Selecting the proper adhesive can be challenging. When manufacturing rapid prototypes, adhesives are used for bonding cured stereolithography (SL) resins together to produce larger prototypes or for adding inserts, name plates or other additional parts to enhance the finished unit. When used in manufacturing, SL parts may be the base of a product that requires long term stability and strength offered by adhesives. There are many variables and chemistries to be considered for any requirement. Before selecting the best adhesive, it is important to look at the entire application and discern what parameters are most crucial.

What Are You Bonding?

First, you must know what substrates you are bonding. Most adhesives will bond well to most metals, but adhesion to highly oxidative metals (e.g. nickel, zinc and tin) is more difficult. Surface preparation, such as abrasion or chemical etch, may be required to produce sufficient bond strength.

Glass and ceramics (especially polished) can be very difficult as they are usually present smooth surfaces. A smooth surface has less area to physically attach to than a rough one, thus producing lower strength bonds. Adhesives, that do not wet the substrate, may not produce a good bond to these materials.

Plastics (polymers synthesized from organic material such as oil) can be the most difficult to bond. Since there are so many different types of plastics, it is impossible for any one adhesive to bond to all types. Surface preparation is also very important for many plastics. Abrading the surface and then washing is sufficient for many plastics. Fluorinated plastics (such as teflon®) must usually be acid etched to obtain any adhesion. Some plastics are best treated with a solvent wash. Knowing the specific plastic helps in determining the best adhesive to use.

Temperature and Environment

The temperature requirements during use are very important in adhesive selection. Higher temperature requirements usually need a strongly cross-linked polymer and therefore, an extended heat cure may often be needed. Also consider if the adhesive will be temperature cycled. Many rigid materials that will withstand temperature extremes will breakdown when subjected to thermal cycling. Also keep in mind that elevated temperatures during processing may affect ultimate performance but many materials can be flashed to very high temperatures (several thousand degrees centrigrade) for short durations without degradation.

Environmental conditions are also a consideration. Many materials absorb moisture and some may actually swell. Strong acids and bases will degrade most polymers, but even weaker materials may affect a polymer over time. Solvents can equally result in swelling or degradation. Chlorinated solvents (methylene chloride or tricloroethylene) will slowly deteriorate most organic adhesives, whereas alcohols or ketones (e.g. acetone) can cause surface distortion to some adhesives. Ultrasonic or heat can accelerate many of these conditions.

Specialty Adhesive Chemistries and Formulas

The physical nature of the bond itself also needs to be considered. How large is the bond gap? Some materials (eg. Cyanoacrylates) work best with very small gaps of 0.002" or less, where most epoxies and urethanes are best for gaps of 0.002 to 0.010". Larger gaps require a material with high intrinsic strength as the material's strength becomes much more important than its bond strength.

There are many different types of specialty adhesive chemistries and formulas to chose from, each with its own advantages and detractions. Some of the offerings are listed below:

Anaerobics
These are excellent adhesives for metal bonding but have limited use in bonding other substrates. Usually supplied as a one part, there may be an activator used when bonding non-metallic substrates. These materials cure best in an oxygen deprived environment. Formulated versions can withstand high temperature, but generic formulas usually don't withstand temperature extremes.

Acrylics
Traditional two part systems are not usually used as specialty adhesives but one part UV cured materials are extensively used. These one parts are fast curing materials that bond well to most substrates. Generic UV cured acrylics are somewhat flexible and handle well in thermal cycling applications but do not stand up well to other environmental stresses. Bond strength is usually good to glass, most plastics but not as good to metals. Newer specialty formulas can yield very unique properties for optical applications.

Cyanoacrylates (CAs)
Also known as super glues, these adhesives use moisture to cure quickly. Usually one part, some types of this adhesive can be used with accelerators. The generic materials have little shear strength but bond to most metals, ceramics and glass. Generic CAs have limited success with plastics and may not have long term stability. Many versions are designed and formulated to bond specific substrates, including plastics. These modified CAs usually solve any stability issues. Most CAs are not good for gap filling and require very thin bond line thicknesses. Their fast cure does make them a good choice for temporary bonds in high speed manufacturing.

Epoxy
There are a wide variety of epoxies for numerous applications. They bond well to most metals, ceramics, glass and plastics. Surface preparation is very important for generic epoxies. They are offered in one and two-part formulations. One-part epoxies require high temperature heat cures or UV initiated cures, while two-part systems can be either heat or room temperature cured. Generic epoxies are usually rigid but specialty formulas allow for many hardness choices. Bonding fragile parts requires care, as many epoxies will shrink and compress parts while curing. In general, epoxies will yield high strength and durable bonds over wide temperature ranges.

Silicones
Offered in one and two-part formulas, these are usually flexible and can withstand high temperatures. Silicones are excellent for protecting fragile parts (coatings and encapsulations), gasketing and mold making. They bond well to other silicones but generic materials do not bond well to other substrates. Specialty formulations may resolve some of the bonding problems. Cured silicones have a tendency to absorb moisture over time but remain durable over long periods.

Urethanes
This chemistry usually yields a strong, tough material that bonds to most substrates. These are used very successfully for plastic bonding. Primarily available in two-parts, there are some specialty one-part versions which cure with UV or solvent evaporation. Uncured urethanes are highly susceptible to moisture absorption that can create gasses that may affect both cure and bond strength. Some urethane formulas are highly toxic. Urethanes are excellent choices for sealing and environmental protection applications.

With all the choices available, it is very important to consider the exact requirements of your application. The right adhesive can affect the long term stability of your finished product. Care should be taken with written information regarding any material. Data sheets provided by the supplier offer a guide to an adhesive's potential; it is always preferable to test any candidate material if at all possible.