Composites Fabrication -- My Shameless Plug

I really really really like composites. Fiberglass, carbon fiber, Kevlar...you name it. I've been a big fan since I first found out that composites were used in exotic supercars with rockin' performance--which means I've been interested in them since I was a kid. I've always loved concept cars (since they're the whole reason to go to the big yearly car shows, right?), but was always upset that the mass-manufactured models of those cars never looked they way they did on the rotating platforms at the show. When I was in high school, I figured that the only way I'd ever get a hold of a "concept" car of my own was to design and build one myself. And that meant I'd have to learn how to build car bodies (and other mechanical, electrical, etc. systems, of course) to do that. After learning about what it takes to make composite bodies, I realized it was within my capabilities as a designer/craftsman, so I set out on a journey to learn, build, and, ultimately, ride my composite creations. It wasn't until I was in college (the first time around), though, that I had any chance to play around with composites, design for them, and really optimize them. And it was at that time that I began to see the possibilities of what could actually be done with composites.
To make a long story short, I've been amassing a lot of information over the past couple decades on how to work with composites. In fact, I've had so much info bumpin' around in my noggin that I decided to write some books on the subject. In academia, we have to do all kinds of publishing to make us look worthwhile to the administration (since the stellar quality of work that comes from our students doesn't look like much on a curriculum vitae, or calculate well into the numbers that the top-of-the-heap bean-counters get all frothy about), so a couple years ago I opted to write a book on composites and share my knowledge with others. It came out in June of 2009 and was titled Composite Materials Fabrication Handbook #1. I aimed the content at those hands-on folks out there who wanted a graphics heavy how-to book with simple projects that they could make in their own cozy workspace. The book has a somewhat basic approach to composites so that even the most apprehensive novice could have some place to start. But, it also contains enough technical "meat" for students to find it worth their while to study from. For that reason, I use it in the composites classes that I teach every summer.
My second book, Composite Materials Fabrication Handbook #2, just came out this week! It's (of course, as stated in the title) the second in a series of composites books that shows more advanced techniques helpful to building composites for high-performance use. I've found that there really aren't that many books out there that discuss (let alone show) some of the ins-and-outs of mold design, moldmaking, compression molding, vacuum-bagging, expandable inserts, and vacuum-assisted resin transfer (VARTM) processes, so I had to throw those into this second book. I think it turned out great!

Having been through the laborious process of writing these two how-to books, I have an even greater appreciation for all those other authors out there who aim to truly educate readers on topics of substance rather than just entertain them with cutesy fictional stories (although such books have their place, I know). I just hope that there are lots of people out there who can benefit from these books...and who can find their creativity opened up to these non-traditional materials. They really are within the reach of the average builder. And once you get working with them, you may just get hooked, like I did.
So, check out my books!
That's my shameless plug for the day.
Enjoy.  

Building a Custom USB Flash Drive -- Part 2: Resin Casting in a Silicone Mold

So, you've got a silicone mold all ready to go?...now you just need to make something in it. This post shows some of the steps to pouring urethane casting resin into a silicone mold. Other types of resin (like epoxy) could be cast into a silicone mold, but some resins cannot. For example, polyester casting resin won't set up well in a silicone mold because the silicone actually inhibits the chemical crosslinking (and hence, curing) of the polyester resin. If you want to cast polyester, you're better off making a urethane rubber mold rather than a silicone one (but you'll also have to use mold release as a surface treatment before pouring the material into the mold since urethane isn't self-releasing like silicone is). Part of my reasoning for using a paper cup as my mold form (aside from the simplicity and low cost of them) was that I could re-insert the mold into a cup of the same size to seal it off before pouring resin into it (as shown below). Without any seal formed around the mold's parting line, liquid resin poured into the mold will just run right out of it. For custom or irregular-shaped molds, it's good to either place the mold back into the original mold forms, or to otherwise seal off the mold with packing tape. If you're using the latter, wrap the whole mold so the tape can stick to itself since it won't stick to the silicone mold or otherwise make an effective seal.

