GB2254570A

GB2254570A - Manufacturing foamed polyurethane

Info

Publication number: GB2254570A
Application number: GB9206880A
Authority: GB
Inventors: Gunter Rebers; Wolfgang Cholewczynski
Original assignee: Kloeckner Ferromatik Desma GmbH
Current assignee: Kloeckner Ferromatik Desma GmbH
Priority date: 1991-04-09
Filing date: 1992-03-30
Publication date: 1992-10-14
Anticipated expiration: 2012-03-30
Also published as: DK47692A; DK173953B1; ITMI920287A1; FR2675147A1; ITMI920287A0; GB9206880D0; GB2254570B; FR2675147B1; DK47692D0; IT1254438B; DE4111458C1

Description

221)4570 1 Process and apparatus for manufacturinq foamed polyurethane The

invention relates to a process and apparatus for manufacturing foamed polyurethane.

For mixing isocyanate and polyol to give a mixture which, when reaction is complete, gives polyurethane, it is known (DE-PS 20 62 008) to use mixing devices which have a mixing screw which tapers conically at least in the region of its section adjacent to the outlet end of the cylinder, the cylinder being of a construction complementary to the screw and the drive coupled to the screw being arranged axially displaceably in the device in a link guide.

Mixing devices of this type are used for example on shoe-soling machines in which soles which have a bottom sole of abrasion-resistant polyurethane and an elastic inner sole of polyurethane are moulded integrally onto shoe uppers. Such machines are constructed as rotary table units and have a plurality of stations with appropriate moulds for moulding the shoe soles, in cases in which initially the bottom sole is moulded and subsequently the inner sole is formed the machines having a last for the shoe uppers to be drawn on, two lateral moulding parts, a base die and a counter-die, one last die and one counter-die each being arranged on a pivotal rotary head which is arranged height-adjustably on the shoe-soling machine.

During moulding of the bottom sole, the two lateral moulding parts in their closed position, the base die and the counter-die delimit a mould cavity whereof the volume is somewhat larger than the volume of the bottom sole of the completely reacted mixture, with the two lateral 2 moulding parts delimiting in their closed position a sprue bore through which the reactive mixture for the bottom sole is introduced into the mould cavity formed. Then, the mould cavity is reduced in size to the volume of the finished bottom sole, with the base die at the same time closing the sprue bore. After sufficient solidification of the material of the bottom sole, the sides of the moulding parts are moved away, the rotary head is pivoted and displaced such that the last, with the upper drawn on, stands upright on the lateral moulding parts which have been brought back into the closed position so that the upper on the last, the lateral moulding parts and the bottom sole on the base die delimit a mould cavity for formation of the inner sole, with the lateral moulding parts delimiting a second sprue bore through which the mixture which when reaction" and foaming are complete forms the inner sole is supplied.

During the mixing of the isocyanate and the polyol, the chemical reactions already begin so that some of the mixture is deposited on the inner wall of the mixing chamber and forms a solid film of polyurethane which becomes thicker over time. This has the result that the gap required for mixing between the cylinder wall and the screw closes up. For this reason, it is necessary to at least partially take off the film in order to maintain a gap with a predetermined minimum gap width. This is effected in that the screw is displaced in the direction of the outlet end of the cylinder, and during this movement takes off completely reacted mixture which is formed on the inner wall of the cylinder. This takes place in the time intervals in which the mixing unit is out of engagement with the sprue bores of the moulds. Isocyanate and polyol are supplied by way of associated valves, in each case in metered quantities, during the individual mixing intervals.

3 Hitherto, isocyanate and polyol have been mixed at screw rotational speeds of approximately 18000 revolutions per minute. At these high speeds, the mixture on the webs of the mixing screw, with the exception of small residues, is hurled away because of the centrifugal forces, so that long service lives for the screw result, i.e. it takes a long period of time until these small residues which remain in the regions between the screw threads being filled with completely reacted mixture, the screw having to be replaced by a new one and the completely reacted mixture being removed in a manner known per se with hot chemical solutions before the screw is next used.

The mixtures produced for the individual layers of the soles have a more or less highly porous structure, i.e. they comprise mixtures which foam to a greater or lesser extent and which moreover have appropriate additives.

Thus, for example, in the case of two-layer soles having a bottom sole and an inner sole and moulded integrally onto a shoe upper, it is required that the bottom sole be compact and abrasion-resistant, while the inner sole, which connects the bottom sole to the shoe upper, is to be elastic and resilient and is thus also called a comfort sole.

It has been shown that these high rotational speeds in the region of 18000 revolutions per minute in practice solve the problems of cleaning the screw and the inner wall of the mixing chamber of the mixing devices, but that the optimum mixing speeds may be very much lower and are dependent on the PUR components to be avoided. This is true in particular for inner sole materials.

Surprisingly, it has been shown that for mixtures foamed to the optimum in many cases the rotational speeds are to be chosen in the range between 6000 and 15000 4 revolutions. At these low rotational speeds, however, neither the previous service life of the screws nor the taking off of completely reacted residues of the mixture from the inner wall of the cylinders can be achieved using the methods explained above.

The object of the invention is to provide a process which ensures optimum manufacture of mixtures of isocyanate and polyol which when reaction is complete give polyurethane and which ensures on the other hand the previous service lives of the screws and also the reliable taking off of completely reacted mixture from the inner wall of the cylinders, and to provide a device for carrying out this process.

