Ejector pin marks; a lucid explanation.
by Tom Berto
I was reading your review of the
howitzer kit today, and realized that I can explain why ejector pin marks
usually end up in the middle of the detailed side of a molded part. Please
pardon me if this is review of something you already know about, but I
don't think it's commonly known and I realized that I could explain it to
It has to do with the molding process, and in particular trying to get completed plastic parts out of an injection mold. Molds have two (sometimes more, but almost always two) main parts to them that form the shape. For simplicity's sake, let's postulate a simple mold with just two halves, one of which has the ejector pin plate. Almost all kit parts are made with molds like this. Parts that have holes in them that are not perpendicular to the parting line would need a sliding part of the mold, usually called a "slider", that pulls off to the side before the mold opens up to release the part, but these are rare in molds used to make model kits.
Considering our simple two-half mold, then: one half will be fixed, attached to the molding machine. Hot plastic comes in from this side, and it is referred to as the "cavity" side. This side doesn't have ejector pins, because it would be hard to get them to work without having the half moving, and it's hard to move the cavity side and maintain the integrity of all the seals that keep hot plastic moving in the intended manner.
Because the cavity side doesn't have ejector pins, the part MUST always naturally pop off this side and stick to the other side when the mold opens. If the part sticks to the cavity side, the molder has to shut down the machine, slide back all the shields, then manually reach in and pry the part off. This is dangerous, and difficult to do without damaging the mold, the cost of which is usually a multiple of the molder's annual pay.
So the part has to stick to the other side, which is referred to as the "core" side. The core side includes reamed holes, inside which are the ejector pins, which are all permanently fixed to a sliding pin plate. After the plastic has been injected and has had enough time to become reasonably solid, the core side pulls away from the cavity side, taking the molded part and the pin plate with it. After a certain amount of travel, the pin plate comes up against a stop and the pins push the molded part off of the core side, usually into the cluthces of a waiting robotic arm that takes the part out of the mold and sets it aside.
The rub is that a plastic part, given a choice, will stick to the side that has more detail. The part shrinks more than the metal does as it cools, so it grips all the metal mold walls that form the ribs and other features. The deeper the relief and the rougher the surface finish, the more it sticks. Since the mold designer wants the part to ALWAYS stick to the core side, the core side is the side with the detail, steeper angles, rougher surface finish, etc. Unfortunately, it's also the side with the pins.
Ejector pins are machined to be at or
slightly below flush with the core side of the mold, which means either
flush or slightly raised ejector pin marks on that side of the plastic
part. Pins that stick out of the core side mold cause big problems - the
plastic shrinks onto the tip of the ejector pin - so the part ejects just
fine from the core side, but doesn't want to come off the ejector pin.
When the robotic arm comes to remove it, there is a mechanized tug of war
between the ejector pin and the tiny, delicate gates holding the part.
Usually, the ejector pin wins and rips the part off the "tree". The part
stays in the mold, and the "tree" is missing ... the instrument panel.
There are a host of variables that can be adjusted - plastic injection rate and temperature, cavity side and core side temperatures, delay before mold opening, injection pressure, etc. One might think that one way to reduce ejector pin mark depth would be to wait a moment longer before opening the mold, so that the plastic is a little harder and so doesn't get indented by the ejector pin. Yes, but the longer the wait, the harder the plastic grips the cold side, so the harder the pins have to push.
I'm certain that there is a lot of tweaking before the production parts are run. I'm certain that there are also a lot of other tricks that molders can use that fall way outside my knowledge of the process. Molding has come a long way in the last twenty years, to the point where it resembles black magic in some ways.
Cheers (and keep up the excellent work!)