Before going hog-wild with encapsulating my fancy-shmancy USB drive in the resin, I figured it was wise to do a test pour with the resin first--just to make sure there were no issues with the mold. I always prefer to do a test part before assuming that things will work our perfectly because there's always so much that could go wrong, and so little working capital to burn on faulty projects. So, to make my test piece, I measured out equal parts of casting resin (since the Alumilite requires a 1:1 mix ratio by volume), and poured them into a cup...

...and then I mixed them thoroughly. As with the silicone resin, I made sure to completely mix this thermosetting resin to avoid soft spots or areas of uncured resin in the final cast part.

Next, I poured the resin in a thin stream through the sprue into the mold. If the sprue isn't wide enough, the surface tension of the resin can actually bridge across the sprue and stop it up so resin won't pour in smoothly. For safety sake, it's good to make sure there's at least 3/8" diameter or more space down the length of the sprue.

Once the cavity of the mold was filled, I let the resin sit for about ten minutes (the recommended cure time for the resin) and double-checked that the residue spilled around the sprue was completely hardened. Thin sections of excess resin like these are usually a good indicator of the state of resin cure within the mold. I then peeled off the cup...

...pried open the mold, and extracted the cast test piece. Aside from the a little bit of "flash" (excess material that formed within the parting line) and a big plug o' material at the sprue that needed to be trimmed off, the piece came out rockin' sweet.

Next, I needed to prepare the USB drive for encapsulation in resin. This required a couple extra steps to keep the casting resin from seeping into the connector end of the drive and filling it up. Resin that gets into the connector end can actually make it impossible to plug the USB drive into a computer in the future--a very bad thing, indeed. If you inspect the back side (or circuit board side) of the connector, you'll notice that there are some very small openings around the wires that attach the connector to the circuit board. These holes can allow resin to leak into the connector, so I used a quick bead of super glue to seal them off quickly and effectively.

Just to make sure there weren't any other areas open for resin leakage, I used some packing tape to seal off the rest of the connector end of the drive. Urethane resin won't stick to the packing tape, so this tape helps promote good clean up of the final part as well.

After sealing up the connector end of the drive, I inserted it into the silicone mold, right where the dummy connector had formed a cavity in the bottom of the mold. This cavity created a good registry for the USB drive so it would fit properly in the mold.

I closed up the mold by putting it into yet another paper cup, mixed up some resin, and poured it into the mold through the sprue.

After waiting long enough for complete cure of the resin, I pulled the newly formed part out of the mold with the USB drive encapsulated in it.

A little bit of trimming with a utility knife was all that was needed to clean up the part. I cut off the sprue plug, cut out the knock-out center (for the finger hole), and removed the tape from around the connector. To get the part prepped for final painting, I sanded the surface of the urethane with 400-grit sand paper, just to ensure that there weren't any chemical residues on the part that could foul-up the paint job.

After a quick paint job, here's what the final part looks like (below)...super-groovy and ready to store up all that important data that I've got kickin' around.

While this drive-forming demonstration shows a somewhat simplistic part, the flexibility of a silicone mold and the robustness of the urethane casting resin can help you build a wide variety of nifty shapes and parts. And since these molds will work for over a hundred castings (if you play nice with them), you could even make bucket-loads of these little gems for whatever purposes you deem necessary. Aren't plastics fun?  I'm a big fan. Have fun, ya'll.


   

