Walk into a supermarket, you most likely to see these slippers, flip-flops hanging like dried fishes on shelves. Commonly, they are known as “rubber shoes”. They are easily considered as cheap and low quality footwear. However, in the world of footwear design, injection is a manufactory technology, which has been taken to a level of style, fashion and design by brands like Crocs, Native, Rains, just to name a few. They are light weight, affordable, stylish and damn comfortable. They are rubberish, and pretty fun to wear or just look at them. They are almost toy like.
So how exactly do you make these “rubber shoes”?
In this article, I am going to introduce you to the world of injection footwear. Simply put, injection is a process of forming a “rubber shoe”. The injection process requires Molding and Machine. Molding is the shape of the shoe, and Machine is where the injection is taking place.
During one of my design and consulting job, I had the opportunity working first hand on a “rubber shoe” from development to production.
The injection molding process is kind of magical. A liquified material and blowing agent(s) are injected into a mold which sometimes could be less than half the size of the final “rubber shoe”. After computerized pressure, temperature, speed and time, “rubber shoe” pops out of the mold once it is open. It is like holding a 3-4 times larger size of a foam in a hand, then release it.
The determine fact for how big the mold used for injection is actually a design fact. Sometime a design requires certain amount of details. And technicians will have to try out different mold size to see which one is able to produce the best details. The rule is to keep the mold as small as possible without affecting design details.
The mold for “rubber shoes” is one of the most complicated ones in footwear manufactory. These are not your kitchen waffle making molds. These molds are so complicated that the creation of “rubber shoe” mold is a project on its own entirely. In order to create the right size “rubber shoe”, the process of managing the expansion-contraction is very complex, which sometimes the expansion-contraction is even affected by weather because of temperature, moisture, etc. That’s why even the largest footwear manufactory usually out-source the making of “rubber shoe” molds, which requires CNC machines to produce. A series of computing, tooling are involved to create CNC machines.
The liquified material, which is a chemical compound called Ethylene Vinyl Acetate (EVA for short). EVA is widely being known as “expand rubber” or “foam rubber” in manufactory. That’s why these EVA shoes are called “rubber shoes”.
Please take a look at these images I took during my visit to factory.
Please leave comments if you want to know more about “rubber shoes”.