NZ193513A

NZ193513A - Production of foamed plastics slabs

Info

Publication number: NZ193513A
Application number: NZ193513A
Authority: NZ
Inventors: R Colombo
Original assignee: Lavorazione Mat Plast
Priority date: 1979-04-24
Filing date: 1980-04-23
Publication date: 1984-07-06
Also published as: IL59701A0; FR2454897A1; AU5708580A; ES8102500A1; AU517132B2; CH638717A5; IT7967870A0; ES8102501A1; ES490338A0; IT1118963B; BE882570A; ES490337A0; GB2047157A

Description

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Complete SpacificatJon FiSsd:^? aih'f QOPI^M •••aOilllli I'l ••■■■■•■■•■ l'l * I ..06.J.UU.19M. /a si Class: iPishticstion Dats: P.O. Journal, Mo: NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION "PROCESS AND APPARATUS FOR CONTINUOUSLY FORMING A SLAB OF SYNTHETIC FOAM" xfc/We, LAVORAZIONE MATERIE PLASTICHE L.M.P. S.p.A., an Italian Joint Stock Company, of Via Nicomede Bianchi 72, Turin, I ta ly hereby declare the invention for whichxfcx^ we pray that a patent may be granted to met/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - 1 935 13 The present invention relates to the art of continuously forming a profiled slab of thermoplastic synthetic foam by continuous expansion of a flow of molten synthetic thermoplastic material containing an expanding agent.

The thermoplastic material is preferably polystyrene, polyethylene, oi^polyproplyene.

The term "profiled slab" means that all longitudinal faces or surfaces of the slab have a transverse profile which is accurately controlled as to shape and dimensions at any point of the length of the slab, the said profile being constant or purposely variable over said length.

Apparatus known until now in this field includes means for the preparation of the molten material under pressure, containing an expanding agent, and an expansion aperture fed continuously by these means. Typically these latter means consist of an extruder with one or more screws, into the cylinder of which opens at least one nozzle for injecting into the cylinder in suitable proportions a volatile expanding agent which is normally gaseous but is liquid at the injection pressure, such as, for example, a "Freon" (R.T.M.) or isopentane. The material may also contain a nucleating agent, such as sodium bicarbonate, in a finely dispersed form. The expansion aperture consists of the nozzle of an extrusion head fixed to the cylinder of the extruder, and the transverse profile of the terminal section of the nozzle is at least approximately complementary to the transverse profile of the profiled slab which it is desired to obtain. 193513 With this apparatus, an accurate control of the profile of the profiled slab is extremely difficult to achieve. In fact, the foam forming the profiled slab being extruded in this manner is hardly "stable", and, on the contrary, is still in a state of evolution which depends on the proportion of the injected expanding agent and on the temperature of the foam, which latter is difficult to control with sufficient accuracy, and in any case, is hardly homogeneous throughout the whole cross section of the profiled slab. The tendency of the profiled slab to change its profile and cr-css-sectional. dimensions may be counteracted by intense- externa i cooling, generally with the use of cooled calibrating nozzles, but the results achieved up till now have not been satisfactor Also the need to form the extruding nozzle with a cross-section of a form corresponding to the cross section of the profiled slab is a substantial disadvantage in that, for every profiled slab, an extruding nozzle must be designed which will not modify prejudicially the optimal counter-pressure needed in the cylinder to obtain the desired quality of foam with the specific extruder used.

Also an extruded slab necessarily has a transverse profile which is constant over the length of the slab, whereas it is frequently desirable to have a profile with local variations, e.g. depressions spaced along the said length.

Accordingly the present invention provides an apparatus for continuously forming a profiled slab of thermoplastic Wvi- 193513 synthetic foam by continuous expansion of a flow of molten thermoplastic material containing an expanding agent, including means for the preparation of this material under pressure and an expansion aperture fed continuously by the said means, characterized in that: the expansion aperture consists of the discharge end of an injection tube fed at its other end by the said means; the said discharge end axially opens into an end zone of a closed-section tubular open-ended forming duct, formed by a plurality ofjjs-opcra-fcc longitudinal walls- each of which extends along the entire length of the duct and sealingly contacts its adjacent walls along the generatrices of the duct; each of the said walls consists of the working run of an endless belt conveyor, in which the outer face of the belt has a profile which is complementary to the corresponding part of the profile of the said profiled slab; and means for simultaneously driving all the working runs of the conveyors towards the other end of the said duct at an adjustable speed; and wherein the means for preparing the material consists of a screw extruder, characterized in that the speed of the working runs of the conveyors is sychronized with the rotational speed of the screw(s). of the extruder. 193513 The term "endless belt" as herein used includes both continuous flexible belts and belts formed by pivotally interconnected plates.

The cooling of the working run of each conveyor may be effected simply by exposure of the back face of the run to the surrounding atmosphere. However, for absolutely reliable operation, suitable cooling means, for example, centrifugal fan blowers or the like, fed with atmospheric air, or if necessary, with conditioned air, may be associated with the working runs.

In principle, three conveyors are sufficient to define by their working runs a closed-section forming duct. In the great majority of cases four conveyors are necessary. More than four conveyors may be necessary when the profile of the profiled slab is particularly complicate, e.g. pentagonal. 1 935 13 An important advantage of the present invention lies in the fact that the profile of the working face of one or more belts does not necessarily have to be constant. For example, this face may be toothed (like a toothed belt). to produce corresponding recesses on the profiled slab, or this face may have other indentations and/or projections uniformly distributed along the entire length of the belt to produce corresponding local projections and indentations, respectively, in the profiled slab. It is practically impossible to obtain these results by the conventional extrusion method.

