P.U.Soling Technology.... Simplified !
by Naveen Kapur

Polyurethane Soles are here to stay, more so because the technology behind the production is now easily accessible, the market for P.U. Soles/ directly Soled Shoes is growing at a fast pace, raw materials and intrinsic Components are easily available, machinery available with back-up service and enigma of P.U. now stands exposed !

Some of the salient features of P.U. Soles are:

• Light Weight.
• Better thermal stability.
• Good Abrasion Resistance.
• Not too highly priced, when compared to TPR Soles, Considering that prices are falling.
• Good flexing resistance.
• Comfortable (Contours more easily conceived)
• High design parameters permissible.
• Good anti-skid properties.
• Long life.
• Possibility to direct-mould a full shoe on the low pressure machine.

Although there are various disadvantages, such as:

• High investment cost of project (machine, moulds and raw materials).
• Non-re-cycling of finished product (although we, at ARYAVRATA, are now introducing an Italian concept to re-cycle P.U.)
• Chemical technology which does take a learning curve; the advantages far out-weight the disadvantages, and is a project with excellent returns.

For a detailed project report, you may contact ARYAVRATA IMPEX.

The basic principle involved in this technology is the mixing of two liquid chemicals, Polyol and Isocyanate, using a highly specialized mixing head and pouring the liquid mixture at low pressure into an Aluminium Mould. Before the mixing, both ISO and Polyol need to be pre-heated and then kept in tanks with finely regulated temperature, Epoxy moulds may be used (one tenth the cost of Aluminium Moulds, but quality suffers, since epoxy moulds do not allow a high grade finishing, does not maintain consistent precise temperature levels) for low quality runs, which do not call for high quality finishing. The liquid mixture / composite, coming into contact with atmospheric conditions, starts to react and foam or solidify and thereby takes the shape of the mould and is thus extracted from the mould in the shape of the Sole, with details of the mould (shape, contours and design) finely etched on the finished product. P.U. being highly adhesive (we all use P.U. adhesives for sole / upper bonding !) has a tendency to glue itself on the moulds, if some layer or a release Agent is not applied or pre-sprayed onto the mould interiors. This Release Agent, is normally Silicon-based (for Automobile P.U. Systems, it is wax-based) and apart from allowing a good release of the final product from the mould, it imparts a finer copying of the mold details to the finished product and more important, Release Agent gives the required feel (soft / hard) and look (matt / shiny), to a certain extent.

P.U. is must be noted, is a highly precise technology and choice of Machine is critical to a project's success. For example, the P.U. in the tanks and in the mixing head, must be kept at a precise temperature, so you need to have circulating heating and cooling systems, which balance out to specified temperature. The ratio between ISO and Polyol decide the eventual density of finished Sole and thereby precise mixing ratio is a critical factor. The metering pumps need to be of a high quality. The locking mechanism for a Rotary table is a very important, to Mixer quality is another critical factor.

With P.U. pouring technology, there are two options, one, in which the mixing head is stationary, and the mould-holders rotate (Rotary Machine); these machines normally have either 32, 40, 60, 90 or 100 stations, as per production required, and are costly options. In the other type, called BANANA (thanks to the shape of mould-holders), the mixer is moved manually, while the mould-holders/ moulds remain stationary; in such a case you could have 6, 12, 18, 24 and upto 32 stations / moulds and is a more economic option. The latter is an excellent option for someone who is starting to get into this technology, the cost being approx. less than one third the cost of a Rotary.

One the moulds front, you could use the mould holder to seat moulds with an option of fabricating a single odd, a pair at a time or a mould-system to directly mould the sole onto the upper ( which is Stroebel - stitched and on a plastic last). One could actually make any article in P.U., as long as the finished product remains within the size of the mould-holder, whether machine is rotary or banana type.

Once the finished Sole is extracted from the mould, the Release Agent needs to be de-greased from the Sole, using a de-greasing machine, so that final finishing on the P.U. Soles, using  lacquers could be done. Lacquers could be applied using Sirem Spraying Machine (for Matt finishes with a variety of finishing techniques, like masking or etching of lines etc.) or a Dipping Machine (for Shiny Soles). Final finishing is also possible using Rollers and specialty Finishing Chemicals and techniques.

P.U. production requires a deep understanding of Chemicals and their combining as also the requirement of an appropriate machine with high precision technology. There are many variables that ensure new problems crop up very often, but harnessing this technology is no longer wrapped in mystic realms, and we would be happy to provide our services to assist you make the right choice.

Naveen Kapur
Email : aryana@ndf.vsnl.net.in

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