Hearing Aid Shell Fabrication - UV Process
The 100th Monkey Phenomenon
You might be wondering what 100 monkeys and UV shell fabrication have in
common - and if you didn't wonder, I would need to wonder about you.
Some time ago researchers found a group of monkeys on
a remote island. These folks wanted to study this particular group because
they had never been in contact with any other monkeys, and they wanted to
know how that might effect their development. The researchers were
startled to learn however, that this group of monkeys were indeed very
developed, and practiced many of the learned traits of other monkey groups
on the mainland, even though they had never interacted. Was this therefore
learned behavior, or some kind of inborn trait?
They decided to perform an experiment.
They took new-born monkeys and separated them into
two groups. One had access to other monkeys to learn from. The others were
an isolated group and forced to learn on their own, or from each other.
The first group developed as expected, but the second group had a
surprise. Researchers found that 1 in about 100 monkeys possessed a unique
desire to be inventive and figure out new and better ways of performing
tasks. This monkey would then pass this knowledge on to the other monkeys
in the group. And even though this process was slower, the 100th monkey
would keep the isolated group advancing in development. And when comparing
different isolated groups with each other, they all advanced at about the
same rate - many times figuring out the same way to do something at the
So What Does This Have To Do With UV Shell
Well, from my standpoint - everything. It takes that 100th monkey to think
in a new direction and try things in a new way. I don't want you to think
I am comparing people to monkeys, but I can always tell when a new thought
process is emerging within our industry because I get phone calls and
emails about it. And one subject that has emerged over and over again
recently is the idea of folks making there own hearing aid shells for
hearing aid fabrication in office - and why not? The benefits are
tremendous and the profits are much higher. So, before we take a look at
how to make our own shells and custom ITE hearing aids, lets look at why
we would want to take on this endeavor.
Here is a list of why you might consider
making your own hearing aid shells:
1) More Profit.
Obviously you would stand to make more money on a hearing aid sale if you
made the unit yourself.
2) More Control: This is
a biggie. You get to control the whole process. From turn around time to
fixing fit problems, you can give your customers superb customer service
as everything you need to fix any problem is right at your facility.
3) Less Turn Around Time: From beginning to end you can easily make a
hearing aid in 60 minutes or much less if you have a production process.
How does that compare to waiting days for your customer.
4) Better Customer Service: You can react to any customer complaint in
minutes instead of days.
5) Less Fit Problems.
This one might not seem as obvious, but the sooner you can work with an
impression, the more duplicatable it is.
6) Easy Process: Making shells, and hearing aids, is not difficult -
especially with the availability of prewired faceplates. Lets face it, if
you've never had electronic training it would be nearly impossible for you
to figure out how to wire a hearing aid circuit, but now you can buy a
digital circuit already wired and tested for you. You just need to learn
the process of putting it all together.
1) Money: It is going to take money to get it going. Not tons of money,
but enough to make you think it through.
2) Personnel: You might need to hire and train folks to help you,
depending on how big your operation.
3) Time: It is going to take more time on all fronts to meet this new
Equipment and Materials:
Obviously, you are going to need some special, and some not-so-special
equipment and materials to make your own hearing aids. Here are some of
the basic pieces required:
Wax Pot for melting wax
Hydrocolloid machine for melting hydrocolloid (larger labs) or a microwave
(for smaller labs).
Water clear hydrocolloid material
Disk Sander for trimming the impression and sanding the shell base.
UV Cure chamber for curing shells
Investment cups for pouring casts
UV Spot Cure unit for curing vents
UV Cure unit for attaching the faceplate
Acoustic testing unit for testing the completed hearing aid
UV materials: Shell material in colors, UV lacquer, faceplate adhesive
Grinding / Buffing station (Redwing Lathe)
Various hand tools, bits, burrs ...
So, If You Want To Make Your Own Hearing Aids Why Are
We So Focused On The Shell?
The hearing aid shell is the whole stumbling block to making hearing aids.
The electronics is easy - just buy prewired faceplates. They are tested
and ready to go. But, the shell - well, if you can't produce the shell
you're not going to be making hearing aids.
Help In Making Your Decision
I can tell you this, making UV hearing aid
shells is not hard work It is a process however, and with every process
comes a step by step procedure. And though it may seem like a lot of
steps, most are very minor and only take a few moments. If you can follow
directions you can do this - I promise.
So, here we go!
Step 1: Working With The Impression
If you've worked in the hearing industry one day you know how
important the impression is. A lousy impression is going to make a lousy
hearing aid fit - plain and simple. There are a lot of little tricks you
can do to help the impression, but if you have large voids you may as well
do the impression again - it will be worth it in the long run. When I
worked for a hearing aid manufacturer, it was a daily occurrence that we
had to call an office and ask for a new impression. And invariably we
would be told to make the hearing aid anyway - big mistake. Many times
these same hearing aids came back for remakes.
(Another reason to consider making your own shells).
Step 1a Making A Cast, or Investment
Once you have a workable impression you will need to cut it down for
making a proper cast, or investment. This is so you have a mold of the ear
in which to check the fit once the hearing aid is completed. The oto-block
is removed and the lateral process is cut with a razor blade to make a
base. The cut impression is then adhered to an investment cup with hot wax
and silicone material is poured over it. Once cured, the impression is
removed and the investment is complete.
Step 1b Trimming the Impression
Now you will need to cut the impression down to the desired size using a
razor blade. What you are doing is trimming closer to the type of hearing
aid model to be made. A Full Shell device will want to be cut revealing
the helix, where a CIC will need to be cut at the canal opening. This is
just an approximate cut as you will take the shell down further after
fabrication. After cutting, you will also need to taper the canal and
detail the impression so the finished shell can fit comfortably in the
ear. Tapering can be easily done using a disk sander.
Step1c Filling Voids
Even the best impression will leave a small void or two. The best stuff
I've found for filling voids is DAP 33 glazing compound. This stuff works
great, is easy to apply, and is inexpensive. Just use your thumb or finger
to work DAP 33 into any void. Smooth along the impression surface and let
it harden for a few minutes.
Step 1d Waxing the Impression
After the impression has been worked it is ready for a coat of wax.
This is done by simply dipping the impression in a hot refined wax, and is
necessary because we need to build up the impression slightly to help give
the hearing aid a snug fit, as well as the small amount of material lost
when we buff it later.
Now there are a few things to consider when dipping
1) Wax adheres in different thicknesses at different parts of the
impression. A concave surface will accumulate more wax than a convex
2) The part of the impression that stays in the wax the longest will
accumulate the most wax. In other words, the canal of the impression is
likely to have more buildup than the base providing the canal was dipped
3) Dipping the impression is a skill that must be practiced and mastered
as the thickness of the wax will have an effect on fit.
This step may seem scary, but it is really just a
matter of practice getting the wax at the right temperature, dipping at
the right speed, and removing the impression at the right angle. And this
will vary depending on everything from the wax pot to the model of hearing
aid to the temperature of the room. But with a little practice, you will
be dipping smooth and consistent impressions in no time.
The wax will harden very quickly, and we are now
ready for the fun part.
For the sake of being long winded, next time we'll take a look at
making the negative impression for the shell, and the actual UV Process.
About the Author
Chris Perkins is the owner of Lightning Enterprises, and facilitates
the Lightning Enterprises newsletter. He has worked in the hearing aid
industry since 1991 in hearing aid manufacturing and product development,
as well as equipment and process consulting.
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