Straight Nichrome Resistance Wire Heater
To bend ACRYLITE® premium sheet up to 1/8" (3 mm) thick, the most commonly used piece of equipment is a straight nichrome wire resistance heater. The unit functions similarly to a household electric toaster by passing electricity through a wire.
You can bend material thicker than 1/8" (3 mm) on this unit, but the material must be flipped over halfway during the heating cycle.
In constructing the heater, use plywood for the base. The top surface should be insulation board or aluminum sheet to provide a smooth surface, helping prevent mark-off marks on the sheet as a result of contact. Aluminum is selected as the metal alternative because it is durable and does not absorb and re-radiate heat.
The heating element is a 17-gauge nichrome (nickelchrome) wire. Attach the wire to a 120-volt, 20-amp variable transformer. Provide a spring as shown to keep the wire under tension. Allow space around the wire for free air flow to aid heat transfer from the wire to the part. Position the wire so it remains at least 1/8" from the sheet during heating. Ceramic stand-offs are useful in supporting 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
To perform multiple line bends, modify the heater as shown on previous page. Where the wire joins the spring, include an insulation ring. Raise the unit’s base so convection air from below flows through slots routed in the base as shown.
Coiled Resistance Wire
Coiled resistance wire can be substituted for straight wire as a heating element. Because coiled wire must be supported and because it is difficult to position the coils evenly, temperatures across the sheet’s width may vary, causing uneven stresses and distortion. See the following section entitled 'Line Bending with Coiled Resistance Wire'.
Tubular Rod Heaters
To heat sheet thicker than 1/8" (3 mm), use a tubular-rod heater with a heating rod like the heating element of an electric stove. Power the heater with a variable transformer. 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 are among the most efficient sources of radiant energy. They look like a thin fluorescent light bulb. These units provide capability to bend ACRYLITE® sheet 1/4" (6 mm) thick or greater. The heater consists of a coiled resistance wire housed in a quartz-coated glass tube.
Use a reflector with a quartz tube heater for most efficient heating. For short heating cycles, mount heaters above and below the sheet. Power the heater(s) with a variable transformer.
Use a cooling jig like the one shown below to assure that your bend is at the proper angle and to prevent the piece from “springing back” into its original shape. This is an all-purpose jig for 90° bends - you can modify it to suit the part. A cooling jig which exposes both sides of the heated sheet to air will accelerate cooling and reduce stresses.
Be sure to follow the manufacturer’s safety recommendations for equipment and materials used with ACRYLITE® sheet.
Bending with a Straight Nichrome Wire Heater
Heat the material until it bends easily. Experience will teach you how long to heat each part. Don’t overheat or lay ACRYLITE® sheet directly on the heating element - doing so will blister and/or mark the sheet.
Bend the heated part to the desired shape and quickly place it in a cooling jig. To retain the bend while you do other work, secure the part in the jig with weights or clamps. Cooling time normally equals heating time.
When line bending with only one heating element, be sure to bend away from the heated side. For example, if the nichrome wire is on the bottom, bend upwards.
Line Bending with Coiled Resistance Wire
Follow the procedures above for nichrome resistance wire line bending. Flip the piece over while heating to prevent uneven stresses in the bend area.
Long Line Bends
When a bend line exceeds 24 inches in length, there is a tendency for the part to bow across the bend. This can also happen when a bend is made close to the edge of a sheet. To minimize bowing, restrict the width of the heated area or, v-groove the sheet along the desired bend line, as shown below.
When a line bend exceeds 36" in length, the part tends to bend unevenly (slight wave) along the bend due to shrinkage. To minimize this tendency, make the bend perpendicular to the manufacturing direction. You can tell manufacturing direction when the label or masking paper is still attached to the sheet.
If there is a label, manufacturing direction is from top to bottom as you read it. If masking paper is still on the sheet, manufacturing direction is from top to bottom as you read the words on the masking.
Sharp Line Bends
For sharp line bends with no deflection, it is a good idea to v-groove the sheet, as shown below. Use a v-rabbet router bit or a table saw, making two opposed 45° cuts. V-grooving the sheet reduces the material cross section at the point of bending, reducing stresses and making bending easier. A wire heater beneath the cut will help create a sharp angle bend without deflections or bowing.
By changing the distance between supports, the worker can increase or decrease the width of the heated area, thus varying the curvature of the bend. In this way, the part can be bent with a gentle curve instead of a right angle bend.
|Bubbling or Blistering||Overheating||Lower heat; reverse or turn over work
Increase sheet heater disrance
|Wet Material||Pre-dry sheet|
|Wrinkling||Narrow heating||Widen heated area
Increase bend radius (bend radius at least twice sheet thickness)
|Bending part the 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 for, heated and unheated areas||Use straight nichrome wire to decrease width of heated 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-see foregoing drawing|
Additional Technical Information & Assistance
For more information or specific questions about your project, contact Roehm America LLC's Technical Service Representatives.