Playing with Composites: A laptop tray with embedded mousepad
The idea for this project came from a lack of being able to throw away a mousepad I created a year ago out of floppy disks. With my standing desk coming in soon, I have a mousepad I am emotionally attached to that I can't get rid of. Also, with the cold weather lately, I have found myself working on my bed more often, and have run into a problem where I can use my mousepad while in the comfort of my covers. Thus, this week I will create a laptop tray that can hold my laptop, has my floppy disk mousepad embedded, and has a slot for my phone because why not.
For the composite, I first had to create a mold. I decided on a one sided composite because a two sided, compression composite would interfere with placing my mousepad. I had to place the mousepad on top of the fibers of burlap, and then cover it with resin in order for the floppy disks to show. Also, when designing the tray in Solidworks and Rhino, I ended up including a circle for keeping a drink in place. I placed it under where the computer would sit, so the tray would have a dual function as a dinner tray. The design is pretty simple, and since I machined the mold out of blue foam, I used the same endmill for all pockets and traces, and only did one pass, a finish pass.
The blue foam had many small frays, even after using a shop vac and shop air to clear the crevices. To smooth it out, I used a heat gun to melt the blue foam. According to how we were taught to build composites, after milling the mold on blue foam, we were supposed to glue to blue foam to a piece of wood to prevent the blue foam from deforming when in a vacuum, then wait 4 hours for gorilla glue to fully dry. After that, you add a layer of epoxy to the top of the blue foam to seal it, and then wait another 4 hours. Ugh.
Instead, I used hot glue to attach a porous-ish board to the bottom of my mold. This took about 10 minutes. I then used a heat gun to melt the outside layer of the blue foam. Do this is a fume hood! Burnt blue foam smells yo. I then added wax to the entire surface to ensure it was sealed. Finally, I sprayed some epoxy-off to ensure the epoxy would not stick to the blue foam when cured.
Also, I cut out 6 pieces of burlap to drape my mold. I used 6 pieces because that was equivalent to about 1/8 of an inch and that was the tolerance I had set to my pockets to ensure my mousepad and phone would fit on my tray. Also, I used a cotton sheet as a breather and absorber. Finally, I used a plastic sheet with holes to ensure my excess epoxy had a way to escape when the vacuum was applied.
I used a two part epoxy mix that consisted of two parts resin and one part hardner by volume. I mixed about 4 portions given the size of my mold. This way, I didn't have to make more in the 15 minute set time. I placed all six of my burlap layers on a table on top of each other, and after mixing the epoxy in a paper cup and stirring it for two minutes, I poured it onto the burlap. I then used a plastic tool to spread the epoxy evenly over the burlap sheet. Each sheet was placed on top of the mold and stacked until all six layers were on top.
On top of all these layers, I placed the plastic sheet with holes, and on top of that sheet, I placed the cotton sheet. I then stuck that entire configuration (including the mold) into a vacuum bag. I used an essentially large ziploc bag. They are the bags you see in infomercials that save space in your closet by removing all the air from your pillows. Anyways, this bag had a fitting made up of two silicone disks sandwiching the bag and being held together by two nuts. The only reason I used this fitting and not the plastic fitting provided was to connect a strong vacuum pump rather than a shop vac.
Between the vacuum bag and the vacuum pump was a resin tank. This prevented the epoxy from moving up the vacuum tube and into the vacuum pump by providing a bucket for the epoxy to fall into before accidentally getting into the pump. This the final measure to keep epoxy from getting into the pump, the first being placing the pump on the cotton sheet away (but still connected) to the mold.
Once connected, I was not able to achieve the -20 inches of Hg that I wanted (It was at about -5). Upon inspecting the bag, I found a two holes to the sides of the vacuum fitting on the bag. I used black mastic tape to try and seal around the fitting. The changed my pressure to about -10 inches of Hg. What I decided to do instead was add another cover over the fitting. I created a mastic tape triangle around the fitting, and used seran wrap to cover the entire fitting. The mastic tape created a seal between the seran wrap and the plastic bag. Although a very ghetto approach, it worked! And brought my pressure to -20 inches of Hg.
It should be stated that, although this worked, the better method would be to remove the fitting, fix the hole, and install the fitting again. Or, to replace the bag completely. However, since I noticed this hole after placing my part (with epoxy) in the bag, I needed to fix it quickly before the epoxy began hardening, and so I MacGyver-ed the situation!
I came back to the vacuum bag 4 hours later, and the epoxy resin was still wet and maleable. I am not sure why. The cure time was supposed to be 4 hours. Perhaps my mix ratio of resin and hardener was off. Either way, since I checked at 3 am and it was still wet, and no one was coming into lab for a while, I kept the vacuum on a few hours longer. I removed my composite at 11 am, meaning I left it in the vacuum bag at -20 in Hg for 12 hours. At this point, the cotton layer and plastic layer pealed right off.
Finally, it was time to embed my mousepad into my tray. I started by hot gluing the floppy disks into the pocket of the tray. I also added hot blue "dams" to the outside profile of the tray. Since I need to create a pool of epoxy for the floppy disks to be fully submerged, I closed off the profile ends with hot glue so as not to use too much epoxy. Finally, I poured the clear CCR Resin and Hardener mix into the pocket of my composite, smoothed out some bubbles that were created from all the holes in the floppy disks, and placed the composite in the fume hood. Since I am using clear resin this time, the cure time is 24 hours.
I came back 24 hours later, and unfortunately, the clear epoxy has poured over into where my phone stand was. It completely filled it on. No more phone stand =(. The epoxy did cure pretty evenly though. I realized I should have used a level to ensure the fume hood was level, and should have make a deeper pocket for the floppy disks. You live and you learn.
Finally, I ended up useing a bandsaw to cut the edges off of my tray. The blue foam was fairly simple to remove once I used a spatula to separate the edges. The whole thing came off in one piece. However, I had a lot of small pieces of blue foam attached to my tray. I tried scraping it off, but there is astill a couple of pieces that I cannot seem to get off.
I by that I mean actually using the tray. I was very satisfied with the space on the tray, and am now able to chill on my hammock, while working on my laptop with a mouse =). Success!
I also rotated the tray 180 degrees and tried using it to eat a healthy lunch. Also, success!