Vibration welding of thermoplastics. Part II: Analysis of the welding process

The vibration welding process for thermoplastics is known to consist of four phases: (1) initial heating of the interface to the melting temperature by Coulomb friction; (2) unsteady melting and flow in the lateral direction; (3) steady-state flow; and (4) unsteady flow and solidification of the film after the vibratory motion is stopped. Simple analytical models are developed for the first three phases. These models are used for estimating the molten film thickness, the size of the heat affected zone, and the weld time as functions of the weld parameters: the amplitude and frequency of the weld motion, and the weld pressure. The steady-state film thickness and the heat-affected zone are shown to be very small.     

Vibration welding of thermoplastics. Part III: Strength of polycarbonate butt welds

In vibration welding of thermoplastics, frictional work done by vibrating two parts under pressure, along their common interface, is used to generate heat to effect a weld. The main process parameters are the weld frequency, the amplitude of the vibratory motion, the weld pressure, and the weld time or weld penetration; The effects of these parameters on weld quality were systematically studied by first butt-welding polycarbonate specimens under controlled conditions over a wide range of process parameters, and by then determining the strengths and ductilities of these welds by tensile tests, A significant result is the apparent existence of a weld-penetration threshold above which high weld strengths are attained, but below which the strength drops off. Under the right conditions, the strengths of polycarbonate butt welds are shown to equal the strength of the virgin polymer.     

Estimates for process conditions during the vibration welding of thermoplastics

The aim of this work is to interpret experimental findings of Stokes and Schlarb on the basis of analytical formulas and finite element calculations. In the analytical approach, some simplifications that had to be made proved to be inappropriate. Only computer calculations may reveal the details of the processes involved in vibration welding. Comparison of the computer calculations with the experiments reveals that even transient effects in viscosity have to be considered. Viscosity is dependent not only on temperature and shear rate, but also on the frequency or amplitude of vibration. After correction for these effects, the correlation between theory and experiments proved to be excellent. Based on the calculations, some recommendations are presented to improve weld quality, as ascertained by the type of fracture in a tensile test, by changing the welding parameters.     

Estimates for process conditions during the ultrasonic welding of thermoplastics

Order of magnitude estimates are presented for processes that play a role in ultrasonic welding. The fact that the sonotrode may not always be in contact with the product being welded, which results in the sonotrode repeatedly hammering the product, is accounted for in this study. The calculations do not use estimates for loss or storage modulus of plastics at 20 kHz around the glass or melting point for amorphous or semicrystalline polymers respectively. The flow of molten polymer in the weld zone is shown to be a laminar viscous squeeze flow driven by the welding pressure. An energy balance is used to show that the heat generated by the internal damping is, in part, used in heating cold material and is squeezed out into weld flash. The theoretical findings are correlated with existing practical pointers on ultrasonic welding in series and mass production in industry.     

Vibration welding of thermoplastics. Part IV: Strengths of poly(butylene terephthalate), polyetherimide,and modified polyphenylene oxide butt welds

In vibration welding of thermoplastics, frictional work done by vibrating two parts under pressure, along their common interface, is used to generate heat to effect a weld. The main process parameters are the weld frequency, the amplitude of the vibratory motion, the weld pressure, and the weld time or weld penetration.; The effects of these parameters on weld quality were systematically studied by first butt welding thermoplastic specimens under controlled conditions, over a wide range of process parameters, and by then determining the strengths and ductilities of these welds by tensile tests. The three thermoplastics investigated are poly (butylene terephthalate), polyetherimide, and modified polyphenylene oxide. Changes in the weld pressure are shown to have opposite effects on the strengths of polyetherimide and modified polyphenylene oxide welds; Also, the weld frequency is shown to have a significant effect on the weldability of polyetherimide. The weldability data for these three thermoplastics are compared with data for polycarbonate. Under the right conditions, the strengths of butt welds in these materials are shown to equal the strength of the virgin polymer.     

