Giving up on Strip Casting (for now)

After several unsuccessful attempts at casting strip material on my Monotype Composition Caster, I’m giving up for a while.

The problems I’ve been fighting with include:

  • Unfused strip material, which easily breaks up into pieces
  • Blowback through the nozzle seat
  • Mould jams (strip material doesn’t advance)
  • Difficulty adjusting the mould clamp
  • Incomplete filling when using wider (6pt) moulds
  • Under-table squirts which foul the nozzle and its seat
  • Temporary misplacement of my collection of rule matrices, reducing the utility of strip casting in general

The first two can be attributed to the temperature of the metal and the mould. Unfused strip material occurs when there is not enough heat to partially melt the end of the previous cast so it can fuse to the new cast. Blowback occurs if the metal and/or mould are too hot and the metal is still molten when the pump disengages from the mould, allowing the pressurized air bubble in the mould cavity to blow the molten metal back out of the nozzle port. I have also recently made some improvements to the cooling water system on the caster, which should help with temperature control.

I found myself sort of thrashing trying to solve these interacting issues and decided it was time to take a step back and think about the problems more rationally. In the meantime I’ve removed the strip-casting attachment and converted the caster back to type casting.

Since then I’ve already found possible (and even likely) causes for some of the problems:

I had already determined how to deal with poor fusion and/or blowback but it is impossible to find the sweet spot between these extremes unless the rest of the casting is going smoothly.

Close examination of the pump piston has revealed that the ports on it which admit the metal into the pump chamber were pretty much completely plugged up, explaining the poor filling on larger moulds. These have now been cleaned.

I had not noticed that the caster was running with one of the gag plates installed (under the pot bracket). These plates cause the pump to start pumping a little earlier in the cycle. This could potentially lead to leakage around the nozzle if the pump trip latch is not also in use. I had tried casting with the trip latch both on and off and neither position seemed to make much difference given the randomness from all the other problems. It should be noted that although the trip latch delays the onset of pumping until the nozzle is seated (producing a burst of metal flow instead of the slow onset of flow with no plates or latch), it does nothing about the delayed end of the pump return stroke. Thus if the mould cavity is very small, or still contains unejected metal from a previous cast, there can still be pump pressure when the nozzle disengages, leading to the under-table squirt. The mould I had squirting problems with was a 2-point strip mould; with a ½″ mould blade stroke that amounts to about the same volume of metal as an 8- or 9-point em quad. Clearly I should not have any of the plates engaged for this. So this might explain the under-table squirting.

The mould jams and clamp adjustment sort of go hand-in-hand. The manuals seem to imply that the spring on the vertical rod that operates the clamp should compress a tiny bit (1/16″ seems to be the target value) during the casting part of the cycle. I was finding, though, that this was preventing the cast material from advancing out of the mould. I have since realized that there are two things that resist the clamp screw: One is the stiffness of the mould cavity sides, the other is the previously-cast material already in that area of the mould. If the clamp is adjusted with too-thin material preset in the mould, the prescribed amount of clamp pressure will close the clamp too far (as only the flex of the mould side resists the clamp force). More importantly, the clamp will not open far enough to freely admit the next cast piece, since the caster only rotates the clamp screw about a tenth of a turn. Next time I’ll have to experiment with this adjustment more. I’m not entirely sure what the effects are of not closing the clamp enough (or not closing it at all), partly because I don’t know if the mould cavity naturally has some taper to it, which the clamping is supposed to take up. Perhaps fixing the squirt issues will allow me the freedom to do such experiments in an effective manner. The mould jams could also be because my home-made spring box does not contain sufficiently stiff springs. Yet another cause could be some dirt incursion when I serviced the mould, so I’ll have to take it apart and re-assemble it again in a cleaner (and cat-hair-free) environment.

I seem to always have something around the shop that I’ve lost (after having stored it in a “safe”/”obvious” place!) and don’t find until long after I’ve given up, and am searching for the next unfindable object. I had a mat holder in this state of limbo, and recently my collection of rule matrices has been unfindable. In the process of looking for my latest unfindable (some large cuts, still not found), I have located the matrices so I can try them with the 6-point mould next time I’m trying strip casting.

I think I have some things to try for all the problems I’ve encountered so another attempt at strip casting is on the to-do list.

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