Plastics Technology

JUN 2017

Plastics Technology - Dedicated to improving Plastics Processing.

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requirements and exercise much weaker process controls. In situ- ations like these, it may be common for operators to continually adjust settings in response to changing part or production condi- tions. While this approach may result in satisfactory production, any problems that occur can be harder to diagnose, especially remotely, when process parameters vary frequently. For example, was the latest parameter change prompted by an equipment problem or a change in part composition or quality? Typically, when an application such as this requires assis- tance, an ultrasonic-welding applications engineer, after asking a few basic questions surrounding the parts (material, joint design, test requirements, and current machine setup) can direct the customer to the proper solution. This approach is especially useful if the troubleshooting can be accomplished directly at the machine, using production parts. An overview of the trouble- shooting/parameter adjustment process is shown in Fig 5. Material-related issues are a frequent source of inconsis- tencies or problems in production. As noted in the following examples, even slight variations in materials can have dramatic effects on weld or production quality: • Polymer changes. As prices fluctuate, it is common for proces- sors to want to switch between similar polymers for economic reasons. However, it is wise to consult with an ultrasonic-welding applications expert before making any change. One example of a common but potentially troublesome change involves moving from an easy-to-weld amorphous material such as ABS, to a much more difficult-to-weld semi-crys- talline polymer, such as PP. ABS requires lower ultrasonic stack output (30-70 microns at 20 kHz) for successful welding than does PP (90-120 microns). If this change results in parts that don't have the strength they had before, or take longer to weld, or if the welds cause damage to sensitive assembly surfaces/components, the issue could be a lack of ultrasonic stack output. An examination of stack components, particularly the horn and booster, is warranted to determine if improvements to either component will allow the application to weld the new polymer efficiently and bring the application back to a "normal" range of success. • High regrind content. Reground thermoplastics, though capable of being melted and reformed numerous times, undergo some degradation of their physical properties with each subsequent melt. The cumulative effect of too much reground material can lead to a failure of parts to meet specifications. For this reason, Branson recommends that no more than 10% regrind be used in parts that are to be ultrasoni- cally welded. In specific applications that demand compliance with rigorous testing and acceptance criteria, producers should strongly consider periodic analysis of production mate- rials to continually validate the quality of mate- rials going into finished parts. • Filler content. Often, fillers are essential to ensure part strength and durability. However, different types and percentages of fillers in parts can affect the success of plastics joining processes. Branson recommends that filler content be kept at less than 30%. Joining parts that contain a higher percentage of filler, particularly long fibers, will sometimes result in fillers accumulating at the weld joint, which can reduce weld strength. Another issue is abrasive fillers. Some fillers that impart added strength or toughness, including calcium carbonate, silica, and talc, can also be abrasive to the contact surfaces of tooling. Prolonged exposure of abrasive parts to tooling surfaces can cause wear that could lead to cosmetic damage to parts and inad- equate energy transfer to part-joining surfaces. Note that while the frequency line at center remains relatively stable, reflecting the output of the power supply, the other parameters vary wildly, as the power supply attempts to compensate for a problem farther downstream in the weld stack. In this case, it is a defective horn. FIG 4 DCX Test & Graph Screen of a Defective 20 kHz Horn PS Test Frequency Memory Amplitude Power XXXX XXXX XXXX XXXX 0 75 150 225 300 375 450 525 600 675 750 Time, millisec Amplitude Phase Current Power PMW Amplitude Frequency Draw from 0 ms to 280 ms Graph Selection Phase X Value 0 Y Value 0 Update Value 90 72 54 36 18 0 -18 -36 -54 -72 -90 20450 20350 20250 20150 20050 19950 19850 19750 19660 19550 19450 120 108 96 84 72 60 48 36 24 12 0 Start Test Reset Overload Status Run Seek Frequency, Hz Power, % / Current, % / Amplitude, % Phase,° Frequency Power Current Amplitude Phase × × × × × Export Graph Data Update Graph Set Default Redraw Graph Result OK-Stored Overload-Cleared @plastechmag 59 Plastics Technology U LT R A S O N I C W E L D I N G

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