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Working With Acrylic Part 3: Joining/Attaching Acrylic

Copyright © 2007 by Davis Multimedia, Int'l. All Rights Reserved.
As Printed in November 2007, Volume 33, No. 5 of The Engravers Journal
Photo courtesy of American Acrylic Awards, Walnut, CA.

   Today’s Recognition & Identification professionals find themselves working with an increasing array of materials. They have found that by offering a variety of materials, e.g. wood, solid surface material, metal, glass, plastic, and by offering a wide selection of products, e.g. gifts, awards, signage, they can appeal to a broader range of customers.
   For sales success and profits, it’s important to keep attuned to trends in the industry. Certain materials and products may be particularly “hot” among customers during a given time period. But it is also important to remember the mainstays. For example, wood plaques have been and probably always will be a traditional favorite among customers, and glass and crystal giftware is certainly here to stay. Another perennial favorite is acrylic.
   Acrylic has been used in the industry for years and has continued to be a popular material choice for applications such as signs and plaques. This material is easy to work with and it can be rotary engraved, laser engraved, screen printed, etc., with attractive, eye-catching results. Other common uses for acrylic include desktop awards, gifts, ad specialties and desk accessories.
   For some of these products, acrylic blanks are readily available from suppliers. For instance, you can purchase prefabricated blanks for items such as plaques, desktop awards and gifts. When it comes to other types of products, however, it may be easier, faster and more economical to create the blanks yourself. For example, if a customer needs a custom-shaped plaque or sign, you will probably find it easier to fabricate the piece(s) yourself.
   This article series focuses on working with acrylic, providing you with tips and techniques for fabricating acrylic blanks. Part 1 of this series examined cutting and finishing techniques (Sept. 07), and the second installment looked at methods for drilling and hole cutting (Oct. 07). This article looks at the process of joining/attaching acrylic.
   There are several instances in which you may need to join acrylic with another component. You may need to adhere one acrylic piece to another acrylic piece, e.g. to attach a base to a plaque or other desktop award, or to attach a sign blank to an acrylic back plate. Joining techniques can also be used for creating unique products, e.g. attaching acrylic pieces together to form a custom award. In essence, joining acrylic gives you the flexibility of creating pieces that cannot be readily machined.
   In some cases, the ability to adhere acrylic to other materials can be useful. For example, you can attach an acrylic “plate” to a wood plaque board or a Corian base. Or you may need to install a sign on drywall or masonry. Still another example is attaching an acrylic control panel to a machine front.
   There are two common methods for joining acrylic. These are chemical methods, i.e. cementing, and mechanical methods, i.e. using screws, bolts or other hardware. When deciding on which method to use, a major consideration is the matter of permanence vs. removability.
   In all cases, you want a strong bond between the acrylic and the material to which it is being adhered. With some pieces, permanence will be a primary factor. For example, if you are creating a desktop award consisting of a plaque mounted on an acrylic base, it is usually satisfactory to join the two pieces together permanently. However, in other cases, removability may be important, e.g. mounting a sign to another material.


Photos courtesy of RS Owens, Chicago, IL.  

