Direct soling via PU injection

  • Special focus: difficult-to-mix PU systems
  • Low-pressure machine with self-cleaning mixing head
  • Machinery for PU boots that are 40% lighter than rubber or PVC

Klöckner Desma is a leading manufacturer of automated lines and robotics for the industrial production of lightweight shoe soles made from polyurethane (PU) and materials combined with polyurethane. The company puts a special focus on manufacturing systems for demanding applications such as, for example, difficult-to-mix PU systems.

PU direct injection machine

PU direct injection machine (photos: Desma)

Direct Injection Process

Direct injection, a process where the sole is moulded directly onto the lasted upper, is used in approximately 10% of the worldwide shoe production. Here a soling material is liquid by heating or chemical reaction and is injected into a shoe mould, from which it is ejected once it has set to the shape of the mould. Three materials, PU, TPU, rubber, either used solely or in combination, can be used in the direct moulding process.

Direct soling via PU injection

Direct soling via PU injection

PU is the result of the reaction between the liquid components polyol and isocyante. These components are pumped into a mixing chamber and mixed by a rotating screw. A chemical reaction takes place and the PU is injected directly into the shoe mould. The moulds are placed on the rotary machine with multiple mould stations. By an exothermic reaction the sole stabilises and de-moulding becomes possible.

PU pouring detail

PU pouring detail

Low-pressure machine with self-cleaning mixing head

PU is a very good adhesive and sticks in the case of direct injection onto the lasted upper, but not only there. To guarantee an efficient production flow even when producing small parts from a difficult-to-mix PU system, the company offers low-pressure machines with mechanical mixing and a self-cleaning mixing head. The mixing head can be equipped with eight different component valves and can mix two or more components in one shot: these include colours and other additives as well as powder in form of PU waste. Changing the colours is done from shot to shot without material waste, according to the company. The throughput can be selected from 20g/s up to 150g/s. The mixing head utilises a tapered mixing screw in a tapered chamber. The screw rotates with an adjustable speed from 6,000RPM up to 21,000RPM and is said to give a shear-intensive mixing. The combination of centrifugal force and shear cleans out material residues from the mixing chamber.

Direct injection by moulding the sole direct onto the lasted upper is used in approximately 10% of worldwide shoe production.

PU boot machinery

Desma was the first company to conquer the market in PU boot machinery. PU boots are 40% lighter than their rubber or PVC counterparts and retain their flexibility even in extreme temperatures. And, thanks to the millions of tiny bubbles enclosed within the polyurethane, they possess good insulation properties, keeping the wearer’s feet cool in summer and cosy in winter, whilst providing ample cushioning for the foot. Due to the materials’ characteristics in terms of chemical and water resistance and anti-static properties, PU boots fulfill all requirements regarding safety footwear.

Desma’s colour modulation technology allows the creation of dramatic colour swirls.

Recycling of shoe soles

An additional option in PU processing is bulk material filling technology. The use of recycled sole material as bulk filling technology becomes possible at PU pouring machines for unit soles. For material cost reduction, inexpensive fillers and PU waste material can be added.

Mixing head for metered addition of solids

Mixing head for metered addition of solids

This offers new possibilities for the designer such as in case of the production of sandals of a cork/PU mixture. The variable bulk material dosage takes place in the mixing chamber above the component valves. Homogeneous mixing is obtained by a special screw geometry and the infinitely variable screw speed. The quick change of process, the possibility of using different PU material systems and of changing between several colours, meet the requirements for a flexible production. Possible applications for the solid additives dosage include insoles, wedge soles, heel fillers, bathing or sauna sandals.

Recycling of shoe soles and bulk material filling

Related Articles & Comments

Comments are closed.