LINE BENDING GIVES SHAPE TO YOUR ACRYLIC PRODUCTS
Problem-free line bending of acrylic sheet requires the right combination of appropriate equipment and proper procedures. Done correctly, line bending provides strong and visually appealing results, adding shape and form to acrylic made products, as well as increasing its usefulness in a broad range of applications.
Straight Nichrome Resistance Wire Heater
To bend acrylic sheet up to 1/8" (3mm) thick, a straight nichrome wire resistance heater is the most commonly used piece of equipment. Electricity passes through the wire in the heater, much like a common household toaster. Material thicker than 1/8" (3mm) can be used on this heater, but the sheet must be flipped over midway through the heating cycle.
To construct a heater, use plywood for the base. The top surface should be aluminum sheet or insulation board. Insulation board provides a smooth surface and helps prevent mark-off (marks on the sheet due to contact). Aluminum is chosen as the metal alternative because it is durable and does not absorb and reradiate heat.
The heating element is a 17-gauge nichrome (nickel-chrome) wire. Attach the wire to a 120-volt, 20-amp variable transformer. Keep the wire under tension with the use of a spring.
Provide space around the wire for free airflow. This aids in the heat transfer from the wire to the part. The wire must remain at least 1/8" from the sheet during heating. Use ceramic standoffs to support the wire, at a consistent height, from the base.
CAUTION: An unguarded nichrome wire is a burn/shock hazard. Provide a guard to prevent contact with the wire.
Modification for Multiple Line Bends
For multiple line bends, modify the heater as indicated. Include an insulation ring where the wire joins the spring. Raise the unit's base so convection air from below flows through slots routed in the base.
Coiled Resistance Wire
Coiled resistance wire may be substituted for straight wire as a heating element. Coiled wire must be supported, however. It is also difficult to position the coils evenly. This causes temperatures across the sheet's width to vary, resulting in uneven stresses and distortion.
Tubular Rod Heaters
A tubular rod heater with a heating rod, similar to the heating element of an electric stove, is used to heat sheet thicker than 1/8" (3mm). A variable transformer is used to power the heater.,Place a reflector, preferably of split aluminum conduit for its good reflectivity, beneath the rod. For shorter heating cycles, mount heaters above and below the sheet. Use water-cooled supports to avoid marks on the acrylic.
Radiant Quartz Tube Heaters
Quartz tube heaters, which look like a thin fluorescent light bulb, are among the most efficient sources of radiant energy. The heater consists of a coiled resistance wire housed in a quartz-coated glass tube. These heaters allow bending of acrylic sheet 1/4" (6mm) thick or more.
Use a reflector with a quartz tube heater for the most efficient heating. For shorter heating cycles, mount heaters above and below the sheet. Power the heaters with a variable transformer.
To ensure that the bend is at the proper angle and to prevent the part from springing back to its original shape, use a cooling jig. An all-purpose jig for 90° bends can be modified to suit the part. A cooling jig that exposes both sides of the heated sheet to air will accelerate cooling and reduce stresses.
Bending with a Straight Nichrome Wire Heater
Heat the acrylic sheet until it bends easily. Don't overheat or lay the sheet directly on the heating element. This will cause the sheet to blister or mark.
Bend the heated part to the desired shape and quickly place it in the cooling jig. To retain the bend, secure the part with weights or clamps. Cooling time normally equals heating time.
When line bending with only one heating element, bend the part away from the heated side. For example, if the nichrome wire is under the sheet, bend upwards.
Line Bending with Coiled Resistance Wire
The procedures for line bending with coiled resistance wire are similar to nichrome resistance wire heating. Flip the piece over during heating to prevent uneven stresses in the bend area.
Long Line Bends
Line bends in excess of 24-inches in length may cause the part to bow across the bend. This can also occur when a bend is made close to the sheet's edge. To minimize bowing, restrict the width of the heated area by using a strip heater or v-groove the sheet along the desired bend line.
When a line bend exceeds 36-inches in length, the part tends to bend unevenly (slight wave) due to shrinkage. Bending perpendicular to the manufacturing direction of the acrylic sheet will help reduce this tendency.
Sharp Line Bends
For sharp line bends with no deflection, v-groove the sheet. Use a v-rabbet router bit, or if using a table saw, make two opposing 45° cuts. This reduces the material cross-section at the point of bending and minimizes stresses. A wire heater beneath the cut will help create a sharp angle bend without deflections or bowing.
Bending with a Tubular Heater and Quartz Tube Heaters
Bend the part as described for a nichrome wire heater. The acrylic sheet should be at least 1/2-inch from the rod to avoid overheating.
By changing the distance between supports, the width of the heated area can be increased or decreased. This varies the curvature of the bend allowing for a gentle curve in the part, instead of a right angle bend.
Written by Grant LaFontaine, Sheet Products Technical Service Manager CYRO Industries. For more information contact: D. Artz, CYRO Industries, P.O. Box 5055, Rockaway, NJ 07866, 800-631-5384, FAX: 973-442-6117, In Canada: 800-268-4743, Web: www.cyro.com.
|Bubbling or Blistering||Overheating||Lower Heat.|
Reverse or turn over work.
Increase sheet heater distance.
|Wet Material||Pre-dry sheet.|
|Wrinkling||Narrow heating||Widen heated area.|
Increase bend radius (bend radius at least twice sheet thickness).
|Bending part wrong way.||Bend away from heated side.|
|Mark off (stray marks on sheet surface)||Surface Contact||Avoid overheating.|
Cover jig with felt.
Reduce areas of contact in cooling jig.
|Bow effect||Stresses from heated and unheated areas||Use straight nicrhome wire to decrease width of heateed area.|
Design reverse curve in cooling jig.
Heat entire part in oven.
Use a V-groove bend line.
|Inherent shrinkage in sheet||Bend sheet perpendicular to direction of manufacture|
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.
For more information, contact Grant LaFontaine at the address shown above.