Bonding FDM Parts
For parts too large to fit on a single build, for faster job builds with less support material, or for parts with finer features, sectioning and bonding Fused Deposition Modeling (FDM) parts is a great solution. There are many methods, and even more materials, for bonding FDM parts.
The primary considerations when selecting a bonding method are the strength of the bonded joint and the compatibility with each FDM material. For strength data, Stratasys conducted lab testing at the University of Texas El Paso to measure tensile strength. Other criteria — including time, cost, operation difficulty, part configuration and general performance — were also considered. Accuracy of bonded parts, however, is dependent on many factors. For example, adhesive characteristics, such as viscosity, will influence accuracy. The skill of the technician, style of joint and type of fixture will have even greater impact.
To assist in selecting the bonding approach most suitable for your needs, the following is a brief evaluation of the common methods for joining parts made in varying FDM materials.
Two-part epoxies are commonly used to bond FDM parts. The epoxy components are mixed and then applied with dispensers, brushes or infiltration. The viscosity will range from thin and caulk–like to thick and putty–like, so application techniques will vary. After application, bonded sections are fixtured or clamped while the epoxy cures.
Epoxies will differ in cure durations, material properties and bond strengths. But in general, they are easy to use. They offer very good mechanical strength, and typically exhibit good temperature resistance and chemical resistance. These adhesives offer the advantage of working times of 20 to 70 minutes, so minor adjustments can be made after mating the sections. However, the tradeoff is long curing times. When cured at room temperature, parts cannot be handled for many hours, and curing cycles will last from one to five days. If heat–cured, the cycle can be significantly accelerated.
Cyanoacrylate is commonly known as super glue. It is a fast–curing adhesive that can be used for quick, easy repairs and light–duty bonding applications. Super glue is simply applied to mating surfaces, and the sections are joined. The adhesive sets in a few minutes. The tensile strength of super-glued FDM parts is higher than that of epoxy adhesives. However, its resistance to high temperatures, chemicals and solvents is poor. Therefore, bonding with super glue may diminish the performance of an FDM parts. So, it is recommended for concept models and form and fit prototypes rather than functional prototypes or manufactured parts.
Solvent bonding works by chemically melting the plastic on the surfaces to be joined. The solvent can be brushed onto sections that are then mated and clamped together, or it can be injected into a pre–mated joint or existing crack. The water-thin solvent wicks into the part surface, which improves the strength of the repair or bond. Several solvents can be used, but the recommended product is SAME STUFF from Micro–Mark. This method produces bonds that are stronger that those of many adhesives. Like super glue, the process is simple, and the bond sets in seconds. Another similarity is that it can be applied to hard-to-reach areas since the solvent will wick into a seam or fracture.
An advantage over super glue and epoxy is that after evaporation the bonded part will contain only FDM material. Although the bond sets in seconds, parts should be allowed to cure for at least eight hours. Also note that if the part is subjected to temperatures exceeding 176 F (80 C), surface blistering may occur. Solvent welding is not suitable for bonding PPSF or ULTEM 9085. These FDM materials are chemically resistant, so there is little reaction to solvents.
Hot air plastic welding
Hot air welding of plastic is similar to oxy–acetylene welding of metal. However, a jet of hot air replaces the jetted flame, and a filament of FDM material replaces the filler rod. To bond parts, a hot air welding tool is slowly drawn along the joint. The heat melts the filament, which then fills the seam. This method produces bonds that are stronger than those of all other methods. It is also fast and inexpensive.
Parts can be put into service as soon as they are cool to the touch. Since the bonding material is a small piece of FDM plastic, the cost is negligible. Another advantage of using the FDM material as the bonding medium is that there is material continuity. The bond has the same properties and characteristics as the part. For best results, hot air welding should not be used on thin-walled sections. Also, the process requires some skill, so the results will depend on the experience and technique of the technician.
Ultrasonic spot welding
This technique is widely used in production processes when creating a permanent bond between plastic parts. The ultrasonic spot welding tool uses sound waves to melt localized areas of the joint. With the availability of handheld ultrasonic welders, this method can also be used in low–volume prototyping or direct digital manufacturing applications. Compared to other bonding methods, there are few, if any, disadvantages to ultrasonic welding beyond the need to purchase the welding tool. Welded areas are stronger than the surrounding material, yet, the tensile strength is not as great as that of hot air welded or unbonded parts. The ultrasonic welder’s horn and horn tips are often interchangeable. A variety of horns and welding tips are available, which will determine the thickness of material that can be welded, the diameter of weld, as well as the type of weld created.
