Materials Update 19761

Material primer: Acrylics 

Molding aesthetically pleasing acrylic parts requires a bit of know-how. According to Peter Colburn, technical manager of molding and extrusion compounds for Cyro Industries (a major acrylic producer), proper knowledge of acrylic's characteristics and molding techniques can help both designers and molders to be more efficient. 

Cost. Acrylic materials are prized for their optical clarity, rigidity, weatherability, and scratch resistance. Compared to transparent plastics such as polycarbonate and polystyrene, acrylics (using average, per pound pricing) fall in the $.90 range, with polystyrene at $.60, and polycarbonate at $1.80. 

When certain performance characteristics such as weatherability or scratch resistance are required, a move to acrylics may reduce costs. With other transparent materials, secondary processes or additives are needed to achieve these goals. Acrylics can eliminate the need for either of these. However, as with some other resins, there are peculiarities of acrylics that deserve special tooling and processing considerations. 

Mold design. According to Colburn, it is important to make sure that molds are designed with the maximum processing window to allow for increased efficiency. For example, high shear conditions that are present with smaller gates may restrict the molder when it comes time to make operations more efficient. While smaller gates may improve efficiencies with crystalline materials through significant shear thinning, taking the same approach with amorphous acrylic materials may simply lead to degradation and surface defects. Colburn recommends that gate sizes be 60 to 80 percent of wall thickness at the gate. 

Part design. Acrylic's rigidity allows the design of a thinner part than with other transparent plastics. Its flexural modulus of 470,000 psi, compared to 340,000 psi for polycarbonate, means that under a given load, PC parts will deflect about 28 percent more than acrylic ones. So in practical use, a thinner part made of acrylic will support a load equivalent to a thicker part made from polycarbonate to reduce material usage. The exact reduction varies according to resin; for example, with a 28 percent higher stiffness than PC, acrylic parts could withstand a 28 percent reduction in wall thickness vs. the same part molded in PC. 

Troubleshooting. Various defects may occur when injection molding parts, and it is often difficult to determine the real cause of a given problem. When troubleshooting, Colburn recommends trying one change at a time, and then waiting for the effect to take place. "This is the key to being successful," he adds. 

For example, when bubbles appear inside molded parts, Cyro suggests the following sequence: 

Tooling concerns. Acrylic materials are generally mild on tooling. They are not abrasive, corrosive, or damaging to tools unless additives are included that may cause some undesired effect, which is rare. Lubricants based on fatty acids are sometimes added, and therefore chrome plating is recommended to protect tool surfaces.—Michelle Maniscalco 

Resin Statistics 

Plastic resin production totaled 6.2 billion lb in March 2001, a decrease of 4.6 percent from the same month in 2000. These production figures were up 6.9 percent from those of February. Year-to-date production in 2001 totaled 17.6 billion lb, a 6.5 percent decrease compared to the same three-month period (January through March) in 2000. 

Sales and captive (internal) use of plastic resins in March 2001 totaled 6.3 billion lb. This is a decrease of 13.7 percent from the same month one year ago. March 2001 sales and captive use was up 2.6 percent over the total of the previous month. Year-to-date sales and captive use in 2001 totaled 19 billion lb, a decrease of 5.9 percent compared to the same three-month period in 2000. 

These figures are based on primary data for selected major plastics materials as compiled by Veris Consulting LLC, for APC's Plastics Industry Producers' Statistics Group.