Assembly
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Adhesive Bonding
Did you know?
Adhesives are commonly used when bonding similar/dissimilar
plastics. Relatively easy to use and inexpensive, adhesives
provide a mechanical bond that distributes stresses over a
large area.
There are five major families of adhesives:
1. Epoxy – usually 1 or 2 parts, excellent moisture
resistance and fair impact resistance.
| a. Advantages: |
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i. Good adhesion
ii. High tensile and shear strengths
iii. Creep resistance
iv. Good rigidity
v. High heat tolerance
vi. Easy to cure |
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| b. Disadvantages: |
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i. Poor peel strength
ii. Brittle
iii. Low impact strength
iv. High cost |
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2. Urethane
– 2 part mix, good moisture resistance and excellent
impact resistance.
| a. Advantages: |
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i. Toughness
ii. Flexibility
iii. Impact strength
iv. Abrasion resistance
v. High peel strength. |
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| b. Disadvantages: |
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i. Volatile
ii. Excess Creep
iii. Poor strength at high temperature
iv. Chemical sensitivity
v. Lacks long-term durability
vi. Usually needs primers
vii. Moisture sensitive in uncured state. |
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3. Anaerobic
– 1 part mix and good impact and moisture resistance.
| a. Advantages: |
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i. Good solvent resistance
ii. Bond flexibility
iii. High peel strength
iv. Good impact strength |
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| b. Disadvantages: |
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i. Sensitive to surface cleanliness
ii. Good gap filling properties |
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4. Cyanoacrylate
– 1 part mix, fair impact and moisture resistance
| a. Advantages: |
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i. High tensile strength
ii. No shelf life |
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| b. Disadvantages: |
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i. Brittle
ii. Not suggested for dissimilar plastics
iii. Poor gap filling
iv. Not recommended for extended water exposure
v. Limited impact and peel strengths. |
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5. Acrylic – 2 part mix, good impact and moisture
resistance.
| a. Advantages: |
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i. Bonds to dirty surfaces
ii. High strength
iii. Superior toughness
iv. Fast curing |
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| b. Disadvantages: |
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i. Strong smell
ii. Problems with flammability
iii. Minimal gap filling |
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How does it
work?
Application methods will determine final properties. When
designing parts for adhesive bonding an adhesive well is typically
used. A solvent well is a groove in the part that will accept
the adhesive and contain it. See figure 7 for optimal joint
designs.
Adhesives can be applied by the following methods:
1. Manually – simple and cheap but slow
2. Semi-automatic – use of more expensive CNC machines
speed up the process and improves consistency and accuracy.
Key Factors:
The following properties should be considered when choosing
an adhesive:
1. Bond strength requirements – see
figure 6
2. Joint design – see figure 7
3. Chemical and environmental resistivity
4. Temperature stability
5. Shelf life of product
6. Dispensing requirements and safety issues
7. Surface preparation.
The following tests can be performed prior to design to evaluate
tensile, shear and peel strengths.
Tips:
The following steps should be taken to ensure a good bond:
 Ensure
clean surfaces
Abrading,
flame treating or other surface treatments can help surfaces
adhere
Control
room temperature and humidity
Mix adhesives
thoroughly before use
The thinner
the better – less adhesive means more strength
Joint design
is critical to bond strength requirements
Special
fixturing may be required for your application
Uniform
pressure is required
Allow sufficient
cure time prior to packaging.
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