The Basics in Joint Design for Ultrasonic Assembly
The joint design of the mating pieces is critical in achieving optimum assembly results. A particular parts joint design depends upon factors such as type of plastic, part geometry, and the requirements of the weld. There are many different joint designs, each with its own advantages. Some of these designs are discussed later in this section.
Three basic requirements are necessary for proper joint design: a uniform contact area, a small initial contact area, and a means of alignment. A uniform contact area means that the mating surfaces should be in intimate contact around the entire joint. The joint should also be in one plane, if possible. A small initial contact area should be established between the mating halves. Doing so means less energy, and therefore less time, is required to start and complete the meltdown between the mating parts. A means of alignment is recommended so the mating halves do not misalign during the welding operation. Alignment pins and sockets, channels, and tongues are often molded into parts to serve as ways to align them. It is important to note that it is best not to use the horn and/or fixture to provide part alignment.
The need for the basic requirements for any joint design can be demonstrated using a flat butt joint. Only the high points will weld on a flat butt joint, resulting in erratic, inconsistent welds. Extending the weld time to increase the melt simply enlarges the original weld points and causes excessive flash outside of the joint. Bringing one of the surfaces to a point produces welds with good appearance, but little strength. When good strength is achieved, excessive flash ruins the appearance of the weld. Figure 1 illustrates the problems encountered with pointed wall parts.
The Energy Director
The energy director was developed to provide a specific volume of material to be melted so that good bond strength could be achieved without excessive flash. It is the joint design that is generally recommended for amorphous polymers.
An energy director is a triangular-shaped bead molded into the part interface. It typically runs around the entire joint perimeter. When ultrasonic energy is transmitted through the part under pressure and over time, the energy concentrates at the apex of the energy director (i.e., where the apex of the triangular-shaped bead contracts the other mating surface) resulting in a rapid build-up of heat that causes the bead to melt. The molten material flows across the joint interface forming a molecular bond with the mating surface.
In terms of the three basic requirements of a joint design, the energy director meets two: it provides a uniform and a small initial contact area. The energy director itself does not provide a means of alignment, nor does it provide a means to control material flash. These requirements must be incorporated into the part design.
The basic energy director design for an amorphous resin is a right triangle with the 90-degree angle at the apex and the base angles each at 45 degrees. This makes the height one-half the base width. The size of this energy director can range from 0.005 to 0.030 high and from 0.010 to 0.060 wide. For polycarbonate, acrylics, and semi-crystalline resins, the energy director is an equilateral triangle, with all three angles being 60 degrees. This makes the height 0.866 times the base width. The base width can range from 0.010 to 0.050.
The most common and basic joint design is the butt joint with an energy director as shown in Figure 2. The width of the energy directors base is between 20 percent and 25 percent of the thickness of the wall. When the wall is thick enough to produce an energy director larger than the maximum size, two smaller parallel energy directors should be used. The height at the apex of the energy director is either half the base or 0.866 times the base, depending on the material. This design produces a weld across the entire wall section with a small amount of flash normally visible at the finished joint. As stated before, the parts should be designed to include a means of alignment. If this is not possible, the fixture can be designed to provide the locating features necessary to keep the parts aligned with respect to each other. Typically, hermetic seals are easier to achieve with amorphous rather than semi-crystalline materials. If a hermetic seal is required, it is important that the mating surfaces be as close to being perfectly flat and/or parallel to each other as possible.
The butt joint with an energy director is well suited for amorphous resins because they are capable of molten flow and gradual solidification. However, it is not the best design for semi-crystalline resins. With semi-crystalline resins, the material displaced from the energy director usually solidifies before it can flow across the joint to form a seal. This causes a reduction in overall strength and makes hermetic seals difficult to achieve. However, sometimes there are certain limitations imposed by the design or size of the part that make it necessary to use an energy director on semi-crystalline parts. In situations such as these, it should be larger and have a steeper angle to give it a sharper point (apex). This enables it to partially imbed in the mating surface during the early stages of the weld, thereby reducing the amount of premature solidification and degradation caused by exposure to the air. The larger, sharper design improves the strength and increases the chances of obtaining a hermetic seal. Experimentation has shown that the larger, sharper energy director design is also superior when working with polycarbonate and acrylics, even though both materials are classified as amorphous materials.
