Introduction: [Video] Large Scale Mold Making - Making the Negative "Tool"

About: Engineer making renewable energy products for African entrepreneurs.

Making tools from the tools you already own... This almost sounds incestuous - almost.

This is the next phase of our fairing construction for the University of Central Florida's 2007 ASME Human Powered Vehicle. Here we will take a positive male mold and begin making a female negative "tool." We call it a tool because it can be used to make many duplicate copies. This tool is in fact a negative female mold.

To reiterate, this whole project is daunting. However, each individual step is not too scary and within the capability of most people handy with a few simple power tools. On the other hand, the price tag is scary. For this half alone, we used 5 gallons of polyester resin (circa $130).

21 minute Video - Lots of Time Lapse (now with captioned instructions)

Video Large Scale Mold Making - Part 1
Video Mixing Polyester Resin
Video - Part Making

Step 1: Making a Stand for Casting

Your mold will be different - but here's the idea.

We want to make 2 female tools - one for each side. To do this, we must split the mold lengthwise (to allow easy mold release). So, the stand needs to hold the entire plug on it's side to present a "waterline" right down it's center. Positioning is critical as we don't want to make a mold that curves around itself more than 90 degrees (making removal impossible).

This stand is constructed using recycled 2X6's as a base and with 2x6 risers. The risers support an MDF board that will contain the plug cut out (recycled is better - these boards are expensive for what they are). The risers are a little more than 1/2 the width of the whole plug (that is, 1/2 the height of the plug when it's on it's side).

To make the cutout - place the plug on top of the stand. Then, use a square positioned against the plug and MDF board - mark the base position with a pencil. Repeat this over the entire plug without moving the plug. Once this is done, drill a hole and cut out with a jig saw.

Once you have your hole -- sand the edges down with a block so it won't be too rough on your plug - this is important for bondo application later. Test fit. Shave down. Test Fit and repeat until you have a good fit. You don't have to be 100% accurate - but you want a waterline that is flush with your MDF surface.

To make a cheap sanding block - use a scrap bit of wood and glue on sand paper - ha! you saved $5 on a sanding block that would likely just break after a few hours of use.

Step 2: Positioning and Drawing a Waterline

Now that you have a good fit in your well sanded stand - adjust your plug until you have an excellent waterline. If there were an order of critical steps - this is the second most critical. You can use towels and bits of foam etc. as shims.

Once you have a satisfactory waterline, mark it with a pencil. This way, the second half will be perfectly aligned too.

I know you won't like this... But... Remove the plug and apply a paste wax to one the side to be molded. It is VERY important that you do this outside of the stand - we want bondo to bond to the stand, but no the mold. So at this time, do not wax the stand. Please, don't cut corners here - it will be worth it in the end.

Replace the plug in the stand, shimming to achieve the correct waterline.

Step 3: Bondo Waterline

Now it's time for bondo!

Apply bondo at the mating line where the plug intersects the MDF. We want to fill in this gap completely. Like all bondo work, apply liberally. Remove the fairing and then sand to get a flat plane with respect to the stand top.

Having a nice rounded edge in your stand allows the bondo to hold tighter due to a higher surface area - see, that little extra works payed off.

Step 4: Apply Paste Wax

Apply Paste Wax

Wax on - wax off. Wrong

Almost right, but we're not looking for a glossy finish. The goal here is to fill in any bumps or trouble areas that may be leftover after bondo work on the plug (time constraints won't let you make a "perfect plug"). So, apply a thin coat - working it in well. Then repeat with a clean rag - repeat this process 3-4 times or until you have a nice fill always using clean rags.

Step 5: PVA - What Is It, Why You Want It, How to Apply It

PVA - Polyvinyl Alcohol

This is a mold maker's best friend. PVA, when dry, creates a brittle film substrate that will tear away from the paste wax allowing easy tool removal. Our resins will not stick to it - in fact, most of nothing will stick to it and other than the alcohol solvent - it's completely inert (unlike MEK-P; more on that later).

