Adhesive Usage in Plastic Fabrication
There are many ways to use adhesives in plastic fabrication. You can walk into 10 different fabricators and see 10 different methods of doing practically the same thing, each achieving the same end result.
Regardless of what one considers the “proper” plastic fabrication technique, the same basic principles are found throughout. Once you have determined what the finished product will look like and selected the fabrication materials, the next task is to choose the right bonding agent for the job, which can be quite confusing given the multitude of adhesive options available (e.g. reactive adhesive, solvent cement, etc.1) To make the selection process a little bit easier, start by answering these simple questions:
- What materials are going to be bonded?
- What are the assembly conditions?
- Clean up and start up of operations interrupted by the
- What are the conditions under which the assembled parts will be placed?
- Do you know your desired viscosity or cure time?
What materials are going to be bonded?
It is always important to verify the materials you are attempting to bond.
- Some plastic substrates (e.g. olefins such as polyethylene and polypropylene)
will not bond together, regardless of what adhesive is used.
- Acrylics have different grades. Some acrylic materials may not bond to solvent
cements due to increased resistance to the cements. Instead, a two-component,
reactive adhesive is more suitable for bonding these surfaces.
- Same plastic material can easily be joined together with the right solvent
cement. However, joining dissimilar plastic materials together will require
the user to better understand the physical properties of these plastics. For
example, there is a substantial difference in thermal expansion among various
plastics. When joints between dissimilar plastics are exposed to rising
and falling temperatures, each plastic type expands and contracts by different
amounts, and a large stress is placed on the joints. The reactive adhesive
and/or solvent cement used in joining dissimilar plastics must be able
to withstand such stress.
Never hesitate to contact your adhesive supplier for their recommendations.
If needed, they should also be able to provide you with data to support their
claims. Most suppliers have selection guides making it easier for you to choose
the appropriate adhesive for your plastic fabrication needs.
What are the assembly conditions?
The temperature and humidity conditions in the fabrication area and the
temperature of the parts to be assembled are important.
- The ideal room temperature for fabricating acrylic sheet is between 70°F
- Low relative humidity condition is preferred. Excessive moisture may cause
the assembled joints to become cloudy.
- It is not recommended to solvent cement in room temperatures under 60°F
or over 100°F. Low temperatures will increase the surface hardness of the
plastic, reduce the solvent cement action and increase the soak time (i.e.
duration where plastic is dipped in the solvent cement prior to bonding).
Crazing, a network of fine cracks on or under the plastic surface, may occur
under such condition. At high temperatures, the solvent
cement easily volatilizes causing the cement to be less
effective in bonding plastic parts.
It is also important to work in a well ventilated area or area with appropriate vapor removal system. Solvent vapors from adhesives are often heavier than air. Some solvents are highly volatile and may be flammable. Excessive exposure to solvent vapors may cause drowsiness, dizziness, intoxication and/or nausea.
What are the conditions under which
the assembled parts will be placed?
Before choosing the proper adhesive for your plastic fabrication project, always consider the conditions where the final assembled parts will be placed. Are the parts going to be outdoor, inside in a machine or in a showroom? What are the temperature ranges and climate conditions? Is it in an area where temperatures are extreme or variable? The right adhesive should be able to handle the temperature and climate stresses as well as maintain an efficient bond.
Do you know your desired viscosity or
The faster you can bond the parts, the more finished products you can produce. This may be true, but are you jeopardizing the quality of your assembled parts? Fast cure time is not always better. If you have a big surface to cover, you do not want the adhesive to cure on one area before you finish applying to the entire area. This often will result in a joint failure due to lack of adhesion in certain areas. You will have “spotty” joint coverage. Do you have to fill any gaps in the fabrication piece? “Water-Thin,” low viscosity solvent cement is the most popular cement used because of its fast cure time and lower cost. However, it will not fill gaps and attempting to use it for gap-filling purpose will result in joint failure (e.g. bubbles in the joint area and weak bonding strength). More viscous
solvent cements or reactive adhesives are recommended
for such purposes.
Reactive adhesives are usually packaged into two components: a base resin and a catalyst. To use the adhesive, carefully follow the mixing ratio and direction as specified by the manufacturer. Adding too much or too little catalyst to the base resin will negatively affect the adhesive performance.
- Assemble: Prepare the plastic parts and ensure that
they are closely fitted and held firmly in place with masking
tape or a clamp. Depending on the type of acrylic
material and fabrication method, the possibility of crazing
may be reduced if the parts are annealed (i.e. the
process of heating and slow cooling to make the parts
tougher and less brittle) prior to adhesive application.
- Mix: Mix the reactive adhesive components together
per manufacturer specification. Read the product guideline
and identify the adhesive’s “working time” (i.e. the
time frame which the mixed adhesive is effective and
can be used for application). The “working time” ranges
from a few minutes to several hours depending on the
base resin and catalyst used.
- Apply: Immediately apply the mixture to the plastic
parts. Lightly press the parts together to remove air
bubbles. Do not squeeze the parts too hard as to force
the adhesive out of the joint.
- Cure: Let the assembled parts cure according to the
manufacturer specification before processing and fabrication.
Some reactive adhesives are packaged as a dispensing
gun with replaceable resin and catalyst cartridges.
The mixing ratio is pre-determined making the
adhesive in this packaging format much easier to use;
just dispense and assemble.
Solvent Cementing Techniques
There are two basic solvent cementing techniques: capillary
action method and the soak or dip method. The capillary
action method is by far the most common method
used due to its speed and ease of use.
- Assemble: The plastic parts must be closely fitted with
no visible gaps and held firmly in place with masking
tape or a clamp.