Hot plate welding of polypropylene. Part I: Crystallization kinetics

The crystalline structure formation in the heat affected zone during hot plate welding has a great influence on the performance of the welded semi-crystalline polymers. The quiescent and flow induced crystallization of polypropylene was investigated experimentally. A simplified, phenomenological crystallization model was developed, which can describe the crystal formation from completely melted and partially melted polymer. Predictions obtained from the model were compared with experimental results.     

Ultrasonic welding of thermoplastics in the near-field

Ultrasonic welding is one of the most popular techniques for joining thermoplastics because it is fast, economical, and easily automated. In near-field ultrasonic welding, the distance between the horn and the joint interface is 6 mm or less. This study investigated the near-field ultrasonic welding of amorphous (acrylonitrile-butadiene-styrene and polystyrene) and semicrystalline (polyethylene and polypropylene) polymers. High frequency ultrasonic wave propagation and attenuation measurements were made in order to estimate the dynamic mechanical moduli of the polymers. The estimated moduli were entered into a lumped parameter model in order to predict heating rates and energy dissipation. Experimental results showed that variations in the welding pressure had little effect on energy dissipation or joint strength; Increasing the amplitude of vibration increased the energy dissipation and the weld strength. For the semicrystalline polymers, increasing the weld time improved strength up to weld times greater than 1.5 s, where strength leveled off. For the amorphous polymers, the weld strength increased with Increasing weld time up to times of 0.8 s; for longer weld times, the power required was too high, causing overloading of the welder. Monitoring of the energy dissipation and static displacement or collapse provided valuable information on weld quality.     

Ultrasonic welding of thermoplastics in the far-field

In far-field ultrasonic welding of plastic parts the distance between the ultrasonic horn and the joint is greater than 6 mm. This study investigated the farfield ultrasonic welding of amorphous (acrylo butadiene styrene and polystyrene) and semicrystalline (polyethylene and polypropylene) polymers. Far-field welding worked well for amorphous polymers. Weld strength improved substantially with increasing amplitude of vibration at the joint interface. Increasing the weld pressure and/or the weld time also resulted in higher weld strengths. Far-field ultrasonic welding was not successful for semicrystalline polymers. The parts melted and deformed at the horn/part interface with little or no melting at the joint interface. A model for wave propagation in viscoelastic materials, which was developed to predict the vibration amplitude experienced at the joint interface, indicates that increasing the length of the samples to a half a wavelength should improve the far-field welding of semicrystalline polymers by maximizing the amplitude of vibration at the joint interface.     

Experiments on the induction welding of thermoplastics

In induction welding of thermoplastics, induction heating of a gasket, made of a ferromagnetic‐powder‐filled bonding material and placed at the interface of thermoplastic parts to be joined, is used to melt the interface; subsequent solidification of the melt results in a weld. Tensile tests on induction butt‐welds of polycarbonate (PC), poly(butylene terephthalate) (PBT), and polypropylene (PP) are used to characterize achievable weld strengths, and microscopy is used to correlate weld strength with the morphology of failure surfaces. In PC, PBT, and PP relative weld strengths as high as 48%, 43%, and 55% of the respective strengths of PC, PBT, and PP have been demonstrated. Relative weld strengths on the order of 20% have been demonstrated in PC‐to‐PBT welds.     

Random glass mat reinforced thermoplastic composites. Part I: Phenomenology of tensile modulus variations

The mechanical properties of random continuous glass mat reinforced composites, as determined by standard tensile tests, are known to have a very large scatter. To understand this scatter, new test procedures were developed to map the local tensile elastic moduli in a large plaque at 12.7-mm (½-in) intervals. Surprisingly, the tensile modulus in these materials can vary by a factor of two over the 12.7-mm distance. The elastic modulus is shown to vary by a factor of three in a 150 × 305-mm (6 × 12 in) plaque. Expressions have been obtained for the average moduli measured by tensile and bend tests. These expressions have been used to compare measured flexural moduli with values predicted by using measured tensile moduli.     

