One of the things I came up with during the construction of the 11-foot Discovery model, is a new way to construct certain kinds of molds. What follows here is sort of a step-by-step explaination of this process. Actually, I guess should say this is new to me and I've never seen it done before. Given the time RTV molds have been around and the number of creative people out there, I doubt I'm the first to do this!
What you see here is representative of almost all RTV molds, with the exception of the brass pins sticking out. It is these pins that have made such a difference!
This particular mold is too large for me to handle and photograph at the same time, so I'm going to switch to the smallest mold--
It is important to remember that when a mold is made, it is upside-down-- the 'top' of the mold is at the bottom of the mold box. What I'm going to show you here works for quite a number of parts, especially those that are flat on top. Remember, these spine modules are hollow, 5-sided boxes and normally I'd be using a two-part 'squish' mold to cast them.
The part is mounted on an angle so that the pour (through the round hole) will travel 'uphill' against the inside of the mold, toward the vent (the 'slit' at the top)
This is the mounted pattern and the mold made from it, illustrating how the part is oriented inside. You can see how the liquid resin would rise during the pour and not encounter any surfaces that would trap air.
Because there is a lot of fine detail on the surface of the part, rubber is rubbed over the entire pattern to insure there are no trapped bubbles.
The part is now ready to have the rest of the rubber poured. Note that the brass pins are in place in the walls of the mold. It is important to drill both sides of the box at the same time so these pins will be dead square to the walls of the mold.
The trick to making this work successfully is how the part is cut out of the mold. Working carefully (and really impossible to photograph when you're working alone!) the idea is to cut along the corner edges of the part, creating 'hinged flaps'. This way, the mold closes where there is the least amount of detail and the line follows a natural transition in the part. Because the mold is made in the 'closed' position, the flaps made when cutting the part out will naturally go back to their original shape and the pins will hold the whole thing in proper registration during the pour.
Here you can see the difference between having the pins in place or not. I'm using the same amount of effort to hold the mold open in each shot!
With the pins in place, the mold is held perfectly closed. Since I shot these photos, I've discovered that putting a rubber-band around the mold before pouring the resin insures absolutely perfect alignment. Small molds, such as this, shouldn't need that, however.
Resin is poured into the round hole until it comes out of the slit. Note that the stand-off used to create the slit was exactly as wide as the flat surface of the pattern. This way, no air could be trapped since the vent goes all the way across the part!
This looks really ragged, but here you can see how the part has formed inside the mold. Molds don't have to look good to work well. In fact, I'd say my ugliest molds work the best! The only thing a mold must be is 'square'. Building a non-square mold box to pour rubber into is just begging for trouble!
Now you can see the entire mold. This kind of mold will work even if there is detail on the inside of the part.
As all of you are aware, flash is one of the banes of working in resin. With this kind of mold, even the flash is kept to an incredible minimum, as noted by the two arrows. Aside from the pour and vent sprues, this is the only thing that needs to be cleaned up on this part!
What you see here is representative of almost all RTV molds, with the exception of the brass pins sticking out. It is these pins that have made such a difference!
This particular mold is too large for me to handle and photograph at the same time, so I'm going to switch to the smallest mold--
It is important to remember that when a mold is made, it is upside-down-- the 'top' of the mold is at the bottom of the mold box. What I'm going to show you here works for quite a number of parts, especially those that are flat on top. Remember, these spine modules are hollow, 5-sided boxes and normally I'd be using a two-part 'squish' mold to cast them.
The part is mounted on an angle so that the pour (through the round hole) will travel 'uphill' against the inside of the mold, toward the vent (the 'slit' at the top)
This is the mounted pattern and the mold made from it, illustrating how the part is oriented inside. You can see how the liquid resin would rise during the pour and not encounter any surfaces that would trap air.
Because there is a lot of fine detail on the surface of the part, rubber is rubbed over the entire pattern to insure there are no trapped bubbles.
The part is now ready to have the rest of the rubber poured. Note that the brass pins are in place in the walls of the mold. It is important to drill both sides of the box at the same time so these pins will be dead square to the walls of the mold.
The trick to making this work successfully is how the part is cut out of the mold. Working carefully (and really impossible to photograph when you're working alone!) the idea is to cut along the corner edges of the part, creating 'hinged flaps'. This way, the mold closes where there is the least amount of detail and the line follows a natural transition in the part. Because the mold is made in the 'closed' position, the flaps made when cutting the part out will naturally go back to their original shape and the pins will hold the whole thing in proper registration during the pour.
Here you can see the difference between having the pins in place or not. I'm using the same amount of effort to hold the mold open in each shot!
With the pins in place, the mold is held perfectly closed. Since I shot these photos, I've discovered that putting a rubber-band around the mold before pouring the resin insures absolutely perfect alignment. Small molds, such as this, shouldn't need that, however.
Resin is poured into the round hole until it comes out of the slit. Note that the stand-off used to create the slit was exactly as wide as the flat surface of the pattern. This way, no air could be trapped since the vent goes all the way across the part!
This looks really ragged, but here you can see how the part has formed inside the mold. Molds don't have to look good to work well. In fact, I'd say my ugliest molds work the best! The only thing a mold must be is 'square'. Building a non-square mold box to pour rubber into is just begging for trouble!
Now you can see the entire mold. This kind of mold will work even if there is detail on the inside of the part.
As all of you are aware, flash is one of the banes of working in resin. With this kind of mold, even the flash is kept to an incredible minimum, as noted by the two arrows. Aside from the pour and vent sprues, this is the only thing that needs to be cleaned up on this part!
