One More Reason To LOVE Sprue Molds

As if my love affair for sprue molds needed more reasons to keep me happy.

Here is one more reason to use a sprue mold if you have the choice.  Smush Mold...resin in 1/2 of the mold and resin in the other half of the mold.  Smush em together, pressurize if possible and cure. 

How many things can go wrong here?  You can trap air in the mold.  You inadvertently orient the mold wrong and in your panic, all the resin runs out.  You wait too long to put the 2 halves together and the resin has already started curing and now you cant get the 2 halves of the mold tightly together giving you an oversize and distorted cast.  Once cast, if there is an air bubble somewhere, its very hard to fix.

Those are the most common things to go wrong.  Here's is how a Sprue Mold overcomes these issues.

The mold is already together when you pour the resin.  Guaranteeing you a close to accurate cast size.  So your biggest worry here is The Shrinkage Rate.  Expect 3%-5% of total size lost.  Since the Mold is preassembled prior to pour, there is no chance of improperly orienting the cast halves. 

But here is my favorite.  A sprue mold relies on the basic concepts of physics.  Gravity.  gravity pulls things down.  So when you pour a cast, gravity pulls the fluid resin down.  It will follow the channels pushing the air out in front of it.  IF something does go wrong, its usually at the end of a pour. 

Recently after I poured a sprue mold, when I took it out of the pressure pot, the air pressure had pushed the resin deep into the mold forcing the air out via the channel.  I had thought the mold full when I put it in the pot only to find that because of the air pocket the cast was no incomplete.  There wasnt enough resin poured into the mold now.

Because of the expereince I have had curing 2 separate colors together, I simply made a little bit more resin.  Carefully put the mold back together with the cast still in it and poured the resin into the mold ontop of the previous cast.  It filled it in beautifully.  You wouldnt know if I didnt tell you.  And if I didnt know, I wouldnt be able to tell.  Its a PERFECT fix.  Try doing that with a smush mold.  Most likely you would scrap the piece and try to make a new one.

Also, in areas that might trap air in the cast, you simply create (cut) a small air pocket where the problem is.  It pushes the air up into the pocket so that the resin can fill in the mold.

Yup, there are so many plusses to sprue molds.  But Smush molds are still easier and involve less trimming and are VERY goos for hi detail parts.  But you will have more scrap if you arent careful.

Resin Mixing Ratios

Every resin has a mixing ratio.  The ratio is how much of Part A to mix with how much of part B.

And then there is the fine print.  The wuh?  Yes...the Fine Print.  Fine Print is what we dont see because we think we know it all.  And then all of a sudden everything goes kerfluey and we dont know why.  Because we didnt see and read the Fine Print.

The Fine Print says...Mixing Ratio "X amount" of A: "X amount" of B......and this is the important part...By Volume "X amount A: "X amount" B by  Weight. 

And some resins, MOST in fact have 2 ratios.  1 ratio is for WEIGHT and 1 ratio is for VOLUME.  The weight ratio is the one that is usually the weird ratio.  100:80 or 100:90.  The volume ratio is usually closer to a 50/50 or 1:1 ratio.  Last week or so when I first made the Roboid molds I was having issues with my urethane resin.  Why?  because I was confusing the volume and weight ratios.  I was trying to measure the weight and instead adjusted the volume and it impacted the resin in a negative way. 

You cant measure weight with measuring spoons.  Measuring spoons measure volume.  And thats exactly what I did...or attempted to do. 

Since I dont have a scale, I have been measuring by volume.  When you get the volume ratio right, the weight ratio will be correct and visa versa.

Thank You Bry

Homeworld wishes to send out a big THANK YOU...a HUGE THANK YOU to Bry Wilkinson for his part in the Pass Around Project.  There are a few more people as well, but Im not quite sure who is all involved.  So I do apologize for not mentioning anyone else.  Im also not sure how others feel about having their names blogged.

