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Manufacturing Processes > Joining processes > Adhesive bonding

 

Adhesive bonding

 

Process description

Joining of similar or dissimilar materials (adherent) by the application of a natural or synthetic substance (adhesive) to their mating surfaces which subsequently cures to form a bond.

Materials

  • Most materials can be bonded with the correct selection of adhesive, surface preparation and joint design. Metals, plastics, composites, wood, glass, paper, leather and ceramics are bonded commonly.
  • Can join dissimilar materials readily with proper adhesive selection, even materials with marked differences in coefficient of linear expansion, strength and thickness.
Adhesive Bonding

Process variations

  • Adhesives available in many forms: liquids, emulsions, gels, pastes, films, tapes, powder, rods and granules.
  • Curing mechanisms: heat, pressure, time, chemical catalyst, UV light, vulcanization or reactivation, or a combination of these.
  • Various additives: catalysts, hardeners, accelerators and inhibitors to alter curing characteristics, silver metal flakes for electrical conduction and aluminum oxide to improve thermal conduction.
  • Adhesives can be applied manually or automatically by: brushing, spreading, spraying, roll coating, placed using a backing strip or dispensed from a nozzle.
  • Many types of adhesive are available:

    • Natural animal (beeswax, casein), vegetable (gum, wax, dextrin, starch) and mineral- (amber, paraffin, asphalt) based glues. Commonly low strength applications such as paper, cardboard (packaging) and wood.
  • Epoxy resins: typically uses a two-part resin and hardener or single part cured by heat for large structural applications.
  • Anaerobics: set in the absence of atmospheric oxygen. Commonly known as thread locking compounds and used for locating and sealing closely mated machined parts such as bearings and threads.
  • Cyanoacrylates: better known as super glues and use the presence of surface moisture as the hardening catalyst. Creates good bonds when using assembling small plastic, rubber and most metal parts.
  • Hot melts: thermoplastic resin bonds as it cools. Used for low load situations.
  • Phenolics: based on phenol formaldehyde thermosetting resins, two-part cold or heat and pressure cured. More expensive than most adhesives, but gives strong bonds for structural applications and good environmental resistance.
  • Plastisols: based on Polyvinyl Chloride (PVC) and uses heat to cure. For larger parts such as furniture and automotive panels.
  • Polyurethanes: similar to epoxies. Fast acting adhesive for low temperature applications and low loads. Footwear commonly uses this type of adhesive.
  • Solvent-borne rubber adhesives: rubber compounds in a solvent which evaporates to cure for minimal load applications.
  • Toughened adhesives: acrylic or epoxy-based adhesives cured by a number of methods and can withstand high shock loads and high loads in large structures.
  • Tapes: pressure sensitive adhesives on a backing strip for light loading applications such as packaging, automotive trim, cable secure and craft work.
  • Emulsions: based on Polyvinyl Acetate (PVA), highly versatile suitable for cold bonding of plastic laminates, wood, plywood, paper, cardboard, cork and concrete.
  • Polyimides: requires very high curing temperatures and pressures. Used in electronics and aerospace industries. High temperature capability.

Economic considerations

  • High production rates possible.
  • Lead time hours typically, but weeks if automated.
  • Time for curing heavily dictates achievable production rate: tapes are instant, cyanoacrylates take several seconds, anaerobics can take 15–30 min, epoxy resins may take 2–24 h, although this can be reduced using catalysts.
  • The viscosity of the adhesive must be suitable for the mixing and dispersion method chosen in production.
  • Very flexible process.
  • Simplifies the assembly process and therefore can reduce costs.
  • Can replace or complement conventional joining methods such as welding and mechanical fasteners.
  • Very little waste produced. Liquid adhesives require accurate metering to avoid excess.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs low to medium. Jigs and fixtures recommended during curing procedure to maintain position of assembled parts can be costly.
  • Equipment costs generally low.
  • Direct labor costs low to moderate. Cost of joint preparation can be high.
  • Finishing costs low. Little or no finishing required except removal of excess adhesive in some situations.

