Plastic consumables for medical applications are often very complex and sophisticated devices. Before these devices can be used by healthcare workers or home care patients, they have passed a long process chain. The process chain often includes production, assembling, packaging, and sterilization. Common processing methods are injection molding and extrusion.
The molding compounds used as a starting material play a very important role in this whole process. Properties like processability, bondability, and sterilizability very often determine the applied methods or the other way around (see Figure 1).
Roehm, for example, focuses not only on material performance at the starting point (processing) and endpoint (use) of the process chain but also on the material performance for essential process steps like assembly or sterilization. As a result, CYROLITE®, an impact-modified PMMA-based copolymer molding compound for medical applications, is available in five different grades. The well-balanced property profile of all grades reflects the general needs for medical plastics. Furthermore, every grade has special features (see Figure 2).
Assembly methods (secondary operations) play a dedicated role in the process chain. In the case of complex (critical) medical consumables, this is often the first step to enable the intended use in the first place. A very common joining method for medical devices is ultrasonic welding. For that reason, Roehm investigated ultrasonic welding of CYROLITE® in an ultrasonic welding study. This article is the first part of a two-part series about ultrasonic welding with a focus on these plastics for medical applications. This article provides an overview about the basic principles of ultrasonic welding. Part 2 will introduce and explain the ultrasonic welding study and offer advice for troubleshooting.
Ultrasonic welding of plastics is a joining technique that is especially well suited for medical applications. It prevents the introduction of contaminants or sources of degradation to the weld, ensuring biocompatibility of the medical device, and allows very fast and highly reproducible production cycles. A well-designed ultrasonic welding process is very efficient and causes minimal distortion and material degradation. Ultrasonic welding of plastics is fast and clean, and it produces an excellent joint integrity. The energy efficient process results in higher productivity with lower costs than most other assembly methods. Restrictions of ultrasonic welding are limited joint areas and that specifically designed joint details are required. The main advantages of ultrasonic welding are:
No ventilation needed.
No contamination introduced.
Can be easily automated and quality controlled by machine parameters.
High weld strength.
Proper optical surfaces.
Liquid and airtight weld lines.
No adhesives and additives.
Short cycle times.
The disadvantages, however, include limited size of joint interfaces and the need for a special design.
The successful outcome of the ultrasonic welding process is highly dependent on well-considered part design and material choice. Therefore, it is very important to carefully design the parts. Failures in this stage very often lead to problems later, which are mostly very difficult to resolve.
Ultrasonic plastic welding usually works by conversion of high frequency (25-40 kHz) ultrasonic energy into low amplitude (1-25 μm) mechanical vibrations. The heat is generated by a combination of surface friction and intermolecular friction in the parts. The generated heat causes plastics to melt at the joint interface between the two parts. The polymer flows and wets the joint interface. Diffusion and entanglement of polymer chains across the weld area results in weld formation after cooling (see Figure 3). Weld energy is the product of the average power dissipated in the joint and the weld time.
In the most common application mode, the movement of the horn is perpendicular to the plane of weld. The plastic parts are fixed in fixtures to keep the parts in alignment. It is important to fix the parts well to ensure good energy transmission and to maintain uniform pressure between the parts during welding. If the parts are not fixed well, a lot of energy can be lost, maybe inhibiting melting of plastics, leading to insufficient parts. The ultrasonic energy is applied by a so-called horn or sonotrode at the top of the parts to be welded. A simplified scheme of an ultrasonic welding setup is shown in Figure 4.
The success of the ultrasonic welding process is highly influenced by the material properties, including the ability to efficiently transmit and to absorb vibration which facilitate the local buildup of heat. In general, a distinction can be made in ultrasonic welding between near field and far field welding. In far field welding, the distance between sonotrode and joint surface is greater than 6 mm. The smaller the distance between sonotrode and joining surface, the better the energy transfer. For semi-crystalline plastics, only near-field welding is applicable.
To concentrate the energy in the weld zone, it is a frequent practice to mold a special asperity into one of the parts. This mostly triangular shaped protrusion is called energy director (see Figure 5). To improve the welding process, a texture may be introduced into the mating surface. The ultrasonic welding process can be described by weld displacement as a function of time. Based on this description, the entire process can be roughly divided into four phases (see Figure 6).
In phase one, after applying the ultrasonic energy, the energy director is forced by welding force into contact with the counterpart and begins to melt. At the beginning, the displacement is rapidly increased and slows down when the energy director spreads out. Important machine parameters in phase 1 are amplitude, horn velocity, and weld force. In phase two, the lower and upper parts begin to melt together through the developing melt layer. The energy director is not completely molten at this point, and the whole process is still unsteady. Important parameters in phase two are still amplitude, horn velocity, and weld force. In phase three, a constant melt layer with a homogenous temperature distribution forms between both parts. The process can be controlled by various parameters as time, energy, force, or distance. After the specific value is reached, the power is turned off, and ultrasonic vibrations cease. The most important machine parameter at this phase is the switch off value (such as time or distance). In phase four, the pressure is maintained, and the melt starts cooling down, forming the welding bond. Important machine parameters at this stage are holding pressure, holding time, and holding distance.
