This removes the need to make bespoke solid gaskets, while tailoring material properties to the exact application
The two common types of liquid gasketing both involve materials that are dispensed as liquids on one part to be joined, and which when cured tend to be softer and more elastic than structural adhesives. That is perfectly appropriate for a substance to be used in compression. But as these two uses actually can require quite different performance, it’s no wonder that two adhesive suppliers (Intertronics and Permabond) both advise a variety of different adhesives for CIP (cure in place) and FIP (form in place) processes, depending on the specific requirements of the application.
Permabond technical sales engineer Ricky Sherred, pictured, right, considers CIP a particular type of FIP gasketing, one that he describes as only having emerged in the last decade or so. In its application, a bead of single-part acrylic adhesive is laid down on a part, and then exposed to UV light for curing, and then the two parts are brought together. Because the adhesive is solid, the item can be reopened later. Sherred says that the advantage of a UV cure here is the process speed: 30 seconds to deposit the material, which is exposed for another 30 seconds or so (depending on depth) and then the parts can be joined.
Of course, the joining material needs to be elastomeric: squishy. Sherred reports that it chooses acrylic adhesives that are soft – with 30-40 Shore A hardness – as well as elastic and, based on tests, only have a compression set of 5%, which means that they spring back time after time.
Intertronics says that it would also typically lead with a UV-curing acrylic, but if costs of material and investment in UV equipment are major considerations for the customer, the team may alternatively suggest customers trial silicone materials. The first gaskets based on silicone elastomers were used on automotive engines in the 1970s, replacing the likes of pre-cut thin sheets of cork, according to learning and development manager Kevin Cook (formerly technical manager).
Silicones, he adds, cure with moisture and are good because, like UV acrylics, they are hard enough to hold their shape, but are also soft enough to be pliable. They are also highly heat-resistant, and can be used in applications up to 300°C, but introduce production volume considerations, as slower cure means that storage space is ultimately needed for curing assemblies.
FIP gasketing is used more for applications where the dispensed path is less straightforward, where the assembly will be opened or closed less often, or where it will be assisted with mechanical fixings, he explains. Cook recommends what he calls an ‘adhesive sealant’. These materials provide some bonding ability, but also facilitate reopening, removal and reapplication if necessary. Suitable chemistries for these applications include epoxies and polyurethanes.
Permabond recommends anaerobic technologies for FIP applications. Offering a more rigid bond, than the CIP acrylics, they also require a tight-fitting application of typically only 0.5mm between the two joined parts. Metallurgy also matters here; the adhesive cures with the metal ions on the surface with which they react, and while mild steel is very reactive, stainless steel or aluminium is less so, slowing down the reaction. (True to its name, the adhesive also needs the absence of oxygen to polymerise). Anaerobic adhesives used for FIP also offer good tensile compression, but are weak in peel – almost by design, he adds, as this facilitates reopening if necessary.
While Sherred observes that anaerobics can cure hard, he adds that softer anaerobics can be specified when producing a gasket to join two dissimilar materials used on a component that is regularly heated – then they could cope with the stresses and strains caused by different coefficients of thermal expansion.
In some cases, material choice might dictate the process. He adds: “For plastic bonding, I would go for cure in place. We do have activators that we can use with anaerobics, but when looking at a lot of plastics in electronics applications, UV cure in place might be more appropriate.”
In applications such as bonding polyproplylene and polyester battery boxes, Permabond is using two-part acrylic polyolefin adhesives to bond. Obviously, they also offer the ability to bond to other nearby materials as well, such as aluminium supports.
He also says that liquid gasketing systems of both types tend to be much thinner than preformed rubber gaskets, as much as an order of magnitude. That can be a problem when replacing or retrofitting an existing part that previously used a rubber gasket, but adhesives’ very thinness helps improve the rigidity of the component around the seal, by offering better load distribution. On the other hand, high-temperature applications might not work with Permabond’s anaerobics that are usually rated up to only 200°C continuous use. In that case, Sherred recommends preformed rubber gaskets or other adhesives.
DISPENSING
However, his adhesive choices are flexible in terms of application method. He goes on to say that both anaerobic and UV-cure acrylics can be set up to be dispensed manually or automatically. “I’ve seen pick and place robots used, or semi-automatic systems that dispense and then are manually relocated and fixed in place. Also, UV curing systems come in a huge range of prices, from handheld lamps for a couple of hundred pounds and up,” says the Permabond technical sales engineer.
Cook offers a slightly different perspective: “In order to achieve repeatable and accurate dispensing, gaskets can be applied using an appropriate dispensing robot. This ensures the right amount of material is used to provide a suitable bead profile to reduce sealing failures, scrap and wasted excess material.”
In terms of dispensing equipment, he points out that its range of offerings for its silicone and UV-cure acrylics include time/pressure dispensing, volumetric pumps and jetting valves that can dispense from larger volumes down to microlitres or nanolitres of adhesives for bond lines as thin as 2-300 micron. He also points out that its gaskets’ viscosities range from self-levelling – which is good for a channel - to thixotropic, meaning beads don’t sag under their own weight.
The adhesive’s viscosity and formulation need to match the application. “A lot of it comes down to the design of how the lid or box forms a seal; is there a groove or a surface? What you don’t want is a very soft sealant with nowhere to go, because mechanical clamping, if not located or spaced to suit having a gasket, could deform the sealing component and gasket enough to cause a leak path,” the Intertronics learning and development manager adds.
BOX: Sealing misconception
A common misconception exists about IP ratings of enclosures for environmental protection according to IEC 60529, states Kevin Cook at Intertronics (pictured). “You can’t make a gasket per se that is compliant with IP67, as it is not the gasket or seal that is tested to the IP rating system, but the final product as a whole. Having said that, achieving IP67 is quite straightforward.
“It requires complete protection from dust and water submersion for extended periods, which means particles down to a certain size are not coming into the system through a seal. That’s pretty easy. IP69K on the other hand is more difficult to achieve as it involves protection against high pressure water jets.”
