Implant Induction Welding of Nylon 6/6 Chung-Yuan Wu Visteon Corporation and Bryan Agosto Ashland Specialty Chemical Co. Abstract associated with resistance in the heating element and hysteresis heating relies on magnetic hysteresis losses of a The implant induction welding technique utilizes a material to produce the heat [2]. Typically, eddy current heating element material at the joint interface to generate heating operates at lower frequency and hysteresis heating the heat. In this study, three factor two level full factorial usually requires higher frequency. In this study, the design of experiments were performed to evaluate two induction welding was operated at 6.5MHz. different types of Nylon 6/6, denoted as A and B, in a lap The basic induction welding machine contains a shear joint geometry. It was found that weld time was the generator, a controller and heating coils. The generator most dominant factor in affecting the weld strength converts line power to a high frequency RF signal through followed by power and pressure. It was also found that the an oscillating tube. The controller controls the power level weld strength was proportional to the heating time at and heating time. The alternating current from the constant power and constant pressure. In addition, final generator passes through the heating coils and creates an sample thickness was inversely proportional to the lap electromagnetic field that heats the heating element, which shear strength. Bending of the joint during testing was the is usually a composite of the polymer to be welded with major failure mode. The maximum achievable strength for metallic filler. Under external pressure, the molten polymer material B is 15% higher than that of material A. in the heating element flows and creates the bond. The Furthermore, vibration welding of these two materials was heating coil is made of water cooled copper tubes and its also performed for comparison. It was found that material temperature is maintained near room temperature. The B achieved 80% higher weld strength than material A heating element has to be placed near the heating coil but using vibration welding. not in contact with the coil to achieve higher heating efficiency. Introduction Previous studies of implant induction welding of PC, PBT and PP [3] indicated that the maximum weld strengths were near 50% of the base material strength in butt joint It is desirable to weld plastic components together configuration. Induction welding studies of 33% glass fiber without adding extra materials at the joint interface. This reinforced nylon 6 containers was also reported [4] enables direct bonding between the two materials to avoid indicated that the burst strength was higher than that of the the material incompatibility and reduce cost. Most welding vibration welded containers. Similar to adhesive bonding, processes require no additional materials at the joint most implant induction applications are designed with a interface such as vibration welding, radio frequency shear joint configuration. Therefore, the lap shear joint was welding and ultrasonic welding. However, in some cases used in this study. where these welding processes cannot be used due to part/joint complexity, component limitation and accessibility of equipment, other welding techniques must Experimental Procedures be considered. Implant induction welding is one of the methods that can be considered. Implant induction welding In this study, the induction welding system was typically operates at frequencies ranging from 40 kHz to manufactured by Ashland Chemical, Emabond Systems 200 kHz or at higher frequency near the mega hertz range. model EA-20, 2 kW operating at 6.65 MHz. The heating It is important to note that most plastics do not heat under element was also made by Ashland Chemical using these frequencies. Therefore, implant induction welding metallic fillers mixed with nylon resin. utilizes a heating element placed at the joint interface to Two grades of unfilled Nylon 6/6 materials, denoted generate the heat and melt the plastics to create the joint. as Material A and Material B, were used in this study. Lap The induction welding operating frequency can be selected shear joint samples were used to evaluate the weldability. based on the heating element composition. There are two Figure 1 shows the experimental set up in which the types of heating mechanisms in implant induction welding, heating element was sandwiched between two Nylon plates joule heating (eddy current heating) and hysteresis heating and placed between the heating coils. The heating coil used [1]. Joule heating relies on eddy current generation in this study was a composite coil that contains a hair pin coil with 600 mm in length and 25 mm wide copper plate 1 to -1, where 1 is the high setting and -1 is the low setting. and a water cooled square copper tube (6.35 mm x 6.35 The R-square values are 88.5% and 85.7% for material A mm). The diameter of the heating element was 2.4 mm and and material B, respectively. The coefficients in Equations the Nylon plate thickness was 3.07 mm for material A and 1 and 2 indicated that time is the most important factor in 3.09 mm for material B. The width of the nylon plate was affecting the weld strength followed by the power. 