The graphs depicted in Figure 3 show the impact of welding a butt joint with an energy director (i.e., a small initial contact) area versus welding a plain butt joint, which is, in effect, no joint at all. As can be seen, the butt joint with an energy director is brought up to melt temperature in a much shorter period of time than the plain butt joint. The butt joint with an energy director also provides a much stronger weld.
The Step Joint
One variation of the energy director joint design is the step joint. Like the energy director, it meets two of the basic requirements of joint design: it provides a uniform contact area and a small initial contact area. A step joint also provides a means of alignment. Figure 4 shows a step joint.
As only part of the wall in a step joint is involved in the welding, its strength is less than that of a butt joint with an energy director. The recommended minimum wall thickness is 0.080 to 0.090.
A step joint may be used when cosmetic appearance of the assembly is important. Use of a step joint can eliminate flash on the exterior and produce a strong joint, since material from the energy director will typically flow into the clearance gap between the tongue and the step. The energy director is dimensionally identical to the one used on the butt joint. The height and width of the tongue are each one-third of the wall thickness. The width of the groove is 0.002 to 0.004 greater than that of the tongue to ensure that no interference occurs. The depth of the groove should be 0.005 to 0.010 greater than the height of the tongue, leaving a slight gap between the finished parts. This is done for cosmetic purposes so that it will not be obvious if the surfaces are not perfectly flat or the parts are not perfectly parallel.
The Tongue-and-Groove Joint
The tongue-and-groove joint is another variation of the energy director. Like the step joint, it provides the three requirements of a joint design (a uniform contact area, a small initial contact area, and a means of alignment). It also prevents internal and external flash, since there are flash traps on both sides of the interface. Figure 5 shows a tongue-and-groove joint.
The tongue-and-groove joint is primarily used for applications where self-location and flash prevention are important. It is an excellent joint design with applications calling for low-pressure hermetic seals. The main disadvantage of the tongue-and-groove joint is that less weld strength is possible because the joint affects less area. The minimum wall thickness recommended for use with this type of joint is 0.120 to 0.125.
Again, the energy director is dimensionally identical to the one used in the butt joint. The height and width of the tongue are both one-third the thickness of the wall. Clearance should be maintained on each side of the tongue to avoid interference and provide space for the molten material. Therefore, the groove should be 0.004 to 0.008 wider than the tongue. The depth of the groove should be 0.005 to 0.010 less than the height of the tongue. As with the step joint, a slight gap designed into the finished part assembly proves to be advantageous for cosmetic reasons.
The Shear Joint
The shear joint is used when a strong hermetic seal is needed, especially with semi-crystalline resins. A shear joint requires that a certain amount of interference be designed into the part. Welding is accomplished by first melting the contacting surfaces. As the melting parts telescope together, they continue to melt with a controlled interference along the vertical walls. A flash trap, which is an area used to contain the material displaced from the weld, may be used. The smearing action of the two melt surfaces at the weld interface eliminates leaks and voids, as well as exposure to air, premature solidification, and possible oxidative degradation. The smearing action produces a strong structural weld.
Note in Figure 6 the depiction of a fixture. Rigid sidewall support is very important with shear joint welding to prevent part deflection during welding. The walls of the fixtured part need to be supported up to the joint interface by the fixture, which should closely conform to the shape of the part. In addition, to make it easier to remove the part from the fixture, the fixture itself should be split so that it can be opened and closed.
A shear joint meets the three requirements of joint design as well. The lead-in provides a means of alignment and self-location of the parts to be welded. Properly designed and molded parts ensure a uniform contact area. The small initial contact area between the parts occurs at the base of the lead-in. In conclusion, choosing the correct type of joint when ultrasonic welding depends largely on factors such as the type of plastic, the geometry of the part, and the requirements of the weld. Analyzing the factors involve will help you choose the right direction, whether it is utilizing the energy director, the step joint, the tongue-and-groove joint, or a shear joint. Incorporating the correct joint design will help ensure the proper type of finished weld.
This material was excerpted from DuKane Corporations Guide to Ultrasonic Plastics Assembly which can be ordered from DuKane Corporation at 630-584-2300, Part No. 703-536, or visit: www.dukane.com/us.