This brief gives advice for:
- Varieties of Domes
- Physical Properties Related to Dome Design
- Dome Design
- Tool Design
- Thermoforming Conditions
- Equipment and Material Supplies
- Additional Technical Information and Assistance
NOTE- This brief is a companion piece to Roehm's Fabrication manual on Thermoforming. The brief you are reading adds to that information with specific data on skylights. Don't attempt to thermoform skylights until you understand the advice in both briefs.
IMPORTANT- Always consult with local building code officials prior to installing skylights made from ACRYLITE® acrylic sheet. Some restrictions may apply.
You can often use ACRYLITE® premium acrylic sheet, made by a continuous manufacturing process, in place of ACRYLITE® GP cell-cast sheet for skylights. ACRYLITE® premium sheet's engineered characteristics require slight fabrication modifications. These changes, such as lower oven temperatures and shorter heating times, reduce cycle time and production cost.
Varieties of Domes
The illustration above shows many possible dome configurations, including free-blown domes in circular, square, and rectangular shapes. Using skeleton mold fabrication, other potential shapes include pyramid and extended pyramid domes. Also shown are cold formed barrel vaults and sloped glazing. You can cold-form ACRYLITE® premium sheet within limitations. The radius of curvature must equal or exceed 330 times the sheet's thickness. Sheet length must not exceed 8 feet. If these criteria are not met, internal stresses may cause crazing (numerous tiny cracks) in the material.
Physical Properties Related to Dome Design
ACRYLITE® premium sheet is safer than glass because of greater breakage resistance. Under impact beyond its resistance, ACRYLITE® premium sheet doesn't shatter into small slivers but breaks into comparatively large pieces. It complies with American National Standards Institute (ANSI) Z97.1-1975, Safety Glazing for Buildings.
Although ACRYLITE® premium sheet's tensile strength is 10,000 psi at room temperature (ASTM D638-room temperature=68°F/20°C), continuous loads below this value can induce stress-crazing. For glazing applications, continuously imposed design loads shouldn't exceed 1,500 psi. In other applications involving continuous loading, loads should be less than 750 psi at 23°C (73°F).
All thicknesses of colorless ACRYLITE® premium sheet transmit 92% of visible light. White translucent ACRYLITE® premium sheet comes in three densities. Each white color's light transmission decreases as thickness increases.
Approximate Light Transmission of White Translucent ACRYLITE® Premium Sheet
|Color No.||Color Name||Thickness|
Other ACRYLITE® premium colors available in 3, 4.5, and 6 mm thicknesses are listed in the chart below. Light transmission of these transparent colors is the same for all thicknesses.
Approximate Light Transmission of Transparent ACRYLITE® Premium Colors
|Color No.||Light Transmission||Solar Energy Transmission|
Wash ACRYLITE® premium sheet with a mild soap (dishwashing liquid) and plenty of lukewarm water. Apply light pressure with a soft, clean cloth. Rinse with clear water; blot dry with a damp cloth or chamois. To remove grease, oil, or tar, use a good grade of hexane, aliphatic naphtha, or kerosene. Obtain these solvents at a paint or hardware store; use as recommended by manufacturers. Immediately wash away oily film residue with a mild soap and water solution.
DO NOT USE: Window cleaning sprays, kitchen scouring compounds, or solvents such as acetone, gasoline, benzene, carbon tetrachloride, or lacquer thinner.
CAUTION: Alcohol may cause crazing.
Dust with a soft, damp cloth or chamois. Dry or gritty cloths can scratch the surface.
Where necessary, wax ACRYLITE® premium sheet occasionally with non-solvented auto paste wax to protect it and maintain its surface gloss. Apply a thin, even coat with a soft, clean cloth; polish lightly with cotton flannel. Then, wipe with a damp cloth to help eliminate electrostatic charges. Keep the surface dust-free.
Store sheets in their original shipping cartons. Don't handle sheets unnecessarily until ready to use them. If storage procedures are correct, pre-drying before thermoforming is rarely needed.
