01 Apr Radio Frequency Welding of Plastisol
Quite often, we find items with a thickness that looks like high-density screenprinting, but with extremely fine detail that belies the process of high-density printing. We can find this process used on clothing, shoes, caps, phone cases, purses, and thousands of other mass-produced items. Look no further than the vinyl covering above the dashboard, or the armrests on your family vehicle or the seat on your bicycle or motorbike. All, and many other products, are produced with vinyl Oranisol or Plastisol over a cloth base and sealed with high-frequency electromagnetic waves or RF welding.
Radio frequency welding (RF welding) has been around since the 1940s, but today is a mature technology capable of welding two pieces of material together at speeds that rival screenprinting, but with a higher cost of production. Still, RF welding has earned a place in the decoration of mass-produced products, as well as waterproofing fabrics.
One of the first uses of the RF welding process was to produce fabrics that were coated with polyvinyl chloride (PVC) or polyurethane (PU) coating over a cloth material to produce a waterproof fabric such as used to produce backpacks, rain gear, tents, banners, and other products.
Vinyl coated polyester is a material frequently used for flexible fabric structures. It consists of polyester material, a bonding agent, and an exterior PVC coating. The polyester base supports the coating, which is applied in liquid form. The polyester base provides dimensional stability, as well as tensile strength without elongation. Depending on the formula of the PVC coating (vinyl Oranisol or Plastisol, the resulting product can be made waterproof, flame-retardant, and resistant to chemicals, dirt, mildew, oil, salts, water, and UV rays. It can be laminated by means of RF welding or hot-air welding, after which it can be sewn.
The tensile strength of the base fabric is determined by the size (denier) and strength of the yarns and the number of yarns per linear distance. The larger the yarn and the more yarns per inch, the greater the finished product’s tensile strength. An adhesive agent acts as a chemical bond between the polyester fibers and the exterior coating and prevents fibers from wicking, or absorbing water into the cloth. This prevents damage caused by absorbed water during a freeze-thaw cycle.
The PVC coating (vinyl Organisol or Plastisol) contains pigments to achieve the desired color. Fabric can also be manufactured with levels of light transmission that range from very transparent to completely opaque. After the coating has been applied to the backing, the fabric is either put through a heating chamber that dries the liquid coating or heated with RF welding to soften the plastics and bond the two layers together.
Product Brand Decoration
For small branding of products where great detail is desired, dielectric sealing or RF heat sealing are commonly used. A metal die is made to direct the shape and detail of the Organisol, Plastisol, or other polymers such as Nylon, PET, PEVA, EVA and some ABS plastics. The depth of the die determines the thickness of the final brand and can vary from paper-thin to depths of 1/4-inch or more. The die is filled with the coating material and partially set with heat. Then the fabric is placed on a table press and the die inverted and placed into position. The upper part of the press is brought into position with pressure to hold the two in place, while high-frequency waves are passed through the materials.
The high-frequency waves cause the molecules in the coating to heat, allowing the coating to create either a chemical or surface bond with the base material. In the instance of fabrics, the coating will flow around the threads and after cooling forms a surface bond. In some instances, depending on the surface energy of the base material an adhesive may be applied to the coating in the die before the RF welding takes place. This is not always ideal, as the adhesive leaves a tell-tale shadow around the image.
Today, you can purchase the designs which are made to your specifications with three-dimensional dies with a varied depth, in single or multi-colors, with adhesive backing, which can be applied to products using a heat press. There are heat presses available for curved or cylindrical items.