FIELD OF THE INVENTION

This invention relates to moulding of articles in plastics and in particular relates to reinforced plastics articles and a method and apparatus for producing the same. The invention is especially concerned with a method and apparatus for producing a endless flexible track for tracked vehicles.

DISCUSSION OF PRIOR ART

Tracked vehicles conventionally comprise two or more driving wheels interconnected by an endless flexible track. It is conventional for this track to be formed from articulated steel elements. The steel tracks are relatively expensive and are very heavy and costly to maintain. In recent years there has been a number of proposals to use rubber as the material for producing the endless tracks. Whilst rubber tracks are far cheaper and incur less maintenance they have proven unreliable and lack the strength and durability to withstand the enormous loads that are inherent with such vehicles.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an improved reinforced plastics article of relatively high durability and a method and apparatus for producing the same.

STATEMENTS OF INVENTION

According to one aspect of the present invention there is provided a method of producing a reinforced plastics article including the steps of positioning a plurality of support members within a mould cavity of a mould and mounting at least one reinforcing member within the mould cavity so that it is secured to or

carried by the support members, filling the mould cavity with plastics material, curing the plastics material and removing the moulded article from the cavity with the reinforcing member embedded in the plastics material.

According to a further aspect of the present invention there is provided an endless flexible track for use on a tracked vehicle comprising of plastics moulding in the form of a closed loop with a plurality of longitudinally extending reinforcements embedded therein.

According to a still further aspect of the present invention there is provided apparatus for manufacturing an endless flexible track in reinforced plastics comprising inner and outer mould sections defining therebetween an open topped horizontally positioned annular mould cavity, a plurality of support means positioned laterally of the mould sections to, in use, hold annular reinforcements centrally of the mould cavity, the support means extending out of the mould sections; whereby in use the mould cavity can be filled with plastics material which after curing can be removed to provide an endless flexible track for plastics material with reinforcements embedded therein.

DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:-

Figure 1 is a perspective view of a track in accordance with the invention when mounted on a tracked vehicle, Figure 2 is a perspective view of part of the track, Figure 3 is a perspective view of a mould in accordance with the invention for producing the track, Figure 4 is a cross sectional view of the track in the mould, the mould being shown in simplified form, Figure 5 is a cross sectional view taken through the mould illustrating means of supporting reinforcements of the track in the mould,

Figure 6 is a partial plan view of one form of reinforcement during the moulding process,

Figure 7 is a cross section of the mould showing the reinforcement in association with the track, and

Figure 8 is a axiometric view of another form of reinforcement for the track.

DESCRIPTION OF EMBODIMENTS

The embodiments of the invention described hereunder relate to a moulded plastics track for a tracked vehicle, the plastics track having embodied therein suitable reinforcing to provide the track with the desired characteristics of durability and strength. The embodiments described also relate to the method of moulding the track and the mould that is used to produce the track.

A typical track 10 is illustrated in Figures 1 and 2, positioned on drive wheels 11 and 12 with idler rollers 13 position there between. Each drive wheel comprises a pair of co-axially spaced drive cylinders 15 and 16 defining an annular gap 17 therebetween. The track 10 is in the form of an endless, flexible belt 21, the underside 22 of which is provided with a plurality of equally spaced centrally positioned downwardly extending guides 20. The guides 20 locate within the gap 17 between the drive cylinders 15 and 16 to ensure that the track remains centrally positioned on the drive wheels.

The idler rollers 13 ensure that the track is kept in tension on the ground engaging side of the vehicle. The upper surface of the belt 21 is provided with a suitably profiled tread 25 comprising inclined tread blocks 26 and 27 that may be arranged in a variety of ways to provide the desired characteristics of the track. It is understood that the precise design and arrangement of the tread is not part of this invention and would be well known to those skilled in the art.

The track is moulded in plastics, especially

polyurethane details of which are contained in the following examples. When the track is to be used on a comparatively large earth moving vehicle it is very important that the track has the required durability and strength to withstand the enormous loads that it encounters during use. Thus it is critical that the track is suitably reinforced to provide these characteristics. A preferred form of reinforcement comprises the incorporation of a plurality of parallel extending spaced wires 30 that are embedded centrally within the belt 21 of the track 10 to extend around the whole length of the track in a closed loop. The closed loop provides the desired hoop stress to give the end product the strength and durability that is required. The reinforcements 30 may also be braced by a plurality of spaced apart transversely extending braces 31 that are secured to the elongate reinforcements by a plurality of means including welding, plastics ties and other fasteners that would be known to those skilled in this art. It is further understood that the precise inclination of the reinforcements would vary and that many variations are envisaged by this invention in its broadest aspect including interwoven or a crossed lattice work of reinforcement incorporating technology taken from other similar fields such as those associated with tyres and hose reinforcement.