Several heating methods are available for thermoforming ACRYLITE® premium sheet into skylights. These include flat, horizontal heaters, constant temperature horizontal ovens, or thermoforming machines. Vertical hot air ovens are seldom used. Because of poor air circulation, they heat unevenly, causing temperature differentials within the oven. This can result in uneven shapes. For a complete review of equipment available to thermoform skylights, see our fabrication manual on Thermoforming.
To heat acrylic quickly, assure the sheet absorbs the wavelength of the radiant heat source. The optimum wavelength for absorption of infrared waves is 3.2 - 3.6 microns for ACRYLITE® premium sheet. This requires emission temperatures of 1000 - 1200°F. If emitter frequency is outside this range, the sheet is transparent to much of the infrared radiant energy. Only surface conduction heats it.
These comments add to statements made in our fabrication manual on Thermoforming.
- Set the oven temperature at about 295°F.
- In addition to foam, consider rubber-backed felt or woven glass fiber cloth on the tool surface to prevent mark-off.
Many machine operation methods work in skylight manufacture. Each has advantages and disadvantages and depends on production volume, piece size, floor space, and dome shape.
Variables in dome design include dimensions, curb design, wind load requirements, and solar gain. Obtain information in publications from:
American Architectural Manufacturers Association (AAMA)
2700 River Road
Des Plaines, IL 60018
Another common term, dome rise, can be defined as:
- The formed height divided by the base dimension for square-base domes
- The formed height divided by the shorter base dimension for rectangular-base domes
- The formed height divided by the diameter for circular-base domes
Forming ACRYLITE® premium sheet orients molecules in the direction of stretching. This structural rearrangement improves certain properties. Strength is enhanced due to biaxial stretching, so you can use thinner material for a curved dome than for a flat skylight. Also, chemical resistance improves.
Heating method provides the basis for tool design. If you use a thermoforming machine, the manufacturer supplies tooling details and also establishes the machine tool design.
Positive pressure and vacuum are two dome-forming methods. Vacuum sealing requires less clamping force but also requires a deeper box than pressure blowing. For custom systems, consider a pressure box.
If you heat sheet from one side, corner areas may require extra heat. Place lamps or reflectors below the sheet to supplement heat from above. If you form double or triple domes, use interchangeable box frames to change dome dimensions.
Often, you can leave polyethylene masking on one side of the sheet while heating it from the other side. Peel masking , which guards domes against scratches prior to installation, from the plastic after it has cooled.
Design tooling so the sheet is heated uniformly, especially in corners. Turned-up corners and wavy edges indicate high fabrication stresses.
Clamp systems should allow preheating of clamping bars. A system temperature of 160°F assures a low stress, straight-edged dome. If automatic clamping is available, a low/high two-stage pressure clamp promotes uniform heating without stresses.
Clamping mechanisms range from pneumatic devices to an inexpensive hand system with several clamps on a side joined by an operating handle. Volume production may justify a pneumatic system. For custom, low volume shops, multiple hand clamps often do the job.
Several methods automatically control dome height or depth.
Electronic Proximity Switch (Capacitive Type): Controls an on/off solenoid in the air line. Mount the switch above the blowing area's center on an adjustable support. Design supports to swing side ways, providing work clearance.
Photoelectric Control: Design a photoelectric light source in the tooling to control height. Breaking the light beam will stop air pressure or vacuum. Consider light beam positioning when designing tools.
Microswitch Circuit: Mount a microswitch on a movable arm to control air pressure or vacuum. The tip of the switch contacts the plastic and may leave a mark.
ACRYLITE® premium sheet's forming temperatures are 290 - 3200F. Don't overheat. Establish heating cycles using temperature indicating tapes that change color as material heats.
If you heat sheet 3.0 mm (.118') thick or greater without a clamping frame, it may shrink up to 3% in the manufacturing direction. (See the discussion of manufacturing direction under "Procedures-Shrinkage" in our fabrication manual on Thermoforming).
In vacuum-forming, provide enough vacuum to keep the gauge above 20 inches of mercury during the forming process. Vacuum storage capacity should be twice that required for the dome.
