Resin casting in a two part mold

things to consider when making a mold

Resin casting in a two part mold

Last week’s post showed how I went about making a two part mold for a dinosaur plastic toy.  While the mold was made with the dinosaur on its side, the idea was to pour resin into the mold while the dinosaur is vertical.  You can see what I mean by the picture at left.  (And pay attention to those undercuts.  You will see why those are important in a minute.)

 

 

dinosaur resin mold

Before pouring the resin, I made sure to bring both parts of the mold together with masking tape.  I then set it upright in a plastic cup.

 

 

 

 

 

 

pouring resin into a mold

I mixed 15 cc total of the Resin Obsession super clear resin and added in a bit of the Alumilite green pigment.  Because the sprue hole was small, I used a toothpick to help guide the resin into the mold.  I poured in some resin, then rocked the mold back and forth, trying to get bubbles to release to the top of the sprue.  I filled resin halfway up the sprue before allowing it to cure.

 

 

 

dinosaur resin casting within mold

This is what my resin dinosaur looked like 24 hours later.  (Perhaps you already notice the two problems with the casting.)

 

 

 

 

 

removing resin dinosaur from mold

Once the resin cured, I was able to peel open the mold and remove the dinosaur.

 

 

 

 

 

 

resin dinosaur casting results

Comparing the casting to the original, you can see where my green dinosaur didn’t fare so well.

Problem 1:  The arms and toes on the feet didn’t fill with resin.  This was a problem that I was expecting.  As shown in the picture above, I needed the resin to go down, then up.  Because of surface tension, resin (amongst other things) does not want to do that.

Problem 2:  The head didn’t fill.  As I was pouring the resin, I didn’t want to have too much extra to sand off from the sprue.  Even though the filled the resin into the sprue, I didn’t fill it all the way to the top.  I didn’t account for bubbles rising to the surface of the sprue, allowing for more resin to fill in.

Overall, I ended up with a zombie dinosaur:  headless and missing a few limbs.

On a side note, flashing is something I talked about before.  It’s where resin leaks out between the two layers of the mold.  It occurs commonly with two part molds, but can clean up nicely.

comparison of cast resin dinosaur to original

This view of the underside shows the resin casting voids.

 

 

 

 

 

 

 

 

comparison of cast resin dinosaur to original

The body turned out well.  I like that the Composimold did a great job of picking up the ‘scaly skin’ detail on the original model.  It transferred to the casting.

 

 

 

 

 

dinosaur toy casting with resin

The good news is that the back half of the dinosaur (cast as the bottom half of the mold) did pretty well.  The tail casted fully and with a lot of detail.  The toes on the back feet weren’t complete for the same reasons the arms weren’t complete but not nearly as bad.

So what’s next?

I think I’m going to try casting the dinosaur as a top half and bottom half.  Maybe then the arms and legs will fill up completely.

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9 Comments

RonnyJ

I think it might turn out better if the two side were cast separately, then glued together with resin, or let one side cure first and lay it in top of the other side when it is starting to cure. I made a sphere this way with two half rounds.

Reply
RonnyJ

(interrupted by Kathy Kat)
The only change would be recasting the two parts separately, without the sprue, making two new molds.

Reply
Katherine Swift

@Ronny, yes, that would be a great option here. Too bad I’m too stubborn to try that yet! 😉

Reply
Andy

To get the resin in the arms and toes you need to create a place for displaced air to travel into (yes, it may also created some flashing you’ll need to remove).
If you search YouTube for the mold company called “smooth-on”, they have a few WONDERFUL 2part mold videos (and other molding videos) 2 of which include howmto avoid such problems as what you experienced.
No, I don’t work for Smooth-on, I just like their how-to videos and I love their silicone mold products for making my molds for my resin products (Oomoo works great!!!)

Reply
Andy

@Ronni, there’s no need to cast 2 seperate pieces and glue then together if the mold is made correctly.
Also the problems of 2 parts glued together are:
1. It will leave a seam line
2. Your resin would have to be poured perfectly (down to the drop) to the the diameter and fit perfect
3. It will NOT be a durable piece

Reply
Andy

@Ronny, there’s no need to cast 2 seperate pieces and glue then together if the mold is made correctly.
Also the problems of 2 parts glued together are:
1. It will leave a seam line
2. Your resin would have to be poured perfectly (down to the drop) to the the diameter and fit perfect
3. It will NOT be a durable piece
I’ve been casting “2 part molds” for years, there are lots of trial and error moments before you “perfect” the skill.
(See the other note I left about the Smooth-on videos if you want to learn how to make excellent 2 part molds.)

Reply
Andy

Here’s an excellent example of the “air release” holes (and “j-frame”) I referenced earlier.
The video quality is so-so as it’s someone’s home made video.
But I think this might help you avoid some of the problems you had with this project. 🙂
https://youtu.be/WsWvMOeX6Zs

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