# Fabricating, Assembling, and Installing

# the Outer Radio Frequency Enclosure

# of the Multiplicity Vertex Detector

Richard Conway

January 5, 1999

µI. Overview of the Outer RF Enclosure

II\. Thermoforming Rohacell

II\. A. Introduction to thermoforming

II\. B. Materials needed

II\. C. Thermoforming procedure

III\. Machining Rohacell

III\. A. Introduction to machining

III\. B. Materials needed

III\. C. Machining procedure

• Cutting the length to spec

• Cutting the width to spec

• Milling out the seal strip grooves

IV\. Assembling the Outer RF Enclosure

IV\. A. Introduction to assembling

IV\. B. Materials needed

IV\. C. Assembling procedure

• Adhere the radiation shielding to the outer surface of the Rohacell

• Adhere the radiation shielding to the inner surface of the Rohacell

• Attach the seal strips

V. Integrating the Outer Enclosure with the MVD

V. A. Introduction to integration

V. B. Materials needed

V. C. Integration procedure

## I. Overview of the Outer RF Enclosure

The MVD\'s Radio Frequency Enclosure has three main parts: the inner
enclosure, the outer enclosure, and the side panels (see Figure 1
below). The outer enclosure functions as an RF shield to help insure the
required 10:1 signal-to-noise ratio is met as well as a barrier to
external temperature and water vapor to ensure that the environment
inside the MVD can be maintained. The two 0.5mil thick aluminum foil
layers provide both the approximately 8dB of RF noise attenuation at
10MHz as well as an air-tight and water-tight seal. Rohacell IG-71 foam,
thermoformed to the correct hemi-cylindrical dimensions, serves as the
structural support for the aluminum foil RF shield, which is adhered to
both large surfaces of the Rohacell by Dielectric Polymer\'s NT 988-2
dry transfer adhesive. Two aluminum O-ring seal strips make the
electrical connections from the aluminum foil to the inner RF enclosure.
They are cemented into a small groove in each of the long edges of the
Rohacell through a conductive epoxy. The whole assembly is held in place
by two aluminum retaining rings that run along the curved outer edges of
the MVD\'s D-rings and by four earthen O-rings that keep the inner
enclosure locked into the seal strips.

Figure 1: The MVD's Radio Frequency Enclosure

(Detail 1 is the Truss Assembly, detail 2 is the Inner Enclosure, and
detail 3 is the Outer Enclosure)

## II. Thermoforming Rohacell

### II. A. Introduction to thermoforming

Rohacell IG-71 polymethacrylimide foam is used as the structural support
for the outer RF enclosure because of its high strength to density ratio
and its long radiation length. It is also easily machined and easily
thermoformed.

Rohacell foam is shipped from the manufacturer in 0.25\" thick sheets.
The sheets must be cut to approximately the desired length and width and
then thermoformed to the correct radius using a forming fixture. The
thermoforming procedure raises the temperature of the Rohacell above its
forming temperature, at which point the chemical bonds that hold the
shape of the foam break down. When the bonds are weakened, the
Rohacell's stiffness and strength are lost and it can be molded into and
held in the desired shape by the forming fixture. When the temperature
falls back below the forming temperature, the chemical bonds regain
their strength, and the Rohacell retains the shape the forming fixture
held it in.

### II. B. Materials needed

· VWR 1655D oven with Watlow 700 controller (or similar oven with air
circulation that heats above 185°C)

· 37.5\" x 30\" x 0.25\" sheet of Rohacell IG-71

· thermoforming fixture (see Appendix A)

· Zetex Plus or similar heat resistant gloves

### II. C. Thermoforming procedure

1\) Place the forming fixture into the oven so that the hemi-cylinder\'s
axis penetrates through the left and right side walls of the oven and so
that the fixture is convex when viewed from above.

2\) Place the sheet of Rohacell into the oven vertically so that the air
can circulate around it and heat it evenly.

3\) Preheat the oven to 120°C.

4\) Once the oven temperature has reached 120°C, let the Rohacell dry
out for approximately 3 hours.

5\) Place the Rohacell into the forming fixture while in the oven so
that it is held in the final shape desired.

6\) Set the oven to 185°C.

7\) Let the forming fixture reach 185°C (this takes approximately 45
minutes if the fixture's initial temperature is 120°C or approximately
60 minutes if the fixture was allowed to cool down while positioning the
Rohacell and it's initial temperature is 100°C) and then remain at that
temperature for 5 minutes. Do not let the temperature of the forming
fixture or Rohacell rise above 185°C. This is no chance the fixture
temperature will rise above 185°C if the oven is set at that
temperature.

