Tuesday, April 30, 2013

Experiments in Reducing Residual Stress with Dukane’s Servo-Driven Ultrasonic Welder

Miranda Marcus & Satish Anantharaman

Even on parts with good weld strength, failure can occur in the field due to residual stresses [A].  With ultrasonic welding, residual stresses are typically in the range of 35 MPa due to the rapid cooling of the small amount of melt [A, B].  Recent experimentation at Turku University of Applied Sciences in Finland have demonstrated that parts welded with a servo welder have significantly less residual stress than parts welded with a pneumatic welder.  This research also demonstrated that parts with shear joints retained less residual stress than parts with energy directors [C].

While residual stress due to cooling is a characteristic of the ultrasonic welding process, orientation induced residual stresses are affected by the hold force applied after the weld phase. Control over the hold force could therefore result in reducing orientation induced residual stresses and thereby minimize the overall stress in the part due to welding. In order to test the validity of this theory, molded polycarbonate parts were welded at a variety of hold speeds and distances. 
For all the samples the weld speed was 2 mm/s.  One part was welded with no hold time.  Three parts were welded at a hold speed matching the weld speed (2 mm/s) with varying hold collapse distances.  The last three parts were welded to 0.5 mm collapse at half, double, and quadruple the weld speed.

Table 2: Hold phase settings used

After welding, the polycarbonate parts were exposed to various mixtures of Methanol and Ethyl Acetate for three minutes. The parts were examined under the microscope for crazing and cracking due to the solvent exposure [13]. The residual stresses were quantified using the chart developed by GE plastics shown in figure 4.  They were then examined for crack formations to determine the level of residual stress at the weld.

Figure 4: Graph showing critical stress levels in Polycarbonate as a function of solution concentration [13]
For these tests, AWS I-Beams with energy directors were used.  This was for two reasons.  First, previous studies indicated that higher stresses occur in energy director parts thereby allowing a greater range of stress to observe.  Second, it is far easier to observe cracks in the energy director parts than in the shear parts due to the part geometry.

Figure 5: Drawing of AWS I-Beam with Energy Director [12]

Some differences in residual stress levels were noted after testing.  Crack formation was seen in varying locations and amounts in the parts.  For the purpose of this paper, number and location of cracks were not considered, only presence or absence.

Figure 6: Cracks shown in a weld joint after exposure to Methanol and Ethyl Acetate mixture.

The results show that increased hold distance may reduce residual stresses at the weld.  Additionally, lower stresses were observed when the hold speed was about double the weld speed.  Further investigation into the effect of hold settings on residual stress in the weld joint is merited based on these results.
Table 3: Stress level as determined by crack formation.

A.  S. Anantharaman and A. Benatar. “Measurement of Residual Stress in Laser Welded Polycarbonate using Photoelasticity” ANTEC 2003.
B. A. Benatar. “Servo-Driven Ultrasonic welding of Semi-crystalline Thermoplastics” 39th Annual Symposium of the Ultrasonic Industry Association. Cambridge, MA. 2010.
C. H. Turunen. “Ultrasonic Welding for Plastics” Bachelor’s Thesis, Turku University of Applied Sciences, Finland. 2011.

Thursday, April 11, 2013

Experiments in Velocity Control with Dukane’s Servo-Driven Ultrasonic Welder

Miranda Marcus, Satish Anantharaman, & Bob Aldaz

Even before the introduction of Dukane’s servo-driven welder, the need for velocity control was recognized.  As Mikell Knights wrote in 2005, “Research has proven that consistency of melting rate has a direct influence on bond strength,”  A linear velocity means a steady melt rate which, in turn, creates a homogenous molecular structure and a stronger weld [A]. 

In past years there has been much effort put into attempting to get consistent velocity control with pneumatic systems [2, 3].  These efforts have been in vain, as it is simply not possible to get precise velocity control with a pneumatic welder [C].   As one writer put it trying to achieve precise speed control with a pneumatic press was “the equivalent of sending a ship on an ocean voyage with a map and a compass from a box of cereal” [B].  Dukane’s servo-driven ultrasonic welder offers clear improvement in process control.

