TYRE CONSTRUCTIONAL METHOD

Abstract: A method of manufacturing a tyre and a resulting product, made from plastics materials, the method comprising the steps of forming an inner core 10 of a relatively soft resilient plastics material, placing the core 10 in a mould 22, 23 having the shape of the finished tyre, filling the mould with a resilient relatively hard wearing plastics material, to form an outer casing 19 around the core 10.

Description: This invention relates to solid tyres for vehicles. The expression solid tyres is to be taken to mean tyres which do not have internal spaces in which air under pressure is maintained. The invention is primarily concerned with solid tyres for vehicles such as fork-lift trucks, cranes, or other mechanical handling equipment or other vehicles not generally used on highways. The advantages of solid tyres are well known in that they do not need to have air under pressure and are thus free from maintenance. This is particularly significant in factory or other environments where objects liable to damage the tyres may be present. Many solid tyres are made from natural or synthetic rubbers, with or without fabric or fibre reinforcements. However, it is common knowledge that tyres of these materials tend to mark floors and in situations such as hospitals and the food industry this is undesirable. The use of plastics can overcome this problem, but many plastics which have acceptable non-­ marking properties, though reasonably hard wearing, tend to be too hard or stiff and softer ones wear relatively quickly. It is the primary object of the invention to provide a method of manufacturing a solid tyre which is particularly efficient and reliable in use and can be efficiently produced. In accordance with one aspect of the invention there is provided a method of manufacturing a tyre from plastics materials characterised by the steps of forming an inner core of a relatively soft resilient plastics material, then placing the inner core in a mould having the shape of the finished tyre, the inner core being located within the mould so that, when the mould is filled with a resilient relatively hard wearing plastics material, an outer casing is formed around the core, the outer casing having at least an outer peripheral wall and side walls. Preferably the inner core includes an annular tubular membrane, within which the relatively soft resilient material is contained. Conveniently, the membrane is formed as a tube with a peripheral joint which is secured before introduction of the relatively soft resilient material. The tube may comprise one or more layers or plies. Preferably also, keying means may be placed on the exterior suface of the membrane for engagement with the outer casing as it is moulded around the membrane. The keying means may be grooves or other indentations or projections on the external surface of the membrane. Reinforcement may be placed in the mould prior to moulding of the outer casing, such reinforcement being preferably at least in the region of the bead of the tyre. The reinforcement may be metal or other material mesh, having holes through which the material of the outer casing can penetrate as the mould is filled. Preferably the plastics material used for the inner core is a polyurethane. Desirably the hardness is in the range 10 - 60 Shore "A" Durometer and the elongation is in the range 200 - 400%. Advantageously the plastics material used for the outer casing is a polyurethane. Desirably the hardness is in the range 60 - 90 Shore "A" Durometer and the elongation is in the range 400 - 800%. The mould in which the outer casing is formed may have means for supporting the inner core away from the sides of the mould, so that upon filling with the material for the outer casing, the inner core is completely or partially enclosed in that material. Gravity casting, injection casting, centrifugal casting or reaction injection moulding methods may be employed in making the inner core. Gravity casting, centrifugal casting, reaction injection moulding methods may be employed in making the outer casing. It is a further object of the invention to provide a tyre for the purposes mentioned which is hard wearing, has a good performance and has a long working life. According to this aspect of the invention there is provided a tyre as produced by the said method and comprising an inner core of relatively soft resilient plastics material and an outer casing of resilient relatively hard wearing plastics material. Varying hardness and flexibility in differing parts of the tyre outer casing may be achieved by using materials of different properties. The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of a finished tyre produced in accordance with the invention; Figure 2 is a part cross-sectional view of a moulding forming a membrane for surrounding the inner core of the tyre of Figure 1; Figure 3 is a plan view showing a mould for producing a finished tyre in accordance with the invention; Figure 4 is a cross-sectional view of the mould of Figure 3; Figure 5 is a plan view of a mould for producing the inner core by an alternative method, and Figure 6 is a cross-sectional view of the mould shown in Figure 5. The tyre illustrated in Figure 1 is intended for use on a wheel having a recessed rim of conventional kind, the rim however being of the split type, including two, three or four components, to enable the tyre to be fitted. For certain sizes there may be sufficient resilience in the tyre to enable the tyre to be fitted without separatioin of the rim parts or with partial separation only. The tyre is typically for use on trucks or other vehicles not primarily intended for use on highways but in which tyre wear must be minimised and in which risk of puncture is eliminated. The tyre will have low rolling resistance and may be used on floors on which it will not leave a mark. The tyre is also resistant to oil, chemicals and solvents. As seen in Figure 1 the tyre comprises an inner core 10 of generally circular section. The inner core includes and is an enclosing annular, tubular membrane 