Making Footwear: Mold Release & Finishing Applications

Shoes are as much about comfort and performance as they are about style and durability. Footwear must not only look good, it also has to perform and last.

Every season brings new designs to keep up with fashion trends and technology improvements. This fast design cycle means manufacturers need flexible processes that respond to the changing market.

Shoes typically have two main parts:

• The bottom or sole often is made of molded foam.
• The upper is made of a range of materials, from leather to synthetic fabric - even plastic.

Processes to manufacture each part are usually automated and strictly controlled. Mastering these operations is the key to making good footwear in a demanding market.

If your shoes have foam soles, the footwear manufacturer likely used a reaction injection molding (RIM) process.

1. It starts with a mold.
2. A mold release agent coats the mold cavities.
3. Reactive chemicals are injected into the mold
4. The foam solidifies.
5. The injection mold opens.
6. The shoe sole is removed.

So that the foam sole easily comes out without defect, the mold release agent must be precisely applied. It is crucial that...

• the surface finish of the release agent is smooth, as it will impact the smoothness of the foam part.
• the inside of the mold is coated evenly, or the foam will stick and tear when it is removed.

Any overspray that doesn’t coat the inside of the mold is considered expensive waste.

Using electrostatic spray guns to apply mold release agents can significantly reduce overspray, waste and defects.

• For manual spraying, Graco has the Pro Xp60 WB Air Spray Manual Electrostatic Gun for mold release applications.
• For automatic spraying with reciprocators or robots, Graco offers Pro Xp Auto and Pro Xpc Auto electrostatic air spray guns, as well as the ProBell rotary bell atomizer.

Using a water-based mold release agent?

No problem. The batch system isolates waterborne material in a cabinet, where it is charged. This allows the mold release agent to be sprayed electrostatically in a safe, efficient manner.

For any given shoe model, the upper is often painted with several different colors or designs. Each must be applied in a precise way to meet the requirements for each style.

For flexibility when changing between many colors and designs, Graco’s ProMix PD uses positive displacement. The pump technology sets the standard for quick and low-waste color changes.

Once the upper is painted, it’s adhered to the sole, completing the footwear. To ensure durability, just the right amount of adhesive needs to be applied. Too much will result in quality defects and overspray waste.

For adhesive spraying, AirPro spray guns can precisely control the amount sprayed, providing a uniform adhesive layer. This allows the shoe to stay intact while reducing overspray waste.