Choosing the Best Plastic Joining Process for the Application
by Bill Heatherwick, Branson Ultrasonics Corporation
This article will help the designer solve the mystery of choosing the best plastic joining technology for each application. With some practical guidance, the optimum process selection can be made from the many joining technologies available. Process selection is not an absolute science, and more than one process can be appropriate. The best process ultimately depends on specific needs, material, size, geometry, and functional requirements.
Parts can be assembled using various methods including mechanical, chemical, and thermal bonding. These are three distinct categories that are commonly referred to when choosing an assembly method. Both the mechanical and chemical methods often require a third consumable component to create the assembly.
There are two types of mechanical assembly methods: fasteners and other mechanical means. Mechanical assembly is normally used to attach two dissimilar materials. The first and most common method used is fasteners, which include screws, bolts, and clips. Other mechanical means include molded features such as snap fits, press fits, and the staking boss, to name a few.
Chemical bonds are created by using adhesives or solvents. This article will discuss plastic joining technologies that rely on a thermal heat source to perform the bond. The discussion further will be limited to compatible thermoplastic polymer materials.
Plastic Joining Application Requirements and Material Selection
When determining the joining technique to be used for an application, it is important to clearly define the needs for which the application is intended. This often will determine the type of polymer chosen. The type of material can be classified by one of two types: amorphous or semi-crystalline. The intent is not to help the designer determine the type of plastic to be utilized but rather, which is the best method for joining them once the material has been selected.
When determining the method for joining two or more pieces, one must first determine the material joining compatibility for joining multiple pieces. Much research has been done with various resin manufacturers, universities, and joining equipment suppliers to determine the compatibility of thermoplastic materials.
The success of the joining process depends greatly on the compatibility of thermoplastic materials that are to be used. The polymer chains of the two molten materials link together resulting in a polymer bond or joint. (Fig. 1) All thermal joining technologies rely on some type of heat source that will result in this type of bond. There are many options available. One method is to use friction as with ultrasonic, vibration, and spin welding technologies. Techniques also exist that use a conductive heat source such as hot plate, infrared, and laser welding.
As a rule of thumb, most materials that are of the same base resin are compatible with themselves. Included for reference is a compatibility chart (Fig. 2).
It is clear from the chart that there is increased compatibility in the amorphous resin category type. This is due in large part to the similar base resins and co-polymers that make up these resins. Also, it is important to note that these resins exhibit a melt characteristic referred to as glass transition temperature (Tg). This is the point or transition in which the resin is going from its solid state to a liquid or molten state. This transition is gradual and is what is responsible for allowing other amorphous resins to be bonded to one another. Further, it is clear from the chart that most semi-crystalline resins are only compatible to themselves. This is due to the fact that the melt characteristics of semi-crystalline resin as they go from a solid to a liquid or molten state and right back to a solid create a very sharp transition (Fig. 3).
Understanding the material compatibility is the first step in determining what process may be considered for the application. If it is required that the application be made of dissimilar materials or materials that are not compatible for thermal joining, then options may be limited to using mechanical fasteners or an adhesive product for the assembly. Several other key material factors can aid in joining considerations. They are the materials melt temperature, modulus of elasticity, and melt flow index. This information is readily available from the various resin manufacturers product data sheets.
- Melt Temperature Higher melt temperatures require more energy. When welding dissimilar resins, similar melt temperatures are required. A difference of 40 degrees F is enough to hinder the compatibility.
- Modulus of Elasticity Generally the stiffer the material the better the energy transmission capability for friction based welding processes like ultrasonic, vibration, and spin welding.
- Melt Flow (Index) The flow rates of dissimilar materials should be fairly close in order to achieve compatibility (i.e., 8 to 10).
The most commonly asked question is how strong will the bond be? This is a complex question that has many parts. To begin, the parent strength or neat strength must be looked at as the basis for determining weld strength. In most cases it is safe to say that most of the commercially available thermal joining processes will be capable of achieving 70 to 80 percent of the parent material.
If fillers are used, these often lead to a reduction in weld strength by almost the same percentage as filler content. Fillers are materials that take up space by volume and add no strength at the intended joint. Only in rare cases, such as those in labs, has the filler material migrated or crossed the weld line. In normal cases they reduce weld strength. For example, if the application is a 30 percent glass- filled Nylon material, the joint strength would be anticipated at 40-50 percent of the parent material, using 70 percent as the baseline or best case.
Most joining techniques are capable of welding a larger area while providing the application with the required joint strength. It is suggested that the material supplier be consulted during development along with the joining supplier to aid in these decisions. Simple weld trials can be performed that may help to determine the materials parent strength.
Understanding the applications performance specifications also is important when selecting the proper joining technology for final assembly. Items that need to be considered are the leak rate, tensile strength, and overall part cosmetics, to name a few. Does the weld need to be flash and particulate-free or can the design accommodate flash traps that can hide this condition?
Joining Process Descriptions
Ultrasonic welding is the most widely used of the welding techniques. The system converts incoming electrical power at 50/60 Hz to a specific output mechanical frequency. The most common frequencies in use today are 20 kHz and 40 kHz; however, 15 kHz and 30 kHz have been gaining acceptance in the industry as applications and uses for this technology continue to grow. The line voltage is converted though a power supply. The output voltage is then transformed into a mechanical vibration or amplitude via the converter, a transducer device with components that vibrate mechanically at the same frequency as the electrical input. This vibration is sent through a mechanical amplifier called a booster, which can increase or decrease the mechanical vibration that is coming from the converter. The horn is an acoustical tool that transfers the vibratory energy to the part being joined (Fig. 5).