   In general, the cementing techniques discussed in this article result in a permanent bond (provided that the proper procedures are followed). The mechanical joining methods are preferred in instances where removability is a requirement. Following is a look at each of these techniques.
Cementing Safety
   When joining acrylic using cements, always think safety. The cements used in this process are harmful if they are swallowed, inhaled for an extended period of time or absorbed through the skin. Therefore, when using cements, always work in a well-ventilated area. Also, use a closed applicator bottle, e.g. a syringe, when dispensing cements and liquid bonding agents to avoid inhaling the harmful fumes. If you are using the “soak/dip” method to join acrylic (to be discussed), be sure to use a tank with a cover.
   It’s also important to obtain and review the Material Safety Data Sheets for each type of cement you will be using, especially methylene dichloride (MDC), which is commonly found in solvent cements. Like many small hydrocarbons that contain halogen atoms, MDC is a suspected carcinogen, and although it’s unlikely to be strongly carcinogenic, it’s important to reduce your exposure to the lowest level possible.
   Cements are flammable, so always keep them away from high heat and open flames, and do not allow smoking in the area where the cement is used. In general, use common sense and always follow the manufacturer’s instructions.
Cementing Tips
   Successful and permanent results can be achieved by following a few helpful tips. First, keep in mind that the type of acrylic you are working with is important. Some grades of acrylic can be cemented with either solvent cements (those that actually soften the material and “weld” acrylic pieces together) or polymerizable cements (those that work more like a glue or adhesive). With other acrylic grades, however, only polymerizable cements will work satisfactorily due to the acrylic’s chemical structure. If you are in doubt, check with your acrylic supplier as to the grade of acrylic you are using and the type of cement that will work best.
   In general, it is easiest to work with cell cast acrylic. This type of acrylic is easily formed using light fabrication techniques and is also recommended for solvent welding. For more information about cell cast acrylic, refer to Part 1 of this article series.
   There are several common pitfalls to watch for when cementing acrylic. One common problem is crazing. Crazing appears as a series of fine cracks on or under the surface of the acrylic. Crazing is most often caused by stress to the material and the tendency of this problem to occur increases when the material is exposed to solvent and/or solvent vapors. Some cements can also inadvertently cause crazing if improper cementing techniques are used. Generally, crazing can be minimized by using the proper fabricating techniques (see Parts 1 and 2 for more information).
   Another common problem associated with cementing acrylic is low joint strength, which usually causes the pieces to “unjoin” after a period of time. Low joint strength can be caused by a number of factors. One common reason is that the pieces being joined do not fit together properly. Whenever you are joining two pieces together, they must fit together accurately and smoothly without having to force them together. In some cases, you may need to remachine the piece(s) in order to achieve an accurate fit.


Figure 1: Solvent welding acrylic using an applicator bottle.

   Other factors that can cause low joint strength include inadequate mixing of the cement, using excessive pressure when joining the pieces together and, in general, poor cementing techniques. For the best results, always take the time to follow the proper procedures.
   When joining acrylic, keep in mind that a proper edge finish on the pieces is important. The edges to be joined should be smooth; however, they should not be polished. Polishing tends to round the edges of the acrylic which can cause an improper fit between the two parts and, ultimately, a weak joint.
   Finishing the edges by flame polishing or by using a sanding machine is not recommended. Both of these finishing operations cause stress which in turn can cause crazing.
   For the best joint strength, the edges on the acrylic pieces to be joined should have a satin finish. This type of finish can be achieved by hand sanding or scraping the edges with a scraping tool (for more information, see Part 1).
   Temperature and humidity are also important to the cementing process. For the best results, perform these procedures at normal room temperature and low relative humidity. Lower than normal temperatures, e.g. below 65 degrees Fahrenheit, reduce the solvent action of some cements, which prolongs the time it takes to harden and also weakens the joint. High humidity/excessive moisture can cause cloudy, unattractive joints.
   Finally, always remove the protective masking from the acrylic prior to cementing. If you leave the masking on the sheet, it and/or the adhesive residue from the masking can be embedded in the joint.
Types of Cement
   There are four common types of cement used for joining acrylic. These include solvent cements, thickened solvent cements, polymerizable cements and flexible cements.
   Solvent Cements—Solvent cements are a popular choice because they are fast and easy to work with. When applied to acrylic, this type of cement softens the material and causes it to swell, which permits cohesion of the two pieces. This process is often referred to as “solvent welding,” a term with which many of you are familiar.
   After two pieces are joined, the solvents in the cement evaporate or dissipate through the material. This evaporation stage occurs rapidly, leaving a hard, relatively clear joint. Solvent cements produce a joint of moderate strength. The most common solvent cement is methylene chloride. Commercial solvent cements are also available.

 

 

Figure 2: An example of the tank used with the solvent soak method. Figure 3: Applying a bead of cement along the edge to be joined.

