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Working With Acrylic Part 4: Thermobending and Cleaning

Copyright © 2007 by Davis Multimedia, Int'l. All Rights Reserved.
As Printed in December 2007, Volume 33, No. 6 of The Engravers Journal

   Signage, desktop awards, badges, desk accessories, key chains—these are just some of the interesting items that can be and are made from acrylic. Commercially introduced in 1938, acrylic is used today in the manufacture of everything just mentioned in addition to products such as formed parts for internally illuminated signs, skylights and aircraft windshields.
   Acrylic is an extremely popular and versatile material. Its many appealing qualities make it suitable for a variety of applications. Acrylic, for instance, is an alluring material, closely resembling fine optical glass in both optical clarity and attractiveness. This material is easily rotary engraved and can also be marked using a variety of other methods, such as laser engraving, sandblasting and screen printing.
   As this four-part article series has shown, acrylic is also easy to work with i.e., it can be fabricated into many different shapes, sizes and types of products. Part 1 of this series looked at cutting and finishing techniques (Sept. 07) while Part 2 examined drilling and hole cutting operations (Oct. 07). Part 3 discussed joining acrylic using both chemical and mechanical means (Nov. 07). This final installment examines techniques for thermobending acrylic and cleaning the finished product.
Thermobending Acrylic
   Thermobending is the process of heating plastic to soften it and then bending it into different shapes. With this technique, you can create a variety of functional products from sheets of acrylic, including freestanding desk and counter signs, “tent” signs, slip-on badges, picture frames, business card holders, desk accessories and display racks.
   There are several methods for thermoforming acrylic, such as vacuum forming, drape forming, free blowing and strip heating. In many engravers’ shops, strip heating is the most popular choice, primarily because the equipment is relatively inexpensive and the process is simple.
   This method of thermobending requires a strip heater, which is essentially a device with a long, straight, electrical heating element (Fig. 1). Strip heaters can be purchased from suppliers of engraving machines and accessories and some acrylic blank suppliers. A bending fixture, which is a device designed for bending acrylic after it has been heat softened and then holding it in the desired bent shape, is also handy. Bending fixtures are available from these same suppliers.
   Before attempting to thermobend acrylic, determine the angle of the bend you wish to achieve. For instance, included angles of 50, 60 and 70 degrees are common for freestanding signs.
   Also, consider the thickness of the sheet you wish to bend. Up to 1/4" thick acrylic can be heated for bending on most strip heaters. If the acrylic is thicker than 1/4" it may not be thoroughly and uniformly heated with the strip heater, which could make bending difficult and/or result in a crooked bend.
   With thicker pieces of acrylic, you may want to “relieve” the bend to avoid this problem. This process involves engraving or routing a V-shaped groove along the bend line which helps to produce a sharp, straight bend. Note, however, that this technique is not recommended when strength is important to the final piece.

   If you do choose to relieve the bend, for the best results, try to use a cutter with an included angle equal or close to the angle of the bend desired. A cutter with a 90 degree included angle and a small tip size e.g., .010", is also a practical cutter to use for this purpose.
   After selecting an appropriate cutter, engrave a groove in the acrylic at a depth of approximately one-half the thickness of the material. Note: Be sure to engrave the groove on the side of the sheet that will be compressed i.e., the inside of the bend.
   The first step in thermobending acrylic with a strip heater is to select an appropriate temperature and time. Most strip heater manufacturers provide time/temperature guidelines with the equipment based on the material thickness. In general, 1/4" thick acrylic can be thoroughly heated at a temperature of around 300 degrees F for approximately eight minutes. This temperature setting and time may vary, depending on your equipment, so you may want to make a few test bends to determine what works best.
   Before heating the acrylic, remove the protective paper masking. If the masking is left on, the heat generated by the strip heater will cause it to decompose, making it extremely difficult to remove later.
   Next, position the acrylic on the strip heater so that the area to be bent runs parallel with the heating element and is directly over it. Some strip heaters have a material guide/stop for this purpose, i.e. you can adjust the stop and abut the piece of acrylic against it so the bend is in the correct place and the heating is even. This is also useful for repeatability, e.g. when you are bending several identical items.
   Also, when positioning the acrylic, place the material on the strip heater so that the heated side represents the outside of the bend, i.e. groove side up if you engraved a groove to relieve the bend. Heating the acrylic in this manner will assure that the heated side will naturally stretch while the cooler side will compress.
   Acrylic sheet manufacturers indicate that during heating, the acrylic sheet should not actually touch the heating element because the plastic may bubble, blister and/or stick. Instead, they recommend that you hold the material 1/16"-1/8" above the heating element. Some strip heaters are equipped with supports for achieving this. If yours is not, try using several wood shims placed on the worktable of the strip heater underneath the
acrylic sheet.
   Thoroughly heat the acrylic sheet until it softens or “wilts.” Heating the acrylic at the correct temperature and for the optimum amount of time are important for achieving good results. If the acrylic is not heated enough, crazing (a series of fine cracks which appear on the surface of the acrylic) will result when the material is bent. If the acrylic is overheated, bubbling and blistering can result.
   Once the acrylic is thoroughly heated, quickly bend the sheet and clamp it in a fixture to allow it to cool. As the acrylic cools, it will attempt to spring back to its original flat shape, so it’s important to hold the sheet in the desired shape until it has completely cooled, which usually takes only a few minutes. To speed up the cooling process, you can spray the acrylic with compressed air. As mentioned, bending fixtures, such as the one shown in figure 2, are available from suppliers. You can also create your own bending/clamping fixtures.

