Rubber is a versatile material and widely used for sole production. It has sought after abilities such as good abrasion resistance, good flexing and cushioning effects. It has a long history in outsole production and many synthetic solutions aim to imitate rubber in different ways. It´s possible to use in raw form as Crepe rubber, but can also be molded and defined to outsoles with near-endless possibilities for design shape and color. Crepe is generally used for more casual and dressed shoes where molded rubber soles are generally for sneakers and sports footwear.
- Good abrasion resistance
- Good flexing
- Cushioning effects
- Many design options
The first known uses of natural latex are by the Olmec tribe of South America, around 1600bce. Olmec means "rubber people" in Aztec language They were the first to know how to harvest the rubber by making small incisions in the bark of the tree and collect the milky substance. They boiled it to produce rubber balls which they later used as part of games.
Aztecs are known to have waterproofed fabrics but it was first 1819 when an American coach-builder, Thomas Hancock, took up the interest of the material it would see further uses. In 1825 he patented a process of making artificial leather using rubber and a variety of fibers. Together with Charles Macintosh, the man behind a style of coat known as the "mac", he developed a water-resistant rubberized "double textured" fabric. A version of it is to this day used for making the "macs".
The process of curing rubber has been done since pre-historic times but the modern process of vulcanization, named after the Roman god of fire: Vulcan, was not developed until the 19th century. In 1844 Hancock filed a patent for "vulcanization of rubber" in the UK. 8 weeks later, but not knowing of each other, Charles Goodyear filed a very similar patent for the vulcanization process in the US. This would later have a big influence on the footwear industry. Although Goodyear tends to be the person credited with the discovery, nowadays the common view is that Hancock had managed to refine his technique earlier and rightfully claimed his patent first.
Crepe - Natural Rubber
Crepe rubber is made from latex, a milky fluid coming extracted from Hevea brasiliensis, commonly known as Rubber tree. This is extract is coagulated with formic acid in a controlled environment. The coagulated substance is then processed in a "creping battery", a series of machines that crush, press and roll the rubber material. The sheets are hung in a heated drying shed and then sorted by grade and rolled up for transportation to the lamination process. The purpose of the lamination process is to achieve the desired thickness of the crepe, suitable for footwear. In a lamination-table setup, layers of crepe are put on top of each other until the needed thickness is achieved. In case of blemishes in a layer of crepe added, they are cut out with scissors. The natural tack of rubber avoids subsequent tearing. The slab of sole crepe is passed through a pair of compression rolls to seal the layers and the pressed sheet is then trimmed to the dimensions required.
There are different kinds of Natural rubber products:
- Pale latex crepe (PLC) is a premium grade, made from raw field latex.
- Thin white crepe (TWC), about 1 mm thick.
- Thick pale crepe (TPC), about 3-5 mm thick
- Estate brown crepe (EBC) is made from a "cup lump" (raw, naturally coagulated rubber from the collection cup) and another coagulum.
- Smoked blanket crepe is made from thick sheets of latex that have been processed in a smoker.
- Flat bark crepe is made from scraps and other poor quality raw product.
Rubber Sheet / Calendar outsole
Raw rubber is firstly prepared together with different agents creating a lump of material. This is processed in a calendar, a roll used to press material to desired thickness and structure. It´s normally used to produce rubber sheets on which cutting dies are used cut the sheets into the right size and shape for soles. Normally the outsole has a distincy anti-slip pattern which is either hot or cold pressed on the full sheet or outsole by outsole. If the pattern is pressed on a sheet instead of individual soles then pattern possibilities are less but productivity increases.
Rubber Sheet / Calendar outsoles are mainly used for vulcanized footwear as the bottom piece with a rubber foxing creating the outsole sidewall.
Rubber outsole pressing
Rubber outsole pressing is a very common process for high-quality cup soles. It´s comparable with TPR but is more durable and has a softer and more tactile touch while a less shiny look. TPR soles are injected as difference to rubber which can´t be injected. Similar to TPR aluminium molds are needed according to desired sole shape and look.
The first half of the production process is the same as when creating calendar soles. Raw rubber is prepared with forming agents and then calendared into rubber sheets. This to have a material in a workable size. The rubber material is cut into shapes according to the mold size and placed in the molds. Sometimes multiple pieces are put into the same mold to compensate for the designed consumption. In order to achieve more than one color on an outsole multicolor molds can be used. It´s also possible to input rubber pieces of a different color to have a logo in a different color. This is depending on the mold design.
The molds are closed and heated, leaving the rubber material to melt and expand. After a couple of minutes the molds are taken out to be opened, the outsoles extracted and left to cool down. Excess material is trimmed off and soles are put in temperature-controlled storage to stabilize dimensions.