Testing your acrylic IQ: understanding the characteristics of a specific compound can mean the difference between success and failure. Here's a look at the benefits and nuances inherent with the ever-popular acrylic. (Plastics).
So says Peter Colburn, the technical manager of molding and extrusion compounds at Cyro Industries. Colburn directs the technical service, new product development, process chemistry, and processing engineering groups at Cyro. He shared with Product Design and Development his insights into this versatile compound, which is more transparent than glass and offers greater stability than other plastics.
Q: What are the advantages of acrylic-based molding and extrusion compounds?
A: Acrylic is inherently more "light stable" and scratch resistant than other plastics. Other plastics require stabilizers or surface coatings to match its weatherability and scratch resistance. Its exceptional weatherability makes it ideal for most outdoor applications where transparent materials are required. Acrylic is also very rigid. 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 injection molding with acrylic compounds affect manufacturing costs?
A: Compared to transparent plastics such as polycarbonate and polystyrene, acrylics are very competitive when considering the balance of price and properties. As with other plastic materials, acrylics are only as efficient as the processes and designs in which they are used. It is important to 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 can restrict the molder when it comes time to make operations more efficient. While smaller gates can improve efficiencies with crystalline materials through significant shear thinning, taking the same approach with amorphous acrylic materials can simply lead to degradation and surface defects. When secondary processes or additives designed to achieve certain performance characteristics such as weatherability or scratch resistance are required, a move to acrylics can reduce costs.
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. As a result, it is seen more often with opaque acrylic alloys than with standard transparent acrylics. Acrylics also can be used with foaming agents to reduce part weight. Again, because of the transparent nature of acrylic, this is not very common, unless one is looking for a visible bubble effect in the part.
Q: How do acrylic materials affect tooling?
A: Acrylic materials are generally mild on tooling. They are not abrasive, corrosive, or damaging to tools unless additives are included that cause some undesired effect. When lubricants based on fatty acids are added, chrome plating is recommended to protect tool surfaces.
Q: How does acrylic meet the challenges of manufacturing thin-walled parts?
A: Acrylic materials are available in a wide range of flow grades, and some are better suited to thin-wall molding than others. The acrylics that are best suited for thin-wall applications have a high melt flow rate and good thermal stability.
Q: How critical are material considerations when designing a mold?
A: A review of material properties when designing molds is a very important step in producing parts that meet the expectations of end-users as well as providing manufacturers with the ability to produce parts profitably. These properties include flow and thermal properties as well as physical strength characteristics. Quite often molders are forced to work with mold designs that do not consider the ability of the material to flow and fill the mold or which fail to cool parts effectively. This results in tools that have very narrow operating windows, limiting the ability of the manufacturer to mold parts properly Then, to earn a reasonable profit, molders must cut corners or abuse materials to reduce cycle times, leading to part performance malfunctions and often creating a perpetual cycle of high scrap problems. Well-designed molds produce faster cycles and better parts. One of the issues mold designers must realize is that what works well with one material may not work with another. Designers must be aware of the limitations of different materials when designing molds. For example, shear thinning of crystalline materials helps fill tools more quickly, leading people to design with smaller gates to create high shear, reduce viscosity, and improve flow. On the other hand, high shear gate designs in tools for acrylics can cause degradation and lead to longer cooling times. Fortunately, there are computer-based simulation and part design tools available that can account for material differences and process effects. However, it is important for engineers to remember that these are only approximations.
More information on acrylics is available by contacting Cyro Industries, Box 5055, Rockaway, NJ 07866, calling (800) 631-5384, writing in 60 on our reader service card, or replying online at www.pddnet.com.
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|Publication:||Product Design & Development|
|Article Type:||Brief Article|
|Date:||Jun 1, 2002|
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