   After an acrylic sheet has been thermobent, you may notice some slight distortion at the ends of the bend, i.e. the material flares out at the ends. This can be corrected by cutting the acrylic and polishing the edges (refer to Part 1 for more information).
   As always, be sure to take the necessary safety precautions when working with acrylic. Acrylic is combustible, so use caution when applying heat to this material and do not heat the acrylic beyond 350 degrees F. Also, the fumes emitted by heating this plastic are potentially very dangerous, so always work in a well-ventilated area.
Storing & Cleaning Acrylic
   If you will be fabricating acrylic in-house, you will very likely have sheets of this material on hand. Following are some tips for properly storing acrylic sheet material as well as some general cleaning procedures.
   Always store acrylic sheets in a well-ventilated, cool and relatively moist environment. Generally speaking, the temperature should not exceed 125 degrees F. Hot, dry environments will cause the adhesive on the paper masking to dry out, making it difficult to remove. On the other hand, excessively moist areas will cause the masking to deteriorate.
   Masked pieces of acrylic are best stored on edge in an A-frame type of storage rack, e.g. on a 10 degree angle from the vertical. If you will be storing the material on a flat surface, avoid trapping dirt and other debris between the sheets. While the paper masking will protect the acrylic to some degree, dirt and other debris can cut through the paper masking and damage the acrylic, especially as the weight of the stock increases.
   Also, stack small sheets on top of larger pieces to avoid “bowing” the larger material. When moving the material, avoid sliding the sheets over each other or across rough surfaces to avoid potential damage to the acrylic.
   Always store acrylic with the original paper masking intact. When it is time to remove the masking, try lifting one edge with your fingernail and then rolling the paper around a wooden dowel or cardboard tube as you remove it. Usually, any remaining adhesive residue can be removed by wiping the acrylic with a soft clean cloth. For particularly stubborn adhesive residue, try wiping the surface with a clean cloth and isopropyl alcohol.
   Unmasking acrylic creates an electrostatic charge on the material. This charge attracts dust, lint and dirt, so it is important to break the charge in order to keep the acrylic clean after it has been unmasked. You can eliminate the charge by simply wiping the acrylic with a damp cloth.
   After acrylic has been fabricated or even after acrylic merchandise has spent some time in your showroom, it often needs a good cleaning before you can present it to your customer. Acrylic can be thoroughly washed with a nonabrasive soap or detergent and water. Loosen dirt and other debris with your fingers and avoid rubbing the dirt into the acrylic as it can be easily scratched. Rinse the acrylic in clean water and dry with a soft, clean cloth.

Figure 1: Strip heating machine from Gravograph (formerly New Hermes), Duluth, GA.

   Grease and oil can be removed by using kerosene. Do not, however, use solvents such as acetone or lacquer thinner to clean acrylic. These solvents will attack the plastic, causing crazing and other surface blemishes. Also, avoid using window sprays, which cause streaking, and kitchen scouring compounds, which can scratch acrylic.
   For minor cleaning, acrylic can be dusted lightly (not wiped) with a soft clean cloth or chamois. After dusting, wipe the acrylic carefully with a soft damp cloth or chamois to break the electrostatic charge. Whenever you use any type of cloth to clean acrylic, make sure it is free of dirt and grit.
   If you notice minor scratches in the surface of acrylic after cleaning, these can usually be removed by hand polishing. Acrylic Idea Factory offers an antistatic spray polish designed for acrylic. Simply spray the polish on the acrylic and wipe with a clean, soft cloth, such as cotton flannel. The company also offers a polish designed to remove minor scratches and adhesive from the paper masking.
   When applying polish, rub along the length of the scratches using a back-and-forth or circular motion. Several applications may be necessary to eliminate the scratches. After polishing, wipe the surface of the acrylic with a clean cloth.
   A good grade of commercial wax (liquid or paste) can also be applied to acrylic to improve its surface appearance and fill in deeper scratches. Apply a thin, even coat of the wax to the surface of the acrylic. Then, bring the acrylic to a high luster by rubbing lightly with cotton flannel. After waxing, blot the acrylic with a damp cloth to remove the electrostatic charge and any remaining dust.
   Acrylic is a versatile material with many different applications. This article series has taken you through the basics of fabricating this unique and interesting material in-house, including cutting, finishing, drilling, hole cutting, joining, thermobending and, finally, cleaning acrylic. Hopefully, the information presented has served to expand your knowledge of this popular material so that you can better serve your customers.
   Try these techniques yourself. While it will take some time and experience to perfect your skills, they can be very valuable skills to have.