In Figure 6 there is an illustration of a different form of reinforcement whereby the reinforcement 40 is formed in a steel band 41 the cross section of which assumes the shape of the track. The band 41 is provided with a plurality of spaced perforations 42 and is positioned centrally of the mould to be embedded within the plastics material during the moulding process. It is understood that the precise orientation and size of the perforations 42 would vary and it is further understood that one or more layers of the perforated band 41 may be embedded within the track.

In Figure 8 there is an illustration of a wire

mesh structure 50 that is used as the reinforcement for the track. The wire mesh 51 is formed as one piece with a plurality of equally spaced circumferentually extending wire reinforcements 52 interconnected by a plurality of equally spaced transversely extending braces 53. The reinforcement is formed into a closed loop and is again positioned within the mould to be embedded centrally of the belt of the track. As mentioned above the braces 53 may be welded to the mesh structure of may be attached thereto by use of other types of known fasteners.

Whilst steel is the preferred material of reinforcement it is understood that other known textile and non-metallic types of reinforcement are also envisaged as being applicable.

The preferred form of apparatus to mould the track is illustrated with reference to Figure 3. The mould comprises an outer annular mould section 60 and a two part inner annular mould section 61 comprising a lower section 63 that is bolted to the floor and an upper mould section 64 that sits directly on the lower section 63. The internal surface 65 of the outer section 60 carries the profile of the tread 25 of the track and the internal wall 66 of the inner sections 63, 64 are profiled to define the downwardly extending guides 22 that are formed on the outer surface of the track. The mould sections 60 and 61 define a cylindrical cavity 70 therebetween which can be filled from the top with polyurethane in liquid form. However, to ensure that the reinforcement is positioned totally within the belt of the track it is important that the reinforcement is accurately and positively positioned within the mould cavity 70 so that the liquid polyurethane flows around the reinforcement to ensure that it is completely embedded within the plastics. As shown in Figure 3 the upper portion 64 of the inner mould section has a peripherally extending annular flange 71 that extends over the mould cavity 70. A

plurarity of spaced small bores 73 are provided on this flange 71, and these bores 73 are arranged to communicate with similar positioned bores 74 in the base of the outer mould section. The steel reinforcement is located centrally in the mould cavity 70 by strands 80 of heavy duty nylon fishing line that extend across the mould cavity through the small bores 73 and 74 to be interlinked through adjacent bores to provide a lattice-work of support to the wire reinforcement to hold it centrally of the mould cavity 70.

It is understood that the linking portion (not shown) that interconnects adjacent bores 73 can be tensioned to tighten the whole assembly. The liquid polyurethane is then poured into the mould to the desired level and the assembly is left to cure. Once curing is complete the upper linking portions of the nylon support line are cut allowing the upper section 64 of the inner mould 61 to be lifted clear of the mould. The outer mould section 60 is then jacked vertically upwards to release the track 10 from the outer mould section 60. The track 10 can then be peeled off the fixed lower inner mould 61. The projecting portions of nylon support line can then be trimmed off the sides of the track producing a track moulded in polyurethane with steel reinforcement centrally embedded within the belt 21 of the track.

It is understood that the technique described above can be used with the other types of reinforcing described above and the method includes techniques for positioning the circumferentially extending reinforcing wires 30 against the nylon support lines 80 and then • attaching the axiarly extending bracing wires 31 as shown in Figure 3. In other embodiments the preformed cylindrical mesh 50 shown in Figure 8 may be positioned centrally of the mould again using the nylon support line 80 or alternatively the circular band 40 with the perforations 42 shown in figure 6 can be similarly positioned centrally of the mould cavity 70.

A schematic illustration of the track with the band of Figure 6 located within a mould section is shown in Figure 7 where the mould components are shown schematically.

In other embodiments steel wire may be used to hold the reinforcements in place and Figures 4 and 5 illustrate such an arrangement whereby the steel wire 90 extends through co-axially opposed apertures 91 in the mould halves, the steel wires being attached to the reinforcement 30 by use of plastics ties or other fasteners (not shown) and the spacing of the steel support wires 90 is arranged to ensure that the reinforcement 70 is adequately supported centrally in the mould cavity. When the moulding process is complete the belt 21 is removed from the mould halves and then the steel wires 90 trimmed to the edge of the track. A cross section of the assembly in the mould is shown with reference to Figure 4.

