Mold making - My first casting

Casting is the process of sticking stuff in a mold and creating a copy of the thing the mold was originally made from.

I'll be using hot glue from a hot glue gun.

I spread a thin layer of soap over the mold to allow the mold to release. Using glue in something you hold pressed together for a while is perhaps a dodgy plan, but hopefully the soap will prevent it's sticking.

I filled both halves of the mold with glue, then pressed them together.

There was a bit much glue, and nowhere for it to go so the glue overflowed between the two halves of the mold.

In future it might be worth drilling a small hole to let out excess glue.

Other than that, it seems to have worked.

The excess was easy enough to trim with scissors.

Leaving a pretty good copy of the original.

Pictured is the copy after it had been heated with a flame to smooth it out a bit.









There were a few lessons learnt here.

a. If there is nowhere for the glue to escape, it attempts an escape between the molds.

b. If you try to mold hot glue in cold molds, it tends to set a bit too quickly mefore you can press the molds together properly.

c. a, and b above mean the copy can be taller than the original as the molds didn't press together completely and there was a gap. The resulting copy was taller than the original by the thickness of the overflow that had to be trimmed with scissors.

All in all a successful experiment.

Mold making - My first mold part 3

Back to part 1
Continued from part 2

I removed the foil and found myself faced with this.

Its a solid blob of set plaster. I looked at it a lot.

There is a slight seam so the plan was to put a knife to the seam and twist, but the plaster its still wet enough that the knife just cuts it. I think it needs to be bone dry and brittle.

From what I've read the drying times vary with the amount of water used in the mix, but more on that later.

I wait.

In a pleasantly surprising sort of way, 24 hours later, the mold came apart.

The plaster had run down the side of the botom mold and stuck to it where there had been no soap. All I had to do was apply enough force with the twisting knife to break the rim that had stuck and it came free.

I need more soap next time. Or rather I need some soap down the sides a bit next time.

Even more pleasant surprise, as the model also released cleanly.

There are a lot of bubbles in the plaster, but I'm reading of some ways to combat that. More on bubbles in a future post.

The roughness visible in the half on the left is a faithful reproduction of the actual roughness of the original.



To round out the trilogy of pleasant surprises, it also turns out that the lumpy bits that were on bottom mold, and their reflection in the top mold serve a very useful purpose. When you replace the two halves, the lumpy irregularities make it easy to align the parts perfectly. Lumpy irregularities turn out to be called "keys" in the mold making world. Keys are often added to aid in aligning the pieces of a mold.

Bubbles aside, the mold looks pretty good. It's quite an interesting thing to crack the mold and remove the original model. It's not often we get to see the negative of a 3D shape. Especially one we make ourselves. The plaster does an amazing job of faithfully reproducing every detail of the original.

Back to part 1

Back to part 2

WARNING !!! Plaster can get very hot when setting 

google search for: plaster of paris third degree burns

Mold making - My first mold part 2

Continued from part 1

The plaster has dried to a clinking sound when tapped with a spoon.

My fist step is to unwrap it, and gently prise the lure from the plaster.

Next is to replace the lure in the mold and lubricate both the mold and the lure to make sure nothing sticks to either.

It turns out, that there is some kind of natural limit as to precisely how much of someone else's lip balm you can use on a mold making project.

I'll be trying soap this time.

I built up a new wall surrounding the bottom mold and model.

With all surfaces soaped, and some luck, the top half of the mold will separate cleanly once it's set.

I mixed up a second batch of plaster and water in random proportions.

After stirring for a minute or two, I poured it over the lure, and filled the foil box.








I have no idea if I need to do all this waiting. I really want a result so I'm going to wait until its totally dry but I think I can get away with opening it tomorrow morning.

Even though I'm doing my usual "fools rush in" approach to learning how to make molds, I am actually studying hard in an attempt learn how to do this properly. I've found I understand a lot more of the stuff I'm researching if I've had some exposure to the materials involved. With that in mind, I should be able to make a decent mold.

But this might not be it.


WARNING !!! Plaster can get very hot when setting
google search for: plaster of paris third degree burns

Mold making - My first mold

I decided to use a failed handmade fishing lure as my first mold subject, not because I wanted copies of it, but rather because I didn't care if it was ruined. I'd make the mold out of plaster, and make the copy by molding some hot glue into shape.

I realized I needed some way of suspending the lure in mid air to make the first half of the mold. Last week I made a failed attempt at my fist mold. I used a bed of leftover mashed potato. By pushing the lure into a bed of mashed potato, it should have been possible to make the top of the mold, then turn it over and make the bottom.

This mold is to be in two parts. I figured the approach would be to find the halfway point of the object, build a box around it, and make it sit in plaster to that halfway point. Then when it was dry, I could build up some new walls, and fill that with plaster. The mashed potato method worked, but the old bag of plaster I had didn't. It never set. I'm guessing it was because it had too much exposure to moisture over the last few years. This time I'm all out of mashed potato, so I'll have to suspend it in mid air.

My first step was to coat the lure in lip balm to prevent it sticking to the plaster.

Lacking any telekinetic powers to speak of, my new approach was to suspend the lure a centimeter or so off the ground in a vice.

Next, I made a box out of foil to hold the plaster, and slipped it under the lure.

I mixed up some plaster and water until it looked about right. (I'll work out what the ratios of plaster to water should be when I look into this a bit more)
And poured plaster into the box until the level reached halfway up the model.

After only about a half an hour it was hard enough that I could touch it without leaving a mark, but it still felt wet and cold, so I thought I'd better leave it until it felt totally dry. So leave it I will.

Now the waiting. I'm still not so good at the waiting.

I really need to get better at the waiting.

Continued - part 2

WARNING !!! Plaster can get very hot when setting 

google search for: plaster of paris third degree burns