--- In STMFC@..., "Steve Haas" <Goatfisher2@...> wrote:
"What I don't fully understand is the concept of "slides" in a mold."
Jeff,
Using an Athearn 40 foot box car kit as an example, the following might
help to understand the concept. Keep in mind that I'm a modeler, not an
injection molding engineer, so my terminology might not be correct, but you
should be able to get the general idea.
The molds for the kit could consist of several movable pieces:
1) A mold for the interior of the car
2) Molds for each side of the car
3) Molds for each end of the car
4) A mold for the roof of the car.
That's pretty close in concept. The part that forms the interior is typically called the "core" (as is any part that forms an interior void in the part.) In model RR practice, the core is typically on the movable half of the mold, which also carries the ejector system. Parts tend to shrink onto a core, so this is the side the part will stay on, to be pushed off by the ejector pins.
By convention, the side opposite the core is the "cavity", even though ours aren't very deep. This is the side that has the sprue where the plastic enters the mold, or a heated sprue bushing (AKA "hot tip") that serves the same purpose. This is on the stationary side of the mold so the machine nozzle can stay against it. Practice varies, but cars with attached roofs usually have the roof detail in the cavity; cars with attached floors have the floor detail. There have been a few models over the years that "shoot through the core", but it makes the mold more complicated, therefore more expensive.
The detail on the sides and ends is oriented 90 degrees to the "parting line", the plane where the mold splits, so the "side cavity inserts" are mounted on "slides", or "side actions", movable pieces that back away from the part as the mold opens. These usually surround the core, so the part is stationary relative to the slides as they open, then once fully open the ejector pins push the part off the core. These four side inserts need to mate with the cavity insert and each other without gaps; +\-.0002" is acceptable tolerance for this fit, and a .0006" gap is too much, and will allow flash to form. Likewise the side inserts need to "shut off" against the core so there is no flash around the open side of the body.
Polystyrene isn't a liquid, it's "plastic". Hot styrene would be perfectly happy to sit there as a blob if left to its own devices, so it takes tremendous pressure to force it to conform to the cavity. That's why these parts are moldings; casting implies a non-pressurized gravity fed part. General rule of thumb is it takes between 2 to 7 tons to hold the mold closed against the pressure needed to fill each square inch of cavity area, the finer the detail, the higher the pressure required. The typical 40' HO boxcar has about seven square inches of area directly opposed to the parting line, and will require somewhere around 50 tons to hold the mold closed. In addition, each side has an additional seven square inches trying to push it away from the core, so the slides, and the wedges that hold them in place, need to be massive. All this extra area doesn't directly try to open the mold, most of it just tries to deform it, with only a portion vectored directly across the parting line, but a fifty ton press is the bare minimum required to mold most HO scale carbodies, and 75T is better.
Of course, all this tonnage (as much as a triple hopper load of coal!) is trying to crush the poor little mold, so it has to be pretty substantial. A typical mold for a 40' HO boxcar might have outside dimensions of 12" x 20", to make a part 1.3" x 5". Typical plate thickness is 2 - 3 inches for each side, plus the height of the part, plus another 3" or so for the ejector box, and weighs somewhere around 700 - 800 pounds.
If I get a chance tomorrow, I'll try to take some pictures.
Dennis
