Building a “Low Buck” Plastic Forming Tool

on 12 November 2010
Created: 12 November 2010

Quite often when working with acrylic and ABS sheet plastics, it is desirable or beneficial to be able to make clean, accurate bends. Examples would be making custom radio side brackets, satellite radio or Ipod mounts, or plexiglass covers and trims for amps, batteries and fuse holders. While commercial solutions exist, they can be expensive, may not be particularly accurate and can take time to warm up. This project can be duplicated with parts that are already lying around your shop, or that can be obtained inexpensively at your local hardware store.

The main part of our plastic bender is the heating element which is made from ni-chrome wire. Ni-chrome is a special alloy of nickel and chromium that has the property of having a high resistance. When current is passed through ni-chrome, it gets hot very quickly. While you can buy this wire at a hobby shop or industrial supply, you can get it easily by raiding your girlfriend’s vanity for an old, worn out hair dryer. The heating element inside has the necessary wire and you’ll get a cool little 12v turbo fan out of the deal.

Other parts you’ll need are miscellaneous speaker wire and terminals, a short piece of ½” aluminum channel, a ¼” by 1” hardware store tension spring, some scrap ½” MDF, and a couple 8-32 x 1” screws, t-nuts and hex nuts. You will probably need to invest less than 10 bucks in parts.

The first step in building the bender is to disassemble the hair dryer to extract the nichrome wire. You’ll find it wrapped in a spiral around a former near the nozzle. Remove it by snipping it with cutters at the original crimps and rivets. Separate out the thickest strands and save the rest for another project.

You might want to also save the motor/fan assembly for a future project. It moves a lot of air at high velocity.

Next, stretch out the wire you just obtained to make it as straight as you can. Cut two strands at least 20” long and chuck one side of each into a cordless drill. Grab the other ends with pliers and spin the drill motor while holding the wires taut until they are twisted together tightly.

Next, using Vice grips to clamp some wires from a 12v battery or power supply (not 120v wall current!)  to the two ends of the heating element, carefully (to avoid getting burned) shorten the distance between the clamp points until the wire barely begins to glow red and then cut it an inch longer than that length. The distance will vary according to your power supply and particular gauge of wire, my project was around 14”

Strip all the insulation from about 2” of 16ga wire. Slide the copper wire into the wrong side of a non-insulated ring terminal sized to fit an 8-32 screw. Insert the ni-chrome into the proper side crimp it tightly. Crimp another ring terminal without the copper wire to the opposite side of the ni-chrome wire. Your element is now compete.

Cut a piece of ½” MDF 3” wider than your finished element (ring terminal hole to hole dimension) and 12” deep. Cut two more pieces the same width but only 5 ¾ “ deep. Draw a center line across the width of the larger piece. Next, drill holes that will accept the barrel of your 8-32 tee nut on the centerline ¾” from each side.

Solder non-insulated ring terminals to the two tee nuts, crimp 10’ of 16ga. wire to each of them, and insert them into the bottom of the MDF.

Slide the end of the element with only the ring terminal onto one of your 8-32 x 1” machine screws. Follow it with two 8-32 hex nuts spun on loose, and finally an 8-32 washer. Screw the machine screw into a tee nut from the top side of the wood. Adjust the screw head to around 3/8” height, and tighten one nut against the wood to lock it. Tighten the other nut against the ring terminal to lock the element in place aligned with the center line.

Crimp a ring terminal to the 2” long stripped copper wire. Assemble it to the MDF just like the other side of the element, but include one loop of your ¼ by 1” tension spring when you tighten the terminal to the head of the screw. Clip the other loop of the spring into the ring terminal crimped to the element. The idea is to have the spring tension the element without carrying the current to heat it. Without the spring, the wire would go slack when it heats. At this point, the wire should be tight and aligned with the previously drawn centerline. It should also be approximately ¼” off the surface of the wood. If not, adjust as necessary by loosening and retightening the screws and nuts or by shortening the spring.

Cut a length of ½” aluminum channel approximately 1” shorter than your finished heating element. It should be just long enough to cover the twisted wire section of the element.

Carefully place the channel under the element with the legs facing up.  Using less than 1” fasteners, nail, or screw (from the bottom) one piece of the 5 ¾” MDF so that the channel rests against it while centered on the element. Attach the second piece of 5 ¾” MDF so that the aluminum is clamped in place. When finished, the element should be centered in the aluminum channel and not touching metal or wood anywhere. 

Next, measure and draw lines parallel to the channel every ¼” on both sides. These will help you judge where to place your part when bending.

To test the bender, hook it up to a fused 12v source capable of at least 10A of current. The wire should glow the dullest possible red. If it glows too brightly, it will bubble the plastic. You can reduce the temperature by adding more 16ga wire to the power leads. If it does not glow brightly enough, you can shorten or increase the gauge of your power leads.

To use, connect the bender to your 12v power source, and place the plastic stock over the element so that the wire aligns with where you would like to have the piece bend. Hold the piece flat to the board, and wait about one minute. The plastic should grow soft and flexible and fold easily on the line. Bend to the angle you would like and hold for one minute while it cools.

One easy project that can be done with a tool like this is a battery cover/fuse block mount. Cut a piece of ¼” plexi large enough to cover the top of your battery and also hold the fuse block. For our project, we rounded the corners and polished the edges for an attractive look. Drill or cut out areas large enough for your battery posts or terminals.

Next, we used a compass to draw a line around the perimeter of the piece and then cut on the line with a razor knife. This piece of the protective paper is then removed and the exposed plexi is sanded to make it white and reflective. If you have a small handheld sandblaster it also would work well to etch the plastic.

A logo was printed out on a computer and attached to the back side of the plexi using two sided lamination tape. This tape is crystal clear and is ideal for this type of job. Another option would be to cut out a design or logo like we did on the outside edge and sandblast the design into the plexi. The sandblasting technique allows the design to illuminate when the plexi is edge lighted.

Remove the protective paper from the plexi and place it on the bender so that the bend line aligns with the heating element. Use the drawn in guide lines on the bender to help make sure it is square. Turn on the bender and wait for the plastic to soften. The thicker the material, the longer it will take to bend. ½” material is the practical limit for this tool, although by making two identical benders and using one on each side of the bend, I have bent up to ¾” plexi. You will know when the bend is ready to be made because the piece will fold with little pressure. Trying to bend too soon can lead to cracking at the joint.

Once the plastic is warm enough, bend and then hold while the plastic cools. Running the piece under cool tap water will accelerate the cooling process.  

For this project, we used a blue LED to make the plexi glow. This was done simply by sanding the plexi with 150grit and using CA glue to attach a grommet behind the fuse holder where it would be invisible. The LED was then inserted into the hole in the grommet and connected to 12V.

Next, mount the fuse holder and make your final connections

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