Adhesive Bonding is a modern joining process in which a liquid or semi liquid substance is applied to adjoining work pieces to provide a long lasting bond. This process is highly useful in bonding dis-similar materials that can not be welded. Materials that have the ability to be bonded together are virtually unlimited. Adhesives used in bonding can exist in many forms and be made from various natural and/or artificial compounds. A hindrance to this process is that adhesive bonds are not instantaneous such as welding or nailing. Adhesive bonds take more time to process, in order to allow the adhesives to cure.
# Uses substances in a liquid or semi-liquid state
# Allows the joining of metals with nonmetals
# Ideal process for the joining of thin materials to other materials
# Time and temperature usually must be controlled
# Helps reduce assembly costs
# Lighter than most other joining processes(Bolting, screws, etc...)
# Creates an "electrical non-conducting joint" in conductive materials
# Enhances the vibration dampening properties of a joint.
The adhesive is applied to either one of both of the materials being bonded. The pieces are aligned and pressure is added to aid in adhesion and rid the bond of air bubbles. The bond is meant to be permanent, unless: insufficient time was allowed for the bond to cure; the surfaces to be bonded were not properly prepared for bonding; the temperature at which the curing took place was unfavorable for the adhesive; or insufficient adhesive was used.
Adhesive bonding is used a lot in joining wood pieces and light-weight material pieces together. The joint is stronger when the adhesive contact area between the two pieces is the greater. To achieve greater sheer stress resistance the bonded materials can include key, corner and shoulder joints.
Setup and Equipment
There are many different ways to apply adhesive to a workpiece. A hot glue gun or caulk gun are used to apply adhesives in a paste or semiliquid form. Liquid adhesives are applied with spray applicators, which can be automated. Adhesives also come in containers with applicator brushes built in for easy application.
Typical Tools and Geometry Produced
Adhesives come as liquids, semi-liquids(pastes), and even solids, such as a glue stick or adhesive tape. Applicators of different adhesives are designed according to the adhesive being used and the size of the area to which the adhesive will be applied. Typical geometries of this process are; thin sheets or foils can be joined to thicker workpieces; component parts can be mounted on an assembly; and fabrics can be joined to solids; etc.
Adhesive bonding can be used to join materials in an infinite number of ways. More common geometries of bonded materials are flat surfaces, corners, contours ,and corrugated backing. The pressure used to bond materials usually ranges from 10 psi (pounds per square inch) to 1000 psi, which will produce a resultant sheer strength (depending on adhesive used) ranging from 900psi and 13,000psi.
Tolerance and Surface Finish
The thickness of the materials being bonded to each other can be thin or thick, as in the making of a violin. The ribs are thin enough that they are bent and then bonded using glue and smaller wood blocks to keep them together, and some violins last for hundreds of years.
There are two ways to hold workpieces, in platens or shaped dies. A specially designed fixture or vise may hold a workpiece with critical dimensions on platens. Otherwise, a shaped die, which contains guide pins, stops, and alignment blocks, may be used for bonding flat workpieces.
Process conditions are measured according to tensile strength, crystallinity and Young’s modulus of elasticity. Success of bonding adhesion is critically influenced by differences of crystallinity and Young's modulus.
* Rubber : 3000 tensile strength, Low status of Crystallinity and Low Young’s modulus( 70Deg F)
* Epoxy : 6000 tensile strength, Low status of Crystallinity and Medium to High Young’s modulus( 70Deg F)
* Phenolic : 5000 tensile strength, Low status of Crystallinity and Low Young’s modulus( 70Deg F)
* Nylons : 8000 tensile strength, High status of Crystallinity and Medium to High Young’s modulus( 70Deg F)
* Spray gun - this is used when the productions requirements are from low to medium. It is a semiautomatic device.
* Roll coater - fully automatic device used to high production.
* Manual - used when output requirements are low and/or if the bond is critical. i.e. making a violin
* Flat honey comb panel - used to press flat surfaces during adhesion process.
* Formed Die - Used to press contour or irregular shaped objects. i.e. the upward curved tips on a snowboard.
Adhesive bonding can be used no matter what the design of the two materials. As long as there is flush contact between the two materials, the bond should cure and join the two materials.
Effect on Work Materials
Adhesive bonding may increase the following:
* Tensile - the ability to hold weight
* Shear - strain in structures due to pressure
* Compression - ability to be reduced in volume
* Impact Strengths - ability to resist shock load
The effects of adhesive bonding vary depending on thew material properties of the workpiece.
* Mechanical : May increase tensile, shear, compression, and impact strengths.
* Physical : Exposure to sunlight and heat may deteriorate adherents.
* Chemical : Solvents deteriorate adherents.
Typical Workpiece Materials
Good workpiece materials: aluminum, steel, plastic, wood
Weak workpiece materials: Brass
Difficult workpiece materials: Copper (due to fast oxidation)
Method of Application
* Manual Application - Rollers, Brush, Film or pellet
* Automatic Application - Brush, Knife coater
* Caulking gun Application - Extrusion
* Robotics Application - Film or pellet
* Spray Application - Air spray, airless spray
* Roll Coater - Bench, Pressure Roll, Dip Roll
The type of application method depends of the type of materials being bonded together, the type of adhesive being used, what is required in production, and the cost.
Types of Adhesive
* Thermoset - Quick set, High strength, Solvent-resistant
* Evaporative - Flexible and Oil-resistant
* Hot Melt - Flexible and Water-resistant
* Film - Used in honeycomb construction
* Pressure sensitive - Versatile and Inexpensive
* Delayed-tack - Short-term bond.
Curing is when the physical properties of the adhesive are changed and the bond becomes permanent. It changes the physical properties of the adhesive through a chemical reaction that is induced by the action of a catalyst, pressure, and/or heat. Curing time also includes cooling time if the adhesive needed to be heated. An example is the curing of epoxy. To get a tensile strength of 6000 psi, the epoxy is cured by clamping it at a temperature of 68 - 165 degrees Fahrenheit.
Power is required when using a spray method of application. When a adhesive's temperature is raised, the viscosity of the adhesive decreases. Viscosity is the resistance of a liquid to flow. Heating the adhesive also results in the atomization energy required to be decreased. This enables a heavier film coating to be used and helps in correcting humidity related problems. (Humidity related problems occur from uneven temperatures.)
Cost and Time
It is sometimes cost effective to use adhesive bonding than other types of bonding, but it does take longer to bond with adhesive than it would be nails, welds, and other types of bonding.
Some of the cost comes from the following areas that also add time to the production process:
* Setup time
* Adhesive application time
* Load/unload time
* Bonding time
* Curing time
* Material costs
* Direct labor rates
* Overhead rate
* Amortization of equipment and tooling
* Adhesive Application time (sec) = A
* Load/unload time (sec) = L
* Platen Velocity (in./sec) = P
* Bonding time (sec) = B
* Curing time (hr) = C
* Platen travel distance (in.) = D
Total Time = A + L + B + C + D/P
Factors Affecting Process Results
The quality of the bond depends upon the type of work material and adhesive. Also, surface preparation, application of adhesive, curing, and equipment used to position heat and clamp/pressurize the workpieces affect the results of the process.
When using adhesive bonding, one should be aware of irritations from the adhesives, the vapors and fumes produced by the adhesive bonding process, burns that can come from the heat treated adhesives and also noise during the production process. Also, one should be aware of environmental hazards, such as how material is going to be disposed, air pollution produced during the production process, fires that may occur during the heat treated adhesives or if adhesives are flammable, and also groundwater pollution is adhesive make their way into water sources.