8\) Set the oven temperature to room temperature (about 23°C) and let
the Rohacell and fixture cool down with the oven door closed.

9\) After the fixture temperature has dropped below approximately 70°C
(about 3 hours), take the Rohacell out of the fixture.

10\) Store at room temperature and a normal humidity level until ready
to machine.

##  {#section .unnumbered}

## III. Machining Rohacell

### III. A. Introduction to machining

The thermoformed Rohacell must be machined to the correct dimensions
before it can be integrated into the outer RF enclosure (see Figure 2
below). Because the Rohacell has a tendency to change shape and size
during its thermoforming procedure, it cannot be machined to the final
enclosure dimensions before thermoforming.

Figure 2: The Outer Enclosure's Rohacell Cover

The change in thickness due to the thermoforming process is
insignificant to the effectiveness or ability to be integrated into the
final enclosure unless the thermoforming procedure causes it to increase
more than 125% of its original thickness. The width has a tighter
tolerance, because it must be held in place by the 1/4\" wide D-rings;
however, the tolerances can still be met using a sharp razor blade to
cut to the correct width . The tolerance on the length error is much
tighter, because the 1/16\" O-ring grooves in the seal strips must match
up with the O-ring grooves in the D-rings. Any mismatch in the grooves
could cause a tear in the airtight laminate of the inner RF enclosure.
But even this tight tolerance can be attained using a razor blade if the
Rohacell is aligned properly in the machining attachment for the
thermoforming fixture.

There is also a small groove that needs to be made in both straight
edges of the thermoformed Rohacell. Each groove will provide a firm
seating for two aluminum seal strips that have the O-ring grooves
machined into them. The tolerance on this groove is very tight, and a
milling machine must be used to achieve the amount of accuracy required.
The mill must have a travel length of at least 30\", and that
requirement is fairly hard to meet.

### III. B. Materials needed

· a formed 37.5\" x 30\" x 0.25\" outer Rohacell cover

· thermoforming fixture

· aluminum D-rings (see Appendix B)

· machining fixture (see Appendix C)

· sharp knife or razor blade

· masking tape

· marker or metal scribe

· a stiff, flat object (e.g. an L-square or a parallel)

· 5/16\" Allen wrench

· 1/8\" diameter end mill bit

· milling machine with an x-axis travel length of at least 30\" and a
digital readout of the milling bed's position (Ivan Taylor, TA-53,
MPF-2)

· milling bed clamps to hold machining fixture in place

### III. C. Machining procedure

#### • Cutting the length to specification

1\) Place the D-rings and struts assembly on a flat table with 2" risers
underneath the D-rings to raise it off the table.

2\) Locate the marks on the inner D-rings and struts assembly which show
where the length of the curved edge of the Rohacell.

3\) Tape the top edge of the Rohacell onto the D-rings so that there is
foam extending past all the D-ring marks.

4\) Adhere two pieces of tape along the bottom edge of the Rohacell near
the D-rings so that the top edge of the tape matches up with the bottom
edge of the marks on the D-rings. Make sure that the Rohacell is pulled
tight around the D-rings before laying down the tape.

5\) Mark the spots on the tape where the D-rings meet the tape in case
the tape is adhered unleveled.

6\) Tape the bottom edge of the Rohacell to the D-rings.

7\) Untape the top edge of the Rohacell from the D-rings.

8\) Adhere pieces of tape to the Rohacell so that they match up with the
marks on the top corners of the D-rings. Make sure that the Rohacell is
pulled tight around the D-rings.

9\) Untape the bottom edge of the Rohacell.

10\) Using the tape on the Rohacell as a marker for where to cut the
length, place the Rohacell in the machining fixture, and finger tighten
the bolts on the fixture.

11\) Finely adjust the Rohacell so that the tape's edge is even with the
flat surface on the top half of the fixture. Make sure that the top and
bottom halves of the fixture are flush by pressing a parallel against
the flat surface of one half and moving the other half until it hits the
parallel.

12\) Readjust the Rohacell if needed.

13\) Tighten the fixture bolts 1/4 turn tighter with the 5/16" Allen
wrench.

14\) Turn the fixture upside down so that the Rohacell is hanging down,
and support the unbalanced bottom half of the fixture with a 4" tall
support block so that the fixture doesn't fall over.

15\) Use a sharp razor to cut through the Rohacell along the bottom flat
surface of the fixture.

