Building the Xenotyrannus Rex Skull

Tackling the issue of turning the original 2013 sculpt of the Rex skull into a much smaller variant for the newer armature. Some of the process in this part of the blog will be new and untested to me so should make for a interesting process.

My first task was to address the requirements of what the skull needed to be...

Light weight

Paintable

Robust/ High strength under pressure

Highly detailed

If one of these factor was not met, it could compromise the overall result of the skull.

RESCALING

Original 2013 X-Rex skull
made from Plastazote and Fimo

The first thing I needed to do was to reduce the scale of the original skull of the 2013 resculpt. The shape and overall design was there but the scale was far too big and was quite weighty being made from Fimo. Instead of taking up time resculpting a new head from scratch I'm going to attempt 3D Scanning the original head to 3D print a core design for me to work from. I'll be using a photographic process that captures up to 50 photos from around the object to create a 3D model for 3D printing.

This was unknown territory for me, as someone who likes to physically pick up and touch things, seeing my sculpt in a virtual space and been unable to reach in and touch it to reshape and resize it was incredibly frustrating for me. This was printed out by MA Product Design student Connor Gallagher, I would lease with Connor with the measurements I wanted the skull to measure, but after the first print we had to increase the scale of the upper jaw to make it fit the scale of the new armature blueprints. 

3D scan created in 3DF Zephyr

Cura software converted the scan into a square grid structure

I must note, if this was printed with a high end printer where either the print lines were invisible or was printed in a resculptable material, such as Cx5, I could quite happily have used this skull for the final armature. The ABS filament material is quite light in weight for what I would require, BUT, I did find the crest snapped off after being subjected to a small amount of pressure (this could be changed in the Cura, either by increasing the thickness of the walls or the number of honeycomb structures inside the print). 3D printed skulls for armatures could well be something I may want to use again in the future.

After the second print was complete (in a little under 2 hours time no less) and I was happy with the fit I could make a junk mold to cast up the Monster clay sculpt to redesign.

New 3D printed copy of the 2013 skull.

Comparison in size between the original (left) and the reprint (right)

JUNK MOLD

Because this is a hard model I will be using silicone to make the mold for recasting the Monster clay version. My previous experiences with silicone molds have primarily been with Smooth-On Silicone, particularly 00M00, however, regardless of its price, I have found the material difficult to work with, mostly because of it viscosity and coloured nature. 

Junk mold progression,
testing out ideas

After some searching I found a silicone product that sounded promising called Platsil Gel 25, it was quite viscous from videos I had viewed online and was somewhat translucent, the only thing I couldn't tell was how soft the silicone would be, to soft and the internal sculpt would be deformed and to hard would mean potentially breaking the Monster clay casting during demolding. I bit the bullet and gave it a shot and much to my relief it works like a charm. 
The one thing I wish I had access to however was a degassing chamber to take out all the air bubbles for a smooth mold, though the viscosity is high enough for most bubble to rise to the surface, not all air bubbles escape the the model. These are minor problems but that's the perfectionist in me.

You'll also notice I have used a 3 piece mold design rather than the regular 2 piece mold, this is due to the shape of the head sculpt itself, the crest is simple enough, but from a previous experience I found that the shape is not really suited for a 2 piece casting, causing no end of trouble for trapping air pockets. since this was a junk casting I knew I had room to make improvements should I find errors in the Monster clay casting providing I made mental and photographic notes. 

Another thing I added was a poor vent, using a piece of wooden dowel approx 6mm in diameter. I wanted to see when it came to pouring resin if this size of hole would be large enough for liquid resin to enter the mold without clogging up the entrance. One of these notes was to cut air vents into the walls of the finished mold rather than lay down piping or wire (it makes the casting processes less complicated).

3 part mold

CLAY CASTING

Melted Monster Clay in molds

When it came time to make the Monster clay casting I had to guess a lot of factors on this, the main one primarily was how long to leave the mold in the oven before the silicone could conduct most of the heat into the center of the cavity to melt the clay. It must have taken about two or three hours at 50c before the mold was a decent through temperature and originally I was placing small chunks of the clay on the top and allowing the heat of the oven to melt it into the pour point, this method was taking far too long, then it dawned on me I could roll u the clay into sausages and simply insert them into the cavity before returning the mold back into the oven for 30 minutes before retrieving for further clay sausages. It took a while but eventually the molds were full of the now liquid clay and after the cooled a little I placed them both in the freezer for 30 minutes to completely solidify.

Demolding clay skull from silicone mold

Quick clean up of clay casting for resculpting.

Once these were demolded I could see the seam lines were extremely thin so clean up was quite easy, another thing I noticed was air pockets had trapped in the finer points on the teeth and the tusk tips, I had expected this and for a junk molding I was prepared to experiment with better ideas down the line.

RESCULPTING

In progress skull resculpt.

Resculpting the new skull was relatively quick at this point, adding clay to areas on the crest to give it a more pleasing crescent shape and removing areas to allow placement of foam for the lip snarl effect and reducing the length of the back of the skull to fit the new armature. Texturing the new skull was going to be a interesting experiment as all my previous sculpts were made from Fimo and I had one one version used a toothed kidney tool and on the other I had manually sculpted in the details. Since I was more familiar with the extremes Monster clay Hard could endure I opted to sculpt the details in and worry about how to keep the said details during casting afterwards. 