If heated sheet contacts a surface, mark-off may occur. To prevent this, cover tooling with thin polyurethane foam, flocked rubber, or billiard table felt.
After forming, cool domes evenly in open air. Diffuse cooling air, if used, to avoid optical distortion.
If post-trimming is required, a carbide triple-chip tooth design saw blade works well. The blade should protrude about 1/8" above the workpiece. A relatively fast cutting rate minimizes frictional heat build-up between sheet and blade.
|Bubbles||Heating too rapidly||Lower heat temperature. Increase distance between heaters and sheet.|
|Uneven heating||If tubular rod isn´t the same red color from end to end, replace rod. Use screening to balance heat.|
|Excess moisture||Predry sheet. Preheat.. Keep moisture-proof masking on sheet until formed. Use older material first.|
|Uneven dome||Uneven heating||Check heater for heating eveness. Eliminate drafts. Baffle heat on all sides.|
|Clamping frame cold||Preheat clamping frame.|
|Bad Surface||Mold surface markings||Use mold covering (foam, felt, flocking).|
|Dirt on sheet||Clean sheet with deionized air.|
|Uneven Edges||Excessive forming temperature differential||Preheat clamping frame. Use slip clamp system (low/high).|
|Raises Corners||Excessive stress||Heat frames before inserting sheet. Add supplimental heat to corners.|
|Cracking in Corners During Service||Stress concentration||Heat sheet evently. Preheat frames. Add supplimental heat to corners.|
Equipment and Material Supplies
The suppliers listed below offer materials and equipment tested and approved by Roehm America LLC. Authorized ACRYLITE® Sheet Distributors may also offer materials and equipment.
|AAA Plastic Equiptment Inc.
P.O. Drawer 163169
Fort Worth, TX 76161-3169
Solder Absorbing Technology
144 Oakland St.
Springfield, MA 01108
|Despatch Oven Company
P.O. Box 1320
Minneapolis, MN 55440
P.O. Box 434
Beaverton, MI 48612
|Thermolable Sensitive Tape
Paper Thermometer Co.
P.O. Box 129
Greenfield, NH 03047
|The Grieve Corporation
500 Hart Road
Round Lake, IL 60073
|Snow Corp., EMC Div.
P.O. Box 9800-T
Fort Worth, TX 7607
(800) 433-2108 or (817) 732-5554
201 Leverington ave.
Philadelphia, PA 19127
214 Dalton, Box 5543
Charlotte, NC 28225
Additional Technical Information and Assistance
For more detailed information, see your local authorized ACRYLITE® Sheet Distributor or contact Roehm America LLC.
ACRYLITE® sheet is a combustible thermoplastic. Precautions should be taken to protect this material from flames and high heat sources. ACRYLITE® sheet usually burns rapidly to completion if not extinguished. The products of combustion, if sufficient air is present, are carbon dioxide and water. However, in many fires sufficient air will not be available and toxic carbon monoxide will be formed, as it will when other common combustible materials are burned. We urge good judgement in the use of this versatile material and recommend that building codes be followed carefully to assure it is used properly.
Like other plastic materials, ACRYLITE® sheet is subject to crazing, cracking or discoloration if brought into contact with incompatible materials. These materials may include cleaners, polishes, adhesives, sealants, gasketing or packaging materials, cutting emulsions, etc. See the Tech Briefs in this series for more information, or contact your ACRYLITE® sheet Distributor for information on a specific product.
This information and all further technical advice is based on our present knowledge and experience. However, it implies no liability or other legal responsibility on our part, including with regard to existing third party intellectual property rights, especially patent rights. In particular, no warranty, whether expressed or implied, or guarantee of product properties in the legal sense is intended or implied. We reserve the right to make any changes according to technical progress or further developments. The customer is not released from the obligation to conduct careful inspection and testing of incoming goods. Performance of the product described herein should be verified by testing, which should be carried out only by qualified experts in the sole responsibility of a customer. Reference to trade names used by other companies is neither a recommendation, nor does it imply that similar products should be used.