Cold Cement vs Vulcanized
In this article we will learn the difference between vulcanized shoe construction and cold cement shoe construction. The vulcanized shoe construction is a much older technology and was mostly dropped in favor of the modern cold cement construction. However, fashions do change and the vulcanized shoe has made a huge come back.
Cold Cement construction allows the use of modern light weight plastic, foam and mesh materials due to the lower temperatures required for bonding. Every modern high performance athletic shoe for Running, Basketball etc… is made by the cold cement process. This is how Nike makes shoes. The vulcanized shoe construction process is the classic way to make a sneaker. This is how Converse All-Stars are made and how Vans skate shoes are made. Due to the high temperature required to vulcanized or cook the rubber outsole the material options are limited, Canvas and suede leather are common.
Cold Cement Shoe assembly Process:
In the cold cement process the shoe upper can be prepared with the strobel bottom. For this classic jogger style the outsole covers the edge upper so a strobel bottom can be used to make the shoe lighter and more flexible.
The Upper is steamed to soften the materials and the last is inserted and pulled tight. Once the last is tight inside the upper a second lasting machine pulls the heel edge. Once the last is secured inside the upper and temporary shoes laces pulled tight the upper is cooled to shrink the upper tight to the last.
The shoe may have a plastic or fabric part installed on top of the tongue to protect the surface from damage and drift during the lasting operations.
While the upper is being lasted the sole unit being prepared. In this case a the rubber sheet sole is combined with the EVA foam cushioning component cemented inside. This is done in a separate process that’s called stock fitting.
Now that the upper is lasted tightly and the outside unit is compete, the two pieces come together. The rubber sole unit will receive a coating of primer and cement. The outsole will get it’s own special primer designed for EVA and rubber. The shoe upper is also prepared with its own special primer and cement.
After the contact cement and primer has been completely dried in the heating tunnels the two pieces are joined together by hand. The skilled worked aligns the upper and outsole together then places the shoe is a hydraulic press.
The shoe will have 3 pressing operations, usually all done with one machine. A vertical press, Toe and Heel Press and Side presses. This insures there is full contact between the upper and outsole. Once the shoe is pressed together it’s often put in the cooling tunnel to set the glue.
After the cooling tunnel a shoe de-lasting machine is used to push the last out of the shoe without wrinkling the upper.
Now the sneaker is complete at this point you can insert the foot bed. The footbed may be molded EVA with a fabric cover or flat sheet cut foam. The flat die-cut footed is usually cemented inside the shoe, while molded footbeds are most often removable.
The new sneaker is ready for a final QC inspection, a quick check for any loose threads, cleaning and packing.
Vulcanized shoe construction:
For the vulcanized footwear construction we’re going to look at at the board lasting procedure. If a shoe is to be vulcanized you will need to use a metal last. The plastic last can not survive the heating required to cure the rubber.
Before the assembly can start all of the outsole components must be prepared. The uncured rubber foxing tape must be made just before assembly. If the rubber parts age they will not bond correctly to the other shoe parts. With the foxing tape we will need the actual bottom of the shoe. This part has already been molded, it about 90% vulcanized, so it’s still a little bit soft can can be bonded in the assembly process.
This part will also have the midsole filler added. The filler material is gray rubber with some air bubble blown inside, it’s made from the recycled uncured foxing tape. For vulcanized construction you cannot use EVA foam because the heat of the vulcanizing process will destroy the foam.
Board lasting operations.
For vulcanized construction the last must be aluminum to survive the oven temperature and the metal lasts heat up and cool down quickly. The first step is to lightly cement the lasting board to the bottom of the last, just enough to hold it in place during the lasting operation. The lasting board is a paper fiber board that provides stiffness to the finished shoe.
The lasting boards are made different materials and can be stiff or flexible, thin or thick. Now the last and upper are taken to the lasting machine. The toe lasting machine pulls the upper down onto the last and securely bonds the two parts together in one operation. Once the shoe heel and waist of the shoe are lasted the upper is ready for the outsole.
The first step of vulcanized outsole assembly process is similar to the cold cement process. The Upper and the rubber sole parts all received their coating of primer and cement. Now the rubber bottom with the cushioning wedge are bonded together with the upper.
The Foxing tape covers the rubber outsole part and overlaps up onto the upper. The Foxing tape must cover 5mm of the upper to have a solid bond. The shoe can now have the extra toe tape added, them a rear logo will be applied to over the joining seam.
With the tape applied it time to trip off any extra rubber with a hot knife and make sure there are no gaps. The sole will get a quick pressing to make sure the parts are fitting correctly.
With the last still inside the shoe placed on a steel rack so I can be wheeled onto the oven.
The shoe will be “cooked” in the vulcanizing oven for several hours, shoe is heated long enough so that the uncured foxing tape and the sole unit will fuse together.
After cooling, the shoe last is removed the footbed inserted and laces are attached.
The shoes are now ready for final inspection, cleaning and packing.
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