Simulation of welding flows in a slit. Part I: Kinematics

The kinematics of ideal welding flows generated by a thin-plate divider, a cylinder, or a slab in a slit channel are studied by using a finite element analysis. The analysis includes simulations of Newtonian and Carreau fluids. There are two flow configurations. First, a single plate-divider or an obstacle was positioned symmetrically in a slit channel with no-slip at the walls. In the second, an infinite number of plate-dividers or obstacles were positioned in parallel, and the boundary walls were infinitely far away. It was found that extensional flow dominates the region near the stagnation points of obstacles and plate-dividers, and that the fluid elements near the weld interfaces have a strain history of both high stretching and shearing. The thickness of the elongated region is reduced as the thickness of the plate-divider increases. Shear-thinning tends to increase the rate of extension. However, its influence on the flow field tends to lessen as the width of the flow channel or the obstacle size increases. A no-slip condition at walls causes slightly stronger elongational flow in the weld interface than does the symmetric condition of perfect slip at walls.     

Ultrasonic welding using tie-layer materials. part I: Analysis of process operation

Ultrasonic welding of oriented polypropylene (OPP) using tie-layer materials has been examined. The thermal cycle at the joint interface was evaluated using a high speed data acquisition system, and concurrent changes in horn displacement (penetration) and the output power were monitored. The model explaining process operation involves four phases, i.e., I–where heating occurs because of the stresses generated in asperities on the contacting surfaces; II–where the whole tie-layer reaches the melting point; III–where the polymer melt is subjected to intense heating from viscous dissipation and is squeezed out; and IV–where the joint cools after welding. In the early stages of ultrasonic welding the heat generated at asperities on the contacting surfaces leads to melting of the tie-layer/oriented polypropylene interface within 50 ms. The tie-layer heats up because of a combination of viscoelastic dissipation and heat conduction from the oriented polypropylene/tie-layer interface, and the rate of temperature rise at the midline of the tie-layer is in the range 200°/s to 400°/s. The reduction in thickness of the test specimens (penetration) is negligible up to the time when the tie-layer melts completely, and then changes rapidly when the melted polymer at the joint interface is squeezed out. The influence of machine parameters (amplitude and contact pressure) and of tie-layer Melt Flow Index is also examined. The total time required for completion of the welding process decreases when the amplitude and applied pressure are increased. The use of low Melt Flow Index tie-layers produces peak temperature as high as 600° at the bondline, and little material is ejected during the ultrasonic welding operation.     

A review of salt scaling: I. Phenomenology

Salt scaling is a major durability issue for concrete, so the phenomenon has been the subject of an extensive research effort. Nevertheless, there is no agreement regarding the cause of this damage, so no means for preventing salt scaling can be identified. One of the primary reasons for this shortcoming is the lack of a critical review of the research in this field. Such a compilation is presented in the present series of articles. In Part I, we review the experimental studies that have revealed the phenomenology of salt scaling. In Part II, proposed mechanisms for scaling are discussed, and the adequacy of these mechanisms is judged based on their ability to account for the characteristics outlined here.     

Support studies for the U-GAS coal gasification process. Part I: Fluidization

This is the first part of a series of papers providing the details of support studies conducted as part of the development of the U-GAS fluidized-bed ash-agglomerating coal gasification process. The fluidized-bed gasifier in the U-GAS process behaves much like a conventional well-mixed bed, having a uniform bed composition and temperature in the bulk of the region, but it exhibits some departure in a small localized area confined to the proximity of the central spout, where a somewhat higher temperature is maintained for producing ash agglomerates. Extensive cold-flow experimental studies have been conducted to predict the solids discharge rates and agglomerates classification from the venturi/classifier system in the gasifier; these studies have resulted in a generalized analytical expression in terms of operating velocities, gas properties, and venturi/classifier configuration for the gasifier scale-up.Large-scale U-GAS gasifiers can have single-cone or multiple-cone grid configurations. X-ray cinematographic studies have been conducted in a program at University College London to compare the fluidization behaviors in the one-cone and three-cone grid configuration systems. The objective of this test program was to visually examine the solids circulation and study the bubble dynamics in the fluidized beds with these different configurations.     

