Pondering Plastisols

My regular readers will know that when it comes to making money with resins, there are more ways to skin a cat than shoot, cool and eject. Dip molding, for example, has a decidedly low-tech feel about it, but looks can deceive.

If you’ve always wondered how “rubber” gloves are made, the process is simple — in principle. Dip a hot (or cold) form in a liquid resin bath, remove, heat gently if cold, and then strip off the finished product. If the male form is ceramic and in the shape of a human hand, you have a glove. Other shapes, like handle grips and smaller thin-wall auto parts, are possible, metal or glass parts can also be coated with the process, and the calendaring community has their unique applications too.

The raw material is a “plastisol,” which is essentially a super-fine dispersion of resin particles, usually PVC, in an emulsifier. Like molding resins, additives are important in plastisols and may include fillers, pigments, stabilizers, etc.

Compared to conventional resin processing, dip molding has some unique control parameters.

Probably the most important issue is viscosity, since the flow of the liquid resin dispersion determines properties including part thickness, gel time, gloss, clarity and mold release. Viscosity, yield, shear thinning and other flow-related parameters are important enough to make viscometry a useful quality assurance procedure for incoming material, something essentially unknown to I/M or extrusion managers.

Molds or part substrates can be hot-or cold-dipped. Hot dipping involves preheated forms on which the plastisol gels on contact. Our old friend the glove is a popular application.

There’s a catch, however: remember the viscosity issue? Heat affects viscosity, and as each preheated mold is dipped and withdrawn, the plastisol gets thicker, affecting the physical properties of the finished product. Cooling of the ‘sol is the answer, making temperature control as important in dip molding as in I/M or extrusion.

Cold dip molders post-heat molds, making it easier to control plastisol viscosity. It’s possible to use thinners, but viscosity profiles in plastisols are basically locked in when the dispersion is formulated. Hot dip molders control the residence time in the tank, as gellation occurs on contact and both hot and cold dip molders need to regulate the rate at which the molds are withdrawn from the tank.

The combination of gellation and withdrawal rate maintains uniform coating thickness and prevents dripping and sag from the mold edges. Cold dip processes usually have slower withdrawal rates.

For a concise video clip of dip molding in action, see Harman Corp. at www.harmancorp.com/videos.aspx