M9T L 7RARIES INVESTIGATION OF ULTRSONIC WELD QUALITY INA BABY TEETHER 3 9080~ 00579i950 4 by LISA CATHERINE GROSSWEILER Submitted to the Department of Mechanical Engineering in Partial Fulfillment of the Requirements for the Degree of BACHELOR OF SCIENCE at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 1989 @ Massachusetts Institute of Technology 1989 All rights reserved Signature of Author Department of Mechanical Engineering May 12, 1989 Certified by Steven D. Eppinger Thesis Supervisor Accepted by Peter Griffith Chair an, Department Committee ARCHJYES MasV10usmS INShUE OF TECHNOLOGY JUL 10 1989 UBRARIES INVESTIGATION OF ULTRASONIC WELD QUALITY OF A BABY TEETHER by LISA CATHERINE GROSSWEILER Submitted to the Department of Mechanical Engineering on May 12, 1989 in partial fulfillment of the requirements for the Degree of Bachelor of Science in Mechanical Engineering ABSTRACT In conjunction with a local manufacturer of children's products and as a follow up to the class "Design for Manufacturability", a study of a baby teething ring is carried out in an effort to improve product quality. Consumer complaints about product failure and European government regulations regarding the strength required to break the ring of the teether are cause of concern to the company. Recommendations are made that improve the strength from an average of 17.2 pounds to 26.1 pounds. These averages are determined from tests done on vinyl strips that simulate an increase in the width of the radio frequency sealed portion of the teether where it is welded into a loop. The width is increased from 1/16" to 1/4". Thesis Supervisor: Steven D. Eppinger Title: Assistant Professor of Management TABLE OF CONTENTS A bstract................................................................................................... .2 Chapter One. Introduction............................................................... 4 Chapter Two. Manufacturing of Teether................................... 6 Chapter Three. Development of Weld Quality Focus............. 7 Chapter Four. Experiments............................................................ 11 Chapter Five. Recommendations.................................................. 17 Chapter Six. Recommendations for Examining Round Seal..19 Chapter Seven. Recommendations for Round Seal Experiments.......................................................22 Chapter Eight. Conclusions............................................................23 Appendix A. Experimental Data.................................................. 24 Chapter One. Introduction This thesis is a result of the class titled "Design for Manufacturability". The class is sponsored by four companies, each producing a product they feel can be improved. The product I am working on is a baby teether produced by a local toy manufacturer. It consists of a ring of eight crystal- clear vinyl water filled beads (see Figure One). Inside of each bead is a brightly colored polyethelyene animal. The beads are interconnected by isthmuses that are divided into two channels by a dot seal. These two channels allow water to flow from one bead to another causing the animals to spin. The teethers are the client's largest selling product at approximately one million units per year. The manufacturer believes that the product's appeal comes from its beauty, clarity and entertainment value. They believe it serves two purposes, a teether and a toy. The product team is divided into three parts. Dave Costello and I are analyzing the production parameters to try to increase the quality of the product, given existing production processes and materials. George Jaquette is performing market research with participants in the manufacturer's research council and with another, unrelated group. Several products with the same consumer appeal are being developed by Steve Betz. Figure One. Drawing of Baby Teether 5 Chapter Two. Manufacturing of Teether The teether is comprised of plasticized vinyl, polyethelyene animals and chemically treated water. Rolls of the vinyl are cut into 12" X 18" squares. A piece of vinyl is laid onto a buffer sheet. The buffer sheet prevents the radio-frequency machine from arcing and acts as a guide for the placement of the animals. The animals are laid onto the vinyl by hand and another sheet of vinyl is placed on top. The buffer sheet, animals, and two sheets of vinyl are placed in a radio-frequency (RF) sealing machine. RF waves are used to create the edge and dot seals in the teether, see Figure Two. This process is used because it is cheap and effective. On the top of the machine is a brass die. The die outlines four teethers and produces as RF seal in the desired bubble configuration. One end of the teether is left open so that they can be filled with water. It also produces a tear seal which allows the teethers to easily be torn away from the waste material. The teethers are stripped from the waste material and are placed in boxes of 1000. This part of the manufacturing process is done by a plastic contractor. The flat teethers are returned to the toy manufacturer where they are placed on a semi-automatic machine that fills them with water, ultrasonically seals the end, and cuts off the excess material. weld. This weld will be referred to as the filling The teethers are carried to seal TEETHER NECK RADIO FREQ.UENCY SEAL - DOT SEAL ') IF Figure Two. Drawing of teether as received from plastic contractor. another machine where they are ultrasonically sealed into the bracelet configuration. seal weld. This weld will be referred to as the round Chapter Three. Development of Weld Quality Focus The toy manufacturer assembles data about product quality from consumers who write to the company returning failed products. The manufacturer believes that 0.1% of their annual