According to a first aspect of the invention there is provided a process for manufacturing foamed polyurethane from mixtures of isocyanate and polyol which, when reaction is complete, give polyurethane, using a mixing device which has a conical mixing screw which may be displaced in relation to the outlet opening in the axial direction between a rear mixing and outlet position and a front cleaning position, in which process the isocyanate and polyol are mixed in the mixing and outlet position at speeds of the mixing screw of approximately 5000 to approximately 10000 revolutions per minute, the mixture is ejected into a mould and finally, in the cleaning position, completely reacted polyurethane which is on the inner wall of the device is taken off and at the same time mixture residues on the webs of the screw are removed, at speeds of the mixing screw of approximately 15000 to approximately 20000 revolutions per minute.

The wording that the mixing of isocyanate and polyol takes place at approximately 5000 to approximately 15000 revolutions per minute means that the rotational speed for optimum mixing depends on the quantity proportions and the chemical composition of the isocyanate and the polyol and their further additives and the predetermined foamed structure of the polyurethane after reaction is complete.

Furthermore, the rotational speed may accordingly be specified in the range from 15000 to 20000 revolutions for the screw during cleaning, it being possible to determine the optimum cleaning speed experimentally again depending on the chemical composition of the isocyanate and the polyol and their further additives.

Because the screw is only operated during the cleaning effect, i.e. for short periods, at the correspondingly high rotational speed, it may also be operated above the hitherto conventional 18000 revolutions per minute, so that, because of the greater centrifugal forces, mixture residues on the screw webs can then be removed even more effectively and the service life of the screw increased.

Furthermore, during mixing the previous noise build-up is reduced because of the low speed. Furthermore, because the rotational speed is only high for short periods, i.e. during the respective cleaning phase, wear on the device is considerably reduced.

As a result of the mixing speed being smaller than the cleaning speed, the thermal load of the PUR components is also very much smaller, which in turn has a positive influence on the starting time of the reaction.

According to a second aspect of the present invention there is provided a device for carrying out the process according to the first aspect of the invention, the device having a conical mixing screw which revolves in a correspondingly conically constructed mixing chamber and may be axially displaced in relation to the outlet 6 opening between a rear mixing and outlet position and a front cleaning position and which has a drive whereof the speed may be controlled by way of a control apparatus and an actual-value sensor for the respective position of the mixing screw in dependence on two predeterminable positions of the mixing screw in accordance with predetermined corresponding time intervals.

The invention will further be explained, by way of example, with reference to the accompanying drawing which shows a section through a mixing device known per se, having a control according to the invention, illustrated as a block diagram, for the rotational speed.

In the figure, the mixing screw 1 has a cylindrical section 2 adjoined in the axial direction by a section 4 which is conical in the direction of the outlet end 3. The mixing screw 1 is arranged axially displaceable by the distance a in the mixing chamber 5.

The figure shows the device in the working position, i.e. during mixing. Isocyanate and polyol are supplied to the mixing chamber 8 in metered quantities respectively by way of the bores 6 and 7 and valves (not illustrated) for metering.

After mixing and conveying of the mixture into a mould (not illustrated), the mixing screw is displaced axially towards the outlet end, until there is a metal contact of the conical section 4 with the complementarily constructed inner wall of the mixing chamber.

The shaft 9 carrying the mixing screw 1 is rotatably mounted in a tubular bearing part 11 by way of ball bearings 10 and carries, on the end remote from the mixing screw 1, a V-belt pulley 12 for driving the mixing screw.

7 The screw displacement for cleaning is performed by way of a pneumatic cylinder.

The movement can be controlled in a manner known per se by limit switches which effect reversal of the direction of movement. As soon as the mixing screw is displaced axially out of its set position, the apparatus 19 increases the rotational speed of the mixing screw and resets it again when the position illustrated is reached.

1 8

Claims

1) A process for manufacturing foamed polyurethane from mixtures of isocyanate and polyol which, when reaction is complete, give polyurethane, using a mixing device which has a conical mixing screw displaceable in relation to an outlet opening in the axial direction between a rear mixing and outlet position and a front cleaning position, in which process the isocyanate and polyol are mixed in the mixing and outlet position at speeds of the mixing screw of approximately 5000 to approximately 10000 revolutions per minute, the mixture is ejected into a mould and finally, in the cleaning position, completely reacted polyurethane which is on the inner wall of the device is taken off and at the same time mixture residues on the webs of the screw are removed, at speeds of the mixing screw of approximately 15000 to approximately 20000 revolutions per minute.

2) A device for carrying out the process according to Claim 1, having a conical mixing screw which is rotatable in a correspondingly conically constructed mixing chamber and axially displaceable in relation to an outlet opening between a rear mixing and outlet position and a front cleaning position and which has a drive whereof the speed can be controlled by way of a control apparatus and an actual-value sensor for the respective position of the mixing screw in dependence on two predetermined positions of the mixing screw in accordance with predetermined corresponding time intervals.

3) A process as substantially hereinbefore described with reference to the accompanying drawing.

4) A device as substantially hereinbefore described 9 with reference to the accompanying drawing.

5) Foamed polyurethane when made by a process as claimed in claim 1.