Key Considerations: Adhesives or Ultrasonic Welding for Plastic Component Assembly?
by Steven A. Williams, manager, Global Product Managment-Ultrasonics
Image 1. Typical of parts that may be joined using adhesives or ultrasonic welds is this two-piece injection-molded housing.
Images 2a-c. Cutaway and red highlighting show the location of the tongue-and-groove joint in a two-piece housing (c). Close-ups of the joints show the difference between a tongue-and-groove joint designed for adhesive joining (b) and ultrasonic welding (a). Note that the ultrasonic joint has been modified to include a triangular energy director, which melts when welded to form a strong bond between the parts.
This article will take a brief look at two of the more popular methods for assembling plastic parts into finished products adhesives and ultrasonic welding and focus on questions product design and manufacturing teams should consider when making decisions about why and when to adopt these assembly methods.
While many important considerations are involved in making a part assembly decision, I find that they generally fall into two broad categories. One set of considerations has to do with the business, its product lines, production needs and the degree of speed, flexibility and scalability needed in its assembly operations. The second, narrower set of considerations goes right to the nature of the part itself: notably, the materials used and the shape, or geometry, of the part.
The first consideration with adhesives and ultrasonic welding is that both are permanent joining methods. Both create a strong bond between components that wont come apart. So, consider: will the part, once assembled, ever need to be disassembled to allow for maintenance, component repair or replacement of a battery or bulb? If so, permanent assembly methods such as adhesives or ultrasonic welding may only be part of the solution. To allow for disassembly (such as to replace a battery or bulb), a product design probably will need to incorporate either mechanical fasteners or snap-fit components.
Of the two assembly methods considered here, adhesives often provide more flexibility in the assembly process. By flexibility, I mean that adhesives can create bonds between plastic components that utilize a wide range of materials and shapes. And, if theres a need to modify the design of one of the plastic components to make one of the dimensions longer or shorter, for example the adhesive joining process remains the same. It is comparatively easy to change components and then adapt the adhesive assembly process. If adhesive dispensing is done manually, simply inform the assembler of the change. Or, if automated, adapt the robots programming to change the pattern of its adhesive dispensing motion.
The notion of flexibility also can make adhesives a good solution for assembly of products in small quantities, including the following:
- Prototype designs
- Product samples
- High-mix production runs that include parts of differing sizes or shapes
For all their flexibility, adhesive joining methods come with some constraints, too. The first involves maintenance. Anyone who has ever used a bottle of glue knows that when the glue is in use, the applicator must be kept relatively clean and the glue applied with consistency and care to ensure a complete and cosmetically pleasing bond. When the number of adhesive applicators is increased, the challenge of ensuring process control grows. Assembly managers must ensure that adhesives are flowing smoothly and consistently to assure part strength, necessitating periodic purging and cleaning of adhesive systems and applicators. When not in use, adhesive applicators must be cleaned and capped to prevent the exposed adhesive from curing and clogging, which can lead to waste or production delays.
Another constraint on adhesive methods is that adhesives are consumables. Every adhesive bond that is made represents an incremental production cost that rises in direct proportion to output. And, if production rises beyond initial cost estimates if, for example, product sales and production ramp up rapidly, perhaps breeding new product variations or options production costs will likely be reevaluated as management sees the product moving from a developmental to a growth phase.
The point of change in production volume and expected sales offers a great opportunity to consider, or reconsider, the benefits of ultrasonic welding. Using ultrasonic welding for part assembly requires some up-front investment, starting with the welder. Then, there is product-specific tooling, which precisely holds the various plastic components in place before and during the welding process. But, this investment is only made once. With these elements in place, assembly costs can be managed, amortizing a single fixed investment over the ongoing assembly volumes for that part. Whether welding 1,000 or 1,000,000 of that part, theres no worry about incremental consumables or assembly costs.
The same favorable economics for ultrasonic welding apply to those who plan high-volume part production from the outset. As soon as a product design is finalized, weld tooling can be completed and high-volume production can begin. The key to amortizing assembly method costs, comparing costs over time and realizing assembly cost savings is to have a firm idea of what annual production volume is going to be. History with Emersons customers demonstrates that those with production volumes ranging from tens of thousands to millions per year often can realize a clear financial benefit with an ultrasonic welding process.