11, also shown in Figure 2. This membrane 11 is formed from a resilient material such as a suitable moulded polymer. The membrane 11 is preferably a multi-layer or multi-ply structure. It has a relatively thin wall section, but at the base of the tyre section it is thickened and includes a separable joint 12 made up of a bead and groove combination, the resilience of the material allowing these to be engaged when required. On the external surface of the membrane 11 are re-entrant or dovetail grooves 13, forming keying means. The inner core is formed by first moulding the membrane, preferably by a low pressure moulding process, then after removing this from its mould, the joint 12 is engaged, thus forming a hollow annular tube. The material for the core 10 is next introduced, through a suitable entry hole, with this being introduced under pressure. Pressurisation provides some degree of pre­ stress within the core. The inner core 10 is produced from a relatively soft resilient material which may be a urethane or other equivalent plastics material. A suitable material is polyurethane being in one example a quasi-prepolymer based on a toluene diisocyanate, a mixture of low molecular weight polyols with some oil extension. In this particular example, the polyurethane mixture was pre-blended in the ratio 1:1 by volume. However, other ratios can be used. This was introduced into the interior of the membrane 11 at 30°C. After filling and plugging, the assembly is then heated or may be transferred to a circulating air oven operating at 35 to 40°C. The curing of the inner core can take place in these conditions over 48 hours. The physical properties of the inner core in this example were as follows: In an alternative arrangement, the inner core may be moulded in a two-part mould 14, 15 such as that shown in Figures 5 and 6 in which the material in liquid form is introduced through an entry port 16, air being allowed to escape through an outlet 17 from an internal generally circular section cavity 18. In place of the membrane the inner core 10 may, in this case, be enclosed in a fabric covering. This is in the form of a band of fabric applied to the outside of the inner core 10 by winding at 45°. A band 2½" wide and of plain weave polyester fabric may be suitable. Wire mesh may be substituted for the fabric band. Such a band is shown diagrammatically in Figure 4 at 29. An outer casing 19 is made from a resilient relatively hard wearing plastics material which is also a utherane material. This material is therefore substantially harder than that used for the inner core 10. This outer casing 14 has a profile corresponding to that of a conventional tyre, having bead portions 20, 21 engageable with beads on the wheel rim. Tread formations (not shown) may be provided, if required on the external surfaces. The outer casing 19 is formed in a mould shown in Figures 3 and 4. This is a circular two part mould comprising components 22, 23. A cavity 24 is defined within the mould, being the shape of the finished tyre. At one side an entry port 25 and at the diametrically opposite side an outlet port 26 are provided. The inner core is supported within the cavity 24 by means of spacers 27. These are positioned at intervals around the mould, six being shown at the internal peripheral surface of the cavity 24. When the inner core has been supported in the cavity on the spacers, the remaining volume is filled to form the outer casing. In this example the mould is placed in a circulating air oven at 100°C. A polyurethane composition based on a toluene diisochanate/polytetramethylene ether glycol prepolymer crosslinked with 44' melthylene bis ortho chlorinaline can be used. Curing takes place at 100°C over a period of a predetermined period. After cooling the finished moulding is removed and any flash trimmed off. The physical properties of the outer casing 19 are as follows: In the example shown in Figure 1 reinforcement 28 is included within the outer casing 19. This may be in the form of wires or other metal strips or sections of metal or other material mesh. The reinforcement 28 is provided in the bead portions 20, 21 and also the section between the bead portions, the latter providing a stiff bridge which greatly strengthens the tyre in this area. It also serves to tend to prevent the tyre from slipping on the wheel. The reinforcement 28 is placed in the mould prior to moulding the outer casing. As the material enters the mould it flows through holes in the mesh or other spaces between portions of the reinforce­ ment. Varying hardness of the portions of the outer casing can be achieved by selection of different material mixes for producing the portions comprising the bead portions, sidewall portions and tread portion but the desired flexibility may be achieved by suitable variation of thickness or combination of these techniques. Reinforcements in the form of wire or fabric mesh are embedded in the outer casing and provide varying strengths over different areas of the tyre, so as to provide greater or lesser flexibility as required. A tread pattern may be provided if required on the outer peripheral face and/or on parts of the side walls. Instead of the gravity casting methods described, centrifugal casting, injection casting, reaction injection moulding or other appropriate method may be used for the formation of the inner core. Similarly centrifugal casting, reaction injection moulding may be used to make the outer casing. The materials chosen may determine or influence the choice of technique used for production. Other materials for the inner core include polyurethanes of the prepolymer, quasi prepolymer or one shot polymer types. Materials suitable for the outer casing include any of those suitable for the inner core as well as thermoplastic grades which can be used in centrifugal casting or blow moulding. In certain cases, including the example shown in Figure 1, the outer casing entirely surrounds the inner core but in other examples, the inner core may be exposed at the bead area, where it is protected by the wheel when the tyre is in place.