Studies using servo ultrasonic welders have shown that the programmed weld velocity can be directly correlated with weld strength [C, D, E, F].  In a study at The Ohio State University it was shown that by using a defined velocity profile with a slower speed during melt initiation and a faster speed in the middle and end of the weld, strength could be increased with less weld time and reduced surface marking [F].  Dukane has provided a unique new weld control to achieve this initiation of melt before collapsing the weld through the use of the patented “melt detect” feature.  This features allows the press to contact the parts and turn on ultrasonic vibration with no vertical movement until a drop in force is detected which indicates that welding has initiated [C]. 

One of the greatest benefits of servo-driven ultrasonic welding is the ability to program a specific weld velocity.  It has often been said of plastic welding that it is more art than science.  With the new controls available, this no longer needs to be the case.  In this experiment, we made both a finite element model and a finite difference model of a shear joint in a standard AWS I-Beam sample to determine the rate of melt formation and then welded parts at varying weld velocities to determine if the modeled weld velocity could be correlated to the optimum weld velocity through experimental data.

Figure 1: Drawing of AWS I-Beam with Shear Joint [12]

For the Finite Element Analysis, a ProE 3D CAD model was made of the AWS I-Beam part consisting of three individual pieces, the top part, the bottom part, and the shear joint.  The shear joint was assigned a heat generation rate based on the following equation:  Q = πƒ•ε²•Eloss, Where ε² is the strain amplitude at the shear joint calculated by amplitude divided by length and Eloss is the loss modulus of the material (Nylon 6,6 in this case).

Figure 2: Screenshots of finite element analysis (modeling half of the part)

The finite difference analysis was performed with the same initial steps.  However, instead of using a CAD model, Mathcad was used for the 1D analysis.

Figure 3: Screenshot of 1D finite difference analysis

Both methods were used to produce a graph of temperature vs. distance at discrete 0.05 second intervals.  The width of the melt layer was determined based on attainment of the melting temperature (354 °C).  From this, the rate of melt formation was calculated.

Table 1: Calculated melt formation rates for Finite Element
Analysis (FEA) and Finite Difference Analysis (FDA).

After determining the theoretical ideal welding velocity to match the melt formation, five parts were welded at each of five velocities: 0.5 mm/s, 2 mm/s, 4 mm/s, 6 mm/s, and 10 mm/s.  After the parts were welded, they were cut in half and each half was pull tested to determine the relative weld strength.

After tensile testing, a clear relationship was observed between weld speed and strength, as expected.  Most interesting was that the maximum weld strength was observed at 4 mm/s and 6 mm/s which were modeled to be the initial melt formation rates in the part.

Another interesting result was that the toughness of the weld, as measured by elongation, followed the same general pattern as the weld strength, although the difference between weld speeds was not as distinct.

Figure 5: Weld strength and elongation as a function of weld speed

A. M. Knights. “Graphical Analysis Helps Find and Fix Ultrasonic Welding Problems” Plastics Technology. Sept 2005.
B. T. Kirkland. “Ultrasonic Welding: The Need for Speed Control” Plastics Decorating. July/August, 2012.
C. S. T. Flowers. “Servo-Driven Ultrasonic Welding of Biocomposites” ANTEC 2012.
D. A. Benatar. “Servo-Driven Ultrasonic welding of Semi-crystalline Thermoplastics” 39th Annual Symposium of the Ultrasonic Industry Association. Cambridge, MA. 2010.
E. M. Marcus, P. Golko, S. Lester, L. Klinstein. “Comparison of Servo-Driven Ultrasonic Welder to Standard Pneumatic Ultrasonic Welder” ANTEC 2009.
F. A. Mokhtarzadeh and A. Benatar. “Comparison of Servo and Pneumatic Ultrasonic Welding of HDPE Shear Joints” ANTEC 2011.