sales are actually returned. failure of the teether. This data suggest two main types of The first is dot seal leakage in which the teether leaks at the RF weld in the isthmus between the bubbles. This occurred in about 20% of the returned cases. The second failure also occurred in 20% of the cases and was classified as round seal failure. The last ultrasonic weld breaks and the teether no longer forms a chain, but otherwise remains intact. Dot seal leakage is believed to occur mainly because of consumer's misusing the product. Should the consumer freeze the teether and handle it before it thaws, the ice that forms in the channel punctures the vinyl creating a leak. The manufacturer has just remachined the platen which creates the dot seal to allow for better flow through the channel. They have not received any information regarding the improvement in quality of that seal. Experimenting with improving the quality of the dot seal would require changing the configuration of the platen which would be time consuming and costly. There is no theory as to why the round seal has been failing. Because round seal failures account for half of the consumer complaints regarding product quality, I am focusing my investigation on its reliability. This investigation coincides with the work being done by Dave Costello. Chapter Four. Experiments After the decision is made to improve the ultrasonic welding process, experiments are performed to determine the weak link in the process. The toy manufacturer's standards require a pull strength of greater than fifteen pounds for a teether. To sell the product overseas a pull strength of twenty pounds is needed. The manufacturer's routine method of measuring the pull strength is first cutting the teether and draining the water from it. After the water is removed, the teether is placed in visegrips which are hung from the bottom of a digital force gauge, see Figure Three. The teether is then pulled until it breaks. The force gauge records the maximum force required to break the teether. Throughout our experiments, the pull test is used to compare the different welds made. To have a consistent baseline case, this process is repeated ten times. The results are shown in Appendix A, Table Al. The average pounds to break is 17.2 pounds, although the values range from fifteen to nineteen pounds. round seal remains intact. In all cases the The teether failed in the area just below the filling seal weld, see Figure Four. To test the strength of the round seal, the broken teether is placed in the visegrips and pulled until it breaks a second time. This time the break is located in the round seal zone. The measured pull strength is recorded at SPRING SCALE -- Figure Three. TEE THER Drawing of pull test apparatus. 12 TYPICAL BREAK Figure Four. -*- FILLING SEAL Drrawing of teether showing typical break. 13 27 pounds. From this we can conclude that the "weak link" in the welding process is the filling seal weld, the first ultrasonic weld. This is reinforced by placing a teether that has not been round sealed in the visegrips and pulling on the teether until it breaks. In all cases the break is in the same location as before with the round seal tests, just below the filling seal weld, see Figure Four. The average breaking strength is 16.4 pounds. Data are shown in Appendix A, Table A2. In order to grip the teether easier in the visegrips, the teethers are pulled off of the machine before the excess material is cut off. Next, experiments are performed to determine the effect that the presence of water had on the ultrasonic weld. Specimens are made by shutting off the water supply to the filling machine. Teethers are sealed, but not filled with water. The data from these experiments are listed in Appendix A, Table A3. It shows that there is no noticeable affect of water on the pull strength of the ultrasonic weld. From our examination of the teethers that failed, we draw the following conclusions. It seems that a crack propagated along the lines left in the seal by the knurl of the ultrasonic welder's horn. Lower pull strengths are observed when the filling seal zone extended over the edge of the teether. Our theory is that the knurling on the horn causes stress concentrations in the vinyl which are greater at the edge of the teether. 14 To test this theory, the teethers are cut and placed on the machine so that the weld falls in the middle of a bubble, see Figure Five. These welds yield pull strengths from nineteen to twenty-seven pounds. The results are listed in Appendix A, Table A4. As a result of our findings, we request that an engineer at the toy company have the plastic contractor make some specimens. They resemble the neck of the teether except for the width of the RF seal. side. It is increased from 1/16" to 1/4" on each Experiments are performed with the specimens to determine the required width of the RF seal at the neck. Using a straight edge and scalpel, the specimens are trimmed and tested with a RF seal width of 1/8" and 1/4". Appendix A, Table A5. Data are shown in Increasing the width of the RF seal by 1/8" roughly increases the pull test results by five pounds. 15 FILLING SEAL Figure Five. Drawing of teether during middle bubble experiments. 