The conveyors are all coupled together to advance at the same speed. Advancement at the necessary speed may be obtained with a variable-speed electric motor, or with the interposition of a speed change gearing. Possibly, the means for driving the conveyors may be coupled (mechanically or electrically) to the motor of the extruder, in such a manner that the advance speed of the working runs of the conveyors is synchronized with the rotational speed of the screw(s) of the extruder and, therefore, is steadily adapted to the quantity of foam being discharged from the injection tube.

The cross-section of the injection tube is of no importance, so that an ordinary tube of circular section is perfectly adequate and it is not necessary to replace this tube by a tube of a larger or smaller cross section depending upon the profiled slab which is to be produced. It follows that the working conditions of the extruder (and hence the quality of the foam) are constant and independent of the cross section of the profiled slab which it is desired to 1935 13 obtain.

In the appended drawings: Figure--1 is a schematic side view of an apparatus according to the invention, - Figure 2 is a cross section taken on line II-II of Figure 1, and Figure 3 is a cross section taken on line III-Ilt of Figure 1.

In the drawings, reference numeral 10 indicates the cylinder of a conventional screw extruder for thermoplastic synthetic foam, provided with an injector 12 for a liquefied expanding agent, for example a "Freon" (R.T.M.). The cylinder 10 ends by an extrusion head 14 to which, in place of the conventional extrusion nozzle, a straight, steel tube 16 of circular section is fixed.

By A,B,C,D, are indicated four endless-belt conveyors each including a continuous, flexible, rubber belt A18, B18, C18, D18, stretched between a drive drum 20 and an idling drum 22. The working runs of the conveyors are indicated by A18', B181, C18', and D18' respectively. Apart from the width and the profile of the belts, the four conveyors are identical to each other. The axes of the respective drums are oriented at 90° to each other and the respective working runs are longitudinally in sealing contact with each other along their longitudinal edges which are preferably chamfered (as shown by 24 in Figure 3) to improve the seal. The working runs thus form a closed-section open ended forming duct 26 which is axially aligned with the injection tube 16 in such a manner that the discharge end of the tube lies within an 1 935 13 inlet end zone of the duct, as is seen in Figure 1. To ensure a seal along the entire length of the duct, the working runs are backed by guide rollers 28. The inner profile of the duct, and hence, also, of the profiled slab to be obtained, is determined jointly by the profiles of the outer faces of the individual belts. By way of example, the outer profile of the belt A18 includes three ridges 30 of semi-circular section, the outer profile of the belt B18 includes a continuous rectangular projection 32, the outer profile of the belt C18 has a continuous triangular ridge 34, while, in contrast therewith, the face 36 of the belt D18 is a planar. All these faces are preferably treated with a non-stick agent, for example a silicone, to prevent the foam from sticking to them.

The drive drums 2 0 of the conveyors are all coupled together by means of pairs of bevel gears 38, so that all the working runs A18'...D18' may be entrained at the same speed towards the discharge end of the duct 26, that is towards the right in Figure 1. In the case shown, the conveyors A ... D are driven by a common electric motor 40 with the interposition of a speed changer 42.

In operation, molten thermoplastic material (for example polystyrene), containing the expanding agent, is fed continuously from the cylinder of the extruder 10 to the injection tube 16 at a controlled temperature (according to principles known per se) whereby a mass of foam 43 under expansion is injected continuously into the inlet end of the duct 26, the mass tending to fill the entire cross section 1 935 1 3 of the duct. As the injection proceeds, the conveyors A ...

D are driven at such a rate that the further, new volumes of foam continue to fill the duct without forming a backward overflow. In other words, the working runs of'the conveyors advance together at a speed equal to the "rate of growth" (m/min) of the slab of foam in the duct 26. Under these conditions, the foam in the duct 26 undergoes a progressive cooling, which may be accelerated and controlled by the use of blowers 44 directed against the working runs of the conveyors. During the cooling process and the consequent consolidation of the foam, the profile of the foam slab is constantly under the control of the working runs which accompany the slab from the start of its formation at least up to the moment at which the temperature of the thermoplastic material has fallen to such a value that the foam no longer shows any perceptible tendency to expand. The length of the duct 26 is chosen so as to ensure this effect, so as to obtain, a profiled set slab 48 which is completely stabilised dimensionally and has the precise desired profile. Possibly, the idling drum 22 may be cooled internally by circulation of water, so that, at the inlet end of the duct 26 each of the working runs A18' ... D181 has a temperature much lower than that of softening of the injected thermoplastic material,. which is particularly advantageous in the case of extraordinarily massive profiled slabs.

Claims

193513 WHAT WE CLAIM 13:

1. Apparatus for continuously forming a profiled slab of thermoplastic synthetic foam by continuous expansion of a flow of molten thermoplastic material containing an expanding agent, including means for the preparation of this material under pressure and an expansion aperture fed continuously by the said means, characterised in that: the expansion aperture consists of the discharge end of an injection tube fed at its other end by the said means; the said discharge end axially opens into an end zone of a closed-section tubular st-eGP&fiZ- open-ending forming duct, formed by a plurality of^aoporato longitudinal walls each of which extends along the entire length of the duct and sealingly contacts its adjacent walls along the generatrices of the duct; each of the said walls consists of the working run of an endless belt conveyor, in which the outer face of the belt has a profile which is complementary to the corresponding part of the profile of the said profiled slab; and means for simultaneously driving all the working runs of the conveyors towards the other end of the said duct at an adjustable speed; and wherein the means for preparing the material consists of a screw extruder, characterized in that the speed of the working runs of the conveyors is synchronized with the rotational speed of the screw(s) of the extruder.

2. Apparatus accroding to claim 1, including means for cooling the working runs of the conveyors.

3. Apparatus for continuously forming a profiled slab of thermoplastic synthetic foam substantially as hereinbefore described with reference to the accompanying drawings. ~ By 4sfe/their authorised Agents.