellet melting and dispersive mixing in twin screw compounding extruders.

The combining of internieshing and none intermeshing screws in continuous compounding machines has been described by Erdmenger and Oetke (12)

THE INVENTION

The use of co-rotating twin screw conipoundirig extruders with a combination of mntenneshing screws and mixing cams for providing good mixing of viscous fluids was established by Nelson(I) and by Erdmenger (2, 3) Such machines have been described in detail by Manas-Zloczower and Tadmor (4) and typical applications by Eise (5).

I)ispersivc and distributive mixing in such machines is described by Wang (6).

The aim of the invention is to reproduce the mixing achieved in corotating twin screw compounding extruders by using the same type of rotors and optionally screw segments in a similar figure-ofeight barrel but which is a very small batch mixing device whilst retaining the ability to discharge the mixed material as one or more strands or as a strip The degree of mtermcsh of the mixing elements and screw segments will depend on the distance between the centre lines of the figure-of- eight chambers. This is determined by the formula and limits described in (4) Axial movement of the material required for uniformity of mixing, and collection of the material with discharge through a die on completion of mixing is achieved by a helical screw positioned axially and peripherally to the mixing elements. The length of the mixer can he increased or decreased to change its mixing capacity by adding extra barrel, mixing, and single screw segments Twin screw segments can be added eg for longer mixer lengths, in order to increase axial movement of material during mixing The screw may rotate in the same or opposite direction to the mixing elements, but co- rotation is preferred as this increases drag flow and hence screw conveying efficiency.

The screw may rotate at a faster, equal or slower speed than the rotors but a faster speed is preferred to maximise drag flow conveying.

Emptying of the mixer can be achieved by either having a valve which when opened allows the molten plastic or other mixed viscous fluid to be pumped by the screw out of the mixer or without a valve by reversing direction of screw rotation. In both cases a die may be used to shape the extruded material Addition of materials eg polymer and additives, to the mixer is through a hopper and feed pipe, assisted by a sliding piston which seals the mixer feed entry when all materials have been added This is preferably situated at one of the mixer ends, such that when material enters the screw it transports it towards the opposite end. It is preferably mounted over a screw or mixer element such that rotation immediately carries the materials to be mixed.

which is normally in particulate form, into the screw channel below. For thermoplastic materials, the barrel is heated to a temperature appropriate to melt the particular type of plastic.

Mixing elements can be in the form of two lobe, or three lobe cams, cylinders with eccentric centres or any combinatiomi which provides good dispersive mixing. Cam type elements can be positioned at right angles to adjoining pairs or mounted at successive smaller angles to form an overall screw type conveying pitch as used in twin screw compounding extruders. The widths may also be varied provided the paired elements are of the same width.

The mixer is most conveniently driven from an electric motor with a variable speed control and optional reversing switch, via a speed reducing gearbox and a "splitter" gearbox that has 3 output shafts, one for each of the 2 mixing rotor shafts and one for the screw The screw can have a channel section typical of the type used in single screw extruders, but to optimise pellet conveying during mixer filling and melt conveying with screw reversed during emptying, the screw channel depth may be varied across its width and its flank radius optimised for pellet conveying in one direction and melt conveying in the opposite direction The Invention will now be described with reference to the accompanying drawings Figure I shows the mixer consists of a barrel (I) having three interconnecting parallel cylindrical chambers (2)(3) and(4) in which the axies of cylindrical chambers (2) and (3) are positioned at a distance less than their diameter and cylindrical chamber (4) is parallel and peripheral to cylindrical chambers (2) and(3) Within each cylindrical chamber (2) and (3) are positioned shafts (5) upon which lit mixing elements (6) with geometries as used in twin screw compounders or screws as used in twin screw compounders, or a combination of the two. A conveying screw (7) is positioned within the tangential cylindrical chamber (4).

Material enters the mixer through the hopper (8) and feed pipe (9) and can be assisted by a piston (10) which also closes the feed during mixing.

