In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

Those highly reflective, crystal-clear plastic parts that sparkle like leaded glass are polymethyl methacrylate. This jewel of the plastics industry is now identified as acrylic (mostly in North America) or PMMA (in Europe and elsewhere).

Otto Rohm, who was half of the Rohm & Haas Co., started research on acrylic resins in 1901 in Germany. Acrylic was introduced in the U.S. in 1931 as fabric coating, paint, and safety glass liners. Sheet materials followed in 1936. Acrylic sheet found wide usage as aircraft window glazing and thermoformed domed canopies during World War II. Rohm & Haas introduced molding grades in 1937 under the Plexiglas trade name. DuPont countered with its Lucite acrylic compounds that same year. Acrylic dentures appeared in 1946, capitalizing on the material’s ability to be cast in plaster molds and its ease of color matching.

Defining Characteristics
Acrylic is a hard, stiff, brittle, crystal-clear, amorphous thermoplastic. Its defining characteristics are its transparency and outdoor weatherability. This combination makes acrylic one of the few plastics suitable for window glazing.

With 92 percent light transmission and a haze factor of only 1 percent, acrylic is the first choice for high-performance optical lenses. Acrylic can transmit light around corners and for long distances. This unique property accounts for its use in all kinds of light piping applications. The advent of computer-aided lens design, coupled with computer-aided machining of lens cavities, allows acrylic to perform in applications requiring precision light transmission.

Acrylic components have survived 20 years of outdoor exposure with no appreciable loss in physical properties. Prior to the introduction of polycarbonate, acrylic found wide usage as automobile taillight lenses. The construction industry specifies acrylic for skylights, greenhouse glazing, building panels, lighting globes, water faucet handles, and outdoor signage.

With a tensile strength in the range of 7000 to 11,000 psi and a flexural modulus of 325,000 to 460,000 psi, acrylic is a strong material. The heat deflection temperature at 66 psi is 175 to 225F. Unfortunately, acrylic has a notched Izod impact strength of only .3 to .6 ft-lb/in. Impact-modified grades raise that value to .65 to 2.5 ft-lb/in, with a reduction in light transmission, tensile, and flexural strength. For comparative purposes, acrylic fits neatly between general purpose polystyrene and ABS.

Acrylic is relatively low in cost with an average large volume price of $.87/lb and $.036/cu in. Toughened grades cost $.066/cu in.

Designing with PMMA
Acrylics have a relatively high melt viscosity, which must be taken into consideration in part, runner, and gate design. Processing conditions must be adjusted accordingly to avoid thermal degradation.

Wall thickness. Flow path lengths of 4 inches have been achieved with a part thickness of .035 inch. A .100-inch-thick part can be at least 6 inches long. There are always exceptions, but the minimum recommended wall thickness is .030 inch. Acrylic can be molded into abnormally thick parts. In years gone by half-inch-thick brush backs were a major market. Optical lenses of up to and greater than .750 inch are a classic example of acrylic’s thick-wall capabilities. These thick parts do, however, require abnormally large gates and runners, in conjunction with extended packing cycles and accurate cavity temperature control.

Radiusing. Acrylic is a brittle, notch-sensitive material. Every effort must be made to avoid sharp inside corners and abrupt changes in thickness. The literature specifies a minimum inside radius of 30 percent of the part wall thickness. Less molded-in stress and greater impact strength will be achieved with a radius of 50 percent of the nominal wall. The best radius would be the largest that is possible.

Molding draft angles. With a good draw polish, inside draft angles can be as small as 1 1/2 degree /side. Acrylic’s low mold shrinkage factor does not pull the part away from the cavity during cooling. A 1 1/2 degree /side draft is recommended on the outside surface of optically clear parts.

Projections. By capitalizing on acrylic’s low mold shrinkage factor, ribs, bosses, and other added features can be designed with a thickness of 75 percent of the part’s nominal wall thickness. In those instances where sink marks cannot be tolerated, the thickness of projections should be reduced to 60 to 65 percent of the part wall thickness.

Depressions and holes. All kinds of holes can be molded with acrylic; however, weldlines can be a problem. Weldlines bend light rays and distract from the appearance of transparent parts. Acrylic is a relatively hard flow material and the length of small unsupported cores must be limited to 2 1/2 to 3 times the thickness of the core pin.

Tolerances. Acrylic is a dimensionally stable material. A commercial tolerance on a .125-inch-thick, 1-inch-long part can be ±.0055 inch. A more costly fine tolerance can be ±.0030 inch. Even tighter tolerances can be held with special molding procedures.

Acrylic is an old, reliable material with properties that are not present in other plastics. Regrettably, acrylic does not get the attention and publicity of the newer engineering materials or the emerging families of alloys and blends. Many young plastics engineers are not familiar with acrylic’s impressive capabilities. This is a unique plastic material that is worth spending some time getting to know. Every design engineer should include acrylic on his or her palette of plastic materials.