16\) Turn the fixture right side up.

17\) Unbolt and remove the Rohacell from the fixture.

18\) Repeat Steps #9 through #15 for the other flat edge of the
Rohacell.

19\) Measure the cut Rohacell against the D-rings and struts assembly to
make sure the length is accurate.

#### • Cutting the width to spec

20\) Measure the width of the Rohacell at several different points,
subtract 29.375" from that shortest measurement. The resulting length is
how much excess should be cut off of one side.

21\) Use the thermoforming fixture and yellow nylon straps to align and
strap down the formed Rohacell, and allow one of the curved edges to
hang over the circular end of the fixture by the length calculated in
Step 20. (The fixture setup is done by laying down the sheet of red
rubber over the fixture surface, then laying the Rohacell on the rubber,
and then tightening down the yellow nylon straps around the entire
fixture. The Rohacell should be aligned so that one of the flat edges
parallel to the hemi-cylinder\'s axis is resting on the aluminum bar
that extends the length of the fixture.)

22\) Use a sharp knife or razor blade to rough cut the excess Rohacell
along the curved edge.

23\) Use a sanding block to even up the edge. Use coarse sandpaper until
there is approximately 1/16" left overhanging, and then use fine
sandpaper to finish the job. To ensure a flat edge on the Rohacell, let
the sanding block rub against the fixture's flat endplate surface while
sanding the foam.

24\) Measure the width of the Rohacell at several different spots along
the curved edges, and make marks near the uncut edge exactly 29" from
the cut edge.

25\) Loosen the nylon straps, align the 29" marks with the other curved
edge of the thermoforming fixture, and retighten the straps. Because the
marks are on the opposite side of the Rohacell as the edge of the
fixture, some guesswork is required, but always err on the side of the
Rohacell being too wide so that you can fix the error.

26\) Use the sanding block to again shave the Rohacell to be even with
the flat surface of the fixture. Start with coarse sandpaper and finish
with fine.

27\) Remeasure the width at several points to ensure its uniformity, and
resand as needed to achieve the 29" width desired.

#### • Milling out the seal strip grooves

28\) Align the bottom half of the machining fixture with the milling bed
and use the bed clamps to hold it in place. Use an edge finder or
indicator to ensure that the fixture is parallel with the milling bed's
axis of travel.

29\) Zero the position of the 1/8\" diameter milling bit by letting it
touch the front and top surfaces of the bottom half of the fixture.

30\) Place the Rohacell into the machining fixture and bolt it in so
that the edge of the Rohacell is flush with the edge of the bottom
fixture half. Tighten the bolts 1/4 turn past finger tight with the
Allen wrench.

31\) Run the milling bit down the edge of the Rohacell at Z=0.058\"
above the fixture\'s bottom surface and Y=0.132\" into the Rohacell,
then set Z=0.067\" and run the bit back across the length of the edge.
This gives a 0.134\" wide by 0.132\" deep groove that will allow the
aluminum seal strips, conductive epoxy, and RF shielding to fit snugly
together in the Rohacell\'s edge. Make sure that the Rohacell lips on
the sides of the groove are approximately the same thickness so that the
seal strips will be flush with both the inner and outer surfaces of the
Rohacell.

32\) Loosen the bolts and remove the machined Rohacell from the fixture.

33\) Repeat Steps #15 through #17 on the unmachined flat edge.

34\) Store the machined Rohacell in a safe place until ready for
assembly. The thin lips of Rohacell at the edges are extremely fragile,
so the machined foam should be handled very carefully to avoid breaking
them off.

##  {#section-1 .unnumbered}

## IV. Assembling the Outer RF Enclosure

### IV. A. Introduction to assembling

Once the Rohacell has been thermoformed and machined to the correct
dimensions, the assembly can begin. The assembly procedure adheres the
RF shielding (aluminum foil) to the Rohacell and permanently attaches
the aluminum seal strips into the grooves in the flat edges of the
Rohacell as well as to the aluminum shielding.

