Victorian Science Fiction: How I Build Them

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Victorian Science Fiction: How To Cast Parts in Resin

I had several of the six leg walkers, so to save work I thought I would cast copies of the pilot house I planned on mounting on top of each.
To see more about the making of the six leg walker you can check it out the six leg walker page.

In order to make a mold and castings, you first need a master to copy.
I have covered this on the Six Leg Walker How To page. But for those who would like a brief run down, you can view the master making page.

To make resin castings requires quite a pile of supplies and tools. The supplies can be obtained from many craft and hobby suppliers. What you will see below were purchased through Micro-Mark . The tools are a hodgepodge, some are available through Micro-Mark . The balance and weight set I picked up at a university surplus sale. But Ebay would also be a good place to look.
 Stuff You Will Need
Here we see the two part room temperature vulcanizing rubber (RTV), the two part resin, the solvent based release agent, cheep brushes and various disposable containers.

The Master:
 Master Front
The master is sealed with primer. The shape of this part is convenient for casting as it is basically a wedge shape, and have only minor undercuts.
 Master Rear
The rear of the pilot house does feature a door knob which would not cast well as it has a large undercut and a very thin support.
 Master Rear Undercut Removed
The best solution was to simple remove the offending part. A quick pull with a needle nose pliers removed the pin that made the handle. This door knob can easily be replaced on each of the copies once they are cast.

The pilot house is also convenient as its base is not a finished surface. This will allow use a way to pour the resin. The simple wedge shape is ideal for a one part mold. I am using a small disposable cup a the mold form. It is important that no part of the master will touch the sides or bottom of the form.
 Centered in Pot
The master is centered away from all sides. I am using a toothpick to suspend the master, and mark it with a pen to assure I have the proper alignment.
 Centered in Pot
And making sure it is clear of the bottom. Most RTV are not see through, so care is needed in planning to assure a good finial mold.

Preparation of the Master:
A Good Coat of Release Agent
I paint the master on all sides with a release agent. One can make a simple solution of dissolved Vaseline in mineral spirits. But here I am using a pre made solution sold by Micro-Mark . Make sure to use a very cheep brush for this as the grease will not wash out.

Preparation of the Form:
Estimating Volumes
One could also coat the form with the release agent, but I am skipping this as I plan to brake the form apart when done. Next the volume of the master most be estimated or measured. By using a graduated cup, such as that used for the form, one can experiment with water to see how full the form must be filled with RTV rubber to properly cover the sides of the master. One a value is found, the portions of each part of the RTV rubber mix are calculated. This is very easy with this particular mix as it is 50/50. Marking the side with bold lines will help remind me when pouring the RTV rubber parts. Because of the small size of this mold, I am not going to measure out the parts in separate containers as the instruction suggest. This "one pot" method works fine for this job.

Pouring the Rubber:
 Needed Items?
Here are some stir sticks needed for the preparation a pouring of the RTV rubber. I have labeled every thing I use here to prevent cross contamination. The stick can be attained from most craft stores or from Micro-Mark directly. I have cut the tip off of some of the sticks to allow me to better reach the corners and bottom of containers.
 Needed Items?
The components of the RTV rubber, "Part A and Part B", are each mixed well in their own container. The components tend to separate over time. Once each is fully mixed, pour one part into the form up to the bold line. As soon as the second part begins to mix with the first you are racing against the clock. Most RTVs have long cure times, but it is good to be mindful of what is happening. Carefully pout the second component to bring the fluid level up to the second bold line. And mix! Mixing is a bit of a problem as you must mix well, but not incorporate too much air. There is no ideal solution, so I usually make sure the mix is complete and try and force the air out as best I can.Most brands of RTV rubber offer the parts in two different colors, this can be very useful in judging if the mixing is complete.
 Needed Items?
By tapping the from on the table, many of the air bubble can be forced to the surface. Alternative include using vacuum chamber and other more high tech solutions. For this job, tapping will have to do.

