The Plastics Distributor and Fabricator - Your Industry Magazine
The Plastics Distributor and Fabricator, Your Industry Magazine
Plastic Spacer
News Features Series Articles Columns
Plastic Spacer
Return Home
Article Keyword Search
Acrylic Fabricating Supplies & Equipment
Acrylic Shapes
Painting Acrylic Sheet
Solvent Cementing
Laser Cutting Thermoplastics
Sawing Acrylics
Acrylic Sheet
 




FEATURES
Select issue:  


Solvent Cementing
When it comes to fabricating plastics, there are many techniques and methods of using adhesives and solvent cements. You can walk into 10 different fabricators and see 10 different ways of doing practically the same thing, each achieving the same end result. Regardless of what one considers the “proper” technique concerning fabrication, the same basic principles are found throughout. Once you know what you desire your finished product to look like, and have chosen your substrates, often next you are left wondering what adhesive should be used. Choosing the proper adhesive can be quite confusing with all of the different options available. To make that decision a little bit easier start by answering a few simple questions.

  1. What substrates are going to be bonded?
  2. What are the conditions under which the cementing will take place?
  3. What are the conditions under which the assembled parts will be placed?
  4. Do you know your desired viscosity or cure time?

What Substrates are Going to be Bonded?

It is always important to verify the materials you are attempting to bond. There are some substrates that will not bond together, regardless of what adhesive you choose. Common examples of this include olefins such as polyethylene and polypropylene. Also, certain seemingly straightforward materials like acrylic have different grades which make it harder to be bonded with certain cements due to increased resistance. You may not be able to use a solvent cement and instead use a two-component adhesive on these surfaces. Remember, it all comes down to the substrate.

You also need to take into consideration the differences between bonding “like” materials to that of bonding “dis-similar” substrates. Without proper consideration, these joints can be much more difficult to bond and may result in failure. However, to counter the myth that “dissimilar” plastics cannot be joined, they most certainly can be. It just takes more understanding of the cements and the plastics to be joined. In most cases there is a substantial difference in thermal expansion between different plastics. When joints between dissimilar materials are exposed to rising and falling temperatures, the materials expand and contract by different amounts, placing large stress on any bond between them. Your cement needs to be able to take that into consideration.

Also, you should never hesitate to contact your adhesive supplier for their recommendations. They should be able to provide you with not only recommendations, but also data if needed to support their claim. Also, many have charts to make it easy for you to decide which adhesive you should choose depending on your substrates.

What are the Conditions Under Which the Cementing Will Take Place?

There are many factors in determining the conditions under which the cementing will occur. Things to consider include: whether the fabrication will take place inside or outside, the temperature during cementing, the humidity level and having proper ventilation.

The temperature and humidity conditions in the cementing area and the temperature of the parts to be cemented are important. The ideal temperature for cementing acrylic sheet is between 70ºF and 75ºF. It is not recommended to cement in room temperatures under 60ºF or over 100ºF. “Water-Thin” or very low viscosity cements should not be used at temperatures below 60ºF. Low temperatures will increase the surface hardness of the substrate, reduce the solvent action and increase the soak time, in turn increasing the possibility of crazing. The most desirable conditions are normal room temperatures and low relative humidity. Excessive moisture may cause the joints to become cloudy. It is often recommended to reduce the cloudy appearance in these conditions add a small amount of acetic acid to your solvent to slow down the process and reduce the blushing effect. An example of this is Weld-On® #5.

It is also important to work in a well ventilated area. Ventilation or vapor removal is necessary, as solvent vapors are heavier than air. Some solvents are highly volatile and may be flammable. If not ventilated, excessive exposure to vapors may cause drowsiness, dizziness, intoxication or nausea.

What are the Conditions Under Which the Assembled Parts Will be Placed?

Before choosing an adhesive, you should always consider the conditions under which the final assembled parts are going to be placed. Are the parts going to be outdoors, or inside in a machine or showroom? If the parts will be outside, what are the temperature ranges and climate conditions? Is it in an area where temperatures are extreme or variable? These are all conditions that must be answered before you choose the proper cement.

Do you Know Your Desired Viscosity or Cure Time?

When determining your viscosity and cure time, many questions must be answered. Remember, faster is not always better. The common thought is the faster you can cement the more parts you can produce. This may be true, but are you jeopardizing the quality of your parts? If you have a big surface to cover, you do not want the cement 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. Also you must determine if you have any gaps to fill. “Water-Thin” cements are the most popular solvent cements due to speed and often price but they will not fill gaps. If used in areas where a bodied cement or gap filling cement is needed, a lack of adhesion and failure may occur. Often these are areas where bubbles are seen because the water thin cement will flow over the top of the part.

Cementing Techniques

The two basic cementing techniques for solvent cementing are the Capillary Action Method and the Soak or Dip Joint Method. Between these two methods, the capillary action is by far the most common method used for both its speed as well as ease of use.