But Bry was instrumental in getting me to do this project.  I originally turned it down.  At the time I didnt think I had the required skills and practices of casting to do a proper job.  But by the time I recieved the parts, I had the right equipment and sufficient confidence that I knew I could do it.  All I needed was the time.   I know I have had this figure here since february of this year, so he has been quite awhile without his figure.  A few years at the very least.  I think the only person left to cast this figure now is him. 

So once again, Thank You Bry for letting us use your figure and to the others who have also so graciously lent their parts to this project, Im sorry I dont have your names.  But Thank you all for your generosity and your patience.  This is awesome.

Tankless Compressor

Our garbage bin at work is a gold mine.  The things people throw away.  As you know I got my compressor from there.  Well now someone has thrown a substantially quieter Tankless Compressor away.  It works great.

A tankless compressor is exactly that...tankless.  It doesnt store air is very lightwieght and compact.  Its smaller than your average vacuum cleaner.  It usually only pumps up to 100 psi.  We need 35-50 psi.  More than sufficient.  It can be used for the exact same things as a simple compressor or for any application requiring 100psi or less. 

So Im running the compressor back out to the garage and now have a tankless compressor to use in the house.  Hmmmm...now if I had a second pressure pot, I could really stack myself up with work.  lol.

Pressure Pot Maintenance Advice

Im known for over tightening things.  Everything from pop bottles to nuts and bolts.  Its something I have to be careful of.  Its a natural habit for me to make sure things are snug. 

When you overtighten things, as with metal fasteners, you are creating stresses and strains.  You are squishing the threads together.  Eventually the metal will tear a little bit and start to gall up and tear in the thread.  This makes it harder and harder to turn till eventually it siezes up.

If you have the same issue as I do and even if you dont, I recommend putting some anti-sieze compound of some type on your lid clamp down threads of your pressure pot.  I use a nickel based antisieze.  There are lots out there all of them just as good as the other.  You have metal turning against metal.  You wouldnt run your engine without motor oil.  This is exactly the same.  They sieze.  Just the results of not using anti sieze are alot less traumatic and costly than running a motor without oil.

You'll be extending the life of your pressure pot substantially.

Field Medic Chiron is Founded in Greek Mythology

I would like to thank Niko Simonson.  It turns out Niko is quite up to date on his history.  After seeing the Field Medic he thought it was inspired by "Chiron".  When questioned about this he gave a quick background.  Bry then pulled a wikipedia.  Thanks also to Bry for forwarding it to me.

But it appears that The Centaur Field Medic is founded in Greek Mythology.  I decided to make this figure in white/red because it seemed to be a good place to start.  It seemed a good partner to the Time Medic once I got into it.  Now I find out I am not quite as original as first thought.  Who knew?  Who says collecting toys isnt educational.

It appears the Centaur Field Medics actual name is Chiron.

"In Greek mythology, Chiron (also Cheiron or Kheiron) (Greek: Χείρων; "hand") was held to be the superlative centaur among his brethren. Like the satyrs, centaurs were notorious for being overly indulgent drinkers and carousers, given to violence when intoxicated, and generally uncultured delinquents. Centaur were also wild and lusty. Chiron, by contrast, was intelligent, civilized and kind.  He was known for his knowledge and skill with medicine."


"According to an archaic myth he was sired by Kronos (Cronus) when he had taken the form of a horse and impregnated the nymph Philyra. "

"Myths in the Olympian tradition attributed Chiron's uniquely peaceful character and intelligence to teaching by Apollo and Artemis in his younger days."

"Ironically, Chiron, the master of the healing arts, could not heal himself, so he willingly gave up his immortality. He was honoured with a place in the sky, for the Greeks as the constellation Centaurus."

"Chiron appears on the cap badge of the Royal Army Veterinary Corps and also appeared on a similar badge worn by the Royal Canadian Army Veterinary Corps."

Smush Molds Vs Sprue Molds - A WINNER!!!