Why you want it
As said above, it allows the mold to release very easily. The resin won't stick to it - the wax won't stick to it. This allows the mold to just peel off with very little effort (hopefully).

How to Apply It
The preferred method is via HVLP spray gun. So here's some technique:
1. Apply several thin coats
2. Apply to the MDF around the seam edge (so we can make a flange)
3. Apply thin coats
4. Allow each coat to dry

Yes, thin coats. PVA will not orange peel so much as it will run. Vertical faces are more likely to run too. Runs will show up in your tool which will translate into your final product. Once you have a significant run - the old solution is to remove the PVA once it dries and start over by re-waxing.

Alternate Methods
I knew other people would be reading this, and I know not everyone has an HVLP setup (in fact, I had to run out to harbor freight and grab that gun today). So - how can the rest of us apply PVA?

A paper towel! And not to advertise or anything, but the recommended brand is Bounty because it's fibers are random (rather than "quilted") and it doesn't fall apart like a wet paper towel would. The trick is Apply one coat quickly. You won't be able to apply a second+ coat because the solvent will re liquefy the old film OR peel away the old film. So, one heavy coat applied quickly.

If you have something about applying with a paper towel - our friend said he's had decent results with a camel hair brush (expensive, I know, but they last "forever" when properly cleaned"). Same rules, one coat - applied quickly.

I've done this without PVA. And it's a NIGHTMARE if your plug doesn't release. It's a terrible feeling when you realize you need to destroy the plug you created.

Step 6: Isophthalic Tooling Resin - Cabosil - Mold Surface

Isophthalic Tooling resin is a fancy name for a type of polyester resin. It's a fun word to say too. It's catalyst is MEK-P - Methyl Ethyl Keytone Peroxide. MEKP More on this later in the step.

Cabosil is a thixotropic thickening agent. Cabosil, unlike micro balloons, becomes a homogeneous component of the resin. Micro balloons does not - but, they provide a lattice matrix (of spheres) which increases strength without increasing much weight (after all - it's mostly air).

Thixotropic means that if you leave the material alone, it will act as a gel. But, if you agitate it by brushing or mixing, it will act like a thinner fluid. This is how we can have a coating that stays were you put it and can be applied like a liquid.

Cabosil is fumed silica - but is not a glass. It is also VERY light. Most of the shipping weight of the 5 gallon bucket came from the weight of the bucket. I could put my hand in the stuff and it was as if nothing was around it. Another name for it is Aerosil - because fumed silica is very much like aerogel - in a particulate form. And if you didn't know before, aerogel has the lowest density of any solid known or made by man.

Lastly, Cabosil is a trade name - Cabosil is to fumed silica as Rollerblades are to inline skates. There is a brand name called "Cab-O-Sil" just like RollerBlade is a brand owned by Nike.

Our mold surface will be formed from a layer of resin that has been thickened with Cabosil and catalyzed by the MEK-P. Application in the next step.

A word about MEK-P

If you read the wiki, you already know it's related to Peroxyacetone (acetone peroxide). And yes, MEK-P will decompose rapidly (some would call it an explosion) if you give it enough of a shock. The MEK-P you buy for tooling is not 100% concentration - so it's quite hard to get it to decompose. That being said - respect these properties.

MEK-P IS A CORROSIVE - IT WILL EAT YOUR FLESH - IT MAY CAUSE OPTICAL NERVE DAMAGE - IT WILL ATTACK ORGANIC COMPOUNDS. This is the most dangerous compound of the process. It's powerful enough that we only need about 10cc per quart (1% of the resin). Epoxy based resins are safer in this regard, but MUCH more expensive. Additionally, this will eat most foams - so do not apply to bare foam plugs.

WEAR GLOVES AND PROTECT YOURSELF. Once you open the bottle, do not touch your face, ears, etc.

Step 7: Mixing Resin

We need to mix our resin now for our first coat. The first coat will not contain any fiberglass, instead - it will be gently brushed on (we don't want to tear the PVA).