16 Chapter Five. Recommendations Currently, the width of the horn is the same as the neck of the teether. The repeatability of the machine is poor and consequently the weld does not always fall in the same place on each teether. It could fall in the middle or off to one side. The critical dimension is the placement of the ultrasonic weld. In order to obtain the higher pull strengths, the weld needs to be placed in the middle of the teether. It can not protrude over one of the edges. It is recommended that the RF weld on the neck of the teether be increased to 1/4". adding an insert to the platen. This can be done temporarily by Widening the RF seal will increase the repeatability of the filling seal process by allowing the weld to be bounded by material on either side, preventing stress concentrations from the knurl to reach the edge. One issue that needs to be considered is how the wider RF edge will affect the loop seal. Issues that need investigation are the creation of flaps caused by the wider edge and consideration of a tapered edge that gets wider towards the filling seal zone. Because the teether fails along the lines left by the knurl, another recommendation is to experiment with removing the knurl on the filling seal. An alternative to increasing the width of the RF seal is improving the repeatability of the weld positioning in the filling seal machine. If the machine can be made to accurately place the weld inside the edges of the teether, the 17 extra width would not be needed. Parameters that should be investigated are the location of the nozzles with respect to the horn and the amount of backlash in the rotating top. Are the nozzles spaced evenly apart around the top? reduce the backlash? 18 Is it possible to Chapter Six. Recommendations for Examining Round Seal Upon examination of the round seal, it can be seen that the teether is not bonded in the middle of the weld, only at the edges, see Figure Six. This might be the cause of the many failures of this weld that result in a single, straight teether that does not leak. The problem with the weld could come from many things and we have discussed many theories. One of them is that the two parts of the teether are not aligned so that the result is a flap that makes it easier to tear the teether apart. Another problem with alignment is the position of the round seal weld in relation to the filling seal weld. Comments have been made about the teether burning when pre-welded material is welded to unwelded material. As a result, the round seal weld is positioned directly over the filling seal weld. Since the repeatability of the process depends heavily on the operator of the machine, the welds do not always entirely overlap. Consequently burning occurs and the gap between the horn and the anvil is increased. This in effect decreases the total power of the process. 19 ACTUALWELD Figure Six. ROUNDSEAL Drawing of typical round seal weld. 20 One of the suggestions we have is to check the height of the six anvils. The manufacturer has determined that the optimal gap between the horn and anvil is 0.012". The reason they do not have the gap set to 0.012" is because the horn overloads approximately once every cycle, and the machine automatically shuts down. This would happen once every cycle if one anvil is taller than all the others. In other words, check the gap on each welding station on the machine. Another suggestion is to check the table the anvils sit on to see if it is level. More power should be used to firmly weld the teether, not just spot weld. 21 Chapter Seven. Recommendations for Round Seal Experiments - Design a pull strength-like test that would measure lateral (tearing) force exerted on the weld. - Tear control teethers and measure the affected area. - Try to increase the area affected by the weld. - Decrease horn gap. - Increase power, weld time. - Vary the position of the round seal in relation to the filling seal weld resulting in the design of a jig to reduce the dependency of the location of the weld on the operator. 22 Chapter Eight. Conclusions In conclusion, experiments have determined that the low pull strength observed by the manufacturer is a result of the filling seal process. The parameter that largely affects the pull strength is the position of the ultrasonic weld with respect to the edges of the teether. One explanation for this is that the knurling on the ultrasonic horn creates stress concentrations in the vinyl as evidenced by the way the teether fails when tested. These stress concentrations are larger when the knurling extends over the edge of the teether. Our recommendation is to widen the edges of the teether from 1/16" to 1/4". Other solutions that should be investigated are increasing the repeatability of the positioning of both welds and removing the knurl from the ultrasonic horn. Recommendations are also made for investigating the quality of the round seal weld. It can immediately be seen upon tearing the weld apart that only the edges of the weld are actually affected by the seal. The middle of the welded zone does not bond. Our main recommendation is that more power be used in the welding process. Comments made about the weld quality suggest that under existing conditions burning occurs when the power is increased. 23 A list of suggested experiments is included to examine the round seal quality. 24 Experimental Data Appendix Table Al. Data from Round Seal Tests Test Number Pounds to Break 1 2 3 4 5 17.7 18.2 15.2 17.4 19.4 18.6 16.5 16.2 15.4 17.3 10 Average Pounds to Break = 17.2 pounds Table A2. Data from Straight Teether Tests Test Number Pounds to Break 1 2 3 4 5 15.6 15.8 16.5 15.9 14.9 17.2 17.1 16.9 17.5 16.6 10 Average Pounds to Break = 16.4 pounds 25 Table A3. Data from Experiments Without Water Pounds to Break 16.1 16.7 16.5 16.1 16.8 Average Pounds to Break = 16.4 pounds Table A4. Data From Middle Bubble Experiments Test Number 1 2 3 4 5 6 7 8 9 10 11 12 Pounds to Break 25.0 23.7 23.8 22.9 22.8 18.0 18.7 24.5 26.6 17.6 19.5 23.3 Average Pounds to Break = 22.2 pounds 26 Table A5. Data From RNR Specimens Width of RF Strip 1/4" Test Number 1 10 11 12 13 14 Pounds to Break 27.4 30.0 24.5 29.9 26.3 26.1 27.2 25.9 24.0 26.8 26.1 27.5 23.7 20.5 1/8" 19.9 20.4 19.2 19.4 20.0 20.9 22.3 19.8 20.0 22.6 10 Average Pounds to Break For 1/4" Average Pounds to Break For 1/8" 27 = 26.1 = 20.4 pounds pounds