GB2047157A

GB2047157A - Foamed plastics slabs

Info

Publication number: GB2047157A
Application number: GB8009166A
Authority: GB
Original assignee: LAVORAZIONE MAT PLAST
Current assignee: LAVORAZIONE MAT PLAST
Priority date: 1979-04-24
Filing date: 1980-03-19
Publication date: 1980-11-26
Also published as: IL59701A0; FR2454897A1; AU5708580A; ES8102500A1; AU517132B2; CH638717A5; IT7967870A0; ES8102501A1; ES490338A0; IT1118963B; BE882570A; NZ193513A; ES490337A0

Abstract

A pressurized melt of synthetic thermoplastic material containing a volatile expanding agent is continuously injected axially and expanded into the inlet end 43 of an open-ended tubular duct formed by a plurality of separate, externally cooled longitudinal walls A18', B18', C18', D18' in mutual sealing contact along the generatrices of the duct. The foam completely fills the cross-section of the duct while the walls are advanced jointly at a speed equal to the rate of growth of the foamed slab being formed and accompany the latter until the slab is stably set. Conveniently, the walls each consist of a working run of an endless belt conveyor. <IMAGE>

Description

SPECIFICATION Process and apparatus for continuously forming a slab of synthetic foam The present invention relates to the art of con tinuouslyforming 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 polys tyrene, polyethylene, or polypropylene.

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.

With this apparatus, an accurate control of the profile of the profiled slab is extremely difficu It 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 cross-sectional dimensions may be counteracted by intense external cooling, generally with the use of cooled calibrating nozzles, but the results achieved up till now have not been satisfactory.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 counterpressure 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.

One subject of the present invention is a process for continuously forming a profiled slab of thermoplastic synthetic foam by continuous expansion of a flow of molten thermoplastic material containing an expanding agent, the process being characterised in that; the said flow is continuously injected axially and leftto expand into one end portion of a closed-section tubular open-ended duct the internal profile of which corresponds the external profile of the finished profiled slab, this duct being formed by a plurality of separate longitudinal walls in mutual sealing contact along the generatrices of the duct; as the injection proceeds all the said walls are cooled from the outside and advanced towards the other end of the duct at the same velocity, chosen such that the flow of foam resulting from the expansion completely fills the cross section of the duct and is accompanied by the said walls substantially without mutual longitudinal slipping while at the same time the said duct is continuously decomposed at said other end by removing the said walls from the flow of foam at said other end and is re-composed at the aforementioned end portion; the length of the duct is chosen such that the flow of foam emerges from said other end of the duct in a dimensionally consolidated set condition.

A further subject of the invention is an 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 said means, characterised in that: the expansion opening consists of the discharge end of an injection tube fed at the other end by the said means; the said discharged end axially opens into an end zone of a closedsection, tubular open-ended forming duct, formed by a plurality of separate 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.

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.

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 atthe same speed. Advancement at the necessary speed may be obtained with a variable-speed eletric motor, or with the interposition of a speed change gearing. Possibly, the means for driving the conveyors may be coupled (mechanically or electricaily) 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 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 Il-Il of Figure 1, and Figure 3 is a cross section taken on line Ill-Ill of Figure 1.

In the drawings, reference numeral 10 indicate 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', B18', 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 900 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 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 Al 8 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 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 20 of the conveyors are all coupled together by means of pairs of bevel gears 38, so that all the working runs Al 8'. ..Dl 8' 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 perse) 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 of the duct. As the injection proceeds, the conveyors A Dare 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" (mimic) 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 termoplastic 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 runs18' ... D18' 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

1. Process for continuously forming a profiled slab of thermoplastic synthetic foam by continuous expansion of a flow of molten thermoplastic material containing an expanding agent, characterised in that: the said flow is continuously injected axially and left to expand into an end portion of a ciosedsection tubular open-ended duct, the internal profile of which corresponds to the external profile of the finished profiled slab, the said duct being formed by a plurality of separate longitudinal walls in mutual sealing contact along the generatrices of the duct; as the injection proceeds, all the said walls are cooled from the outside and advanced towards the other end of the duct at the same velocity, chosen such that the flow of foam resulting from the expansion completely fills the cross section of the duct and is accompanied by the said walls substantially without mutual longitudinal sliding while at the same time the said duct is continuously decomposed at said other end by removing the said walls from the flow of foam at said other end and is recomposed at the aforementioned end portions; the length of the duct is chosen such that the flow of foam emerges from said other end of the duct in a dimensionally consolidated set condition.

2. 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 open-ended forming duct, formed by a plurality of separated 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.

3. Apparatus according to claim 2, including means for cooling the working runs of the conveyors.

4. Apparatus according to claim 2 or 3, 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.