Arrangement of sprues on molds

Dave, it depends on several issues.  First, how big is your injection molder?  It may have enough space for 2, 3, 4, or even more sprues.  So theoretically, you could set up your machine to make an entire kit all at once every time the machine fires.  Or, you could set up and make several copies of sprue "D" each time, and it takes 4 set ups or 4 machines to make an entire kit.  The mold is formed from hardened, machined tool steel, for long life, and is hard to make changes to unless the change involves removing some material, but there is no going back if you do that.  You can set up a mold with inserts, so that for example, the different noses for a Mustang could be interchangeable in the mold, since the rest of the pieces in that sprue could be the same.  This would be more expensive up front in making the mold, but allows the mold to be more flexible.  Injection mold design has a certain level of art in it, to optimize the flow pattern so all of the sprues are filled, but still make the mold as busy as possible, since the major cost of the kit is the mold.  Hope this answers your question, but there are I'm sure several videos on Youtube that would explain the process in more detail.

John

Thanks for the info. I should of thought about checking out Youtube to begin with.

Dave

Hi ;

    Having worked in plastic at one time I can tell you this .Molding machines are very expensive .Now the mold for say a simple destroyer hull won't cost much . If you do the hull in halves . If you choose to go to slide molding , well that's a bigger more expensive machine and mold too . Our molds in the sixties ran from over forty-five thousand to one hundred thousand apiece ! Because of that cost we had to start out with a used machine that was compatible !

      It is simpler to put like pieces on the same mold then Block them out when needed .That's what we did . Remember the technology has made millenial jumps since then . I saw a picture of a LEGO brick molding machine ( for the regular brick) . It cycles ever twelve seconds and is half the size of our old beast . It runs thousands a day too !

 A machine can pay for itself quickly , the molds not as fast . I asked a friend who got the picture for me what he thought . He told me that LEGO spent almost two hundred thousand to get that mold just right for their product ! That's a lotta bricks to pay that bill !

One thing I don't get is why on certain manufacturers are the parts for let's say the pit spread over several trees instead of being grouped as some do. It's a real headache when there are 10 to 20 trees to go thru.

One particular example is the 1/35 Eduard Hetzer  that has a complete interior and has something like 15-20 trees having the interior parts scattered thru out multiple trees. I remember setting them out over the floor to be able to jump from one to the other gathering the tiny parts. Much easier if kept grouped.

If the parts are in multiples of two, usually there will be one on the sprue- two sprues make two; two on the sprue- two sprues make four etc.

Wait until/ if you get into Dragon tanks, in particular Shermans. They'll just add sprues for the various models, I think the last one I bought had Firefly, A2, A4 and "Normandy added armor" all in the box.

Pretty handy for spares.

I have an Eduard 1/72 Fokker triplane. The instructions show two sprues (or runners, if you like) - A and B. What's actually in the box is one sprue. Half of it's marked A and the other half B. They're connected, side by side, by one big pour stub. Both obviously came out of the mold at the same time.

I think this molding method is quite common: the whole kit is squirted int one mold, and the "individual sprues" are busted apart so they fit in the box.

Which brings me to one of my minor peeves. Why, oh why do kit reviewers thee days religiously recite the number of sprues in a kit? That number (a) doesn't mean anything whatever, and (b) provides the reader with no useful information whatever. (That beautiful little Eduard kit has either one or two sprues, depending on how you count. And what conceivable difference does that make?)

FSM's reviews make more sense, in that they always give us an actual parts count. That number certainly can be misleading, but it does tell us something useful.

I imagine creating the layout of parts is like working a jigsaw puzzle, or laying out the artwork for a printed circuit board.  Lots of trial and error and intuition required.

Hi Proff !

I am with you on that .I don't care how many sprues there are . I want to know how many total parts there are ! T.B.

Parts count is only slightly more helpful. My resin  1/350 Combrig Scharnhorst probably has less than 100, although I'll need to make several times more than that before I'm finished.

Some of these new tank kits get to 1,000 pretty fast. But it's 36 of this, 72 of that, 128 of this, 256 of that and so on.

A Sherman I am working on has more parts that the guns on my big Victory.