- Inject Cement: The cement is dispensed with a syringe
or applicator bottle with a needle along the edge of the
joint. For water thin cements, it is recommended to use
a 23 or 25 gauge needle, and for slightly thicker
cements, use a 14 gauge needle. The capillary action
will draw the cement between the parts.
- Cure:An initial bond will begin to form in as quickly as
30 seconds and the time for the joint to set will vary
from approximately 2 to 5 minutes depending on the
solvent used, temperature and humidity. Wait 3 to 4
hours before processing and fabrication. High bonding
strength is reached within 24 to 48 hours and will continue
to build over several weeks. In the soak or dip
method, the plastic parts must also fit closely together.
- Soak/Dip: Pour a moderate amount of solvent cement
into a pan. Dip the edge of one of the parts to be joined
into the cement. Only dip the edge. Exposing too much
area of the plastic to the cement will result in a weak,
slow setting joint. Soak thin sheet plastic for approximately
20 seconds whereas thick sheet plastic should
be soaked for 30 seconds, depending on the solvent
cement being used. Remove and hold the sheet at a
slight angle to allow excess cement to drain off.
- Assemble: Carefully place the soaked edge precisely in
place on the part to be joined. Hold together for 30 seconds
without applying pressure. This allows the solvent
cement to work on the surface of the piece that was not
dipped. After 30 seconds, apply slight pressure to
squeeze out any air bubbles. Be careful not to apply too
much pressure as the cement can also be squeezed out
resulting in a “dry” joint (i.e. weak bonding strength).
- Cure: Maintain the joined parts in firm contact with each
other for 10 to 30 minutes and do not move the parts.
Similar to the capillary action method, wait 3 to 4 hours
before processing and fabrication. High bonding
strength is reached within 24 to 48 hours and will continue
to build over several weeks. Other cementing tools
include eye droppers for use with “water thin” cements
and brushes or rollers for more viscous, bodied cements.
Potential Solvent Cementing Problems
A common problem in plastic fabrication, particularly
when using acrylics, is crazing, a network of fine cracks
running on or slightly under the plastic surface. Acrylic is
prone to physical stress during original sheet casting and
fabrication of finished products (e.g. machining and polishing).
If subjected to high stress for a long period of time,
acrylic will eventually craze. The tendency to craze also
increases when stressed materials are exposed to solvent
cementing. To minimize crazing, avoid placing additional
stress on acrylic parts during solvent cementing.
- Carefully prepare the joining parts. Parts must fit accurately
and smoothly without being forced.
- Annealing of the parts before cementing may minimize
- The joining edges should be machined smooth to fit
each other, but should not be buffed since this tends to
- Avoid flame polishing or disk sanding acrylic parts
where solvent cement contact may occur. Flame polished
edges will craze upon contact with solvent cement.
Another common problem related to solvent cementing
is poor bonding strength. The main causes are improper
fit of parts, excessive clamping pressures, and/or poor
technique. Properly follow the cementing techniques mentioned
in the previous section should improve the bonding
strength between the assembled parts.
Many, but not all, adhesives can be harmful if sufficient
concentrations are inhaled for extended period of time,
absorbed through the skin or swallowed. When using adhesives
in your plastic fabrication project, safety should always
be the number one priority. Always
practice proper safety procedures:
The most important safety precaution is to use common
sense. Treat all adhesives with respect and use only for
their intended purposes.
- Wear appropriate chemical-resistant
gloves when handling adhesives
to avoid skin contact. If the
skin does come in contact with an
adhesive, the exposed area should
be washed thoroughly with soap
and water. Follow additional safety
procedures as provided by the
- Work in a well-ventilated area and
avoid breathing the adhesive
vapor. Vapor exposure can be
reduced by using the adhesive
product in a cartridge format or
applicator bottle with easy
open/close access. When the soak
or dip solvent cementing method is
used, ensure the workplace is properly
ventilated as solvent cements
can easily volatilize from the soaking
pan with a large surface area.
- Adhesives are flammable. Always keep them away from
open flame or sources of high heat. There should be no
smoking in the area where the adhesives are stored or
- Take special care when mixing chemicals (e.g. multicomponent
adhesives, raw chemicals, etc.) Pay attention
to the proper mixing order and carefully follow each
product’s direction of use. Deviation from the proper
mixing order may result in a violent chemical reaction
with potential for explosion or fire.
- Keep copies of the Material Safety Data Sheets (MSDS)
for all adhesives and other chemicals in an easily accessible
location. The MSDS contain detailed information
about the Manufacturer, Hazardous Ingredients,
Chemical Physical Data, Fire and Explosion Hazard
Data, Health Hazard Data, Reactivity, Spill/Leak
Procedure, Special Protection Information and Special
Precautions. Consult the MSDS whenever you have
questions about the products being used.
Written by SCIGRIP Americas, the manufacturer of
Weld-On® Industrial Assembly Adhesives and Solvent
Cements for plastics and other materials used in the fabrication
industry. For additional information, contact
WELD-ON Assembly Adhesives, Customer Service &
Technical Service, 500 Distribution Parkway, Collierville,
TN 38017, 901-853-5001, Fax: 901-853-5008, E-mail:
email@example.com, Web: www.scigrip.com.
1 Reactive adhesive: A two- or three-component adhesive system that
chemically reacts when components are mixed together, to form an adhesion
with the surfaces of the plastic joints. Solvent cement: An adhesive,
formulated with resins and solvents, initially dissolves the plastic surfaces
and then “welds” the plastic joints together (i.e. the adhesive become a
part of the plastic).