We have a fascination with gloss. Think of shining shoes, polishing apples for the teacher, don’t touch my car, squeaky-clean and more. Somehow, shiny noses are the other way around, which is why we have face powder. Anyway, plastics give us a variety of gloss options. The injection molder relies on the mold surface, but in extrusion we must deal with the natural gloss of the material.

Glossy isn’t the same as smooth. For example, leather-grain surfaces can be embossed into otherwise-glossy extruded sheet. Remember that, like most embossing, the pattern can be reduced or even washed out when the product is reheated above the embossing temperature, as is necessary for thermoforming. Gloss is also dependent on angle of testing, and isn’t the same as haze or transmission; these are optical properties, and deserve more attention than is possible here.

Fortunately, most plastics are inherently glossy. The non-crystalline (amorphous) polymers are also inherently transparent, but even for the semicrystalline resins, the crystallites are usually so small that they don’t affect gloss much.

We can change gloss with additives. Fillers like calcium carbonate will do it, and mica will give a sparkly opacity, which is not quite gloss, but has its own appeal. With fillers, particle size and shape are very important, so they must be selected and specified with some care and pretesting.

Some polymers aren’t simple homopolymers but have components that affect gloss; one of the most important of these is high-impact polystyrene (HIPS), made by including rubber chains either in the actual polystyrene chain, or as an additive during or after polymerization. For these, there is a balance between impact strength and gloss: More rubber is duller but tougher.

In the extrusion process, melt temperature is the most important controller of gloss. The hotter, the better, but beware the visible discoloration and invisible brittleness that may appear  if the surface gets too hot. Measuring melt temperature isn’t simple, and its value and accuracy depend on how and where it’s measured (at the screw tip is the least reliable, but it’s still better than nothing).

One way around this overheating is to laminate or co-extrude a surface layer of a glossy polymer over a a tougher but less glossy core. Most refrigerator door liners used to be made that way, until the makers realized that the glossy surface was more brittle and showed scratches more easily, so the dull surface became preferable. It may also be preferred for some picture frames or other wood-like surfaces, or insulation that is pulled through a conduit, where too slick a surface increases friction.

The lamination process is still useful for applying special surfaces, and is thus competitive with co-extrusion. Making the laminating film separately costs more, but thickness is better controlled, so material savings may make it cheaper. One of my clients for many years was a maker of formed products, where a pre-extruded printed film was put over a thin, 25-mil sheet during extrusion. This veneer was then rolled up and stored for use in the same extrusion line to cover one or both surfaces of a thicker sheet, often black, which contained all the edge trim and the trim from forming the end products. Most of the time, the materials were similar, so adhesion of the film to the hot, newly extruded surface(s) was good, and no adhesive was needed. When using films that didn’t adhere well, a heat-reactive adhesive could be coated onto the pre-extruded film, or the laminate was preheated just before application, or both.

External glazing is another technique, where the extruded surface is passed under radiant heat (a bar or lamps) or even a flame for some profiles (flamepolishing). If it’s a gas flame, connect the gas source to a switch that shuts off the gas if the profile stops moving, thus avoiding fires or escaping gas.

Another way to get the same effect is to increase the distance from die to cooling surface. The surface stays molten longer and has more time to level out, just as a painted wall doesn’t show brush strokes, as they level out before it’s completely dry. (OK, I know wallpainters use rollers now and not brushes, but the principle of leveling still works, and they still use brushes for moldings and corners.)

All these glazing methods may embrittle the surface just like running a hotter melt temperature. Many years ago, I saw a line that flowed carbon dioxide or nitrogen over the surface in the glazing area, thus reducing the oxidation and making the surface less brittle and less likely to crack in service. It worked, and maybe someone out there is still using it, but the result has to be worth the cost.

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years, as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or e-mail him at [email protected].

Griff will present his one-day practical seminar, "Introduction to Extrusion," in Los Angeles on Nov. 15 and Houston on Dec. 5. Topics include the ten (11) key principles of extrusion, plastics chemistry for non-chemists, a review of extrusion hardware, the limits to production rate, quality control of raw materials, simplified rheology, start-up and shut-down procedures and troubleshooting common extrusion problems. E-mail him at the address listed above for more information.