Wednesday, November 7, 2012

My first attempt at mold-making

Having finally gotten the CNC up and running, I'm kind of interested in seeing how to manufacture it cheaply and quickly.  The design of the machine centers around 3D printed plastic parts that bolt to aluminum cheeseplates.  These printed parts are the weakest parts of the machine; they are great in the sense that you can build a mill without owning a mill, but bad because the strength simply isn't there.   They are also time-consuming to print: most parts take 30min each or so, meaning that to make a copy would take a couple days or so.  The partially assembled machine is shown below, everything white is 3D printed, and most of the parts are not actually visible.

Enter casting.  Cast parts can solve a few of these problems.  Cast parts are likely to be much stronger than the 3D prints because the parts will be solid material and less prone to delamination than the current parts.  They can also be made relatively quickly using epoxy or polyester resins, which set very quickly.  Not much faster than the individual prints take, but multiple parts can be cast in parallel, so an entire axis could be cast at once and be ready in a an hour or so, instead of closer to 8 right now.  Cast parts can be further strengthened by the addition of chopped glass fibers, yielding FRC or FRP (fiber-reinforced composite/plastic) parts, although this will add considerable complexity to the process and likely require vacuum-assisted resin-transfer molding.

A downside is that the casting materials and resins can be quite expensive.  Wanting to get my feet wet, I turned to OOGOO, a Sugru substitute made from Silicone caulking, corn-starch and food coloring (actually I used Mio water flavor to color the first batch).  Below you can see the first test, including all ingredients and the first part still en-molded.

It is important to get the right caulking.  You want clear 100% silicone caulking, with no anti-fungal additives, and preferably a brand that mentions that it will stink of acetic acid (or is 'acid-cure').  'Neutral-cure' is a no-no, as is acrylic caulking.  At first I tried GE Silicone II but this failed to properly set.  GE Silicone 1 is what I eventually ended up using with good results, which is good because it's about $4 per 300 mL, making it one of the cheapest caulks there.

I don't have photos during the process because it was astonishingly messy, but can say that I mixed roughly 2:1 cornstarch to silicone by volume after adding mixing a few drops of water and coloring to the silicone.  You don't have to add coloring, but it helps you tell if the stuff is well mixed once you add the cornstarch.  Mixing the cornstarch is hard work, particularly in quantity, but you want a consistently colored, pastry like result.  Basically as you mix, you want the stuff to kind of tear and crumble at first but eventually reform and blend together.

Be quick though, because with these ratios you have limited working time.  After about 10 minutes the stuff sets up into a putty that becomes harder to work with and by 20 minutes it's effectively rubber.  However you can always mix more and it will bond just fine to Oogoo that has already set.  With my first batch made, I globbed it onto one of the pre-redesign CNC parts, ever cautious that it would dissolve it to nothing or light on fire.  After about 30 minutes I cut open the mold with great trepidation:

I needn't have worried, the ABS part was just fine (as are PLA parts, by all appearances).  Separating the mold showed that the Oogoo is perfect for this sort of thing:

Here you can see the pattern still embedded in half of the mold and then fully separated.  I used vegetable oil as release agent on the pattern prior to making the mold; this worked well, but it is hard to get the Oogoo around the part without also rubbing oil on the Oogoo surfaces that should not have release agent on them while building the mold.  I will have to learn how to deal with this for the next part.

Dimensional tolerances appear to be quite good, there is a firm friction fit between the patten and mold material.  In the photo immediately above, you can see that I wasn't able to get the Oogoo all the way into the slot of the part, but this could be easily fixed with a handsaw on the finished part.

With the pattern removed, the rods can be reinserted into the mold to create the impression for the mounting holds.  Provided the casting resin has low-enough viscosity, it will then easily flow around these posts to create the mounting holes.   I found it interesting to see that the acetic acid produced by the curing caulking actually corroded the post patterns, which you can see above by the discoloration of the steel posts where they were in contact with the Oogoo and by the rust-stains on the Oogoo itself.

Anyway, this process seems like it has great promise.  It's easy to work with, very cheap (less than $8 per pound) and seems to give quite accurate results.  The final molds are predominately silicone too, which has high heat resistance for the eventual polyester resin casting (I have seen large amounts of resin actually auto-ignite due to the exothermic reaction as the resin 'kicks').  I intend to keep playing with this stuff and hopefully end up with a reasonable method for performing casting of printed parts in the near future.

1 comment:

city said...

thanks for sharing.