Building a Custom USB Flash Drive -- Part 1: RTV Silicone Moldmaking

Just about everybody’s got a USB drive these days, but so few of those drives has any kind of “cool” factor to it. Why? Because they’re made to appeal to the wide middle-ground of consumers rather than to the quirky, one-of-a-kind demands of unique users—the design equivalent of campaign managers aiming their candidate’s political ads at those numerous, undecided moderate voters rather than the smaller, polarized pockets of right or left wingers. So, what’s a person to do if they don’t want a USB flash storage device that looks uber-boring--like everyone else’s? Well, you could spend inordinate amounts of time scouring the internet for some obscure manufacturer out there making something that fits your particular style, or…you could just make your own (if you know how). And that is the exact point of the next couple of blogs: to show how to 1) make a mold using a model and silicone resin, and 2) to cast urethane (or other) resin around a USB flash drive to make a truly unique data storage device.
One of the classes I teach here at Metro State (IND 1130 – Plastics: Materials and Fabrication) focuses on different methods for using plastics in industry. We discuss several plastics forming methods used in mass-manufacturing and the design constraints involved with each of them. But since we don’t have the time or money to let students design their own real-world parts using injection molding, blow molding, or extrusion processes (not to mention a boatload of others), we show them how to use smaller-scaled forming and fabrication methods found in a typical industrial design prototyping shop. In a hands-on way, these methods still help students understand the overarching considerations needed to design for thermoplastics and thermosets, mold a wide range of shapes, and develop a sensitive eye for quality design through ideation, development, and craftsmanship within a shop setting. One of the projects I use to help my students understand thermoset resin forming methods is to have them create a cast product with an embedded component. In the case of this semester, I'm having them design and construct a functional USB flash drive, using the innards from an existing drive with a new design cast in resin around it.
As an example for my students (and for this blog posting), I've designed a simple, unique drive that has a big hole in it for my finger. This allows the drive to be easily grabbed (or yanked) out of the computer with my big digits. The painted, ready-to-rock drive is shown here: 

To get started in building this custom USB flash drive, I took an existing drive and pried it open with an X-Acto blade (see below). Most of these units are just snap-fit together, so they're easy to take apart once you get into the seam with a blade edge.

The next step was to extract the circuit board. This isn't too difficult since these drives typically have the board sandwiched between the two snap-fit plastic case halves without any glue. Once the halves are apart, the board comes right out. Note: It's a good practice to "ground" yourself before handling any circuit board directly--which can be done by touching a metal water pipe, door knob, or other static-draining object (such as your friend's nose) to rid your person of those damaging stray electrons.

Next, to make sure I designed the custom drive to the right size, I took some good measurements with the ol' calipers. This helped me to be sure that I had enough space on each side of my design to keep the board from popping through the casting resin around the sides of the completed part.

I also traced the outline of the board just to give me an actual-size sketch that I could use as an underlay for developing sketches. Underlays are great since they help keep you honest with your proportions and size--something I'm sure I've mentioned in a previous posting.

After creating a few sketches, I chose my favorite design and then traced it onto some Renshape (a high-density urethane foam for sculpting and prototyping) to create the "pattern" (or original shape around which the mold is formed). Patterns for RTV silicone moldmaking can be fabricated from a wide variety of non-porous materials. Such materials include wood (as long as it's the closed-cell variety), clay (firm plasticene works best), plastic, metal...even cheese, if you're so inclined. RTV silicone doesn't stick to very many materials, especially if they have smooth surfaces and lack any significant porosity.

Silicone will precisely copy the surface quality of whatever it's molded around (down to a microscopic level), so I make sure to use good craftsmanship in finishing the surfaces of any pattern I make for a silicone mold. With the Renshape material, 220-grit sandpaper gets the surface close enough to the quality I need, especially considering that I'll have to fine-sand the surfaces of any cast parts made in the mold prior to final painting.

Next, I put a small plug of plastic (cut from the end of a rod of 3/4" diameter acrylic rod) in the center of the finger-hole in the pattern. This helped to keep the RTV silicone from flowing between the two sides of the pattern and locking it into the mold. Open shapes like this take some special attention to keep them from becoming one with the mold.

For the connector end of the pattern, I cut a piece of plastic that closely matched the USB's connector to act as a dummy piece to mold around. I cut this piece at least long enough to adequately form space in the mold for the actual USB circuit board that will be encapsulated in resin.  

Next, I used a little dab of hot-glue to attach the USB connector dummy piece on one end of the pattern, and then added a small wooden dowel to create a pattern for the "sprue". (The sprue is the hole through which the resin was poured into mold cavity.)  

I then took the entire pattern assembly and hot-glued it to the bottom of a paper cup. This cup acted as a mold form into which the silicone could be poured without worry of spillage. Paper cups make a great, quick mold form...as long as you can find ones that are the right size. For long, narrow objects, cardboard paper towel rolls (or toilet paper rolls) make a great mold form. Otherwise, small Tupperware containers can do the trick, too. If you can't find something that is the right size for your RTV mold form, you can fabricate one using cardboard, foam core, or other stiff material, as long as you seal it completely so the silicone won't ooze out of any cracks or holes. It's never good when silicone oozes out of a hole your form.

