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Ask the Expert
by Ken Holt, Herrmann Ultrasonics, Inc.

Ask the Expert
October-November2007



Question: Fractures in Welding
While using ultrasonics to weld our parts, we see fractures occasionally. The part is made of a high temperature amorphous plastic and the cracks always occur in the same area. What is happening here?

The first step in analyzing this failure is to isolate the area(s) that is involved and look at its relationship to the molded partís gate and vent areas. It is typical with the higher temperature plastics, i.e., PEEK, PEI, PSU, etc., to see weak weld (or knit) lines opposite the gate area. This commonly is due to an incomplete fill of the mold cavity during the injection molding process. The molding and mold temperatures of these high temperature plastics are quite specific for optimum mold fills. The producerís processing sheets need to be adhered to stringently. Mold heating may be required for good fill. Mold design also should be reviewed for proper venting and gate size for the part configuration.

This review and the appropriate remedial actions should resolve the problem. However, ultrasonic processing steps that also can be taken include the following:

  • Minimize welding amplitude used while still retaining enough to produce a good weld.

  • Eliminate any "over welding" conditions such as welding beyond the designed in weld collapse.

  • Use higher force to minimize weld time. This step may not always yield good results but often reduces weld times enough to alleviate the damage.

Question: Benefits to Computer I/O Devices
The newer ultrasonic welding equipment uses various computer I/O devices. Are the benefits equal to the costs?

As I am currently employed in this industry, I do have a bias on this matter. Just as with automobiles, the design process, molding machines, and even our work desks have benefited from advanced computerization. So too have ultrasonic welding machines. This is particularly true, but not limited to, those running high speed, complex weld processes on multi-cavity molding processes.

There are a vast number of reasons why faster computers and resultant controls provide benefits, which all relate to increasing the bottom line. The most obvious reasons and discussions follow:

  • Reduction of set up times. Through the use of preprogrammed welding processes that can be easily recalled, set-up can be reduced to simple tooling changes and lack of "try this and see what we get" testing. Companies welding medical parts are almost required to have such control.

  • Quick and accurate comparisons to previous runs of the same parts. By comparing data from the last run to that being generated currently, it is easy to compare the results.

  • Dialing in of a new process. By using the graphing function of the most advanced computer controllers, the weld process can be evaluated. Optimization of the process through the use of visualization of the weld process is key to setting up new weld parameter sets for new parts, or for parts that are undergoing redesign. A joining velocity graph is useful in looking for abrupt changes in welding velocity that would indicate stalling of the weld during the process or reaching the end of the designed weld joint prior to end of weld. Linearity of the graph is desired. (see below)

  • Cavity to cavity variations. Changes in the same part configuration are easily seen when looking at outputs from the welding controller. If there are problems with the welding of certain cavity combinations, these can be diagnosed by looking for changing weld times, differing heights/depths of the final welded parts, etc. This can indicate whether mold changes need to be made or if the ultrasonic weld process can be adjusted to compensate by using various weld process controls. Some systems allow welding to more than one weld parameter such as welding to an absolute distance but stopping the process if a specified energy level is met.
 

Question: Weld Results with Different Colored Parts
We get differing weld results when we run different colors of parts. Why is that?

If you are controlling your weld process strictly by time, these variations are common and different color parts frequently need slightly different weld processes due to differing physical properties of the colored material. Try instead welding by an energy parameter or a distance parameter and see if you canít minimize the variations in the results.

Using a distance welding mode in order to match the geometric requirements of the part and its weld joint design should provide for a repeatable process across color changes by matching the weld process to a specific geometry and not to a time constraint. As an example, if the parts to be welded have a joint design .5 mm tall, then that amount should be welded and the time required will be seen to vary. This can be set up to allow variances over a prescribed limit window. Weld time variations using a distance weld will be seen over cavity to cavity variations as well as over color changes. Allowing this time value to float is the result of using the "closed loop control" of distance welding.

Alternatively, some welding programs of plastic parts can similarly use the energy weld mode. This mode also allows the time to "float" or vary. Welding by energy typically works best on either soft plastic parts, i.e., LDPE, PP copolymer, etc., or on thin films and fabrics. As energy is a product of power and time (watt x seconds), it also is a closed loop welding mode, taking into account the various amounts of power used to weld specific parts.  

Ken Holt is the applications manager at Herrmann Ultrasonics, Inc. and can be reached by e-mail at ken.holt@herrmannultrasonics.com.