Since no material is introduced into the joint, there is little change to the accuracy of the part or its properties. This makes ultrasonic welding ideal for medical applications that must consider the quality of the part as well as its suitability of human tissue contact.
When higher strength is needed, ultrasonic welding may be used in combination with other methods. Tack weld individual pieces to fix their position and then apply adhesives, solvents or other bonding agents. This approach is especially useful for bulky or awkward assemblies. Ultrasonic welding is fast and very inexpensive. Once the welding operation is complete, the part can be immediately put into service. And since there are no consumables required, the only expense is that for direct labor.
Although this approach is a joining method, not bonding, it can be an effective alternative. There are a large number of mechanical fastening approaches and hardware options that can be used when joining FDM parts. One unique approach to mechanically join sections is to insert fastening hardware in the FDM part during its build process. When it emerges from the Fortus machine, the fasteners are integrated within the part.
Gluing PolyJet Parts
Objet’s over molding capabilities have vastly reduced the need to mock up products by gluing molded parts together. But in some cases, gluing is needed to create a part larger than the build tray, or to combine 3D printed parts with other components. Fortunately, parts 3D printed from PolyJet photopolymers can be glued together quickly and easily, using simple, inexpensive household glue.
For large models, the design can be separated into several parts for 3D printing, using a software cutting tool to ensure that the parts can be connected accurately. Parts can be attached using instant glue, or glue that requires an activator can be used to control bonding time and allow for repositioning.
Pre-printing parts for gluing
When printing big parts, you can separate your design into several parts and then glue them together to create the full assembly. You can do this using your CAD software before the design is complete, or by using STL file manipulation software after the CAD design is already completed. It is important to consider the following when gluing several parts together:
- Finding an easy way to connect the parts together accurately.
- Maintaining the dimensional properties of the glued parts as intended by the CAD design.
The Magics software cutting tool provides an easy and intuitive way to glue together multiple parts while achieving these goals. The Magics software cutting tool has several options available. The following procedure is recommended when using this software tool:
- Orient the part in view that coincides with the natural coordination system of the part (that is, front view, back view, left view, etc.). In this manner, you can get a clear perspective of where you draw the cutting line in relation to the part.
- Draw the cutting line. It is recommended that you start with a point outside the part body and end with a point on the other side of the part body, thus creating a straight line across the part.
- It is possible to choose between a triangle (most suitable to glued parts), square (most suitable to molds), jigsaw, or user–defined cut. You can also control the size and offset of the cutting type.
- Make sure that you add a clearance between the cut parts. This leaves enough space for the glue and ensures that the final part maintains the designed dimensional properties. Note that the clearance value varies depending on the type of glue used and the recommended default value is 0.1mm. Two separate STL files are created when the cutting operation is complete, and they can be printed freely on the tray.
Post-processing parts for gluing
The following process is recommended before gluing the parts:
- After the parts are printed, use the water jet station to clean the parts.
- Once the parts are cleaned, dip them in 2 percent NaOH solution diluted in water for 20 to 40 minutes. This removes all support material residues.
- Wash again with water
- Dry the parts in free air, or use pressured air to speed up the drying process.
Recommended Glue Types
The glue types most commonly used by PolyJet experts are super glues (also known as cyanoacrylate). Generally, cyanoacrylate is an acrylic resin that rapidly polymerizes in the presence of water (specifically hydroxide ions). ALTECO- ACE –D is usually used with an activator to control the bonding time. An activator spray can also be used to harden the residues as they spill onto the model. They can then be sanded off, thus preventing excess glue from spreading on the parts.
To get an instant bond, the glue should be applied on the desired surface and then sprayed with the accelerator. For more information on the glues and accelerators, access the following links:
- Glues: http://www.alteco.com.sg/industrial_gluerange_ace.shtml
- Accelerators: http://www.hobbylinc.com/htm/hot/hotqa-6.htm
Other Glue Types
- LOCTITE 401 – medium viscosity
- Permabond Ultra Fast 792 – ultra fast cure time, general purpose
- Al-fix – comes with activator
- Kleiberit 851.0 – with easy to use applicator cap
- Sico Met 8300 and High Speed Accelerator Spray — good for elastomers
- Permabond Black Magic 737 for flexible parts