Under the capillary method, the parts must be closely fitted with no visible gaps. Often pieces are held firmly in place with masking tape or a clamp. The cement is dispensed with a syringe or applicator bottle with 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 thickened cement use 14 gauge. The capillary action will draw the cement between the parts. 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 strength is reached within 24 to 48 hours and will continue to build over several weeks.

In the soak or dip method, the parts must also fit closely. Pour a moderate amount of solvent cement into a dip or soak pan. Dip the edge of one of the pieces to be joined into the solvent. Only dip the edges, exposing too much area to the solvent will result in a weak, slow setting joint. Thin sheet should remain in the solvent for approximately 20 seconds and thick sheet should be soaked for 30 seconds, depending on the solvent being used. Remove and hold the sheet at a slight angle to allow excess solvent to drain off. 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 to work on the surface of the piece that was not dipped. After 30 seconds, apply slight pressure to squeeze out any air bubbles, however too much pressure will squeeze out the cement resulting in a dry joint. When the pieces are joined, allow to maintain firm contact for 10 to 30 minutes, not allowing the parts to move at this time. Similar to the capillary method, wait 3 to 4 hours before processing and fabrication. High strength is reached within 24 to 48 hours and will continue to build over several weeks.

Other methods for applying water thin cement can be done using eye droppers and body cement can use brushes, rollers, etc.

Potential Cementing Problems

When determining potential causes for joint failure, there are many factors that may come into play. Preparation of the joint is very important when cementing parts. The parts must fit accurately and smoothly without forcing. If the parts do not match or fit well, the edges to be cemented should be machined smooth to fit but should not be buffed since this tends to round edges. More specifically, flame polished edges will craze upon contact with a solvent cement. The proper preparation of edges is necessary to ensure that little or no internal stress is present in the material. Such stresses will cause crazing on contact with solvent cements, which brings us to our two most common cementing problems. The two most common problems found with cementing are crazing and poor joint strength. Crazing is a network of fine cracks running, on or slightly under, the surface of plastic materials. Acrylic plastic will eventually craze when subjected to high stresses for a long period of time. The tendency to craze is greatly increased when the stressed material is exposed to solvent or solvent vapors. Stress is often introduced into a part during casting of the sheet or during fabrication operations such as machining or polishing. The possibility of crazing may be greatly reduced by minimizing internal stresses through annealing of the parts before cementing. Also to avoid crazing, avoid flame polishing or disk sanding where solvent contact may occur. Potential culprits for weak cement joints are improper fit of parts, inadequate mixing of cements, excessive clamping pressures or poor technique.

Safety Precautions

When using any cements and adhesives, safety should always be your # 1 priority. It is always important to be in the habit of practicing proper safety procedures. Many, but not all cements may be harmful if sufficient concentrations are inhaled for extended periods of time, absorbed through the skin or swallowed. To limit the amount of skin contact with the materials, wear gloves when handling solvents. If the skin does come in contact, the exposed area should be washed thoroughly with soap and water. As stated earlier, all cements should be used in well ventilated areas, again remembering that most vapors from the cementing process are heavier than air. Often volatilization can be reduced by using products in cartridge forms which reduce the need to hand mix components, or by the use of a closed applicator bottle. Both of these applications keep the solvents themselves, as well as their vapors, in a more controlled environment. Other methods such as the dip and soak method discussed earlier are more prone to producing objectionable vapors.

Some types of cement are also flammable. All cements should be kept away from open flame or sources of high heat. There should be no smoking in any area in which the cements are stored or used.

Special care must be taken when mixing chemicals, which not only goes for mixing 2- or 3-component items but also for mixing raw chemicals. Attention to the proper order must be taken into consideration. Deviation from the proper order of mixing may result in a violent reaction causing potential explosion or fire.

You should always have a copy of the Material Safety Data Sheets (MSDS) for all cements and adhesives that you keep on hand. They should be kept in a location that is easily accessible and should be consulted to whenever in doubt about a product. The MSDS will include sections on: the Manufacturer, Hazardous Ingredients, Physical Data on the product, Fire and Explosion Hazard Data, Health Hazard Data, Reactivity, Spill or Leak Procedures, Special Protection Information and Special Precautions.

The most important thing to always remember is to use common sense. Treat all adhesives and solvent cements with respect and use only for their intended purposes.


Written by Chad Hogancamp, Industrial Adhesives Manager, IPS Corporation, manufacturers of Weld-On® industrial cements and adhesives for plastics and other materials used in the fabrication industry.


For more information, contact IPS Corporation, 455 W. Victoria Street, Compton, CA 90220, 800-421-2677, Fax: 310-898- 3392, E-mail: chadh@ipscorp.com, Web: www.ipscorp.com.


References used: Working with Acrylics, Craftics Inc. For your free copy, call Craftics Inc. at 505-338-0005.

Return Home | Back To Top
Plastic Spacer

 
Copyright © 2024
Plastics Distributor® & Fabricator Magazine
P.O. Box 669
LaGrange, Illinois 60525-0669
All Rights Reserved.
Header Image courtesy of Nylatech, Inc.

Phone: (708) 588-1845
Fax: (708) 588-1846
Email Us