Making Sole Moulds-3 : Types of Sole Moulds
Types of Soles Moulds
The soles can be directly moulded on to lasted to shoe or it can be produced as a single unit (unit sole), later attached to lasted upper. When the sole is directly moulded on upper, the first requirement is accuracy of foot/last. Because it forms part of the sole mould. Tight closure of the mould minimizes sprue under high moulding pressures. The mould also has to be strong and durable to withstand repeated use under the given moulding temperature and pressures without distortion or damage.
Vulcanized Rubber Moulding
The moulds comprises a bottom plate, side moulds with nip plates, the foot (metallic last, if sole to be attached to shoe directly) and a false top plate for double-density soles. Side moulds are heated to 150 to 180 degrees centigrade. Moulding pressure in this is around 5 Mpa.
To withstand the high moulding pressures steel moulds are also used to reduce mould cost and aluminum moulds can be used with hardened coating on the mould faces. The sole and nip plates are most important and are likely to be machined from high grade steel. For multi color, there will be 2 or more intermediate moulds where different colored material is moulded and partially cured, prior to combining the parts and re-pressurising to complete the moulding and vulcanization.
Metal foot used in the moulding machine are not heated but become warm (150 degrees) when used.
Thermo plastic Injection moulding/PVC
Higher grade aluminum, have good thermal conductivity and resistance to the corrosive character of the PVC, when it decomposes. It is not hard wearing however and the mating matching surfaces of the mould may need to be tipped with chromium plated steel because of high locking pressure is used. On some machines the side moulds are made up of aluminum alloy and the sole-plate is made stainless steel.
All moulds for injection-on moulding consists of two side moulds and a bottom plate (If the sole is moulded directly to lasted upper). The side moulds are either hinged at one end so that can be closed and opened by coupling a push-pull mechanism to them or it may contain separate mechanism also. The sole or base plate is either fixed or removable.
The last and sole mould are usually unheated although the mould can be warmed before production. The sole mould kept at ( 20o- 40o C) during production by a forced cooling system, using a network of waterways within the mould carrier.
Closure of the mould usefully follows of two sequences. Either the side moulds close on to the sole mould and then the foot with upper lowered to sit on the moulds , or the foot will be positioned over the sole mould and then side moulds closed around them.
Two color moulding
There are three basic method used to mould design.
- By using false foot or two top plates
- By using two bottom plates or
- By using two cavities on both sides of the plate.
In first case the first injection forms the wearing surface, this is done by using a false foot as the top plate, which fits lower in mould well than the true foot. The first compound is injected and allowed to solidify. The false foot is lifted up and rotate the through 180 degrees so the actual foot forme can be bought down to form the top closure. The second compound is then injected into the cavity made between the top surface of the first compound and the shoe bottom.
In the second method the first colour is injected to the shoe bottom and allowed to solidify. The base plate then moves down and the second compound is injected to form.
In the third method in which the base plate of the mould is a double mould plate having two cavities on opposite sides. The first compound is injected on to the shoe bottom and allowed to solidity. The foot can be lifted and the base plate revolved through 180 degrees to give an enlarged cavity when re-clamped. Next the second compound is injected to form the wearing surface.
PU moulding moulds
Moulds used for PU moulding are basically the same as for injection moulding-on thermoplastics. Although injecting pressures are lower, clamping pressure must be high to prevent sprueing. The mould is heated to 40-600C and the metal foot form may be heated. For rubber on PU moulding, the false lid above the rubber out sole would be heated to improve bonding to the vulcanized rubber insert.
In case of PU pouring machines the core and cavity does not contain any sprue etc., because the material is poured when the moulds are open.
EVA compound are composed primarily of Ethyl Vinyl Acetate resin in which the ethylene content varies. In general EVA soles are processed by Injection process. It is curious fact that unlike all the other foamed materials in which expansion occurs in the moulding, in injected EVA this expansion takes place outside the mould (i.e when the mould opens) for this mould cavities must be smaller than the final dimensions required and material expansion degree must be calculated previously with precision. It is important to have knowledge about the type of material to be used is essential before constructing the moulds. The expansion of the piece takes place during the mould opening phase and when the cross-linking of the material has almost completely finished. The mould cavities are generally smaller than the final dimensions of the product.
In India first EVA was produced in 1989 by National Organic Chemical Industries NOCIL under the trade name PILENE EVA. There should be provision for sufficiently large flash groove of 1-3 mm, a distance from the cavity the filling us of the groove channel ensures proper feeding and also acts as plug for the decomposed gases. The top plate should be as light as possible to facilitate immediate opening of the mould but strong enough to resist deformation and bending on application high pressure. It should cover the total area.
Blown PVC (Popularly known as Air-Mix): The construction of Air blown mould is same as other moulds where as here air is also injected in to the mould for proper expansion (air will aid in expansion) of sole. Air also gives glazy effect on the sole surface and acts as a cooling media. Air entered into the mould will escape from the mould through the vents (air holes and air pins) provided at the cavity plate. Air entered into the mould from top plate and reaches to bottom of cavity plate and comes on to the top cavity surface through holes provided and escapes from sides of the cavity plate through a vent. A rubber ring is provided on the top surface to avoid leakage of air from the top surface.
Note: Technology has changed a lot now. Please contribute to expand the list of moulds
Contributor: A.V.Suresh, Footwear CAD Engineer