by Kenneth A. Holt, Herrmann Ultrasonics, Inc.
Although injection molding and ultrasonic welding are both well known and documented processes, the interplay of the two oftentimes is not understood. Hopefully, by discussing problems and solutions in this interplay, this article may aid those who produce assemblies that are ultrasonically welded.
Ultrasonic welding is a common method of assembly due to its accuracy, repeatability, low cycle times, energy efficiency and low relative costs. However, the success of the process is strongly dependent upon the input of well-molded parts. Both the molding process and the design process have direct influences upon the success of the ultrasonic process. Understanding the interdependency of the design, molding, and welding processes is imperative to be successful in this segment of the industry.
Design Considerations
Much has been written about proper design techniques for plastic parts, and those responsible for design should assure they have the technical knowledge before designing a thermoplastic part to be ultrasonically welded. The influences of good plastic part design can be shown to also affect the other joining processes, e.g. hot plate, laser, and vibration welding.
Proper integration of a suitable welding joint into the part is of utmost importance to the joining success. The most important step to this integration is to solicit good resources from the ultrasonic welding industry and/or to get references on good design resources. Certain advantages exist in each type of joint design and manufacturers of the equipment give little advice on cost of design and usage. However, the most important assets of all good joint designs can be examined.
- Part to part location using part details such as tongues and grooves, mating steps, or posts and holes.
- A small initial contact area between the two parts in order to hasten the welding process and minimize exposure of the upper part to horn vibration and applied force.
- Freedom of vertical movement between the two parts at the initiation of the ultrasonic vibration, which reduces exposure time.
- The ultrasonic joint should be kept as close to the horn/sonotrode contact surface as possible to avoid the attenuation of the vibrations to the weld joint. The terms near field (6mm) are utilized in the industry to describe this measurement.
Equally important to the success of the joining process is the usage of standard “good design practices.” The use of good practices insures well-molded parts that can be consistently joined. Special attention should be given to those details that minimize part warpage, sink marks, internal stresses, and mold filling difficulties (e.g., weak knit or weld lines, incomplete filling, etc.).
Wall thickness should remain consistent throughout the part to avoid problems with sinks, voids, and warpage of the part. If this is not possible, a variation of less than 15 to 25 percent is generally acceptable, provided the transitory zone is gradual (versus an abrupt change). Any condition that causes a decrease in the contact patch of the horn or sonotrode on the part, such as sink and warpage, will yield a lack of force (a primary welding parameter), thereby leading to incomplete welds.
Use of proper radii to reduce stress concentrations will assure less molded in stress in the part and will allow easier fill into the mold. Radii with values of 50 percent of the wall thickness are suggested to avoid the notch effects of sharp corners that concentrate stress and reduce part strength. Areas of highest stress levels, e.g., gate areas, have been seen to fracture upon exposure to ultrasonic vibration and joining forces.
The ultrasonic energy and weld force can cause fracturing in areas of weakness such as weak knit or weld lines around bosses, holes, or other flow restrictions. This is commonly seen in the process of emplacing ultrasonic inserts, with interference, or with shear type joints, which can place sections under a great deal of tensile force.
Gate sizing to half the nominal wall thickness will lessen mold-filling difficulties. Proper vent sizing and location will minimize short shots, burn marks, degradation, and high residual stresses, resulting in a more robust process and part.
Resultant Part Shape or Part Geometry Considerations
Even though the design of a plastic part is done well and most of the problem areas are addressed, conditions in the molding process that exacerbate or introduce problem areas into the final part still may be encountered. What was “designed out” in the pre-production phase may be reintroduced. The following are examples of encountered problems and their root causes.
