Polyurethane is a polymer commonly used for producing insoles, midsoles or outsoles in the footwear industry. This article is describing thermosetting polyurethane used for outsoles but there are also thermoplastic polyurethane (TPU) widely used as well with slightly different properties. PU is used since it´s generally lightweight, has good abrasion resistance and is very flexible. Both PU and TPU is also widely used in for coatings of materials, where TPU can be used without any backing.

The first polyurethan was developed during 1937 by chemists in Germany. Since then it´s undergone many improvements to the product we have today. PU are generally coming in two forms, either solid or a blowing agent is used to decrease the density to achieve a lighter material. This is know as cellular PU or foam rubber. Read more about foam rubber later in this article.

Generally PU is either used through a Direct Injection Process (DIP) or by pouring.

Chart of different sole materials and durability. Source: www.polyurethanes.org

Foam rubber

Wedged foam rubber sole from Dr Sole

Foam rubber is made by blowing an agent into the base material. This expands it by creating a cellular structure, it lowers the density and making it spongy and more light. With a less solid material the durability is affected negatively. Today the most common polymer to use is Polyurethane.

Foam rubber is sometimes referred to as rubber sponge, expanded rubber or blown rubber.

Foam rubber sole from Dr Sole

Solid PU vs Cellular PU (foam rubber)

Solid PU generally has a more shiny appearance than cellular PU which can be controlled by anti-shine agents. The big difference is in weight and abrasion resistance.

Left: Solid PU. Right: Cellular PU

PU Direct Injection

The direct injection process pushes liquid PU into a closed mold, creating an outsole around the shoe. This creates a very strong bond between the outsole and shoe. Due to the high extrusion pressure the PU gets a good grain structure and obtains good physical properties.

PU Direct Injection Process (DIP)

PU Pouring

By mixing Polyol and Isocyanate, with PU the right consistency and properties to pour the material into an open aluminum mould through a mixing head. The molds are closed, heated for the PU to set and cooled until opened.

There are two options of PU pouring machines:

Rotary machine or Conveyor type fabricated mould holders machine. where the mixing head is stationary, and the mould-holders move . It requires less workmanship but relatively big investment in molds and space. A rotary PU pouring machine typically have 32, 40, 60, 90 or 100 mold stations. It´s common that not all stations are having a mold but it´s depending on the current production need.

The banana machine has gotten it´s name from the shape of the mold rack. The mixer is moved manually where the molds are stationary. It´s generally a smaller machine but requires more manual support. It normally has 6, 12, 18, 24 and up to 32 mold stations.

Banana PU pouring machine