Briquetting process eases handling of powders, granules, regrind.

If you have to handle resins, additives, or regrind in the form of powders or fine granules, there's a way to eliminate problems of dusting, caking, and bridging without the heat history imparted by extrusion pelletizing. Briquetting is a method of compacting particulate materials under moderate pressure into a pellet-sized "pill," thereby rendering the material free-flowing and increasing its bulk density.

A `NEW' OLD TECHNIQUE

Briquetting is not a new method, though it's probably unfamiliar to most plastics processors, says Richard Komarek, president of K.R. Komarek Inc., Elk Grove Village, Ill., a maker of roll-press briquetting equipment. This process has found a number of niche applications in post-finishing of thermoplastic powders--e.g., acetal copolymers, ethyl cellulose, nylon, polycarbonate, and polyphenylene oxide (PPO). Komarek thinks that if the process were more widely known briquetting could probably provide material-handling solutions to a wider plastics audience. He suggests handling of reground or shredded scrap as one example. Powdered thermosets--such as phenolic, epoxy, melamine, urea, and urea formaldehyde--are other likely candidates for this "cold" densification process. Still another possibility, Komarek says, is briquetting dryblends of resin and additive where either one or both is in powder form. "Briquetting the resin/ additive mixture immediately after blending can improve quality control by preventing segregation or settling out during conveying or storage, disturbing the resin/additive proportion," he explains. He also notes that briquetting need not be viewed only as a substitute for extrusion pelletizing--it can also improve feeding of powders to the extruder. Ability of the briquetting process to serve wider markets is abetted by improvements in the equipment over earlier designs.

HOW IT WORKS

A roll-type briquetter has two rotating rolls with half-cavities or pockets cut into their surfaces. The rolls are indexed so that the half-cavities of opposing rolls come together as they rotate. Loose material is fed between the rolls in the nip region, filling the cavities just prior to closure. Pressure applied by the rolls compresses the material into a solid mass. As the rolls turn further formed briquets discharge from the pockets as the half-cavities move apart.

For very heat-sensitive materials like epoxy powders, rolls can be water-cooled to counteract heat build-up. However, Komarek says polyolefins do not lend themselves to this process. Their lubricity requires high compaction pressures, which can generate enough heat to exceed the resins' low melting points.

Roll presses can make many different briquet shapes and sizes--down to 2 g (0.07 oz). Throughputs can range from 20 lb/hr for bench-scale models to 100,000 lb/hr for large systems.

IMPROVING THE TECHNOLOGY

The versatility of today's roll-type briquetters is afforded largely by the ability to optimize pressure according to the properties of the materials to be compacted. Either too much or too little pressure can degrade briquet quality. Earlier spring-loaded roll presses did not provide much control of compaction pressure. Modern roll presses have hydraulic pressure control, Komarek says. One roll is mounted in fixed bearing blocks and the other in movable bearing blocks. Hydraulic cylinders press against the bearing blocks that support the movable roll. Hydraulic pressure is maintained by nitrogen gas accumulators, which require no pump or motor. Roll pressure is controlled by simply adjusting the gas pressure in the accumulator.

Another critical factor in producing good-quality briquets is ensuring that the proper amount of material is fed into each cavity. Earlier machines used gravity feed in which material was simply allowed to drop into the roll cavities. Most modern systems use adjustable-speed screw or auger feeders, Komarek notes. Better feeding control often permits higher roll speeds and thus greater throughput rates.

Besides controlled feeding, screw feeders provide the advantage of precompacting and de-aerating the incoming material. In addition, screw feeders allow the alternative of feeding a vertical pair of rolls from the side, rather than feeding a horizontal roll pair from above. Horizontal flow of materials to the rolls, free of the effects of gravity, provides better control, Komarek claims.

DIFFERENT TYPES OF ROLLS

Roll presses come in two main types. One type has cantilevered rolls (Fig. 1a), which are mounted outside their bearing blocks. Thus, the forming rolls protrude from one end of the press. This design allows the rolls to remain more nearly parallel as compaction forces spread them apart, resulting in more uniform briquet volume. These units usually have larger diameter shafts with greater torsional rigidity, thus supporting higher roll speeds. The open-end design also permits easier roll changes; however, it limits roll face widths to about one-third of roll diameter. Cantilevered rolls in a vertical arrangement are most common in plastics applications.

[Figure 1a ILLUSTRATION OMITTED]

The other type of briquetting press has symmetrical rolls (Fig. 1b), which are mounted between their bearing blocks. This type accommodates wider rolls with more cavities for higher throughputs. This design also can apply greater roll pressure for the same roll diameter. And symmetrical mounting lets the rolls skew to maintain more uniform pressure across the roll face, which achieves more uniform briquet density.

[Figure 1b ILLUSTRATION OMITTED]

Briquetting rolls themselves are classified into three types:

* Integral rolls (Fig. 2a) and their shafts are formed in one piece, usually with a band of stainless steel or other corrosion- or abrasion-resistant metal welded onto the working face. Because they have no joints, integral rolls are most often specified for food or pharmaceutical uses where cleanability is of paramount importance.

[Figure 2a ILLUSTRATION OMITTED]

* Integral rolls also can easily be cored for steam heating or water cooling.

* Solid rolls (Fig. 2b), sometimes called "tires," are the most common type. They consist of replaceable rings keyed or shrink-fitted onto their shafts. This allows rings and shafts each to be made of the most suitable materials, whereas integral rolls often require some compromise in material properties to satisfy both roll and shaft specifications. Replaceable rings also allow for machine changeovers at lower cost than with integral rolls.

[Figure 2b ILLUSTRATION OMITTED]

* Segmented rolls (Fig. 2c) have a number of working-face sections that are mechanically clamped onto the shafts. This design further simplifies frequent machine changeovers. But it is also the most expensive design--making it most appropriate when briquetting hot or highly abrasive materials that require specialized roll surfaces or frequent replacement due to wear. However, soft plastics can extrude into the cracks between segments, causing briquets to stick in the machine and not eject cleanly.

[Figure 2c ILLUSTRATION OMITTED]

Equipment suppliers can perform test runs on samples of your material to determine the best roll, press, and briquetting conditions for the applications. (CIRCLE 18)