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Testing Your Acrylic IQ -- Answers to Common Questions

A basic primer for molders and fabricators.

By Peter D. Colburn

Acrylic offers numerous advantages for applications requiring exceptional optical clarity, high impact resistance, rigidity and outstanding weatherability. Proper knowledge of acrylic’s characteristics and proper molding techniques can help molders become more efficient and profitable.

The following Q&A offers some insights.

Q: What are the advantages of acrylic-based compounds over other plastics?

A: Acrylic is inherently more light-stable and scratch-resistant than other plastic materials. Other plastics require stabilizers or surface coatings to match the weatherability and scratch resistance of acrylic. This is why acrylic is primarily used in applications that take advantage of its superior clarity, scratch resistance and UV stability characteristics. Acrylic is also more transparent than glass, lighter in weight, and is used in many applications where glass has been traditionally used.

The exceptional weatherability of acrylic also makes it ideal for most outdoor applications where transparent materials are required. Acrylic is very rigid, providing stability in design.

Perhaps the most outstanding feature of acrylic materials is their optical clarity. Acrylic transmits more light than any other material. This makes it an excellent choice for lenses and other applications where optical quality is required.

Q: How does acrylic meet the challenges of manufacturing thin-walled parts, for example with DVD replication?

A: Acrylic materials are available in a wide range of flow grades, some of which are better suited to thin-wall molding than others. Acrylics that are best suited for thin wall applications have a high melt flow rate and good thermal stability.

A good example of this in practical use is with DVD replication. This requires that the acrylic be molded into a 0.6mm thick by 12cm diameter disc in a 5.6-second cycle time without degrading the material. It also requires producing a high-quality optical part with exceptional surface replication to allow reproduction of the digital information on the stamper.

Q: How well do acrylics work in gas-assisted molding operations?

A: Acrylics work well and have been used successfully in gas-assisted molding operations. Typically, such operations are used to minimize part weight by blowing a gas bubble inside a part. This process is most often used for opaque materials where the bubble cannot be seen, so we see this more often with opaque acrylic alloys than with standard transparent acrylics. Acrylics can also be used with foaming agents to reduce part weight. However, because of the transparent nature of acrylic, this is not very common.

Q: How critical are material considerations when designing a mold?

A: Make sure that molds are designed with the maximum processing window to allow flexibility and room for increased efficiency in manufacturing. 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 shear thinning, taking the same approach with amorphous acrylic materials may simply lead to degradation and surface defects.

In applications such as displays, the inherent rigidity and strength of acrylic allows the design of a thinner part than with other transparent plastics. This allows the end user to use less material.

Physical strength characteristics should also be considered. The notch sensitivity of acrylics means that parts should not be designed with notches. This can be overcome to some extent with the use of impact-modified acrylics. The high optical quality of acrylic also amplifies the importance of a mold's surface polish and detail characteristics since acrylic transmits more light than any other material.

Q: What material advancements can we expect from acrylic compounds in the next five years?

A: One of the key areas that acrylics are focused on at this time is optical media storage, such as DVDs. Acrylics were initially used in laser discs because of the high transparency and low optical distortion of the product. The inherent properties of acrylic, such as naturally superior transmittance and extremely low optical distortion, mean that current limitations of optical storage capacity could be exceeded and at a lower cost than currently used materials.

Peter D. Colburn, Technical Manager, Molding & Extrusion Compounds (M&EC), CYRO Industries, directs all performance aspects and operations of technical service, new product development, process chemistry and processing engineering groups. He also provides technical support for CYRO’s M&EC manufacturing and toll manufacturing facilities, as well as direction of pilot plant operations for all M&EC products. For more information contact: (800) 631-5384 or visit www.cyro.com.

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