### IV. B. Materials needed

· a partner

· formed and machined outer Rohacell cover

· two 44" x 34" 1/2mil sheets of aluminum foil at least

· two 2mil sheets of Dielectric Polymers NT 988-2 dry transfer adhesive
at least 39\" x 30\"

· the thermoforming fixture

· the large rubber sheet used in the machining procedure

· a sharp razor blade

· a tape measure

· masking tape

· thin, clean cloth gloves (clean room gloves)

· a flat, cutting board surface (cardboard works well)

· a strip of the dry adhesive's backing at least 30" x 1/2"

· a straight-edged strip of metal 1/8" wide

· the rolling bar (a smooth cylinder at least 35" long and between 1"
and 3" diameter)

· conductive epoxy

· a few cotton swabs or a small plastic spatula to spread out the epoxy

· two inner seal strips (see Appendix D)

· two outer seal strips (see Appendix D)

### IV. C. Assembling procedure

#### • Adhere the radiation shielding to the outer surface of the Rohacell

1\) Lay down one of the sheets of aluminum foil on the cardboard and
tape the edges taut.

2\) With your partner\'s help, cut out one of the sheets of adhesive and
lay it down centered and smoothed on the foil. The best way to lay the
adhesive down smoothly is to adhere one corner and smooth the rest
outward from that corner.

3\) Run your finger along the edge of the adhesive\'s backing to ensure
that it is firmly adhered to the foil.

4\) Peal the backing off the adhesive. This is best done by pealing from
one corner and making sure that the backing stays as parallel as
possible with the cardboard, so that the aluminum foil is not stretched.

5\) Align one of the flat, grooved edges of the Rohacell with one of the
shorter edges of theadhesive. Place the corner edge of the Rohacell on
the adhesive approximately 1/2" from the adhesive\'s edge so that the
larger radius surface of the Rohacell can be rolled onto the adhesive.
Make sure to line up the Rohacell so that it will not roll off the
adhesive.

6\) Put a straight-edged strip of the adhesive backing over the 1/2" of
exposed adhesive to prevent it from adhering to the cardboard or the
inner surface of the Rohacell.

7\) While holding the Rohacell upright so that it does not pull on and
stretch the foil, have your partner cut the masking tape from the foil
around the area where the Rohacell is adhered.

8\) Roll the Rohacell onto the adhesive keeping the foil as taut as
possible while your partner cuts the masking tape from the foil around
the area you are rolling.

9\) Repeat Step #6 for the exposed adhesive at the other grooved edge.

10\) Cut the excess foil from the two curved edges of the Rohacell. To
avoid tearing the foil, press down on the smaller radius surface of the
Rohacell and cut through the foil at the point where the Rohacell is
contacting the cardboard.

11\) Cut a piece of the adhesive backing into a strip that is at least
30\" x 1/2" and make sure that one edge is straight.

12\) Have your partner hold the Rohacell upright so that the groove in
the Rohacell is parallel with the cardboard.

13\) Align the straight edge of the backing with one of the straight
edges of the Rohacell and adhere it.

14\) Align and tape down the 1/8" wide metal strip with the straight
edge of the Rohacell and backing.

15\) Use a sharp razor blade to cut through the backing, adhesive, and
foil along the outer edge of the metal strip, while keeping the strip in
place with your finger.

16\) Remove the metal strip and peal off the backing (parallel to the
foil).

17\) Make several passes with your fingertip to fold the foil over the
Rohacell lip and into the groove.

18\) Use the razor blade to firmly adhere the foil to the inside surface
of the groove.

19\) Repeat Steps #11 through #18 for the other flat edge\'s excess
foil.

#### • Adhere the radiation shielding to the inner surface of the Rohacell

20\) Repeat Steps #1 through #4 to lay down the foil and adhesive that
will be adhered onto the smaller radius surface of the Rohacell.

21\) Cut the masking tape off of both long sides and one of the short
sides of the foil.

22\) Adhere a strip of adhesive backing over approximately 1/2" of the
adhesive on the shorter untaped foil edge.

23\) Align and adhere the edge of the Rohacell with the edge of the
adhesive backing so that the adhesive can be rolled onto the smaller
radius surface.

24\) With your partner holding the Rohacell upright, use the rolling bar
to roll the foil and adhesive onto the first few inches of the smaller
radius surface of the Rohacell. Keep the foil tight while rolling.

25\) Untape the last edge of the foil, have your partner hold the loose
foil up and off of the cardboard, turn the Rohacell so that the bare
surface is facing you, and have your partner climb up onto the table for
more ease in holding up the foil.

26\) Continue rolling the foil onto the Rohacell until you have rolled
it past the other grooved edge. Have your partner keep the foil taut as
it is rolled on, and put tension on the foil along the length of the
rolling bar to minimize wrinkles.

27\) Adhere a straight-edged strip of adhesive backing to the exposed
adhesive at the grooved edge of the Rohacell to keep it from adhering to
the larger radius Rohacell surface or the cardboard.