Among the machine parameters which can be adjusted for the ultrasonic welding process, three parameters turned out to have the biggest influence of the welding process result in most cases. These are:
Weld time: Weld time mostly reaches an optimal time (e.g., in one study 0.8 seconds for PMMA and 1.0 second for ABS was found). Weld times above the optimum result either decreased or only a slightly increased weld strength.
Weld pressure/weld force: Weld pressures that are too low lead to poor energy transmission; pressures that are too high result in molecular alignment and decreased weld strength.
Amplitude: Heating of the weld interface can be controlled by varying the amplitude of vibration. Amplitudes that are too high lead to molecular alignment and significant flash generation (equal lower weld strength). Amplitudes that are too low produce nonuniform melt initiation and premature melt solidification (a form of sticking). Higher amplitudes generally mean greater stress for the parts.
For a satisfactory result of the ultrasonic welding process, a suitable joint design is essential. The main factors influencing the joint design are:
Placement of joining area.
Requirement for the weld line.
Edges and corners in the joining area should be rounded to avoid crazing and destruction of parts. During the design of the parts for ultrasonic welding, the joining process should always be kept in mind. The joining area should be mostly perpendicular to the horn axis, lie in a single plane, and be parallel to the horn area. For the best energy conversion, the horn should be placed directly above the joining area, and the distance between horn and joining area should be short. In general, the closer the horn is placed to the joint minimized the energy that is lost through absorption.
As mentioned before, it is highly recommended to integrate an energy director into one part. The energy director allows concentration of the ultrasonic energy and enables to initiate the melting process in a short period of time.
In most cases, it is best to place an energy director symmetrically in the middle of the joining area. The height of the energy director should be between 0.30 and 0.80 mm (see Figure 7). In general, it does not matter if the energy director is on the lower or the upper part. In the case of dissimilar plastics with differing stiffness, the least stiff plastic should contain the energy director.
The joint design is always a balance between several needs and opportunities regarding the overall device design. Very often a good mechanical and optical performance of the weld is favored. A variety of joint designs can accommodate specific requirements. But there are three basic joint designs known (see Figure 8) that can give a first hint as to which joint design may be the best for the device. One of the simplest joint designs is the butt joint with an energy director as shown in Figure 8a. The step joint with an energy director (see Figure 8b) is a good choice for devices where optical appearance is important, because it prevents flash at the outside of the joint. In the tongue-and-groove joint shown in Figure 8c, the welding area is completely hidden from the exterior. For this design, no fixtures are necessary, but the close tolerances required make parts difficult to mold. In general, the volume of the energy directors should be calculated to fill, upon melting, as much of the cross section possible without producing flash.
Properties of plastics vary over a wide range. In each case, the specific properties of the materials used must be taken into account, and the welding properly must be adjusted to ensure a satisfactory ultrasonic welding process. The most important properties are:
Thermal and rheology properties.
Fillers and additives.
The mechanical properties determine the efficiency of energy transmission. Good energy transmission is related to a high shear modulus G and a low mechanical loss factor. As a rule, stiffer (higher modulus of elasticity) resins transmit ultrasonic energy to the joint interface better than resins with a lower modulus of elasticity, although there are exceptions to this. Plastics with higher melting or glass transition temperatures and/or higher specific heat capacity require more ultrasonic energy for welding.
Fillers and additives can have a huge impact on the result of the ultrasonic welding process. As a matter of fact, every additive or filler reduces the amount of polymer chains available in the welding area.
Fillers and additives can include:
External release agents can cause serious problems; internal are mostly no problem (fine dispersion).
Internal lubricants cannot be removed and will reduce the coefficient of friction at the interfaces of the parts to be welded. They can upset the ultrasonic process.
Plasticizers can be a big problem; experimentation prior to welding is recommended.
Impact modifiers can reduce energy transmission (higher amplitudes necessary) and can reduce the amount of polymer at the joint face.
Colorants mostly do not interfere; for some, adjustment of parameters can be necessary.
In low concentrations, fillers can increase weld strength by increasing E-modulus.
When a material contains moisture, this can have a negative impact on the strength of weld. During the welding process, water will boil at 100 °C and will negatively influence the outcome of the process. Parts should not be welded directly after the injection molding. The parts should be allowed to properly cool before ultrasonic welding. If possible, the whole ultrasonic welding process should be done at constant conditions (cooling time of parts, environment, temperature, humidity, etc.). Generally, best results are always achieved by welding similar materials together. In welding dissimilar plastics, plastics should be similar in chemical structure and should have a similar melt temperature range. Examples for ultrasonic welding of dissimilar plastics — PMMA copolymer, PC, MABS, SAN (all medical grades) — will be discussed in Part 2 of this article.
- M. J. Throughton, Handbook of Plastics Joining: A practical guide, 2nd edition, William Andrew Inc., 2008.
- DVS , “Guideline 2216-1 Ultrasonic joining of molded parts and semi-finished products made of thermoplastics in series production,” German Welding Society, 2018. (in German)
- AWS, “Specification for Standardized Ultrasonic Welding Test Specimen for Thermoplastics,” American Welding Society, 2010.
This article was written by Dr. Dirk Heyl and Andrew Sneeringer, Technical Marketing Managers for CYROLITE® advanced medical acrylics. For more information, visit here .