Next, I marked a line along the edge of the cup right along the edge of the pattern. This marked the place where I would cut the mold so I could remove the pattern and any other pieces I cast in the mold in the future.    

Since everything was now set with the pattern and the mold form, I could move forward with the pouring of the mold. The RTV silicone resin I used for the mold, along with the urethane resin ("Alumalite" brand casting resin), was purchased from a local vendor, Plasticare, here in Denver. They're good guys who've always been helpful with product and application info and give us good deals on materials.

As a thermoset resin that requires precise chemical ratios for good setup, silicone is best measured with a scale. We just use a simple digital scale in our shop (as shown below). It's always good to make sure you've measured your resin exactly as prescribed by the manufacturer. Tweaks in a mix ratio should be reserved for adventurous cooks in a kitchen--not in a plastics lab. I've seen too many projects ruined by short-cutting students who figured they had a "good enough" eye to guesstimate the proper mixing proportions. I don't like to take chances with silicone because it's a spendy chemical. You should also notice that I've got gloves on (and an unseen coverall for my clothes)--these chemicals don't come out of clothes very easily, and can often cause skin irritation or allergic reactions in some users. Once they've set up, they're actually pretty inert, but as individual components, they can be messy and somewhat hazardous.

Next, I made sure to mix up the resin completely. If the resin is only partially mixed, it will have soft spots in it that can ruin the integrity or usability of the mold. For this reason, I always use clear plastic cups (preferably ones made of PETE plastic--for its chemical resistant properties) so I can tell if I've completely mixed up the material on the sides and bottom of the mixing cup.

I really like to make silicone molds that have as few pores as possible because such molds make better, imperfection-free parts. Unfortunately, stirring up all that silicone resin puts a lot of air into the mix. That air forms bubbles, which form lumps, bumps, and nuggets on the surface of any parts made in the mold. So, to get rid of the air in the resin, I like to use a vacuum-chamber, like this one from our shop (shown below). Please note that this piece of equipment isn't necessary, it's just helps produce superior results. I've seen plenty of usable molds made without a vacuum. But I've seen even nicer molds made with a vacuum. (In the prototyping world, a vacuum-chamber is typically used for molds that are going to be used by a client themselves, or where a client wants very good surface quality on the final cast parts made from that mold.)

The vacuum-chamber pulls a vacuum of about 26" Hg (which is pretty good considering our elevation in the Rocky Mountains) and allows any air in the resin to expand and rise out of the liquid. I put the mixed resin onto the platform in the vacuum-chamber...

...put the cover on...

...and then flipped the switch on the vacuum pump. In less than a minute, the chamber was completely evacuated, and the bubbles within the resin started to form and brew up like a good root beer float:

Once the bubbles maxxed-out, the resin began to sink (as shown below) and the residual bubbles kept rising in the liquid until all the air was gone.

After all the bubbles had expanded out and the surface of the resin was smooth, I shut off the vacuum pump, removed the cup of resin, and poured it into the mold form (my paper cup) around the new USB drive pattern. Any bubbles that are introduced into resin by pouring it into a mold form quickly rise out of the resin because they tend to be rather large compared with the bubbles created when mixing the resin.

After pouring the resin into the form, I let it completely cure overnight. Depending on the type of silicone, silicone may cure within a few minutes to a couple of days, so I just follow the manufacturer's recommendations for the cure time with whatever resin I'm using.

Once the resin was fully set up, I cut the cup along the parting line that I'd drawn earlier. I made sure to cut into the silicone at least deep enough to be able to find my parting line again after I'd peeled back the cup.

With the cup removed, I used the knife to carefully cut into the silicone until I reached the edge of the pattern. This part can be kind of tricky if you've got a pattern with all kinds of crazy shaping to it. Regardless, it's important to cut through the silicone as cleanly and thoroughly as possible to get the pattern out of the mold.

Once it was completely cut, I opened up the mold...

...and extracted the pattern. I checked the mold surfaces for any imperfections or tweaks, and then declared it "good". The next step was to seal the mold back up and make a test casting in it!

In my next post, I'll explain the process of pouring urethane resin into the mold to create new cast parts. I'll also show how to prepare the USB drive prior to putting it into the mold for encapsulation with resin.
Peace out, my friends.