Factors For Selecting Acrylic

(Author: Savey Bakarne | Article Views: 13 | Word Count: 498 | Published: Dec 20th 2007)

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When selecting material on basis of environmental conditions features that figure into the selection are chemicals likely to be encountered, sterilization methods, humidity, temperature, and thermal cycling. Additional important factors are mold and part design, processing requirements, and assembly methods. When selecting the best grade of acrylic to use, these same factors must be considered. The higher molecular weight grades are more resistant to crazing from chemical exposure and mechanical stress, but have lower melt-flow rates. But as the level of impact modifier is increased in impact-grade acrylic, other properties such as clarity, light transmission, and tensile strength often diminish. It is difficult to predict processing requirements as flow patterns and mold-filling problems can result from large part surfaces, deep ribs or flanges, asymmetrical geometries, and unbalanced thick and thin sections. While experimenting with several different grades in the prototype tool is recommended. Molds are designed for acrylic, ABS, and polycarbonate parts typically accommodate a shrinkage level of 0.006 in allowing for convenient trial runs of acrylic in molds used for ABS or polycarbonate parts. But direct comparison of the parts molded from different materials can help determine the best one for the application.
When compared with other polymers, acrylic is relatively easy to process. It can be molded with little or no residual stress and is available in formulations specifically designed for injection molding or extrusion in a wide range of melt-flow rates. When processing is done under normal conditions, acrylic produces melts that are typically higher in viscosity than those of many other thermoplastic polymers. Also the higher molecular weight grades are generally recommended for extrusion. As acrylic has higher viscosity flow properties, injection molding runners and sprees used to process it need to have larger diameters than those handling polystyrene or polyethylene. Parts with large injection molded or parts with thin-wall geometries require a high melt-flow rate. On processing acrylic in molds built for polycarbonate or polyester, lower injection pressures should be used, as it is less likely to warp than polyesters and is more forgiving in molds not designed with well-balanced gates.
As acrylic has a higher melt viscosity than polystyrene or styrene alloys thus it requires higher injection pressures when selected in place of those materials. Also higher clamping forces may also be required. Acrylic is a hygroscopic material thus it absorbs water and must be dried prior to molding. But if it is molded while wet, it exhibits moisture splay, leaving streaks, bubbles, and a rough surface on the part. Thicknesses for acrylic parts range from 0.040 to 0.500 inches. The thick or thin part can be achieved with special designs or processing methods such as injection/ compression molding. Main consideration is consistency and any changes in thickness should be gradual, and feature radiuses edges.

About the Author

Savey Bakarne is a professional retail display specialist and teaches store owners how to gain profits from utilizing the proper Custom Retail Store Displays.

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