   Thickened Solvent Cements—These types of cements have basically the same chemical makeup as solvent cements except they include acrylic sheet chips or acrylic molding resins as a thickening agent. The “thickening” serves as a carrier for the cement. This type of cement has a longer solvent action, due to slower solvent evaporation, and therefore takes longer to dry. However, thickened solvent cements produce higher joint strength and have excellent weatherability characteristics.
   Polymerizable Cements—Polymerizable cements are more difficult and time-consuming to use than solvent cements. These cements typically consist of an MMA monomer, a polymer solution and a catalyst. In simple terms, these cements are a type of epoxy.
   Polymerizable cements have little or no solvent action. Instead, this type of cement is more like an adhesive. When using these cements, a deliberate gap is left between the two pieces to be joined together. The cement acts like an epoxy and fills in the gap between the two pieces, forming a new polymer in the joint to hold the parts together. Polymerizable cements have maximum joint strength and are a good choice for exterior use. Additionally, they are often recommended for joining acrylic to nonacrylic materials.
   Flexible Cements—Flexible cements are commercially available. These types of cements include contact adhesives such as those used for floor tile and counter tops, and silicone adhesives.
   Flexible cements are often required in order to create a permanent bond between dissimilar materials. For example, acrylic and wood expand and contract at different rates which causes stress on the bond.   Flexible cements compensate for some of this stress. However, the joints created with flexible cements are usually not transparent.
Cementing Acrylic to Acrylic
   When fabricating items such as desktop awards and plaques, you will need to cement acrylic to acrylic. There are several methods for doing this.
   Solvent Capillary Method—The solvent capillary technique involves using the solvent cements described earlier. This is one of the most common methods used for joining acrylic to acrylic because it is both easy and fast, i.e. it typically only requires 2-5 minutes set time and 15 minutes to fully cure.
   The first step in this method is to remove the paper masking from the acrylic. If necessary, clamp the pieces to hold them together during the set-up procedure. When clamping, be sure to use only light pressure. Also, there should be no gaps between the pieces and it’s easiest to work with the joint positioned horizontally.
   Next, apply the cement along the edge of the joint. The most convenient way to do this is by using a hypodermic needle, eye dropper or similar dispensing instrument (Fig. 1). When you apply the cement along the edge, capillary action will draw it between the two parts. Be careful not to spill the cement on the areas of the acrylic that are not being cemented. If you do, quickly wipe up the excess cement with a damp cloth. Allow the cement to dry before unclamping and/or handling the piece.


Figure 4: Using a holding fixture to hold together pieces of acrylic.

   If you are working with larger pieces of acrylic that cannot be easily clamped or held together, try using masking tape to abut the pieces. Do not use cellophane tape as the solvent in the cement may dissolve it.
   Solvent Soak Method—As the name suggests, this method also uses solvent cements and involves soaking the acrylic in the solvent. For this technique, you will need a tank apparatus consisting of a tray and a cover with a cut out opening to insert the part to be cemented. Place a rack, e.g. made of wire, in the bottom of the tank to support the part and then fill the bottom of the tray with the solvent cement. Figure 2 shows an example of this type of assembly.
   Insert one of the pieces to be cemented into the tank so that it contacts the solvent cement. The soaking time will depend on the grade of acrylic you are using. In general, soak the piece until the edge softens to a “cushion.” Note: You may want to mask the area near the edge to be cemented to prevent excess softening.
   Once the edge of the acrylic has softened, remove it from the solution and press it against the second piece of acrylic. The softened edge forms a cushion against the piece and adheres to it through solvent action. Allow the pieces to dry for 24 hours.
   Using Thickened Solvent Cements—These types of cements require a slightly different technique. The cement you are working with is somewhat syrupy, more like glue.
   As with the other methods discussed, remove the masking from the acrylic. The edges to be joined should have a satin (nonpolished) finish and there should be no gaps between the two parts.
   Using a syringe, eyedropper, etc., apply a small bead of cement to the edge of one piece (Fig. 3). Gently join the pieces together and clamp or hold them together until the cement sets. Remember, always use light pressure. Excessive pressure will cause the cement to seep out between the pieces. If this happens, immediately wipe up the excess cement with a damp cloth. Thickened cements will cure in approximately two hours.
   Using Polymerizable Cements—As mentioned earlier, these types of cements work more like an adhesive rather than a solvent and, therefore, require different techniques.
   For the best joint strength, a slight gap should be left between the two pieces. The characteristically thick cement will then polymerize and fill the gap without leaving any voids.
   For maximum strength, the cement should be free of air bubbles. After thoroughly mixing the cement, allow it to stand for about ten minutes to allow any air bubbles to rise to the top. Be aware, however, that after the cement is mixed, it will fully harden in about 45 minutes. For optimum results, the cement should be applied within 20 minutes of mixing.
   To apply the cement, you can use a standard dispensing device such as a polyethylene syringe. Also, it is usually convenient to hold the acrylic pieces in a fixture to prevent movement during cementing and drying. Figure 4 shows an example of a holding fixture.


