MAS.863 How To Make (Almost) Anything – Fall 2014

Richard Li

Composites Manufacturing

Week 8

Filamentary composites are fascinating materials that take advantage of the synergy of the constituent materials. As a common example, carbon fiber reinforced plastics (CFRP) are made of carbon fibers glued in place within an epoxy matrix. Carbon fibers are highly engineered fibers that have high mass specific strength and stiffness while epoxy matrices provide the material to carry shear and allow effective transfer of stresses between fibers. Through this interaction, very lightweight high performance structures can be fabricated as is typically desired in aerospace, racecars, and sporting goods. But beyond having impressive mechanical properties, CFRP are also making an entry into the music industry. As an example, carbon fiber cellos are prized instruments that emanate loud and full sounds. More importantly, CFRP is more durable to the elements (corrosion) and have high fracture toughness – thus rendering the instrument more resistant to drops or crack initiation and propagation.

I’ve been itching for a CFRP instrument for some time, and my grad stipend only goes so far in buying one. Owning and playing a guitar already, I wanted to add another one to my collection: the ukulele.

The ukulele is a great example of an application for CFRP. As a small instrument that is meant to be portable and played outside on a Hawaiian beach, a lightweight and environmentally resistant material would be highly desired. Additionally, as a first initial shot at making a more complex CFRP geometry, this would serve as a great starting point for experimenting with the correct processing in making CFRP instruments before scaling up to larger ones such as a guitar! It’s a long and ambitious journey for a week, but the composite fabrication part will be documented here at the very least.

Designing the Ukulele and Mold

Designing a ukulele for CFRP is not as straightforward as replicating geometries of a traditional wood Ukulele. Considerations on drapability (which will be discussed later), seams, and final edges will have to be considered carefully to ensure a good looking part. In this case, I started off with the KALA Soprano Ukulele and modified the geometry to incorporate smoother edges and rounded corners. One of the biggest dilemma I had in making this was deciding between using one large positive foam mold on which I could lay up my fibers, and then use the foam to my advantage as a sandwich structure core that will help carry shear. However, after much discussion with David Constanza (complex geometry composite layup and infusion master), it seemed that the best approach would still be using two negative molds in order to achieve better finishing and have easy mold reusability. Having a positive mold would result in a large seam where the fibers have to overlap. Thus a negative mold was designed so the instrument assembled at the front edges as is typically done for stringed instruments already. However, the ambitious part is to make the entire body/bottom half as one single piece.

With the part finally drawn in SolidWorks, I discovered Mold Tools, which are incredibly helpful in designing the parting surfaces and automatically generates the entire surface that can be imported into MasterCAM for toolpath generation. A great tutorial on that can be found here:

http://grabcad.com/questions/tutorial-solidworks-mold-tools

The G-code generated for this via MasterCAM used a ½” diameter ball endmill for both a rough and finish pass in order to achieve the smooth and “drapable” corners. More detailed tips on programming in MASTERCAM can be found in the pages from previous weeks.

Making the Mold

The stock material was prepared for two 2” thick foam sheets by putting Gorilla Glue on one side and smearing it evenly. Water was sprayed on both surfaces afterwards.

The two surfaces were clamped together overnight to ensure good adhesion

Stock was placed at the origin on the ONSRUD router, and g-code was imported to start the cuts.

Afterward the finishing pass, the mold looks ok, but still has a lot of fuzzy on the surface. Sandpaper did not help much since the material was very soft.

Entropy SuperSap 100 Glue was then used to laminate and fill in the surface for 4 hours before more sanding was done again. Finally, generous portions of Paste Wax was applied to all the mold surfaces.

Making the Body

Carbon fibers sheets were first cut to size using serrated fabric scissors to prevent fiber slip. Care was taken to prevent inhalation and skin irritation using masks and gloves.

Vacuum assisted resin infusion (VARI) was desired as the resin application method because of the high quality and low void fractions that could resin compared to wet layup (which may entrap air pockets as wet fabric are placed). However, when the dry carbon fiber weaves are placed within mold, there were drapability issues that prevented the fabric from sitting nicely on the sharper corners even with the help of blue flash tape.

As a result, it was decided at the last minute to switch to a wet layup method as wetter fabric allows for better draping and conformation of fabric to mold. Entropy CCR Epoxy/Hardener was mixed as that provided a nice clear matrix, and the longer 1-hour work time needed to apply resin to all the plies. Shear mixing was done in cups and wooden sticks slowly to prevent void formation.

Resin was then applied onto the weaves and squeezed on both the front the front and back sides.

A marked improvement in drapbility was noticed for the wet plies. 5 were placed inside the mold before being covered with porous peel ply (the red sheet) and the bleeder breather material (which wicks up excess resin, and also allows for air transport in the vacuum).

Here is the bleeder breather tucked in:

The entire assembly was then vacuum bagged, and the sides of the mold were held down as the vacuum evacuated the entire bagging:

Soon, all the excess resin was wicked up to the bag surface, and creases in the bagging was given to allow for conformal normal pressure on the layers.

As this utilized the clear epoxy, its cure time is significantly longer (24 hours) compared to the 4 hours of the Entropy Super Sap 100. However, the conformality looks decent overall from visual inspection.

Making the Soundboard, Neck, and Headstock Front Panels

The parts facing the front are to be waterjet out of a flat CFRP laminate. As a result, a wet layup using the same methodology was applied onto a flat metal plate. Note that metal plate was cleaned thoroughly, and the vacuum tape was applied directly to the top surface of the plate. The interior was the rubbed with paste wax again prior to a lay-up of 6 wet carbon fiber plies, and then the peel ply on top.

Bleeder breather was trimmed and placed on top:

And finally vacuum bagging was stuck onto the vacuum tape, sealing the whole lay-up between the bag and the metal plate.

Resin wicked up to the surface quickly once again, and the assembly is left to cure for 24 hours.

Next Steps

Coming soon - the remaining steps are to:

1. Carefully remove the flat laminate and the body from the molds.

2. Waterjet out a slightly oversized soundboard, neck, and headstock front panel out of the flat laminate.

3. Fill the inside of the neck, heel, and headstock with expandable polyurethane foam such as this: http://www.uscomposites.com/foam.html

4. Use a diamond saw to trim down the excess from the bottom so that it has a nice flat rims that can come up and meet the soundboard, neck, and headstocks for gluing.

5. Line the inside of the rims with the kerfed lining for musical instruments such as this: http://www.stewmac.com/Materials_and_Supplies/Bodies_and_Necks_and_Wood/Mandolin/Kerfed_Lining_For_Mandolin.html

6. Attach small bracing/stiffeners on the inside of the body and under the soundboard.

7. Epoxy the front and bottom halves of the ukulele together.

8. Sand off the excess from front halves until the edges are flush with the sides of the instrument.

9. Coat all surfaces with filler, epoxy, and clearcoat until all the surfaces are glossy and ready.

10. Use a diamond grit core drill to make holes into the headstock for the tension adjusters, and then screw in the components: http://www.amazon.com/Generic-Ukulele-Chrome-Machine-Mounting/dp/B00EQ29KNG/ref=sr_1_3?ie=UTF8&qid=1414970338&sr=8-3&keywords=ukulele+parts

11. Glue on the fretboard, bridge, and string the instrument.

12. Tune the strings and jam on!