The vibratory energy required for welding is referred to as amplitude. In most cases semi-crystalline or engineering resins require more amplitude (in the range of 80-120 microns peak-to-peak) versus amorphous resins (typically 40-80 microns peak-to-peak). There are some basic limitations to ultrasonic welding based on the shape and length of the weld:
- The height variation to the weld plane is 0.4″/10.2mm
- Maximum diameter of a round weld seam is 12″/304.8mm
- Maximum dimensions of a weld seam is 14″ x 3″ or 355.6mm x 76.2mm
Types of welding joint designs include energy director, shear joint, and spot welding. Many design variations are available to meet the requirements of the application. In applications that require the weld seam larger than the maximum, an option may be to use a segmented or discontinuous weld seam. Another possibility is to use localized welding, which may be done through the use of multiple welding units.
Vibration welding is a joining technology where the required melting temperature is generated by friction in the weld area. In general, one of the two parts to be joined is kept stationary while the other is moved in a reciprocating motion relative to it, at a predetermined displacement, frequency, and force. Upon completion of the melt, the parts are re-aligned and allowed to solidify, forming a bond. This process is capable of producing continuous weld seams. However, the weld seam must be designed for the proper movement of the parts during welding.
In the case of high frequency vibration welding (240 Hz), 1.8mm of motion peak-to-peak is needed and with 4mm of motion peak-to-peak with low frequency welding (100 Hz). If the parts are properly designed it is possible to weld inclines of up to 10 degrees in the direction of vibration with no issue. Horizontal and vertical weld planes also are possible if they are in the direction of vibration. (This is commonly referred to as a slip plane.) The two parts must have freedom of motion in order to be joined successfully. Today welders come in a variety of sizes and configurations that can weld small parts with a weld area of 5 cm2 to large parts that can have a weld area of 750 cm2.
Hot plate welding is a joining technology where the heat is transferred directly to the surface to be welded. In general, both parts to be joined are pressed against the heat platen at a given temperature for specific amount of time, and or melt displacement. This displacement is controlled by the tooling (melt stops). The parts are removed from the heat platen and the platen is then removed. The parts are brought back together under force to fixed stops (seal stops).
The parts are then held together and allowed to solidify, forming a bond. In hot plate welding, generally two types exist: high temperature and low temperature. The plastic melt temperature will dictate the type to be selected. The dividing line for high and low temperature is 650 degrees F.
Infrared welding is a non-contact joining technology where the heat is transferred directly to the surface to be welded via radiation or convection heating. In general, both parts to be joined are brought into close proximity (0.2 – 0.5mm) to the infrared heat platen for specific amount of time. The heating time will be dependent on the emitter intensity and absorption rate for a given thermoplastic. This distance is controlled by the tooling stops. The parts are removed from the infrared platen and the platen is then removed. The parts are brought back together under force to fixed stops (seal stops). Next, the parts are held together and allowed to solidify, forming a bond. Different types of IR emitters are used commonly in manufacturing today, including metal foil or glass bulb. These emitters are usually in the medium wave range as most resins absorb best between 2-4 µm. This is an excellent process to use with semi-crystalline materials that tend to stick to a heated platen. This process also is typically 40-50 percent faster than standard hot plate welding.
Laser welding is a process by which laser energy is generated using high power diodes in the range of 780 -980nm. This is often referred to as Through Transmission Infrared Welding (TTIr). Parts to be welded are clamped together while the laser energy is transmitted through one piece; this energy is absorbed in the second piece. The absorbed energy creates heat that allows the joint to soften and form the bond. There are multiple technologies in the market today that include Simultaneous Through Transmission Infrared Welding (STTIr), Trace Welding, and Wide Beam Scanning to name a few. This welding process is often associated with applications that require flash- or particulate-free assemblies.
The other critical requirement is that the parts be molded to tight tolerances. When parts are assembled and there are small gaps or locations where the parts do not touch, this effects the ability for the laser to transmit through this air gap. This can be compensated by using STTIr, which allows for material displacement or melt down.
With spin welding the required melting temperature is generated by friction in the weld area. In general, one of the two parts to be joined is kept stationary while the other is moved in a circular or spinning motion relative to it, at a predetermined velocity or revolutions per minute (RPM), down speed, and pressure.
Upon completion of the melt, the parts are allowed to solidify, forming a bond. There are generally two types of weld systems available in the market today – non-orienting and orienting technologies. Non-orienting is typically driven by a pneumatic air motor. This relies on tool weight to carry the revolutions required to perform a weld. The alternative is electric drive or servo drive systems that allow for final part orientation. As a rule of thumb, as the part gets smaller in diameter, welding needs to be performed at a higher velocity or RPMs. Conversely as the part gets larger in diameter, it will require less velocity but will require more torque. Custom systems have been made to weld large parts of 1100mm in diameter. The technology relies on parts that are round in geometry at the weld joint. The most critical dimension is the parts concentricity or roundness. This seems to be the single biggest factor effecting the successful implementation of this joining technology.
It is clear that there are many different welding technologies that can provide the end user with an assembly that will meet a specific need; and advantages and limitations exist for each joining technology . Using them as a basic guideline or starting point will aid you in selecting the best process based on the material and requirements for a particular application. Individual joining suppliers should be contacted for assistance in assessing needs and requirements to further help in the plastic joining selection process.
Bill Heatherwick is the market segment manager for Branson Ultrasonics Corporation. Founded in 1946 (a subsidiary of Emerson Co.), St. Louis, Mo., as part of the Industrial Automation Division. Along with more than 60 years of joining technologies, innovations, and global leadership to the many market segments it serves, Branson is dedicated to supplying its customers with the best solutions for their particular application. For more information, call (248) 299-0400 x.108, email firstname.lastname@example.org, or visit www.bransonplasticsjoining.com.