Recent developments in hot plate welding of thermoplastics

This paper considers the hot plate welding process applied to three injection molded thermoplastic materials: polypropylene, high impact polystyrene, and poly(phenylene oxide). Weld quality was assessed by tensile testing. The process was found to be suitable for all three materials, although tolerance to variations in process parameters varied. The parameters in the heating phase, i.e., hot plate temperature and time, were the most important for achieving good welds, whereas parameter variations in the consolidation phase were relatively unimportant.     

Process data acquisition in vibration welding of thermoplastics

A measuring method is presented for process monitoring and process analysis of linear vibration welding of thermoplastics. The method works by recording and evaluating time-dependent signals that describe the process—such as the frequency-dependent signals of the displacement of the two parts being joined, the tangential force in the welding plane—and the nonperiodic signals of the welding process, i.e. the normal force in the welding plane and the melting or joining displacement of the parts being joined. It is possible to determine the energy input into the welding zone as a function of the selected machine parameters and the process sequence over time.     

Comparison of orbital and linear vibration welding of thermoplastics

This work was conducted to determine if there were any benefits with orbital vibration welding compared to linear vibration welding. The experiments were conducted using standard full‐factorial designs with each process and each material. Four materials, polypropylene/polyethylene copolymer (PP/PE), polycarbonate (PC), acrylonitrile‐butadiene‐styrene (ABS) and Nylon (PA), were studied with each process. The equipment used was a modified Branson VW‐4 with an orbital head that had isolated magnets. The same machine was used to weld with both linear and orbital motions. This was achieved by modifying the controlling parameters of the drive. It was found that compared to linear vibration welding, orbital welding had a reduction of cycle time by 36% and 50% in Phase I and Phase III, respectively. It was also found that orbital welding dissipated 56% and 100% more power than linear vibration welding in Phase I and Phase III, respectively. In addition, it was seen that orbital welding was able to universally join unsupported walls with higher strengths and better consistency compared to linear welding. Other benefits included: a difference in the appearance of weld flash and small increase in weld strength. Some of the limitations of orbital welding that were identified included the effects of disengagement and residual stresses. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Flame-retardant thermoplastics. I. Polyethylene–red phosphorus

Red phosphorus is a highly effective flame retardant for polyolefins. The low additive levels of red phosphorus in polyethylene make it an attractive route to nonhalogen flame-retardant systems. The mode of action of the red phosphorus has been investigated. Results indicate that the red phosphorus is effective both in the vapor and condensed phase. In the gas phase, PO species produced from the combustion of red phosphorus quench radical processes. In the condensed phase, the red phosphorus substantially lowers the heat of oxidation and traps radicals. This improved thermal stability results in a decrease in fuel production during burning.     

Heating and bonding mechanisms in ultrasonic welding of thermoplastics

An experimental study of the heating and bonding mechanisms in ultrasonic welding is described. Polystyrene specimens were joined under a variety of welding conditions while the temperatures at the interface and within the interior of these specimens were measured. The power input, amplitude of vibrations, and amount of deformation during welding were measured concurrently. In general, the rate of heating at the interface is greatest at the beginning of the weld cycle, but slows markedly after the interface temperature reaches approximately 250°C. The interface temperature peaks well before the weld is completed. Temperatures within the body increase most rapidly at temperatures near the glass transition temperature. Welded specimens were broken on a special testing apparatus under combined torsional and compressional loads to determine the weld strength. The results show that weld strength is dependent on the amount of energy input and the degree to which material flows out of the interface region. Possible mechanisms for heating and bonding during ultrasonic welding are discussed in light of the observed behavior.