MINIMIZE IMPACT OF STRESS ON ACRYLIC SHEET THROUGH ANNEALING
Fabrication techniques such as thermoforming, cementing, machining, line bending, buffing, flame polishing, and screen printing can cause stress on parts made using acrylic sheet. Annealing, the strengthening of acrylic sheet through controlled heating and then cooling, minimizes the effects of fabrication stress.
Internal stresses in the sheet can cause crazing (the appearance of numerous tiny cracks in the material) when acrylic comes in contact with solvents such as glass cleaners or paints. Annealing helps to reduce crazing or large scale cracking by reducing internal stresses and increases the strength of cemented joints.
Choosing an Oven
Commercially available, forced-air circulation ovens designed for the annealing and heating of parts are recommended. Good forced air circulation ensures uniform temperatures essential to the annealing process. The oven's air velocity should be between 150-250 feet per minute and should be controllable to within +/- 10°F (+/- 6°C) to avoid uneven or excessive heating.
Temperature control selection effects oven performance. Controllers monitoring oven temperature and maintaining constant voltage into the heating elements are considered to work best. Percentage timer controls, which regulate the percent of time heaters are on, can be used, but may not provide uniform heat. Proportional time controls with step switches to vary heat output may also produce uneven temperatures.
It is possible to anneal with a restaurant-type oven obtained from a restaurant supply firm, but be sure the oven incorporates air circulation and accurate temperature control systems.
Also, select oven size carefully. Be sure to provide large doors for large parts, and smaller doors to avoid heat loss when inserting small parts.
Before the annealing process begins, the part must be clean and dry. Spray masking, protective tape, paper masking, and other material must be removed to prevent it from baking onto the material. Plastic masking may remain in place.
To anneal acrylic sheet, such as CYRO Industries' ACRYLITE® FF, heat it to 180°F (80°C), just below the deflection temperature, and cool slowly. Heat one hour per millimeter of thickness. For thin sheet, a minimum of two hours is recommended.
While cooling times are generally shorter than heating times, thermal stresses may occur if cooled too quickly. A minimum cooling time of two hours should be used. For thicknesses above 8mm, the hours required to cool equal the thickness in millimeters divided by four. Do not remove items until the oven temperature falls below 140°F (60°C). Removing a part too soon can offset annealing's positive effects.
Sheet Thickness Heating/Cooling Time
It is important to ensure sheet parts are adequately supported during annealing. Raised sections may need independent support to prevent sagging. Lack of proper support can also inhibit relaxation.
During fabrication involving surface machining, stresses occur only at or slightly below the surface. Therefore, the entire thickness need not be annealed. Heat for at least two hours, and cool as usual. If holes were drilled throughout the sheet, position the part so heated air flows through the hole.
Annealing after cementing requires an interim wait of at least six hours to avoid bubble formation which is caused by rapid solvent evaporation in the joint.
Written by Grant La Fontaine, Sheet Products Service Manager, CYRO Industries.
For more information, contact D. Artz, CYRO Industries, 100 Enterprise Dr., P.O. Box 5055, Rockaway, NJ 07866-5055, 800-631-5384, FAX 973-442-6117, Web: www.cyro.com.
NOTE: Acrylic sheet is a combustible thermoplastic. Precautions should be taken to protect this material from flames and high heat sources. Users should undertake sufficient verification and testing to determine the suitability for their own particular purpose. Be sure to follow manufacturer's safety recommendations for equipment and materials used with acrylic sheet.