Line Bending


Equipment

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.

Cooling Jigs

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.

Procedures

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.

Gentle Bending

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.

 

Troubleshooting

Problem Cause Solution
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

 

Equipment and Materials Suppliers

The suppliers listed below offer materials and equipment suitable for use with ACRYLITE® premium sheet. Authorized ACRYLITE® Sheet Distributors may also offer materials and equipment.

Variable Transformers

Superior Electric Company
383 Middle Street
Bristol, CT 06010
860-585-4500
Fax: 860-589-2136
superiorelectric.us

Staco Energy Products
301 Gaddis Blvd.
Dayton, OH 45403
937-253-1191
Fax: 937-253-1723

Nichrom Wire/Strip Heaters

Pelican Wire Company
6266 Taylor Road
Naples, FL 34109-1896
941-597-8555
Fax: 941-597-9783
pelicanwire.com

Ceramic Stand-offs

CHROMALOX
Edwin L. Wiegand Division
Emerson Electric Co.
103 Gamma Drive Extension
Pittsburgh, PA 15238
412-967-5148
Fax: 412-967-5148
chromalox.com

Equipment

Edge Finisher Corporation
16 Stony Hill Road
Bethel, CT 06801
203-796-7924
edgefinisher.com

FTM, Inc.
6160 Cobblestone Road
Placerville, CA 95667
530-626-1986
Fax: 530-642-2602
fabricationtoolsandmaterials.com

C.R. Clarke & Company
4407 Vineland Road - Suite D5
Orlando, FL 32811
800-676-7133
crclarke.co.uk

 

Additional Technical Information & Assistance

For more information or specific questions about your project, contact Roehm America LLC's Technical Service Representatives.

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Fire Precautions
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.

Compatibility
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.