THE BASICS: Solvent Cementing

WARNING! This article may hurt the feelings of fabricators who would rather look for excuses than learn real solutions to common cementing problems.

Now that we have that out of the way, I will attempt to dispel a few of the misconceptions about what causes the most common problems fabricators experience while solvent cementing. Unfortunately many fabricators tend to blame the material or glue - neither of which usually deserves the blame.

First, the basics - solvent cementing requires a chemical reaction to take place. This reaction requires time - it can not be rushed or sped up. Often this is difficult to accept given today's high volume production shops where you are constantly trying to meet customer's deadlines.

This chemical reaction requires the softening of the surfaces to be joined. This softened surface is referred to as the "cement cushion". The cushions are joined together and as they evaporate or dry, the former wet surfaces become one piece of plastic. Understanding this basic principle will help provide explanation to the problems fabricators usually face. Here's were we start hurting some feelings.

Bubbles in cemented joints are rarely caused by the cement or by the plastic. Joint bubbling is most often caused by microscopic air bubbles trapped in poorly or inadequately prepared surfaces. Parts to be cemented must be machined to the closest possible tolerances, with sharp tools. They should not be polished or belt sanded prior to cementing. These methods generally create the most amount of stress on the plastic and leave the surface vulnerable to the microscopic air pockets when they are softened by the solvents.

Another cause of bubbles is poor or improper application. This is when, looking for speed in assembly, fabricators try to shortcut the forming of the cement cushion. If the surfaces to be joined are not machined absolutely perfect, the capillary action method may not draw cement to the entire surface, leaving microscopic areas of air. It is always better to keep the pieces slightly apart, apply the cement to both surfaces and allow them to soften for 20 to 60 seconds (depending on length and width of the joint) and then, very gently clamp them together with only enough pressure so as not to squeeze away your entire cushion. You may sacrifice some production time but you will have far less scrap.

Hairline cracks at or just below the surface of the cemented joint are not caused by the cement. Crazing is caused by the stress put on the plastic during machining and other fabrication steps. To minimize the stress, always make sure cutting tools are as sharp as possible. Also, avoid working in temperatures below 65° F. The single most important step any fabricator can take to reduce or eliminate crazing is to anneal or "heat treat" the parts before cementing. It is often an overlooked step because of the time involved and the cost related to annealing equipment. Most manufacturers of acrylic sheet recommend annealing machined parts to relieve internal stress prior to cementing. Contact your sheet supplier for their recommendations and specifications.

Cloudy joints are usually the result of excessive humidity in the work environment. In the case of joint "whitening", taking steps to reduce humidity in the work area will greatly reduce the chances of cloudiness in the cemented joint. If that is not possible, using a cement with a slower evaporation rate will help reduce the effects of humidity.

Joint Bubbling

1. Anneal parts to be joined prior to the cement application when possible.
2. Reduce clamping pressure (ideal pressure is 1 psi).
3. Increase the amount of the cement used on the joint and wet both surfaces prior to joining when possible.

Crazing

1. Make sure the cutting tools are sharp.
2. Machine parts to closest possible tolerances and do not polish.
3. Anneal parts prior to cementing when possible.
4. Reduce clamping pressure to 1 psi.
5. Always work in temperatures above 65 F.
6. Take steps to maintain constant humidity in working environment.

Cloudy Joints

1. Usually caused by excess humidity.
2. Use cement with a slower evaporation rate.

WE WANT TO HEAR FROM YOU! The Basics was developed to address common problems that fabricators face every day. We have talked with several leading suppliers in the industry and they have shared some of the regular questions they receive about their products and various fabricating techniques. Based on these conversations, we have several topics under development, however, we want to hear from you too. Send your questions to: The Basics, c/o The Plastics Distributor and Fabricator Magazine, 2701 North Pulaski Road, Chicago, IL 60639 or e-mail us at pdfm@plasticsmag.com. We look forward to your input.

For more information, contact Don Kiepert, President of Belco Chemical Ltd., manufacturers of PLEX-WELD™ acrylic and colored laminating cements, cold chemical dye, and PVC-106 cement. They are available to answer your solvent cementing questions. You can contact them at Belco Chemical Ltd., 6484 Congress Drive, West Bend, WI 53095, 262-675-2499, FAX 262-615-0629.