Adhesive assembly processes range in complexity. The most basic may consist of a relatively simple fixture for one part and a hand-held adhesive dispenser. An individual assembler may lay down a bead of adhesive on one component and then affix the mating component by hand, either pausing to hold it while it sets or attaching a clamp or fixture to hold it steady during the curing process.
A more complex adhesive assembly process may involve automation. Again, a base fixture will be needed to hold one part, plus any clamps or other means to hold the part while it cures. The expense of a robot will need to be included, but robotics tend to be very flexible: Users can change the programming, change the fixtures, change the adhesives and assemble a number of different parts with different geometries using the same robot.
Perhaps the biggest factor with an adhesive assembly process is the cycle time required. The adhesive assembly cycle isnt done when the when the two parts are brought together; typically, a full-strength adhesive bond requires curing time for each part. By contrast, ultrasonic welding provides a permanent, welded bond in one second or less. As soon as the welded part is removed from the weld tooling, the weld cycle is complete. A new part can be loaded and welded immediately.
Materials selection is an important variable in the effectiveness of assembly processes. In general, it is more difficult to bond dissimilar materials rubber to plastics or plastics to metals, for example. In such cases, mechanical fasteners or adhesives are probably the best places to begin.
When it comes to fully plastic assemblies, similar thinking applies. Material selection may be somewhat more diverse when adhesives are used in assembly, because adhesives are more likely to achieve bonding between dissimilar plastics. There are some exceptions a few polymers that may react chemically or degrade in the presence of certain adhesives but relatively few.
When it comes to ultrasonic welding, like or similar polymers tend to weld better than dissimilar polymers. However, some dissimilar polymers also may be welded if they have similar melt temperatures and melt flow characteristics. In addition, amorphous polymers tend to weld better than semi-crystalline polymers, since they have more gradual melt curves and more predictable melt flows between parts, which help to create more consistent bonds. ABS, polystyrene and polycarbonate are examples of amorphous materials that weld very well.
Semi-crystalline polymers are more challenging to weld because these materials tend to melt and solidify more abruptly. These characteristics that can make achieving a consistent melt and melt flow more difficult, making it correspondingly more difficult to get a consistent bond. Examples of semi-crystalline materials that are more challenging to weld are polyethylene, polypropylene and nylon.
The use of adhesive joining methods allows for considerable variation in the geometry of parts. As long as the edges to be joined offer surface area for the adhesive to be placed, its a workable method for joining.
Part geometry imposes a few more challenges when it comes to ultrasonic welding, since the structure of the part itself must adequately transmit the energy received from the horn down to the weld joint. As any engineer knows, some part shapes will inherently do this better than others. A great example of an easy-to-weld shape would be a cube with walls that are rigid enough to direct energy straight to the weld joint. A more difficult shape to weld would be a sphere, since one half would tend to flex under load and therefore not transmit the energy as efficiently.
Easy-to-weld parts tend to have the following characteristics:
- Relatively flat surfaces (limited contours) so that good horn contact can be achieved
- Surface area on the top of the part over the weld joint area
- Side walls with enough rigidity to transmit energy to the weld joint
- A properly designed weld joint
Every part is unique, of course, so the only way to know whether any design will work with any assembly method is to speak with a knowledgeable professional who can help evaluate the design, consider the assembly needs and find the right solution.
Design with the future in mind
One of the wisest things for any product design and for a companys bottom line is to make design choices that keep the assembly options open to both adhesive and ultrasonic welding methods. Perhaps the easiest way to do this is to design a simple tongue and groove joint into the mating surfaces of the components that will compose the part. This type of joint offers an inherent alignment feature the groove thats ideal for capturing adhesive and aligning the tongue of the mating surface or for making a strong ultrasonic weld.
Should production needs or volumes change, it is easy to convert a tongue-and-groove part from adhesive assembly to assembly using ultrasonic welding. All that is required is to add an energy director a small bead of sacrificial weld material to the bottom of the existing tongue. Typically, this can be done with a modest steel safe change to the mold. Then, during the weld process, the energy director on the tongue melts neatly into the groove, resulting in a very precise weld joint that offers high strength and good sealing properties.