The barrel (l)is heated by heating elements (I I) and the temperature controlled by a controller via a therniocouple. Shafts (5) are arranged to rotate at the same speed and in the same direction. The screw (7) rotates in the same direction as the 2 shafts (5). In the embodiment shown in figure I, emptying is by reversing the rotational direction of the screw (7) so that the screw pumps the mixed material out of the mixer via the shaping die (12). The die (12) is of a form to provide the required shaped extrusion for testing.

Figure 2 shows an alternative arrangement in which the screw (7) is nominally the same length as the mixing chambers (2) and (3), and the screw chamber (4) is terminated with a valve (I 3) which is opened to allow discharge of material when mixing is complete without reversal of the direction of the screw (7). For this arrangement the hopper (8) and feed pipe (9) are positioned at the opposite end to the valve (13) arid die (12).

Figure 3 shows an arrangement in which the single screw (7) in figure 1 is replaced by two intermeshing screws (14) to provide more positive pumping and complete emptying of the mixer In figure 4 the mixer drive (I 5)is connected to the mixer via a splitter gearbox (13) with 3 output shalls (1 8) with a torque measuring device (16) which continually records the torque provided by the drive motor so that technical data on the mixing process can be obtained The mixing procedure for the arrangement in figure 1 is described as follows The barrel (I) is heated to a temperature appropriate for the plastic to be mixed and the drive started to rotate shafts (5) such that the screw (7) will convey the material away from the entry' point and away from the die (12) Plastic pellets and additives to be mixed are added via the hopper (8) and feed pipe (9) and fall doii on to the cams (6) beneath The cams (6) drop the material into the screw (7) as it rotates The screw (7) conveys the materials away from the feed section towards the opposite end of the chambers (2 and 3) It will then either melt in the channel of the screw (7) or be transferred to the Intercoimectmg chambers (2 and 3) along the length of the barrel (1) where melting will also occur The materials will be continuously transferred between chambers (2) and (3) and screw (71, between spaces between pairs of cam elements (6), and between adjacent mixing elements (6) Eventually with assistance from the piston (10) the mixer will he completely filled and the plastic being mixed contamed within the barrel (I) by the piston (1 0j The plastic will be completely melted and the additives mixed into the molten plastic. Oiice the material is completely melted the rotational speed can be increased to give the required rate of shear Dispersive mixing is provided by the interaction of the intermeshmg cam elements (6) and distributive mixing is provided by the axial movement by the screw (7) and the counterfiow between adjacent pairs of mixing cams (6) This provides both the dispersive and distributive mixing features of the continuous processing in a co-rotating twin screw compounding extruder.

When mixing is completed the screw (7) and mixing elements (6) direction is reversed and the screw (7) conveys the mixed material to the outlet and extrudes it out through the die (12) Durmg this emptying period, the rotating cams (6) transfer material from the chambers (2) and (3) to the channel of the screw (7) For the arrangements in figure 2, when mixing is complete the valve (12) is opened to allow material to discharge.

Claims

A batch mixer for mixing and compounding small quantities of viscous materials such as thermoplastics using intermeshing rotating elements combined with a parallel peripherally mounted rotating screw 2 A hatch mixer according to claimi in which the intermeshing rotating elements are of a cam or screw type as used in co-rotating twin screw compounding extruders 3 A batch mixer according to claim 2 in which the elements rotate in the same direction 4 A hatch mixer according to claim I in which the screw rotation is reversible to enable it to both participate in mixing arid also discharge the product on completion of mixing.

A batch mixer accordmg to claim 4 in which the screw is specifically profiled lhr pellet conveying when filling and viscous fluid pumping when discharging 6 A hatch mixer according to claim 4 in which a second screw intenrieshes with the screw that is parallel to the mixing elements 7. A batch mixer according to claim I fitted with a die to form the discharged material into an extruded shape.

8 A batch mixer according to claim I which is heated at a controlled temperature.

9 A batch mixer according to claim 1 with a feed opening 1 0. A hatch mixer according to claim 8 with a plunger to assist in feeding material into the feed opening.

II A hatch mixer according to claim 3 with a valve to control discharge