Instructables

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

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)



Related
Video Large Scale Mold Making - Part 1
Video Mixing Polyester Resin
Video - Part Making
 
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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.
good job and well written, thanks
Squee7 years ago
That looks dreadfully exciting... I'm kinda glad I dropped engineering so I'm allowed to do things the "wrong" way again :-D
Ferrite7 years ago
Why do you have to make a negative mold? Can't you make the final parts form the positive mold that you started with and then attach them togeather. It seems like that would save a lot of money.
trebuchet03 (author)  Ferrite7 years ago
We were trying to suck out as much resin as possible... Last year, we didn't make a negative mold -- we just used the male plug... And the damn thing imploded.... We couldn't apply a decent vacuum. As for saving a lot of money.... Yes and No. It would save us somewhere around $300 (10 gallons of polyester resin, chopped glass, very small amount of cabosil). Mind you, $500+ was spent on raw foam. But now, we have a reusable (with high service life) tool that can be modified to make similar (but different) shapes. We can even do smaller body panels off the tool and then pull them together to make a drastically different shape. So, in the long run - when we go back and optimize the design, we don't need to spend the $500+ on raw foam plus bondo, water jetting and TIME (a big deal)! So short term, it would save us ~$300 out of a $2500 budget :p That being said -- there's plenty of ways to do this ;) In fact, I have lots of goodies to experiment with off of suggestions at the Maker Faire :D I'll be sure to post them as I work them out -- these are the type of things you discover in your garage to save a few bucks :D
westfw7 years ago
Do you let the thickened resin cure any before applying the unthickened resin (eg between step 8 and 9)? How about in between the layers of glass? Do you know how the PVA used as a release is different than the PVA used to make slime? (to make slime, I make a 4% solution in water, starting with solid PVA. This gives me a liquid like runny syrup, which is significantly thicker than the PVA solution I see at tap plastics (and it's clear rather than green.)) Is it a different MW of PVA, or additional additives in the release version, or ...
trebuchet03 (author)  westfw7 years ago
Do you let the thickened resin cure any before applying the unthickened resin (eg between step 8 and 9)? How about in between the layers of glass?

I asked that same question - no, no cure time. Once you mix the catalyst in the initial batch of resin, you're committed to finish completely - no stops.

Do you know how the PVA used as a release is different than the PVA used to make slime?

I don't know, but I'll ask today :)
Hi, The caption when adding the first lot of resin says to add, unthickened resin. Does that refer to not adding cabosil or does it mean don't add catalyst? I was thinking that maybe the first lot of resin is put on without catalyst to give ye time to get the first layer of glass on and that the rest of the resin would contribute it's catalyst. Doubtful, but I just wanted to make sure. Cheers Diarmuid
trebuchet03 (author)  dwrenne7 years ago
Just no cabosil (or whatever thickening agent you're using) ;) Always use hardener/catalyst in this stage ;)
Just a quesiton: When I interned over at a fire retardant development lab last summer, they were really worried about Cabosil and made me wear a really thick respirator mask. Just from what I've heard it's like breathing in razorblades, and it's pretty much lighter than air. Shouldn't you be wearing a mask??
trebuchet03 (author)  T3h_Muffinator7 years ago
Cabosil is given a 1 on the hazardous material index system (HMIS). Where 0 is harmless and 5 is highly dangerous. 1 Meant that it was in irritant - so don't get it in your eyes. the MSDS shows that it's not a mutagen but does say to use an eye washer should you get it in your eyes :P

On the subject of silicosis (this has been mentioned a few times now).... Silicosis requires the crystalline form of silica - like quartz. Fumed silica is colloidal - as such OSHA classifies fumed silica as nuisance dust and does not require personal filtration if the dust level is maintained at a low enough level - I think that is why there's a big health concern, someone hears silica and immediatly assumes silicosis.

Fumed silica isn't lighter than air -- but it is a particulate form of aerogel - a material with the lowest density compared to other solids. So it will float in the air for awhile, but eventually will settle out.


Now - comparing dust you find in your house to Cabosil.... Floating dust in your home is actually quite hazardous due to radioactive decay. Our soft tissues (that is, not our skin) don't block alpha particles very well.... If you ever get a chance to measure the radioactivity of household dust -- be prepared to be amazed :P Cabosil is completely inert (after all, it's pure silica sand :P) and doesn't have that effect.

Here is the MSDS


If your bossman required that you wear a respirator - it was probably a liability thing ;) OSHA requires that you either have taken measures to reduce the dust level through filtration or dilution (aka fans) OR that you wear an approved dust mask. If you were in a closed environment and a spill were to happen (say, someone put an air hose in the bucket), the dust level would quickly go above OSHA standards for use without a mask :P

I'm honestly not concerned for my own health with respect to this. The big hazard working with polyester resin the the MEKP and resultant styrene. So either have good ventilation or organic respirators :D
Alright, cool. Thanks for the info!

As for the safety precautions in that lab, they're pretty anal about safety. I suppose it was understandable for them to make me wear a mask.

Wait, MEK is worse than Cabosil?? Oh god.... I practically played with that stuff all day....

Oh well, now I know =P
trebuchet03 (author)  T3h_Muffinator7 years ago
MEK is a bit "safer" than MEKP ;) And some people (like me) are more sensitive to it than others. I had to deal with an MEK chemical spill (on myself!) and was able to taste it about 10 minutes later.... not one of my better moments, but that's what safety equipment is for :)
Oh, and other than that, awesome Instructable =)
carlson7 years ago
Great video, thanks for posting.
LasVegas7 years ago
This is an absolutely fascinating series, trebuchet! I'm anxiously looking forward to the next installment. I'm curious though... Which of the group is you? ;)
trebuchet03 (author)  LasVegas7 years ago
I'm wearing the white/tan visor - the back of my shirt in the first 2/3 of the video says "A New Definition of Speed" and has a picture of a yacht - which happens to be the fastest (in it's size class) privately owned mega yacht. Everyone except the gentleman wearing the Lockheed Martin Shirt is on the HPV team. Everyone excluding the Lockheed gentleman and myself are in the senior design class at my school. Oh yeah, tomorrow - we vacuum bag :) I've got a total of 11 more 1 hour tapes to burn :D So expect more soon :)