GB2276831A

GB2276831A - Twin-screw extruder with low friction material for mixing foundry sand and binder

Info

Publication number: GB2276831A
Application number: GB9407043A
Authority: GB
Inventors: Michael Cornelius Ashton
Original assignee: CASTINGS TECHNOLOGY INT
Current assignee: CASTINGS TECHNOLOGY INT
Priority date: 1993-04-08
Filing date: 1994-04-08
Publication date: 1994-10-12
Also published as: GB9307417D0; AU6507094A; GB9407043D0; WO1994023930A1

Abstract

The extruder (10) comprises in lets (20, 20A, 20B) for sand, hardener and resin, a chamber (16) containing co-rotating intermeshing screw members (12, 13), arranged to intermix the ingredients in close co-operation with the wall of the chamber and each other and together defining a path through which the ingredients pass, and an outlet (21) to discharge the mixture, wherein the surface of the screw members (12, 13) and/or of the facing wall of the chamber (16) is formed of a material having a low coefficient of friction whereby the ingredients are mixed without adhering to the screws (12, 13) or the wall. The low coefficient of friction material may be a high MW polyethylene or polypropylene of 3 - 6 million g/mol. A further use for the extruder is in the dressing of seeds with herbicide. <IMAGE>

Description

MIXING The invention relates to apparatus for and a method of mixing a minor proportion of one or more ingredients with a major proportion of one or more other particulate ingredient(s) in such a way as to efficiently distribute or coat the minor ingredient over the particles of the major ingredient. The invention is of particular value in mixing ingredients in the foundry industry and for this reason will be described in that context although it may be employed in other industries as well.

A foundry uses moulds in which to cast articles of molten metal. A mould comprises sand or like refractory particles bonded together by means of a chemical binder into a selfsupporting shape appropriate to the article to be cast. The binder, which may be liquid or solid, and may be two-part, e.g. a resin plus a hardener, must be mixed with the sand grains in such a way that it will bond them together. Conventional methods of mixing binder with sand grains are not very efficient and problems arise. Adequately uniform distribution of the binder throughout the sand is not always achieved and this is often compensated for by the deliberate addition of excess binder. The sand-binder mixture tends to adhere to the parts of a mixing device over a period of use so that throughput of the ingredients to be mixed declines and the set calibrations are adversely affected.

The invention is based on the first realisation that by mixing the ingredients in a high intensity extruder and providing the screw member any /or the barrel thereof with a surface of a low coefficient of friction better mixing can take place. The invention is based on the second realisation that by using screw members of a defined shape further benefits can be obtained.

In one aspect the invention provides apparatus comprising at least one inlet for the ingredients to be mixed, the inlet communicating with a chamber containing a screw member arranged to intermix the ingredients in co-operation with the wall of the chamber and together defining a path through which the ingredients pass, means to rotate the screw member, and an outlet to discharge the mixed ingredients, characterised in that the screw member comprises two screws which intermesh and that the surfaces of the screws and/or of the facing wall of the chamber are formed of a material having a low coefficient of friction whereby the ingredients are well mixed without adhering to the screws or the wall.

The screw member, therefore, comprises two screws which intermesh and thereby in use closely wipe over the surface of each other and that of the inner wall of the chamber.

In another aspect, the invention provides a method of mixing at least two ingredients, the method comprising feeding predetermined proportions of the ingredients to the inlet of a mixer and discharging the mixed ingredients from an outlet from the mixer, the mixer comprising a chamber containing a screw member arranged to intermix the ingredients in co-operation with the wall of the chamber and which together define a path through which the ingredients pass, characterised by using a screw member comprising two screws which intermesh and in use closely wipe over the surfaces of each other and that of the inner wall of the chamber and by passing the ingredients through the path at least one wall of which comprises a material having a low coefficient of friction.

As indicated above, the inlet to the extruder may, if desired, comprise two or more separate inlets. Thus, for example, where it is desired to mix sand and a binder, they may be fed through separate inlets into the extruder.

Because of the low coefficient friction of the material bounding all or part of the path through which the ingredients pass, the ingredients being mixed have poor adhesion to the screw members and wall and so do not adhere thereto; hence the calibration and uniform mixing of the ingredients are not adversely affected by continuous use. Wear of parts is minimised and the close-fitting tolerances ensure a self-cleaning that prevents build up of particles of the ingredients in the mixer. It is no longer necessary to overdose the binder ingredient to compensate for its poor distribution in the sand as happens with conventional mixing operations. A better and more efficient mixing action is achieved by the 'kneading' of the sand and binder compared with the more conventional stirring action by which these materials are conventionally mixed.

Several materials may be used to provide the surface of low coefficient of friction. It is a much preferred feature of the invention to use a material which additionally has high wear resistance and general toughness to resist the onerous conditions of use. Such materials are polymers of high molecular weight, especially ultra high molecular weight, say from about 3 to about 6 million g/mol. One such material is polyethylene, another is polypropyiene.

The material may be used as a lining to a metal screw member or the wall of a metal barrel or both, or the entire parts may be made of the material. The coefficient of friction of the material is preferably less than 0.5 and will normally be in the range 0.08 to 0.5, for example 0.1 to 0.15, the coefficient being measured as the ratio of tension to load when a test specimen of the load is slid over a steel plate of roughness 2 to 2.5 microns.

In order that the invention may be well understood it will be described by way of example only with reference to the accompanying diagrammatic drawings in which: Figure I is a side elevation of one apparatus of the invention suitable for mixing sand and a binder to make a foundry mould mixture; Figure 2 is a plan view thereof; Figure 3 illustrates various stages in the mixing steps.

