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