Start by mixing a pot of resin - without hardener. Slowly add Cabosil thickener and mix in. Mixing involves pulling up resin from the bottom as it will be thinner than the top. Keep mixing in more Cabosil until you get "peaks" on your mixing stick - just like whipped cream peaks. Another analogy is that it should be the consistency of tooth paste. Further instructions with video can be found in the Mixing Polyester Resin Video Instructable.

At this point, you should be ready. Once you add your hardener - you're committed. You will need to finish your layup soon. Keep an eye on your environment - you should be out of the sun and the sun should not reach your work during the process. Do not leave your resin in the sun and do not leave a pot in the sun. This will reduce pot time (the working time of your uncured resin).

Once your resin starts generating the necessary exotherm - it will get hot and become a chain reaction of curing. So know your resin's cure time (this will be shorter in your pot), be aware of your environment (ambient temp, sun position, etc.) and work quickly.

Now, add your measured amount of catalyst (in our case - MEK-P). Mix into your resin pot well. You will notice that our resin changes color - from pink to a brownish color. This is good. If you pull out your stick, and it's red - that means you need more mixing.

In the next step, I'll explain some finer points of application.

Step 8: Apply First Resin Coat

Okay, I lied - but only a little bit. This batch of thickened resin will be applied with brushes to the surface of the plug. We don't want to tear the vinyl film we sprayed down, so apply almost no pressure.

I say I lied a little because this coating (similar to a gel coat) will hide some mistakes (like minor PVA runs). This coating is what makes the finish closer to a glassy smooth.

We're using "throwaway" 3" brushes. Apply liberally in a cross hatch motion at first. You want to get an even coat over the entire plug (half). At the seam where the plug intersects the MDF - build up a little bit of resin. This will help the fiberglass to transition from the plug to the MDF to make a flange. Keep in mind though, that AIR is a BAD BAD thing. So pay attention to any bubbles the form and remove them with long brush strokes.

Now, you'll notice that some areas have a kind of textured look. To remove that, use long fast brush strokes over those areas and the resin seems to re liquefy (it doesn't) and the texture goes away.

For this half of our plug, we applied 3.5 quarts of resin mixed with Cabosil. 3 probably would have been enough, but it's better to have extra than not enough ;)

REMEMBER: Once you mix your catalyst/hardener into your resin - the clock is ticking, there is no time to stop - proceed to the next step.

Step 9: Applying Glass and Resin Application

Here is where it's nice to have a few people. Well, 2-3 at least. One to mix resin, one to apply glass and resin, one to remove air bubbles. The first step of glass applying is to cover the plug with fresh resin -- not mixed with any thickening agent. Don't mix it into the thickened resin "gel coat" - otherwise you ruin it's purpose of providing a glossy surface.

Just smear it over with a brush or paint roller (whatever is available at the time). For the next part - you'll want to use a paint roller.

Now it's time to break out the glass. Chopped fiberglass is easy to tear because there's no weave. So, tear off large chunks and place them on your plug. The "wet" surface should hold it quite well. Apply some resin (you don't need to completely cover the entire piece now) and keep adding glass until the entire surface is covered. Once the plug is covered -- roll on some resin. Then, use a roller to remove air bubbles. Look closely inside the/beyond the glass for bubbles and gently roll them away.

The rollers we're using are designed just for the purpose of removing air bubbles from fiberglass.

We need a flange! The flange goes around the entire mold. So be sure to let the chopped fiberglass go down onto the MDF - this is why you applied wax and PVA to the edge near the plug. This is also why you built up some of the "gel coat" at the edge of the plug/MDF - easy fiberglass transition. Be sure to remove air bubbles from here and make sure that the edge is nice and even.

Repeat this - non stop for as many layers that you need to get the desired strength. We're doing 4 - and it used up 5 gallons of resin. This mold is going to weigh around ~50 lbs! But, it will have an indefinite lifetime (well, very long service life) as long as it's kept out of UV radiation.

