When using adhesives in powder coating applications, proper assembly and fixturing techniques are necessary to avoid bond failures. This involves knowing which products are recommended for bonding before and after powder coat.
To begin with, there are the general procedures to follow to ensure that you have a good bond before your parts go through the oven. The points to remember are:
Bond before powder coat
Choose the proper adhesive
Carefully dispense and mate the part
Fixture and build handling strength before the oven
Choosing the right adhesive is key. When you powder coat, ideally you are choosing adhesives that specifically are attuned to the temperature of the oven and bake cycle. In this situation, there are six things to consider before even selecting the adhesive. This includes the type(s) of substrates, weight of the bonded assembly, temperature of the oven and bake cycle, type of powder coat (i.e. Kynar), tack time, and UL requirements. Parker LORD has basically four different adhesives that can go through a powder coat bake process in the oven. The all have a temperature resistance of 400 degrees F or 204 degrees C for up to 90 minutes without the bondline degrading while it goes through the oven.
Adhesives to Consider
A unique property of Parker LORD’s technology is that it will come out stronger from the oven than if it had cured at room temperature. For example, we have both a tensile strength and a tensile strength post-bake. Maxlok Series is 2868 psi tensile/3159 psi tensile post-bake. LORD 400 Series is 2536 psi tensile strength/3226 psi tensile post-bake. LORD 810 is 8841 psi tensile/2088 psi tensile post-bake.
Maxlok is considered the superior acrylic adhesive for powder coat bake. It has superior tensile strength. You want to use this if you have large or heavy parts, or anything that is galvanized or has zinc in it. Maxlok is good for oven temperatures of 350 degrees F or higher and with longer bake cycles. It has superior temperature resistance, which makes it ideal for the hotter and longer bake cycles.
The 400 Series is the next best acrylic adhesive option and will work for most applications. The Signlok 400 Series is similar and was developed for signs. LORD 810 is a highly flexible metal bonding adhesive designed to eliminate read-through and is recommended for thin-gauge substrates.
Check Adhesive Ratio
The next step in the process is that you need to make sure your plungers are level, which is done by dispensing some of the adhesive before using. Always check the ratio of the adhesive before using. Color irregularities, in particular, are an indicator that something is off. All of LORD’s adhesives are rated for ambient temperatures, approximately 72 – 74 degrees F. The open time is determined based on the ambient temperature. So, with every 20 degrees above ambient temperatures you will cut your open time in half. If you do not meet your open time you won’t have adhesion. For every 20 degrees below ambient temperatures, you want to double your open time and handling time, which means you need to keep parts clamped longer.
Fixturing and Mating
Once they are mated, it’s important that the parts don’t come apart. It’s important that you see a smooth even layer, with no spatter parts. If they do come apart, the adhesive must be cleaned off with an alcohol wipe and the adhesive process must be started over. Parts can be repositioned by sliding, but they should not be separated.
For clamping, there are two recommended methods, either start on one end and work your way down or start in the middle and work your way out. Poor clamping can cause bowing and stress between the parts, as well as entrapping air.
It is necessary to determine the amount of handling strength you will need. Handling strength will vary, depending on the weight of the part. The heavier the part, the more handling strength you’ll need. Clamps are just option for fixturing. Rivets and weights are also options. Try to get your parts as flat and as smooth as possible before they go in the oven, because the heat of the oven will soften the adhesive. Keep the parts clamped until the handling strength is reached.
After the parts come out of the oven, check for popped areas or seams. If you suspect that there is entrained air, grab a wedge and jam it in to the bondline. Put the bondline into peel so you can see what is there. It’s best to do this only on a test part, since you would have to scrap this part.
Bonding after Powder Coat
Decide what you’re bonding to – paint or metal. Bonding to bare metal is the preferred option. With bare metal you must mask off the bond area or scuff to base metal. When bonding directly to paint, the strength of your bond is only as strong as your paint. Epoxy-based paint yields best bond performance. It has a faster cycle time and reduced material cost (no masking). Once your parts are prepped for bonding to a painted part, follow the same procedure as outlined for fixture and mate. Make sure you know the handling strength you will need.
When choosing the best adhesive, there are several options. The Maxlok and 400 Series work well for bonding to paint and can also cross bond to bare metal as well. Other adhesives that work well with paint include LORD 7542, which is UL approved and lower viscosity, and LORD 7545 which is a higher viscosity.
If you do decide to scruff, make sure you scuff to the bare metal and follow with an alcohol wipe. The same procedures should be followed from then on with mate and fixture and handling time.
To learn more, watch our webinar about using our adhesives with powder coating, https://www.lord.com/event/sign-webinar-series