The simplest unit for the production of polyurethane soles, polyester as well as polyether, consists basically of two elements. A low pressure open pouring dispensing unit the mixer of which is attached at the end of a lever which can be moved from mould to mould by the operator, and a set of static clamping units equipped with moulds and arranged in a semicircle. The moulds are pneumatically driven and, their opening and closure are done by the operator. The moulds can also be equipped with lasts where the uppers of the shoes are fitted.
The figures below show the units supplied by different manufacturers. The number of moulds those plants can accommodate vary from 6 to 30 units. The hourly output range lies between 50 and 250 units, depending on the model. Single layer soles as well as shoes can be manufactured. Because of their simplicity, those plants are the cheapest of the range of PU shoesole plants.
Higher production requirements resulted in the automation of the moulds movement which, eventually were fitted on a caroussel or turntable. Such a sole production plant is shown below. The caroussel can accommodate up to 30 moulds. The polyurethane components are mixed with a rotating mixer prior to being poured in the moulds. The head is cleaned without solvent.
The mixing head is mounted on a trolley which automatically advances to the open mould. The pouring sequence starts and stops automatically, then the trolley and the head slide back and the pneumatically operated mould closes. The caroussel turns and an empty mould aligns itself with the mixer. Up to 500 pairs of soles, made from systems with a demould time of 3 minutes, can be produced depending on the machine type. The metering pumps are of high precision, and the polyol to isocyanate ratios are digitally controlled
Mainly the sports shoe market is packed with all kinds of shoes with bi-colour soles. There is no doubt that the upmarket and most expensive brands include polyurethanes in the dense sole as well as the foamed interlayer. The sketches below (design Gusbi), show two different methods of producing such soles as well as the corresponding shoes. In both plants, the chemicals are mixed by a rapidly rotating, about 25 cm long screw, which also injects the mix in the mould. Eventually, the screw is housed at the front of the machine.
In the first sketch, the turntable revolves twice to mould the complete sole on the upper by means of a twin-head injector. During the first round, either the dense, sole, layer is injected, followed by the interlayer which is injected in the gap betwee the upper and the already moulded solid sole.
In the second sketch, the turntable revolves once to mould the solid sole, with one injector. The second injector moulds the interlayer between the sole and the upper. Of course the process can be reversed by moulding first the interlayer onto the upper and then the dense or solid layer in the gap between the interlayer and the mould.
One of the liquid injectors in the plant above can be replaced by, a more expensive, thermoplastic injection moulding machine, essentially for the moulding of the solid layer with TPU. In this case the density to the layer is 1.2 and its surface is devoid from defects which might arise when using a liquid injector. The latter is used for moulding the low density PU interlayer. In extremis it can be used to mould the solid layer with the injection moulding machine injecting an interlayer in EVA.
