FOCUS ON JOINING TECHNOLOGIES
■ What is the type and extent of load on a bond ( dynamic , heavy-duty , heatresistant )?
■ Where will the bonded metal be used later ?
Material issues
Rubber-to-metal bonding ( image : DLR Elastomer Engineering .
It can be challenging to choose the best metal adhesive , given the vast number of options . Each chemistry has its own unique benefits . These include :
■ Structural adhesives , such as structural acrylic adhesives and epoxies , are suitable metal adhesives for forming strong bonds that resist stress , temperature and chemicals .
■ Anaerobic adhesives / sealants are used as threadlockers , pipe thread sealants , retaining compounds or FIP gaskets . Anaerobic adhesives are only used as metal adhesives and sealants . They do not cure on other substrates such as glass and plastic .
■ Cyanoacrylate adhesives develop strength very quickly . These bonds perform well in shear and resist non-polar solvents . Consider structural adhesives if you require an adhesive for metal that provides impact resistance or resistance to polar solvents .
■ UV Curable adhesives are suitable for invisible bonds between glass and metal . These are used extensively to bond metal hinges , knobs and fixtures to glass doors .
■ MS Polymer adhesives are suitable for thin sheets of metal , such as truck panel bonding . The low shrinkage provides no read-through ( you can ’ t see where the adhesive is placed through the panel ), providing a good aesthetic finish .
However , before deciding on a suitable adhesive , it is worth asking the questions below :
■ What kind of metal are you bonding ?
■ What is the metal surface ( oxidised , oily , surface-treated , painted )?
■ What is the geometric bonding surface ( flat , angular or punctual )?
■ Are there any processing specifications or constraints ?
Above and below : The 2023 European Adhesive & Sealant Conference and Expo was held in Tarragona ( Spain ) from 13-15 September 2023 .
For demanding bonded joints or for difficult surfaces , pre-treatment of the component surfaces is often necessary . The materials are cleaned and activated or modified so that adhesives can adhere to them for a long time . The ageing behaviour must be analysed and the service life estimated . The dispensing and application technology are part of the automated joining process . Rapid curing processes are often used to reduce cycle times .
The type of material to be bonded has an impact . For example , stainless steel may need to be cleaned to remove dirt , dust , fingerprints and machine oil . Steel surfaces are typically coated before or after bonding to prevent corrosion , either with zinc ( galvanized steel ) or some type of paint . If coated before , the adhesive bonds to any coating rather than the steel itself , which affects surface energy and how well the adhesive wets out . Other adhesives and tapes are designed to survive the coating or painting process .
Aluminium resists corrosion because a thin layer of aluminium oxide forms almost immediately when the bare metal is exposed to air . Aluminium often needs to be cleaned or abraded to remove dirt , dust , fingerprints and machine oil . Metals that can corrode over long periods of time , such as copper and its alloys brass and bronze , require special consideration when selecting an adhesive .
“ Oxidations as well as oils and greases , form separating layers on which adhesives can bond well . However , this layer in turn does not adhere sufficiently firmly to the metal ,
Hot-melt adhesives are also common . These are applied in a molten state approx . 150 ° C - 190 ° C and the parts must be joined before the “ melt ” solidifies . An example of this is the hot glue gun . Adhesive tapes are suitable for bonding similar and different materials such as metal ( e . g . steel , stainless steel and aluminium , also for powder-coated surfaces ) with plastic , wood or glass .
56 | ismr . net | ISMR November 2023