75 mm and the length of the nylon plate was 125 mm. The Pressure has less effect on strength compared to time and overlap for welding was 25 mm and the heating element power. The coefficients of the cross terms are much was placed in the center of the overlap as shown in Figure smaller than that of the major parameters indicating there 1. After welding, the sample was cut into a 25 mm wide is no interaction between the major parameters. By test coupons. Only the middle sample was tensile tested on removing the cross terms in the Equation 1 and 2, the Instron 4466 using a crosshead speed of 50 mm/min. coefficients in the regression equations did not change and Three factor two level full factorial design of the R-square values were slightly reduced to 85.9% and experiments (DOE) was used to evaluate the lap shear 83.9% for material A and material B, respectively. strength. The three factors were: heating time, weld Figure 2 shows the effect of weld pressure and heating pressure and power level. The high setting for heating time time for both materials. It is noted that high weld pressure was 8 seconds and the low setting was 6 seconds. The high always produced higher weld strength especially at short setting for power level was 100% (2 kW) and the low heating times. However, at 8 seconds of heating the weld setting for power was 80%. It is noted that this power is the strength was nearly independent of pressure. It is also induction generator output power not the power absorbed noted that increasing the heating time increases the weld by the heating element. The weld pressure was read from strength. However, the rate of increase was reduced at 8 the pressure gauge on the welder, and the high setting was seconds of heating. Furthermore, the maximum achievable 0.41 MPa that is equivalent to a force of 1310 N and the strength for material B is about 15% higher than that of low setting was 0.14 MPa that is equivalent to a force of material A. The bending during tensile testing of induction 436 N. Each condition was duplicated three times. In welded samples was the main failure mode as shown in addition to the tensile strength, the final sample thickness Figure 3. Thus, proper joint design should be considered in was also recorded to estimate the bond thickness. Since the the induction welding to eliminate the bending such as heating element flows under pressure, by using the tongue and groove design. conservation of mass, the bond width could be estimated Figure 4 shows the effect of vibration weld pressure from the thickness measurement. on weld strength with 1 mm of meltdown and peak to peak Vibration welding of butt joint samples was also made amplitude of 1.75 mm. It is clear that low weld pressure for purposes of comparison. It is difficult to compare a butt produced higher weld strength for both materials. The joint to a lap shear joint; therefore, the focus was in the maximum weldable butt joint strength for material A and material weldability. A Branson Micro welder was used material B was 29.5 MPa and 54.9 MPa, respectively. The with constant peak to peak amplitude of 1.75 mm. The material B achieved 80% higher weld strength than that of weld pressure was varied from 0.5 MPa to 4 MPa and the the material A. Figure 5 shows the effect of vibration weld meltdown was varied from 0.5 mm to 2 mm. Again, the meltdown on weld strength using 1 MPa weld pressure and tensile testing speed was 50 mm/min. peak to peak amplitude of 1.75 mm. Evidently, the meltdown has no effect on weld strength. Again, the weld Results and Discussion strength for material B was 80% higher compared to the weld strength of material A. The full welding matrix and the lap shear weld As mentioned earlier, for induction welding, sample strength for 8 welding conditions are shown in Table 1. thickness was measured after welding and correlated to the The DOE analysis resulted in the following regression weld strength. Statistical analysis resulted in the following equations for strength. regression equations for sample thickness. Material A - strength: Material A - thickness: Strength = 2427 + 372P + 993T + 681P − 76P × T Thickness = 6.68 − 0.0821Pr − 0.105T − 0.0863P r r (1) + 0.0287P × T + 0.0246P × P + 0.0296T × P (3) − 80Pr × P − 151T × P −118Pr × T × P r r − 0.0179Pr × T × P Material B - strength: Strength = 2721 + 431P + 1208T + 722P −167P × T Material B – thickness: r r (2) Thickness = 6.68 − 0.08Pr − 0.112T − 0.0842P − 44Pr × P + 9T × P −134Pr × T × P (4) + 0.033Pr × T + 0.0258Pr × P + 0.0358T × P In the above equations, weld strength is given in − 0.0125Pr × T × P Newton, T is the weld time, P is the power, and Pr is the weld pressure.