RF Welding and Compression Molding for Shoes
Radio frequency welding (RF welding) and compression molding are widely used to make modern athletic shoes. Both processes are used to create logos, design details, and even the shoe structure. In this article, we will review how Nike™ and Adidas™ are using these techniques.
Compression molding and RF welding both use heat and pressure to remold shoe materials. Welding can create unique design effects and save labor costs.
Man-made and natural materials can both be used but they require different tooling and RF welding techniques. Man-made materials such as mesh, PU, and foam can melt, mold, fuse, and stretch. Natural materials, such as leather and canvas, have some stretchability but will not remelt or fuse without the use of a synthetic backing material.
The factory will use EVA, PU, PE, and PVC foams to make the backing layers. These foams all accept the compression and with the right head and pressure, they will not rebound after RF welding.
RF Welding Tooling:
Logo welding equipment is usually Computer Numerical Control (CNC) cut from a soft, conductive, and rust free metal. Usually brass. Larger pressing tools can be CNC cut from flat aluminum sheets. Pressing tools that require a flat surface to press against can be made one-sided. The one-sided welding tool is limited in the dimensions it can create. A two-sided or “matched welding tool” can create 3D pockets and larger features. After molding, these pockets can be filled with foam or silicon.
Two classes of equipment are used to weld and emboss. The radio frequency or RF welder uses focused energy to locally heat the shoe materials and the machine comes in many sizes. A small, tabletop unit is great for logos, while a square meter sized machine is best for clothing and entire shoes.
Let’s look at some popular shoes and discuss how they are made.
Nike Air Zoom Pegasus ’92
This sneaker uses a combination of RF welding and compression molding to create a unique upper design. Nike has laminated a thin layer of soft, 4mm PU foam between a semi-elastic lining and a jersey knit polyester outer shell. The upper is a single u-shaped piece of material. A ribbed embossing tool is used to crush the PU, creating the design. The factory makes the Swoosh and Air Pegasus logo in a separate molding operation.
The factory die-cuts the swoosh logo from flat TPU stock. A small RF welding machine is used to mold the logo’s 3D shape while simultaneously melting and bonding it to the fabric surface. The Air Pegasus logo is made by a similar process with higher heat to create the details. The Air Pegasus logo is made from metallic TPU film.
The tongue is completed by screen printing the logos and then welding the edge design. The tooling required to build this shoe is not expensive when compared to the outsole cost but must be CNC cut from brass.
Nike Flywire 3.0
The Nike Flywire shoe is made by welding die-cut TPU parts directly to knit mesh. To make this style, the flat, die-cut parts are loaded onto a flat plate. A second flat plate is loaded on top and pressed with high heat and tons of force. The TPU melts into the fabric surface for a permanent bond.
TPU parts can be die-cut or laser cut for more complicated shapes.
Adidas Stan Smith Premium Leather
I really like what Adidas has done with this version of the classic Stan Smith. The Adidas designers have taken advantage of the Stan Smith’s toe cap pattern design to make a molded, one-piece heel, without any breaks. In this shoe design, Adidas shows the mold-ability of PU coated action leather. The heel logo detail is made by compressing the leather panel with a matched mold. When using a matched mold, the inner surface follows the outer surface to stretch the leather. After molding, the factory will fill the empty space with silicone gel or foam padding.
Shoe Material Design Guide
Do you want to be a professional shoe designer? You must learn how to select and specify footwear materials correctly. The Shoe Material Design Guide details all the shoe materials you will need to make modern athletic, classic casuals and high fashion footwear.
Each chapter covers a specific shoe material type. You will learn how each material is made, the options available to you, and how to specify the material correctly. Inside you will find chapters on leather, textiles, synthetics, laces, glue, reinforcements, hardware, logos, midsoles, outsoles, and more!
See exactly how each material is used inside real production shoes. We have included annotated cross sections of over 30 different shoe types. Look inside basketball shoes, running shoes, track spikes, hiking boots, work boots, high heels, cowboy boots and many more!
You will also find information on topics such as material testing, sustainable production, exotic materials and more. Written as a companion to our best selling How Shoes Are Made, The Shoe Material Design Guide digs deeper into the world of footwear materials and design. Softcover, 330 color photos, 28 chapters, 195 pages.