This is a capital M in 36 point cut in half to show the huge air bubble in the body of the type.
My Monotype Composition Caster is a British-made one, with a serial number indicating fairly late manufacture (1960’s) so it should in theory be fitted will all the latest bells and whistles. But for some reason it is missing the pump modifications that are required for casting larger size display type. As a result when I try casting 36-point type I end up with hollow type whose face caves in after only a few impressions. Properly cast type of this size would have thicker solid walls and its core would be a froth of small bubbles.
There are three main requirements to casting sufficiently solid type:
- The molten type metal is injected into the casting mould by a single stroke of a piston pump, and this stroke must be able to pump enough metal to fill the type.
- The pump stroke must be fast enough that the mould fills before the metal in the injecting nozzle solidifies, stopping any further flow. Fast injection also replaces the large single air bubble with a froth of smaller bubbles, which is much stronger.
- The final pump pressure must be high enough to compress the bubbles to a small size. There is little chance for the air that starts in the mould cavity to escape, so the only way to reduce the bubble size is to compress the bubbles using several atmospheres of pressure.
My caster had the high-strength spring required for item (3), but did not have the extended-stroke pump required for (1) or the pump latch required for (2). As a result I am now working on installing these two items on my caster. This is proving to need a combination of scavenged parts and home-made parts.
The extended stroke piston consists of a piston rod modified to allow it to go further down during its stroke, along with a piston made shorter so it does not bottom out on the extended stroke. I have several of these shorter pistons collected as spares from various sources, but I find I have to make my own piston rod.
As for the pump latch mechanism, which holds back the pump piston then suddenly releases it, I finally have one to install, thanks to Rich Hopkins. This too needs some work though, as a couple of related parts are missing and the pump latch has some screws snapped off in their holes.
Finally, installing the pump latch, once I have it ready to go, requires disassembly of part of the pump operating head, so I decided that while I’m at it I would take the whole pump head apart and clean all the parts of the accumulations of hardened oil and gunk.
I will have more to say on each of these three phases in future posts.
Regarding “There is little chance for the air that starts in the mould cavity to escape,” isn’t this why Hartzell offered to vent molds? I’ve seen vents made in shop with the careful use of a Dremel. Great post! Thank you for sharing.
Terrence,
The Hartzell venting allows air to escape from the jet cavity but not, in an obvious manner, from the mould cavity proper.
However, on thinking about things more, I suspect that the injected metal enters the jet area as a laminar jet with a round cross-section, and that the momentum of the metal carries it through the slot into the cavity proper with little turbulence, so that, at least for wider sorts, some of the slot remains open to allow air to escape the cavity proper, into the jet area, and out the Hartzell vents.
This thought came to me when reading about how the injection occurs in a Ludlow caster: the mouthpiece slot is narrower than the mould width (6 or 12 points) so the metal shoots clear up from the mouthpiece and strikes the matrices, pretty much filling the mould from the top down, and air can escape through shallow grooves all along the edge of the contact area between the mouthpiece and the mould.
The advantage of a non-turbulent straight jet of metal coming out of the nozzle and through the jet slot would explain why some minor nozzle blockage, or use of the wrong nozzle or wrong crossblock hook, has such an impact on type quality.
I have at least one mould where I think Hartzell made their vents too wide/deep. Ideally the metal that escapes these once the mould is filled will solidify partway along the vents, plugging them up. On this one mould, the vents are wide enough that the metal doesn’t immediately solidify, so I get a spray of fine metal droplets accumulating on the side of the pump that faces the nozzle. This could also mean I’m running my caster too hot, but the vents certainly have the look of being made using a hand-held grinder of some sort and so would vary from one mould to another.