Poor dimensional stability. Poor dimensional stability results in parts fitting together differently, or into the ultrasonic tooling differently, from run to run, batch to batch, or cavity to cavity. Of course, parts always need to fit together the same way to allow a consistent and reliable weld. Different part “fit up” will result in differences in welding results and many times is seen in the welding data as varying collapse distances, peak power outputs, weld times, and as a result, energy levels. For example, a tight fit between the two parts will require longer weld times that may result in part damage. An increase in welding amplitude may overcome this problem, but is not the preferred cure. Too loose of a fit may result in joint designs not coming together as designed. Changing part dimensions or varying dimensions of parts coming from multi-cavity molds also will affect the way parts fit into the tooling. This may result in inconsistent welding data, marking, or damaging of the parts by the tooling and welding process. This is commonly seen in parts coming out of multi-cavity molds and can be sometimes addressed by using PE film between the tooling and part.
The culprits of this instability are many and diverse; however, varying parameters of the molding process will produce such part geometry differences. A varying shot size will produce parts with some sink marks (affecting horn contact surface) or some flash (tight part to tooling fit) in areas. In extreme cases, weld lines (knit lines) may not be strong enough to withstand the force and vibration of the welding process. Stock and mold temperatures, varying regrind percentages, differing melt flow rates and shrinkage rates of materials may all cause this condition. A good stable molding process repeated over many runs and well maintained equipment run by good personnel are all steps in the right direction.
Warpage. Warpage is a condition whereby a feature is not in an intended shape, such as a flat ledge curving upward at its ends, or a wall bowing inward instead of being straight. This can influence the welding by prohibiting the proper engagement of the joint design features.
For example, if a shear type joint design is designed to yield 0.5mm interference between a Housing and a Cap, and the Housing bows outward 0.25mm in its middle point, this area will lack strength. However, a fixture wall providing additional alignment may suffice to solve this. Another example that is seen commonly is if a horizontal wall, on which an energy director joint design sits, bows upward at the ends of its length. The middle area will not weld equally compared to the ends. In such cases, an increased trigger force may overcome this condition although stresses will remain in the welded area.
Warpage is commonly seen in the first shots off a new mold when the process is not “dialed in,” and there is residual stress in the parts. Parts with severe warpage are marked by the welding process in areas where the warp is toward the tooling face. An example would be an upward facing convex surface, supposed to be flat, whose ends are under more compressive force, exerted by the horn, than the middle area. The ends are “shined” or scuffed by the tooling.
If warpage is not caused by design faults, it can be minimized by changes in the molding process. Increased injection molding hold times to allow the parts to solidify “flat,” and proper feed amounts will help in many cases. Also, utilizing higher stock temperatures, and lower injection pressures to reduce molded in stresses also will decrease warpage. In addition, gussets and ribs in the part design may help with such problems.
Sinks marks. Sink marks, or uneven part surfaces, are a result of either too small amount of material being shot into the mold or a result of a larger wall thickness in areas of the part shrinking more locally than a surrounding thin wall area. These areas see a reduction in the amount of horn contact and force applied by the ultrasonic horn. This reduces the heating of the plastic and amount of weld generated. Also, as above, the areas left to contact the horn may have large force loadings and be marked.
Insufficient shot size is most commonly seen as the cause of sink marks. Cored out sections of thick walls may be used to prevent this if molding changes cannot alleviate it. Again, correct stock temperatures and feed rates are key. Proper venting of the mold must also be assured.
Many difficulties seen in the ultrasonic welding of thermoplastic parts can be seen to result faults of design, molding, or both processes. Knowing where to look for the root cause is key to a solution.
References/Suggested Readings
1. Joel Frados, ed., Plastics Engineering Handbook of the Society of the Plastics Industry, Inc., 4th ed. (Van Nostrand Reinhold Co., pgs. 641-667.
2. Paul Tres, Designing Plastic Parts for Assembly (Cincinnati: Hanser Gardner, 2003), pgs. 8-19.
3. Designing With Thermoplastics (Dow Plastics, 1992), pgs. 55-62.
4. Troubleshooting Chart for the Injection Molding Process from GE Plastics, exact source unknown.
Kenneth Holt has been involved in the plastics industry for nearly twenty years, eleven of which have been in the ultrasonic welding field. Having been involved in all the aspects of application development and de-bugging of hundreds of such projects gives him unique insight into the real problems experienced most frequently in the process. Holt is employed by Herrmann Ultrasonics, Schaumburg, IL.