Another means of removing the support wires from the moulded track is to allow the liquid polyurethane to partially cure, then release the wires from the mould and pull them clear of the mould with the track still in the mould in partially cured form. In the partially cured form the polyurethane will adequately support the steel reinforcement in the desired position and thus it is possible to remove the support for the reinforcement without causing relative movement between the reinforcement and the polyurethane. Once curing is complete it is then possible to remove the track without having to trim off the support wires.

Polyurethanes suitable for use in this invention include but are not limited to polyols of polyester, polyether or caprolactone types which are reacted with di-isocyanates including but not limited to tolylene diisocyanate (TDD, 4,4'-diphenylmethane diisocyanate (MDI), dicyclohexyl ethane diisocyanate (HMDI), iεophorone diisocyanate (IPDI), and the like, together with a selection of many alternative curing agents

including but not limited to diamines such as methylene dianiline, 4,4'-methylene-bis (2-chloroaniline) (MOCA) , or m-phenylene diamine, or short chain polyols such as ethylene glycol, diethylene glycol, propylene glycol,l,4-butanediol, 1,6-hexanediol, or trimethylolpropane. The reaction may take several forms. In one form all the ingredients are liquified and mixed at once and poured directly into the mould, where reaction proceeds with the assistance of appropriate catalysts and/or temperature. In a second form the polyol and diisocyanate are pre-reacted to form a prepolymer. Separately, this prepolymer is prepared and mixed with the curative and admitted to the mould where reaction proceeds. This can occur at room temperature or at higher temperatures, depending on the selection of appropriate curative. In general different properties will be obtained under different processing conditions. In a third method the reactive functions of the polyol/diisocyanate prepolymer may be temporarily blocked by reacting them with appropriate molecules so that the reaction product dissociates at a predefined temperature or temperatures. This enables the prepolymer and curative to be premixed, to yield a single component urethane liquid which is heat activated.

While various mixtures of these ingredients and processes may lead to a wide range of properties in the resulting polyurethane polymers, it will be recognised that a more restricted range will be appropriate for application in traction drive belts. Properties of outstanding wear and tear resistance and tensile strength must be combined with suitable flexibility and resilience for the size and shape of the particular belt. In general we have found that belts with Shore Durometer hardnesses between 50 and 95 are the most suitable for heavy duty applications. Polymer ingredients must be chosen to optimise all physical properties within this range for the given belt

dimensions and service.

EXAMPLE 1

A prepolymer was made from polytetrameythylene glycol ether having a molecular weight of about 1000 with a quantity of TDI to yield a prepolymer with an available isocyanate content of about 3.2%. Such a propolymer is sold commercially as Adiprene L83. This was preheated to a temperature of

85 C and degassed under vacuum. 100 parts by weight were then mixed with 10.3 parts by weight of MOCA previously melted at a temperature of 120 C, and the mixture was poured into a mould preheated to a temperature of 100 C, and cured at that temperature for 3 hours. It was then de oulded and postcured at room temperature to yield a belt of about 83 Shore A hardness with good wear and toughness.

EXAMPLE 2

A prepolymer made from poly(ethylene adipate) prepolymerised with TDI, sold commercially as Cyanaprene A8, was used. This was prepared in exactly the same way as in Example 1, using 0.5 parts of MOCA as curative per 100 parts of prepolymer. This belt was post cured for 16 hours at 85 C, and yielded at polyester polyurethane belt again of 80 - 82 Shore A hardness with excellent properties.

EXAMPLE 3

A blocked isocyanate polyurethane sold commercially as Monothane A8 was used in this example. The viscous liquid was brought to a temperature of 85 - 90 C before pouring into a mould preheated to the same temperature. The temperature was then gradually increased to 135 - 140 C over a period of two hours to allow any entrapped air and moisture to volatilise, and then was held at that temperature for four hours before de oulding. It was further postcured for 10 hours at

100 - 110 C. This process also yielded a belt of 80 Shore hardness.

EXAMPLE 4

A "one-shot" polyurethane was used in this example. Z-thane Z-326 is a commercially available high reactivity system comprising as Part A and MDI prepolymer, and as Part- B a mixture of polyether polyols and selective catalysts. The two components of this system were preheated at 40 - 50 C and were metered via accurate metering pumps in a ratio of 89 parts of Part A to 100 parts of Part B through a mixing device and thence into the mould which had been preheated to 85 - 90 C. This material cured in 12 minutes and was demoulded after 1 hour. It was further postcured at 70 C for 4 hours to yield a belt of 78 - 80 Shore A hardness having good wear properties and toughness.