There are several reasons why clamping during adhesive cure is important.
Firstly, and most obvious, is to prevent the components falling apart whilst the adhesive is curing. Adhesives have different cure speeds. Some products (e.g. cyanoacrylates) cure in seconds and can be easily held by hand until handling strength is achieved. Handling strength is the point at which the bond holds itself together firmly enough to be lightly handled. Other products such as epoxies, can take several hours. It might get a bit boring and uncomfortable holding a joint by hand for this long! RTV silicones can even take days. Although many of these types of products have a high initial “grab” strength which tends to hold the joint fairly well without clamping, providing it is not in a load-bearing joint.
Many adhesives act as lubricants when in their liquid state. Which is quite helpful if you’re trying to screw parts together. However, unsupported joints with liquid adhesive will be very prone to slipping out of alignment or even apart completely. For this reason, it is a good idea to clamp parts to stop this movement. High-viscosity single part heat cure epoxy adhesives don’t present as slippery. However, they can become slippery when heated up in the curing oven. So even if you don’t think the parts will slide apart, err on the side of caution and clamp together during cure.
UV-curable adhesives can be quite challenging to secure as a clamp will effectively create a shadow area which won’t cure properly. It is very important that if you cure in two stages (so removing the clamp halfway through to cure the shadow area) that you are not premature with the removal of the clamp, the joint can accidentally “pant” open slightly and suck in air around the edges, this can lead to a “crazy paving” effect. This is easily prevented by increasing the length of time the assembly is exposed to the UV light or increasing the intensity to enable a faster cure.
Disturbing a joint during initial cure is not a good idea as a similar thing can happen with all adhesives, the joint can “pant” open and air can suck in. This reduces the joint strength and in the case of two part no-mix acrylic adhesives, “panting open” can actually affect the product cure. To prevent this happening, do not disturb the joint until it has reached its handling time (which is normally listed on a product’s technical datasheet).
Clamps can be very basic and cheap – bulldog clips are quite good for simple joint configurations. Of course, many joints are quite complex or the substrates are too thick to use a stationery clip. More heavy duty G-clamps could be the order of the day for larger assemblies or specially-made jigs may be necessary. Clamping for long periods can delay production or take up a lot of room in a production facility. So it is worth exploring the option of using a quicker curing adhesive. Many can offer the strength performance benefits required for your application but help reduce clamping time.
For more help and advice, please contact Permabond.
