The Gumboot Manufacturing Process
The building of hand-made natural rubber gumboots is a very labour and capital-intensive process requiring multiple handling of most of the components. Most Skellerup gumboot styles have in excess of 19 individual components for each half pair of boots.
There are 7 major steps when manufacturing a pair of natural rubber gumboots (and several other ancillary ones). They are as follows:
1. Mixing. The natural rubber is combined with other ingredients such as carbon black, oil, fillers and curatives and mixed either in an enclosed mixing machine or on an open top mill. Extreme accuracy is required when weighing out the ingredients at this stage to ensure the desired physical properties of the mixed compound is achieved.
Once the compound has been mixed it is stored to allow it to ‘settle’. The compound is tested to ensure it complies with the pre-determined specifications.
From the store the uncured rubber is issued to 4 pre processing departments prior to the assembly stage.
2. Heel Moulding. The heel of the gumboot is compression moulded. This is done by placing a small piece of pre weighed uncured rubber (a slug) into a mould cavity that is subsequently squashed under high pressure and concurrently heated.
3. Outsole Calendering. Sheets of uncured rubber (pigs) from the store are warmed up on a mill to make them soft and are then fed through the outsole calender. A calender is made up of a series of metal rollers that process the uncured rubber to a certain gauge and in some cases (as in this instance) roll a pattern onto the surface of the rubber. Heated knives are used to cut out the desired sole size subsequently by cutting around a template. (Note for gumboots with a moulded sole such as the ‘4 X 4’ this process is not required).
4. Upper Calendering. As before ‘pigs’ from the store are warmed on a mill then fed into the upper calender (when we refer to the upper this relates to the thin outer layer of rubber on the gumboot). Unlike in the outsole calendar, a perfectly smooth finish is left on the surface of the rubber. The continuous thin sheet of rubber is wound up onto drums. An assortment of components is then cut out from the continuous sheet of upper rubber.
5. Skim and Friction Calendering. Skim is the canvas lining inside the gumboot. It has a layer of rubber skimmed onto the surface that makes it form a very strong bond to the other rubber components. Friction is canvas that has had rubber forced into the weave of the canvas and is used as a reinforcing layer internally in most of Skellerup gumboot styles. Unlike in Upper Calendering, a roll of canvas is fed into the first 2 rollers where either the rubber is ‘skimmed’ or ‘frictioned’ onto or into the canvas. The Skim and Friction is then rolled onto the drums. An assortment of components is then ‘clicked’ out from the continuous rolls of skim or friction using a clicking machine.
6. Boot Assembly. The assembly chain is a long narrow table with a chain revolving around the outside with the lasts sticking up in the air with 22 people standing on station each doing a particular operation. Every pair of gumboots is assembled on a ‘last’, and is done by laying on the successive layers of skim, friction, uncured upper rubber components etc to the last and finally a sole and heel is affixed. Because the rubber is uncured it is sticky so once it is laid in the desired position, it stays there.
7. Vulcanisation. The lasts are removed from the assembly chain and packed on a boot trolley and pushed into a large oven called a vulcaniser where the boots are cured by heat and under pressure. Once the vulcanisation is complete the boot trolleys are removed and the gumboots are pulled off once the aluminium lasts have cooled down.