28\) Set up the thermoforming fixture with the large rubber sheet on
top, and place the shielded Rohacell on the rubber sheet.

29\) Use a sharp razor blade to cut the excess foil off of the two
curved edges of the Rohacell. Put pressure on the Rohacell close to the
area being cut off to make a clean, straight cut.

30\) Repeat Steps #14 through #18 to adhere the shielding to the insides
of the grooves, but keep the Rohacell on the thermoforming fixture to
cut the foil on the two straight edges to the correct 1/8" width.

31\) Use clean, white gloves or a clean, soft cloth to smooth the foil
out on the Rohacell and remove any fingerprints.

#### • Attach the seal strips

32\) If you are satisfied with the way the shielding rolled onto the
Rohacell (satisfied with the amount of wrinkling), then mix up a small
(approximately 1oz.) amount of the conductive epoxy.

33\) With a cotton swab, spread a very thin coat of the epoxy on the
flat surfaces of one inner and one outer seal strip that will touch each
other when assembled.

34\) Attach the seal strips together, clamp them, and let them dry. Make
sure to wipe off any excess epoxy.

35\) Mix up more of the conductive epoxy.

36\) Put the epoxy on the lower surfaces of the seal strips.

37\) Push the seal strips into the groove in the Rohacell. Start by
pressing one end of the seal strip into the groove and then move down
the length of the strips pushing the rest in. Be sure that the seal
strip with the O-ring groove that penetrates through the side walls of
the strip is nearest the axis of the hemi-cylinder with the cut outs
pointing towards the axis.

38\) Clamp the seal strips in the correct place and let the epoxy dry.
Wipe off any excess epoxy.

39\) Repeat Steps #32 through #38 for the other seal strip groove.

##  {#section-2 .unnumbered}

## V. Integrating the Outer Enclosure with the MVD

### V. A. Introduction to integration

The integration of the Outer Enclosure with the rest of the MVD is one
of the last MVD assembly steps. Only the Inner Enclosure is installed
later in the process. Consequently, the integration procedure must be
very safe and reliable so as to insure non-interference with the MVD
electronics.

Fortunately, the integration of the Outer Enclosure with the rest of the
MVD is fairly simple and straightforward. The procedure involves four
steps: firmly securing the MVD to protect against bumps, seating the
enclosure in the pocket formed by the MVD's aluminum frame (the D-rings
and struts), aligning the O-ring grooves in the seal strips with the
O-ring grooves in the D-rings, and then screwing the outer enclosure
retaining rings into place.

### V. B. Materials needed

· ten 1/4" long aluminum 4-40 flat head screws

· a small flathead screwdriver

· two outer retaining rings (see Appendix E)

· an assembled Outer RF Enclosure

· the holding fixture (see Appendix F)

### V. C. Integration procedure

1\) Secure the MVD in the holding fixture.

2\) Place the assembled Outer Enclosure on top of the inner set of
D-rings and in between the outer set of D-rings.

3\) Align one of the seal strip edges so that the O-ring grooves in the
seal strips run smoothly into the O-ring grooves in the D-rings.

4\) Screw two flat head 4-40 screws through one of the retaining rings
and into the two tapped holes in the D-ring closest to the aligned edge.

5\) Repeat Step #4 for the other retaining ring holes closest to the
aligned edge.

6\) Make sure that the retaining ring makes good contact with the
radiation shield. If the rings cause a little compression of the
Rohacell, then good contact is insured. If the Rohacell is not held
firmly in place, then put a strip(s) of indium wire in between the loose
retaining ring(s) and foil and repeat Steps #3 through #5.

7\) Make sure that the O-ring grooves in the other seal strip edge of
the Rohacell match up with their corresponding O-ring grooves in the
D-rings.

8\) Screw in the rest of the screws starting with the ones closest to
the seal strips and moving towards the middle of the Rohacell.

## Appendix A: The Outer Enclosure Thermoforming Fixture and Machining Attachments

## Appendix B: The Endplate's D-rings

⇑ The Inner D-ring ⇑

⇑ The Outer D-ring ⇑

## Appendix C: The Outer Enclosure Machining Fixture

## Appendix D: The Outer Enclosure Seal Strips

⇑ The Inner Seal Strip ⇑

⇑ The Outer Seal Strip ⇑

## Appendix E: The Outer Enclosure Retaining Rings

⇑ The Outer Enclosure Retaining Ring ⇑

Appendix F: The MVD Holding Fixture accomodating the integration of the
Outer Enclosure