Beech mixing sticks turned into predatory teeth.

As for the teeth I was at a crossroads... when Ray made his creature teeth he either used cast steel/ Aluminium to make the sharp dentures or monkey teeth from taxidermy, since I couldn't find a taxidermist that would have such items and not having access to a forge I decided to try cast the teeth in the skull as I did once before, but the last time I wasn't happy with the shape I used and now, even if I did use them again the scale would be all wrong. I didn't want to make teeth out of cocktail sticks as I did back in 2012 as over time when I made test animations I discovered the back teeth would get crushed and splinter. I then thought of a plan, to create teeth in wood that would be thick and strong to the scale of the skull, then I would cast those teeth with the skull so it came out as one piece and with air vents at the tips of those teeth would be completely solid.
I used beech mixing sticks and made two types of teeth, long curved teeth and short hooked teeth using the oval and rounded needle files, it was quite fiddly but eventually I made all the set required for the skulls. Sculpting the gum lines and the roof of the mouth, the skull was now ready for casting.

Finished skull sculpt.

Lower jaw details

Upper jaw details

MOLDING

Mold in production, with the
addition of more key points.

It took me a while to decide on which number of pieces I was going to make this version of the skull in, I didn't want to loose the details on the skull and began to over thing the complications, it wasn't until after I made a spare junk casting from the previous mold that I remembered about the seams on the monster clay, and the same result happened with the polyurethane resin, so naturally, I opted for the three part mold again.

Preparing the third
section of the mold

I made some slight modifications this time round, adding more plug sockets into as many spaces as possible and even added additional sizes, from a mid sized bottle top used from a sushi snack pack and a small ball tipped tool. I now could begin molding, making sure to apply plenty of wax mold release so the silicone didn't stick to itself (I have a sneaking suspicion that the oils from the Newplast clay I use for mold making actually leave a residue that makes silicone bonding to itself less likely, but why run the risk).
Now that I had all three sides made I began to make my air vent insertions on the tips of the tusks, the teeth and the back of the jaws and crest, for the lower jaws tusks I created the vents to connect each of the tips to allow the liquid resin to flow neatly upwards without going around a unnecessary vent system, Also now that I knew how big a pour vent I needed to have in order for the resin to pour in smoothly I cut a wide enough vent between the two halves of the upper parts of the mold.

RESIN CASTING
Now that I had my mold I could begin casting the poly resin junk casting, this cast allows me to check all my vents are cut correctly, work out if any spaces require additional vents and to remove any debris from the mold itself. Thankfully of the first try everything came out perfect, even the teeth were perfectly cast. I even made a second casting in hopes that it could be used as a back up copy or experimental ideas. Clean up on these castings was exactly like I had seen in the previous junk mold, they were tissue thin and were easily removed with a slight brush of a tool or even my finger, I didn't even have to much, if any, in the way of sanding down imperfections from vent outlets or seam lines.

Demolded upper jaw, complete with resin filled vents.

Both parts removed and ready for cleaning

Cleaned skull 

SOLID VS. HOLLOW CASTING

Hollowed out view of the skull

There was one more thing I needed from this skull, and that was to be light weight, in its current state both solid pieces totaled up to be around 36 grams, what I needed was to try and reduce that weight down to at least 20 grams without sacrificing the structural strength of the skull as a whole. Using the first test casting from the junk mold, I tested out how much material I could remove before the structure either fractured or completely caved in under pressure. After a few hours worth of boring out a central channel to the "snout" of the crest and removing the remaining poly resin with various Dremel bits, I discovered that you could actually remove quite a lot of material before the structure started to show signs of stress under pressure.
The only disappointing factor was none of the Dremel bits had the appropriate length to reach all the way into the snout of the skull without resorting to removing the bulk of the skull from the snout with a jewelers saw, which when gluing back together left a noticeable line between the two halves, so I decided to leave the remaining resin inside the snout. The skull over all is still much lighter than its solid counterpart, BUT it is a shame I couldn't make it as perfect as I know I could make it. One thing I have noticed that is currently concerning me, is that two of the teeth on the second casting have broken off after been struck from the side, this could be due to two possibilities, that the resin is not as strong as I suspected on such a small scale, OR that there was a tiny collection of air pockets trapped in the teeth that left them vulnerable to shattering, but after careful examining I thinking its more the former that is the real problem here. I'm going to investigate the issue to see if I can find a possible solution to this problem, the last thing I want is the teeth breaking off mid shot because I have either knocked them or they are under too much strain during a bite sequence.

MOUNTING
Now that the skull is currently finished (I still have yet to add the ears and the wires for the lips) I can now mount the skull and jaw onto the armature parts, This took a lot of readjustments to get the fit right, mostly on the armature parts since they were made first and the skull was already an existing property. but once things were starting to slot into place without too much wobble I was able to insert plastazote filling into the skull to pad out the armature fixture and secure the jaw armature in place inside the jaw casting before I cemented it in place with Araldite epoxy, I am keeping the upper jaw "loose" for the moment until I can create a solution to the fragile teeth problem.

The next things for me to tackle now are the replacement teeth solution, the insertion of the wire for the lip snarl animations and the 3mm ball joints for the ear animations, and of course priming and painting.