Typical applications

  • Building and structural applications
  • Electrical, electronic, automotive, marine and aerospace assemblies
  • Packaging and stationery
  • Furniture and footwear
  • Craft and decorative work

Design aspects

  • All levels of complexity.
  • Can be used where other forms of joining not possible or practical.
  • Joints should be designed to operate in shear, not tension or compression.
  • Adhesives have relatively low strength and additional mechanical fixing recommended on highly stressed joints to avoid peeling.
  • Most common joint is the lap or variations on the lap, for example, the tapered lap and scarf (preferred). Can also incorporate straps and self-locating mechanisms. Butt joints are not recommended on thin sections.
  • A loaded lap joint tends to produce high stresses at the ends of the joints due to the slight eccentricity of the force line. Excessive joint overlap also increases the stress concentrations at the joint ends.
  • For lap joints, the length of lap should be approximately 2.5 times that of the thinnest part for optimum strength. Increasing the width of the lap, adhesive thickness or increasing the stiffness of the parts to be joined can improve joint strength.
  • Adhesive selection should also be based on: joint type and loading, curing mechanism and operating conditions.
  • Can aid weight minimization in critical applications or where other joining methods are not suitable or where access to joint area limited.
  • Inherent fluid sealing and insulation capabilities (electricity, heat and sound).
  • Life prediction at operating temperature and should be assessed.
  • Adequate space should be provided for the adhesive at the joint (~0.05mm optimum clearance).
  • Adhesives can be used to provide electrical, sound and heat insulation.
  • Can provide a barrier to prevent galvanic corrosion between dissimilar metals or to create a pressure tight seal.
  • Design joints using minimum amount of adhesive and provide for uniform thin layers.
  • Jigs and fixtures should be used to maintain joint location during adhesive curing.
  • Provision for the escape of gases and vapors in the design important.
  • Minimum sheet thickness =0.05 mm.
  • Maximum sheet thickness, commonly =50 mm.
  • Unequal thicknesses commonly bonded.

Quality issues

  • Excellent quality joints with little or no distortion.
  • Residual stresses may be problematic with long curing time adhesives in combination with poor surface condition of base material, but otherwise not problematic.
  • Dissimilar materials can cause residual stresses on cooling due to different expansion coefficients especially if heat is used in the curing process.
  • Problems encountered with materials which are prone to solvent attack, stress cracking, water migration or low surface energy.
  • Problems may be encountered in bonding materials which have surface oxides, loose surface layers or which are plated or painted (de-lamination may occur from the base material).
  • Stress distribution over the joint area more uniform than other joining techniques.
  • Joint fatigue resistance improved due to inherent damping properties of adhesives to absorb shocks and vibrations.
  • Heat sensitive materials can be joined without any change of base material properties.
  • Adhesives generally have a short shelf life.
  • ptimum joint strength may not be immediate following assembly.
  • Various adhesives can operate in temperatures up to approximately 250°C.
  • Control of surface preparation, adhesive preparation, assembly environment and curing procedure important for consistent joint quality.
  • In surface preparation important to remove any contaminates from the joint area such as oxide layers, paint and thick films of grease and oil to aid ‘wetting’ of the joint. Mechanical abrasion (grit blasting, abrasive cloth), solvent degreasing, chemical etching, anodizing or surface primers may be necessary depending on the base materials to be joined.
  • Adhesive almost invisible after assembly. Joint surface free of irregular shapes and contours as produced by mechanical fastening techniques and welding.
  • Joint inspection difficult after assembly and NDT techniques currently inadequate. Quality control should include intermittent testing of joint strength from samples taken from the production line.
  • Quality control of adhesive mix also important.
  • Consideration of joint permanence important for maintenance purposes. Bonded structures are not easily dismantled.
  • Joint strength may deteriorate with time, and severe environmental conditions (UV, radiation, chemicals, humidity and water) can greatly reduce joint integrity.
  • Flammability and toxicity of adhesives can present problems to the operator. Fume extraction facilities may be required and safety procedures for chemical spillage need to be observed.
  • Rough surfaces preferred to smooth ones to provide surface locking mechanisms.
  • Fabrication tolerances a function of the accuracy of the component parts and the assembly/jigging method during curing time.

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