Submerging the Master:
 Placing the Master into the RTV Rubber
Now after the air bubbles have been worked out but before the RTV rubber can start to cure, place the master into the RTV rubber mix. By slowly dipping one corner into the rubber and slowly working it down into position air will not be trapped against the master. An alternative is to quickly dip the master and pull it back out. One can then use a brush to assure full coverage of the master with rubber by working it into all the detail areas. Then the master can be returned to the form full of RTV rubber. Make sure to place the master as planned. The marks on the supports help to guide you.  Check the level of the RTV Rubber
I use tape to help hold thing in place. Checking the level of rubber in the form I see that the master is not quite fully covered. I have save chunks of the same rubber from other attempts to make molds. By ripping off small chucks and adding them to the mix, I can raise the level. The cured rubber will bond to the new mix as long as it is not contaminated with such things as grease or dust. I often "recycle" old molds this way. Time to set it aside
Once satisfied with the level of the rubber and the position of the master, the form is set aside to cure. This is a function of the RTV rubber used and environmental conditions. In this case 24 hours will due.

Finishing the Mold:
Busting it out
After the cure time, it is time to free the mold from the from. Some molds require the form to help them keep their basic shape. This is not the case here, so I simple peal the form off the cured RTV.
Just Rubber

Cutting away unwanted parts leaves the completed mold. This can be saved to use in the making of the next mold. Cutting thing away
The RTV rubber is quite fragile and very easy to cut. Care must be taken not to tare it in handling.
Removing the Master
Take care to gently squeeze the sides of the mold to allow air to pass between the master and the mold rubber. Small slides with a razor help to free the was for the removal of the master.
the Master in Process
Once the master is removed the mold is inspected. The opening at the top does not appear large enough to allow an easy pour, so it is enlarged.
the Master in Process
A small sharp scissors work well for this.
A look inside
It is very difficult to photograph, but you can see the detailed impression left in the internal cavity of the RTV mold.

Casting in Resin:
The Release Agent
Now with the completed mold it is time to start the casting process. The mold is sprayed with a release agent to prevent the resin from sticking to the mold. The release agent was purchased from Micro-Mark . I spray several other molds to accommodate any extra resin. A tray (an old cookie sheet in this case) is use to contain any spillage.
The Stuff You Need
Assembling the required items to cast in resin. A pan balance is very useful in weighing out of the resin. A good set of scale weights is also a big help. The resin comes in two squirt bottles. Disposable cups and a stir stick are needed to mix the resin.
The first step
The cups are put into balance with the built in slides on the balance.
Weighing Out the Part A of Resin
A small weight (in this case 5 grams) is placed one side of the balance, and the scale brought back into balance by dripping the part A resin into the cup opposite the weight.
Then a second equal weight is placed on the left pan and the scale brought back into balance by adding the part B resin. The resin is then quickly mixed and poured into the molds.
Filling the Molds
Once mixed the resin must be poured quickly to fill the molds before the resin starts to gel.
Filling the Molds
A second, third or even forth mold is filled with the extra resin. Here we see some windows for my Martian buildings.
The Resin is Curing
Soon the resin begins to gel, as seen by the growing fogginess in the originally clear resin.
The Resin is Almost Cured
The curing time is a function of both the resin and the environmental conditions. Here the cure time is about 15 minutes.
Freeing the Parts
By working the sides of the mold the casting can be slowing freed from the mold. The RTV is very flexible, but care need to be taken not to rip it.
Freeing the Parts
With care, by working around and round the mold, the parts seem to free themselves. Freeing the Parts
The same method is used with the pilot hose, but as it is a much deeper mold the progress is slower and requires gentle pulling of the casting to free it.
Freeing the Parts

Inspection and Consideration:
With the part freed, it is time to compare the result to the master.
Comparison Front
Comparison Rear
Defect Found
It appears an air bubble was trapped within the mold when filled with resin.
Trimming the Mold
By trimming the rubber and extending the slit to the corner of the cavity the trapped air should be vented during the pour.
Defect Fixed
Now comparing a second casting to the first we can see that the problem is avoided.
Finial Processing:
With a successful mold, one can now cast as many parts as will be needed.
Defect Found
The bottom of each casting is filed and sanded level and each of the castings needs to be washed to remove the release agent. Disk soup and a tooth brush does the job well.
Defect Found
The door knobs which had been removed from the master can now be added to each casting
Defect Found
The castings are secured to a stick for priming. Defect Found
The pilot houses are now ready to be used on the models.

To return to The Building How To page, simply click here
To return to The Building How To Six Leg Walker, simply click here
To see the finial models go to me Battles page VSF Battles.
Or you can go to the Model Gallery Page.
Or to head back to the The Victorian Science Fiction Page.

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This work is licensed under a Creative Commons License.

Creative Commons License
This work is licensed under a Creative Commons License.