Havent I had this post before?  Yes...but I'm always thinking.  Especially when I have issues.  And this post about Shrinkage Rate got me thinking...

Some of my pieces are bigger than they should be.  Again, these are silicone molds, not steel.  Pressure can distort your mold and cast.  And some of these pieces being bigger are causing fitting issues.  Whats up?


And then it hit me...Smush molds DO NOT give a true cast.  It may look close, but it will invariably always be bigger.  When there is a 3%-5% shrinkage rate, how is this possible?  With "Smush Molds" you have exactly that.  2 halves of the mold are filled and then smushed together.  NOW...my guess is, you have slightly overfilled the mold cavity.  So when you put the 2 mold halves together you are forcing the mold cavity to try and hold as much of the resin as possible.  And slightly distorting your mold and cast piece.  The "extra" resin get forced into the only available space left...The space in between the mold.  This then separates the mold ever so slightly and keeps the mold from fully being closed and gets added to the cast piece thickness. 

Oh it isnt much...but it can be.  Maybe only .006"...I had one piece that was .033"  thicker.

With a "Sprue Mold", the mold is already together.  The resin isnt "stressed" into any cavities.  There is no real pressure put on the mold so there is much less distortion.  Much like water running into a crack.   And because of this, you WILL then fully be able to notice your 3% Shrinkage Rate.

So...

IT IS IN MY OPINION THAT...While smush molds require less work at the mold stage and are easier to make and use...And that they do allow for parts with greater detail to be made much easier,  SPRUE MOLDS GIVE A TRUER MORE ACCURATE CAST.  

Shrinkage Rate

What is "Shrinkage Rate? "  Is it important?

Shrinkage rate is the amount a cast will deviate usually in a descending format from the original master.   When you make a cast inevitably it will shrink.  Engineers and scientists and people with lots of signed pieces of paper on their walls have determined that there is roughly  a  3% -5%  Shrinkage Rate for resins.

And I say "POPPYCOCK!!!"   I dispute this based on 1 thing and 1 thing only...imperfection.

Not to be arrogant and not to cause trouble.  Ever here of a joke that goes...what 10 000 lawyers at the bottom of the ocean?  A good start.   Well next in line are Engineers.  Why?  Because 99% of Engineers have never been outside of their office and seen people putting their calculations to work.  In todays imperfect world their is only one constant.  Their is only one thing in life that IS perfect.  Its math.  Numbers are beautiful.  There is no grey.  Only black and white.  Right or wrong.  And you can make numbers do whatever you want them to.  Their only flaw is the people using them.

On paper...ON PAPER...everything looks good.  Paper doesnt have any factors.  No outside influences or interferences.  Paper doesnt have  "variables".    In the real world, there is EVERYTHING fighting against you.  I did construction.  Sheetmetal.  Man...  Everyones blueprints were based on them being the first ones on site.   Well now I have to work around the electricians work or this beam got missed and is in my way...lots of little things.  According to the blueprint everything fit fine.  Um...yeah.

Engineers are the pain in my ass.

The same applies to casting.   You can say there is a 3%-5% shrinkage rate.  Sure.  Based on WHAT?  Is that for 2 tablespoons of mixed resin or 2 cups or 2 gallons?  Whats the temperature outside?  Hows the humidity?  How big is the actual piece you are making?   AND THE BIG QUESTION>>> How much pressure are you putting on the mold halves when casting?  How far above sea level are you if at all?  What method(s) are you using to cast? Smush?  Sprue?

These are soft silicone molds...not steel injection molds.  There is give and play in the mold.  You can squeeze and flatten the mold if too much pressure is used.  The mold can separate and the pieces can ACTUALLY GET BIGGER if not enough pressure is put on the mold halves.  So where is the shrinkage rate then? 

These are important things to consider.   And thats why sometimes the pieces need to be fitted.  Because shrinkage rate is based on perfect conditions.  We live in a imperfect world.