Stuck on You

Without a doubt, bonding is becoming a more important and prevalent tool in the hands of automotive manufacturers and refinishers alike. Automotive OEM’s can take advantage of the increased design flexibility, improved ride dynamics, and reduced variability afforded by the use of adhesives, while collision repairers are learning that profitability can be increased at the same time insurance Key Performance Indicators (KPI’s) are improved.

If you are just getting started with adhesives in your shop, it can seem a bit daunting. “Glue” may seem more appropriate to the school room than the shop floor. After all, nuts & bolts are understandable, and you trust welding – adhesives can seem a little like magic.

However, the pass/fail rates of most welding certification programs show that welding doesn’t always deserve that trust. A repair process that includes the best of both philosophies, where appropriate, offers the best solution.

Luckily, despite the large number of products and applications, the general procedures for using adhesives is fairly universal and not all that different from anything else.

Getting Ready
As with any new process or tool, the first task is to understand WHEN and HOW to use it. Just like your tool box contains impact wrenches, screwdrivers, hammers, and pry-bars, your adhesive cabinet will contain different products useful in a variety of situations: tapes for attaching body side moldings, sprays for adhering carpet, and cartridges for bonding panels. But, just as you can’t afford to buy every option from the tool catalog, avoid loading your shop up with too many different adhesives. While there are many that can be purchased, generally a smaller subset will get most jobs done. Work with your supplier to identify just what repairs you are interested in starting with, and then grow your portfolio from there.

Work with your local jobber and adhesive manufacturer’s representative to learn the use of these new tools. Depending upon your interest and needs, your training options can range from a 10 minute one-on-one introduction, to a four hour seminar. In many cases, I-CAR Alliance Credits can be earned at the same time. Get hands-on with the rep during training, and take the time to practice the new techniques a few times before you expect perfection.

Getting Organized
Before you start a repair that will use adhesives, do yourself a favor and make sure that you have all the components that you’ll need to complete the repair. If you are using a fast curing product that measures its working time in seconds, rather than minutes, you won’t want to have to go looking for something unexpectedly. The first few times you do the repair, review the instructions provided in charts and directions-for-use before you start. They will help you remember all the necessary accessories – from applicator guns and nozzles, to abrasives and clamps.

Getting Started
The first steps in any adhesive repair involve preparing the surfaces of your workpieces. Start by washing them with soap and water to remove standard grime, like dust and dirt. Follow that up with a good surface cleaner that can remove any wax, grease, or tar that remains. These can all disrupt the adhesive’s ability to bond to the surface. Make sure to remove other items, such as paint overspray, rust, scale, or mold release agents that may not have the same bond strength to the surface as your adhesive. For instance, foam attachment tape will adhere very well to the paint overspray on the back of a piece of trim molding. However, should that bond-line be stressed, the paint overspray is likely to release from the molding, and take the tape with it. Non-woven scuff pads as part of the surface preparation work well for removing this type of contamination.

At this stage, dry-fitting the parts is a good idea. Don’t assume that the parts are going to come together and fit perfectly, even if they are new. Before you start applying anything, make sure that you know how you are going to install the part and hold it in place during the curing process. This is especially important for larger panels, such as truck bedsides and quarter panels.

Once the part has been fitted and is clean, refer to the adhesive manufacturer’s directions-for-use to see if additional surface preparation is needed. Adhesives typically use a combination of mechanical and chemical forces to create the bond needed. Abrasive scratches (in the grade range of 36 grit to 180 grit, typically) give the adhesive more surface area to grab onto. In the case of plastic repair, adhesion promoters can modify the surface energy of the workpiece to encourage the creation of bonds at the molecular level. Depending upon the repair type and the materials involved, both abrasive and chemical treatment may be required for a robust repair. Again, working with your adhesive supplier will get you the appropriate recommendations.

Finally, make sure that the parts are brought up to a reasonably warm temperature: 75°F is generally recommended, but anything above 60°F - 65°F will most likely be OK. Heat allows adhesives to flow into the nooks and crannies of a surface and energizes the activity at the molecular level. If the temperature of your parts and adhesive is too low, the final bond may be compromised, and it will take longer to cure.

Getting it Right
Now that the parts are ready to go, it’s time to move on to the adhesive. Depending upon the delivery format of the product, you may have some additional steps. Removing the liner from a strip of foam attachment tape is pretty straightforward, as is shaking the rattle-can of aerosol spray adhesive. However, they’re no less critical than the equalization steps for a 2-component cartridge. Again, the manufacturer’s Directions-For-Use will lay out what needs to be done, and the accessories needed to be successful.

For instance, let’s take a look at that 2-component adhesive cartridge. Before it can be used for the first time, the plunger seals at the back need to be equalized before the mixing nozzle is installed. This is accomplished by placing the cartridge in the recommended applicator gun, and caulking a small amount of material to ensure both components of the adhesive are being extruded from the cartridge. If this is not done, the mix ratio of the delivered adhesive could be incorrect, causing the final bond to underperform.

Once the cartridge has been equalized, install the recommended mixing nozzle. Small differences in the flow characteristics of different models of nozzles can impact the mixing of the components. As the adhesive was developed for use through a specific mixing nozzle, substituting just any mixing nozzle can have disastrous effects. Similarly, disregarding the nozzle and trying to mix the product by hand will most likely cause defects. This is one place where you will most likely not be rewarded for your thrift. Use the recommended mixing nozzles and make sure that you won’t have to experience a costly re-do due to poor mixing.

Now that both the surface and the adhesive are ready to go, it’s time to bring them together in a step called “wet-out.” Adhesives need intimate contact with the surface – if they’re not touching, they’re not bonding. For a tape, this usually involves firm rolling pressure. For a liquid or paste product, this means using a brush or spreader for force the adhesive into any and all surface scratches and pores.

Once the surface has been “wet-out,” additional adhesive can be added to create the bridging or bonding layer between the two parts. Align the parts, and bring them together using firm, even pressure along the entire bondline. Depending upon the clamp or set time of the adhesive, the parts will need to be fixtured together for a period of time until the adhesive’s handling strength has been reached.

When replacing a door skin, this fixturing can generally be accomplished by hemming the door outer about the door inner, avoiding the use of clamps. Similarly, the increasing availability of Squeeze Type Resistance Spot Welding (STRSW) equipment is allowing for a self-fixturing process.

Weld Bonding
Weld-Bonding refers to the practice of using both adhesives and STRSW along the same joint. Auto manufacturers have long acknowledged the benefit of bonded joints, but have struggled with the need to allow for the adhesive’s need to cure before moving the structure. Weld-Bonding has allowed them the best of both worlds: the instant handling characteristics of a spot weld, with the continuous seal and joint stiffness inherent in an adhesive bond.

When using Weld-Bonding in the repair shop, it is important take into account the conductivity and thickness of the adhesive between the panels. If the adhesive layer is too thin, it will be difficult to wet-out to both sides. If the adhesive layer is too thick, an appropriately sized weld nugget will not form. Either way, the joint strength could be damaged, rather than reinforced. Look for adhesives that identify their suitability for weld-bonding and include spacer beads to provide optimum bond line thickness.

Once the adhesive has cured long enough to reach its handling strength, any fixturing devices can be removed, and the job can move on to the final refinish steps. With a bit of practice, the benefits of adhesive bonding can be taken advantage of in any shop, with great results.

Jason J. Scharton is the global marketing manager for the 3M Automotive Aftermarket Division. He can be reached at (651) 733-2569 or j.scharton@mmm.com.