Step 10: Clean Up

This took about 10-12 hours of work. Mind you, actual layup of resin/glass happened in less than an hour. So theoretically, you could take a finished plug and have half of your cast cuing in a day. But believe me, it's grueling. What you see here was broken up over two days (one day working 2-4 hours, the other 8).

Tomorrow (next step) we continue!

Clean up is simple.
Your spray gun can be cleaned with water - and then a little acetone to displace the water (we don't want rust). Water is fine as PVA is water soluble (ethanol is a polar substance ;) ).

Cleaning of brushes and other tools is done with acetone. Wash in a bucket - then pour off the acetone into a flash pan (a pan with large surface area to promote evaporation) OR dispose of at your local hazardous waste collection facility.

Interestingly, our plug got hot... HOT. Too hot to touch in fact which can cause problems if your foam can't handle it. It can also lead to shrinkage problems later. So for tomorrow, we know to mix a little less. That's just how these things work when you're using a new type of resin - even a different brand ;)

Step 11: Day 2

I hope you slept well :)

Day two starts by taking a putty knife and prying under the flange surface up to the plug -- to release. We won't be removing the tool from the plug - more on that later.

Next, use bits of wood to push the plug out of the stand to release and support it a few inches above the stand.

Step 12: Trimming and Sanding

With the plug supported above the stand - mark a line on the flange three inches away from the plug all the way around the plug.

Then, take a jigsaw and cut along this line. Use a fine blade -- one for metal works well. A wood blade is not suitable and will chip bits off the flange (the polyester resin makes a stiff structure at the cost of it being more brittle).

Sand away any burrs on the entire plug and on the flange edge.

Step 13: Remove Plug - Clean Stand

Now it's time to remove our plug. At this time do not lift by the flange - we want to keep the tool on the plug. Once the plug has been flipped over - you can grab the flange - but only when the tool acts as a bucket for the plug.

Set the plug/tool aside and clean the stand with a putty knife. We're just removing any excess PVA and glass bits so it won't interfere with the fresh coats of PVA later.

Step 14: Replace Plug

Put the plug back into the stand -- but this time face the un glassed side up. A bit of foam in between the stand and tool is a good idea ;)

Why we didn't remove the tool from the plug
Look at your plug/tool again. The tool we made yesterday now creates a perfect waterline with a perfect flange surface! No more bondo - no rearranging the stand etc. This is a big corner cut that saves so much time without sacrificing quality.

Step 15: Repeat Yesterday

Now it's time to:

1. Wax (don't forget the flange)
2. Apply PVA
3. Apply "Gel Coat"
4. Apply Resin
5. Apply Glass Mat and Resin
6. Clean Up

The same exact thing as yesterday - except the tool isn't so close to the ground anymore.

So your tool shrunk a little

Look at that picture -- it looks like our tool shrunk a little below the waterline. This exposes a curve greater than 90 degrees (uh oh). The solution - prop it up as much as possible (as shown) and then apply a thick coat of the "gel coating" to this edge to make up the difference. Otherwise, our tool will break when we try to separate.

Step 16: Final Notes

Ask questions! During the entire process I was asking questions of our composites mentor (alternative methods, why certain things are done etc.). I know for certain I missed a few things (but I can't remember exactly). So, ask away - I'll answer to the best of by abilities and I can elaborate further into certain processes etc.

As we didn't need them -- You may consider adding a index keyways to your tool. It's as easy as attaching something like a golf ball cut in half to the base of the stand before you start applying wax, PVA etc. This creates a bump in the flange that will translate to both sides if you follow the method in this instructable.

Another way to make a key is to drill holes in the first completed tool flange before starting on the second - this creates impressions for the second tool to sit in.

Finally, it would be beneficial to add bolt holes for storage. Once both parts are cured - drill holes every 16 inches or so (especially on the corners) so you can add bolts for easy storage later. These tools will not fit inside each other.

How Long to Let the Whole Thin Cure?
As long as reasonably possible. The final bits of curing take some time to work their way out (Just like the hoover dam that is still curing today). Meanwhile, leaving the tools on the plug a few days will minimize the effects of shrinkage.