Claims: 1. A method of manufacturing a tyre from plastics materials characterised by the steps of forming an inner core 10 of a relatively soft resilient plastics material, then placing the inner core 10 in a mould 22, 23 having the shape of the finished tyre, the inner core 10 being located within the mould 22, 23 so that, when the mould is filled with a resilient relatively hard wearing plastics material, an outer casing 19 is formed around the core 10, the outer casing 19 having at least an outer peripheral wall and side walls. 2. A method as claimed in claim 1 characterised in that the inner core includes an annular tubular membrane 11, within which the relatively soft resilient material 10 is contained. 3. A method as claimed in claim 1 or claim 2 characterised in that the membrane 11 is formed as a tube with a peripheral joint 12 which is secured before introduction of the relatively soft resilient material 10 under pressure. 4. A method as claimed in claim 2 or claim 3 characterised in that keying means 13 are placed on the exterior surface of the membrane 11 for engagement with the outer casing 19 as it is moulded around the membrane 11. 5. A method as claimed in any one of the preceding claims characterised by reinforcement 28, placed in the mould 22, 23 prior to moulding of the outer casing 19, such reinforcement 28 being at least in the region of the bead of the tyre. 6. A method as claimed in any one of the preceding claims characterised by placing means 27 for supporting the inner core 10 away from the sides of the mould 22, 23, so that upon filling with the material for the outer casing 19, the inner core 10 is at least partially` enclosed in that material. 7. A method as claimed in any one of the preceding claims characterised in that gravity casting, injection casting, centrifugal casting or reaction injection moulding methods are employed in making the inner core 10. 8. A method as claimed in any one of the preceding claims characterised in that gravity casting, centrifugal casting, reaction injection moulding methods are employed in making the outer casing 19. 9. A tyre produced by the method as claimed in any one of claims 1 to 8 characterised by an inner core 10 of relatively soft resilient plastics material and an outer casing 19 of resilient relatively hard wearing plastics material. 10. A tyre as claimed in claim 9 characterised in that the plastics material used for the inner core 10 is a polyurethane. 11. A tyre as claimed in either claim 9 or 10 characterised in that the plastics material used for the inner core 10 has a hardness in the range 10-60 Shore "A" Durometer and the elongation is in the range 200 - 400%. 12. A tyre as claimed in claim 9 characterised in that the plastics material used for the outer casing 19 is polyurethane. 13. A tyre as claimed in claim 9 or 12 characterised in that the plastics material used for the outer casing 19 has a hardness in the range 60 - 90 Shore "A" Durometer and the elongation is in the range 400 - 800%. 14. A tyre as claimed in claim 9 characterised by a tubular membrane 11 encases the inner core 10 to form part of that core. 15. A tyre as claimed in claim 14 characterised in that the membrane 11 includes a separable joint 12. 16. A tyre as claimed in any one of claims 9 to 15 characterised by reinforcement 28 whch is moulded within the outer casing 19. 17. A tyre as claimed in claim 16 characterised in that the reinforcement 28 is provided with holes through which the material of the outer casing 19 flows as it is moulded. 18. A tyre as claimed in claim 9 characterised by a fabric band 29 wound around the inner core. 19. A tyre as claimed in claim 18 characterised in that the band 28 is a polyester fabric.

Inventors: WHITE, LEWIS WILLIAM
Applicants: WATTS INDUSTRIAL TYRES LIMITED
Application year: 1986
Main classification: B29D30
IPC codes: B29D23, B29C33, B29L30, B29C70, B29D30, B29C39