Mixer 10 comprises a housing II defining a barrel 16 containing two side-by-side longitudinally extending rotary twin scroll screws 12 and 13. The screws carry respective feeder blades 14A and 14B and mixing paddles which are spaced apart along the length thereof. The screws 12 and 13 are arranged so that they are closely meshed, one with the other. The screws lie in the barrel 16 which has a substantially figure of eight cross-sectional shape to accommodate closely the two screws. Because of the way in which the scroll screws intermesh and because of the shape of the barrel, materials introduced into the mixer are well mixed in their travel from inlets 20, 20A and 20B. As can be seen in Figure 3 the blades 14A and 14B of the adjacent screws 12 and 13 intermesh as the screws rotate to provide very good uniform mixing of the ingredients as they are carried along in the path or clearance between the screws and the inside of the barrel.

The screws 12,13 are driven by a belt drive 17 powered by a motor 18, e.g. a 15 KW A.C.

motor, through a synchronisation gearbox 19.

Preferably the housing 10 is in two halves which may be separated by moving one horizontally away from the other for access to the screws 12,13.

In accordance with the invention the scroll screws 12,13 and the barrel 16 are formed from a material having a low coefficient of friction and which additionally is very tough and wear resistant. One such material is ultra high molecular weight polyethylene, e.g. having from about 3 to about 6 million g/mol.

In use of the invention grains of sand are introduced into inlet 20, a hardener into inlet 20A and a resin into inlet 20B and passed along the apparatus. As they travel, the grains and binder (hardener plus resin) are brought into contact with each other between the screws 12, 13 and the wall of the barrel 16, so that they are well intermixed. By the time the grains of sand are discharged through the outlet 21 they have been provided with an even coating of binder and substantially all grains are so coated. Substantially all of the sand is discharged and little or no deposits are left in the apparatus, on the screws 12,13 or the wall of the barrel 16.

The screws adjacent the said inlet can be self-regulating thereby dispensing with the need for a sand gate on the sand supply hopper.

The invention reduces cleaning time needed after use of the apparatus, and provides a lighter, cheaper mixing apparatus that is easy to install, maintain and dismantle. The rate of throughput remains substantially constant. Because the distribution of binder is better, consumption can be reduced, so lowering the cost of addition and the need to remove excess binder when reclaiming used sand. The rate of emissions of binder by-products is therefore reduced. Burn-on of sand is reduced and undesirable metal/mould reactions are minimised.

Emissions from binders are reduced at mixing, pouring, and reclaiming. Reclamation is easier (less binder in sand) and casting knock-out is easier (less binder in sand).

The invention is not limited to the embodiment shown. The apparatus may be heated or cooled as appropriate. The screw members may be arranged to counter or contra-rotate or rotate at different speeds. They may be of conical section. The method may be applied to other materials, e.g. dressing of seeds with herbicide, or the like.

Claims

I. Apparatus (10) comprising at least one inlet (20,20A,20B) for ingredients to be mixed, the inlet communicating with a chamber (16) containing a screw member (12 or 13), arranged to intermix the ingredients in close co-operation with the wall of the chamber and each other and together defining a path through which the ingredients pass, means (17,18,19) to rotate the screw member and an outlet (21) to discharge the mixed ingredients, characterised in that the screw member comprises two screws (12,13) which intermesh and that the surface of the screws member (12,13) and/or of the facing wall of the chamber (16) are formed of a material having a low coefficient of friction whereby the ingredients are well mixed without adhering to the screws (12,

13) or the wall.

2. Apparatus according to Claim I, characterised in that the material comprises a high molecular weight polymer.

3. Apparatus according to Claim 2, characterised in that the material comprises plastics of ultra high molecular weight of about 3 to about 6 million g/mol.

4. Apparatus according to Claim 3, characterised in that the material comprises ultra high molecular weight polyethylene or polypropylene or the like.

5. Apparatus according to any preceding Claim, characterised in that the screw members (12,13) are each formed of the material of low coefficient of friction.

6. Apparatus according to any preceding Claim, characterised in that the housing (II) defining the chamber (16) is formed of the material having the low coefficient of friction.

7. Apparatus according to any of Claims I to 5, characterised in that the housing (II) is formed of metal and the wall of the chamber (16) has a lining of the material of low coefficient of friction.

8. Apparatus according to any preceding Claim, characterised in that the housing (II) comprises two housing halves one of which can be moved horizontally away from the other to allow access to the screw members (12,13).

9. A method of mixing at least two ingredients, the method comprising feeding predetermined proportions of the ingredients to the inlet (20,20A,20B) of a mixer (10) and discharging the mixed ingredients from an outlet (21) of the mixer, the mixer comprising a chamber (16) containing a screw member (12 or 13) arranged to intermix the ingredients in co-operation with the wall of the chamber (16) and which together define a path through which the ingredients pass, characterised by using a screw member comprising two screws (12,13) which intermesh and in use closely wipe over the surfaces of each other and that of the inner wall of the chamber (16) and by passing the ingredients through the path at least one wall of which comprises a material of low coefficient of friction.

10. A method according to Claim 9, characterised in that a first ingredient is in a minor proportion and a second ingredient is particulate and makes up the major proportion, and the passage through the mixer coats the first ingredient over individual particles of the second ingredient.

11. A method according to Claim 9 or 10, characterised in that the major proportion ingredient is sand and the minor proportion ingredient is a binder for the sand.

12. An apparatus according to Claim 1, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

13. A method of mixing at least two ingredients substantially as hereinbefore described with reference to and as shown in the accompanying drawings.