 

 

Figure 5: The steps involved in creating an open "V" butt angle joint using polymerizable cements.

Figure 6: The steps involved in creating an open "V" butt joint using polymerizable cement.

   Polymerizable cements can be used on both angle joints and butt joints. The first step in creating an open “V” angle joint is to position the parts. When doing so, it’s a good idea to offset the pieces, allowing a lip of about 3/16"-1/4" (Fig. 5). In addition, position the pieces with a small clearance (not greater than .015"). It may be helpful to use a piece of paper or small wire as a spacer when positioning the pieces.
   Once the pieces are in position, clamp them together, e.g. in a holding fixture (Fig. 4). Apply the cement and allow it to dry. After it has dried, machine off the excess material and sand and buff the edge to finish it.
   To create an open “V” butt joint, try creating a cement “dam” with masking tape. Figure 6 illustrates one method of doing this.
   Position a piece of 1" wide masking tape, adhesive side up, on a work table. Then place a piece of 1/4"-5/16" wide tape on top of the 1" wide strip, adhesive side down. This will prevent the cement from coming into contact with the masking tape adhesive. Position the acrylic pieces to be joined on top of the tape as illustrated. Using a syringe, apply the cement in the gap, allowing for some overflow to compensate for shrinkage. As you do so, keep the dispenser nozzle deep inside the joint to ensure that it is completely filled with cement.
   Let the cement cure for 24 hours. Then, machine off the excess cement and finish the piece, e.g. through sanding and buffing.
Cementing Acrylic to Non-Acrylic
   It is more difficult to achieve a strong, permanent bond between acrylic and nonacrylic materials. As noted earlier, dissimilar materials expand and contract at different rates, which puts stress on the bond. Generally speaking, flexible cements work best when joining acrylic and nonacrylic materials. However, these joints are not usually transparent, so keep the dimensions of the cemented area as small as possible. To avoid problems, you may want to experiment with flexible cements before using them on actual products.
   Besides cementing, another option is to mechanically attach a piece of acrylic to the non-acrylic material to serve as a “base,” and then cement the acrylic piece to the acrylic base. Or, you may want to use mechanical fastening techniques, which are covered in the following discussion.


Figure 7: Common mechanical fasteners used for joining
acrylic.
Figure 8: General size guidelines for drilling holes in acrylic.

Mechanical Fastening
   As noted, mechanical fastening is usually the best choice for attaching acrylic to non-acrylic materials. Also, as mentioned earlier, most mechanical fasteners provide the advantage of exceptional holding power but easy removability. Popular mechanical fasteners include panhead or roundhead screws, sheet metal screws, ovalhead screws, bolts with finishing washers and threaded rods with acorn nuts. Figure 7 illustrates some of these fasteners.
   When using bolts, screws and other hardware to join acrylic with other materials, keep in mind a few tips. First, don’t use fasteners that require countersinking, i.e. drilling a hole with a tapered top so that a screw can lie flush with or below the material surface. Drilling holes for countersinking may fracture the acrylic. In addition, there is minimal room for expansion and contraction, so you run the risk of fracturing later on.
   When drilling holes in acrylic for screws and other fasteners, drill them slightly oversized. This allows for thermal movement and helps avoid stressing the acrylic. An appropriate hole size can be based on the size of the acrylic and potential temperature fluctuations. Figure 8 provides some general guidelines for hole sizes in acrylic.
   If possible, holes should be drilled at least 1/4" from the edge of the material to avoid chipping and fracturing. For more information on drilling, refer to Part 2 of this article series.
Finally, when attaching screws, bolts, etc., do not over tighten them because some room is needed for expansion and contraction. A good rule of thumb to follow is to fully tighten the screw, then loosen it about 1/4 turn.
Conclusion
   This article has focused on the various methods you can use to join acrylic to create interesting plaques and signs, as well as other types of awards, gifts, ad specialties, etc. Knowing the basics of working with acrylic can be very valuable information to have. Primarily, it allows you to create custom pieces on an as-needed basis. This can save you time and money.
   All of the techniques discussed in this article series do require experience but, in time, you can master these methods. Next month, the conclusion of this article series will address thermobending and cleaning techniques.


 

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