What is "flash"

[SpencerC]

When talking about molded plastic what does flash refer to? As in "there are several knockout marks and a little flash" when referring to the parts on a spruce. And also what are knockout marks?

[Scale-Master]

It's sprue, or sprues, not spruce. Spruce is a kind of wood.

Flash is excess plastic that flows out of the mold on the edges of parts. More common on older kits, but still happens even on new kits.

Knock out marks or ejection pin marks are the usually round or half moon shaped depressions where the mold ejects the parts during production.

Edited June 2, 2014 by Scale-Master

[10thumbs]

Correct.

This squishing out of material from a mold happens when steel or other metal is poured into a cast as well, especially when the mold is two or more pieces.

Imagine pressing the buns of a juicy Hamburger together and the good stuff that squishes out....that's good flash.

I deal with flash in my job. Lots of work, but necessary to get the piece polished out. Resin products will have flash showing too.

The Lindberg 64 Dodge I'm working on has a lot of flash on the smaller pieces.

Michael

[Harry P.]

It's sprue, or sprues, not spruce. Spruce is a kind of wood.

Flash is excess plastic that flows out of the mold on the edges of parts...

Usually the result of old (worn) molds that don't seal well anymore face to face, allowing the molten styrene to "squish" out wherever the mold halves have a gap (a gap that should not be there!).

The more tightly and accurately the molds are machined and polished, the less flash you'll see. That's why flash is generaly more common on kits made from old, worn molds (reissues). A brand-new kit can sometimes have some, but it shouldn't, if the molds were made correctly.

Here's an example of flash...

[Harry P.]

And also what are knockout marks?

More commonly known as "ejector pin marks."

A model kit is made by injecting molten (liquid) styrene under pressure into a two (or more) piece mold, the liquid styrene flows into and fill the hollows in the mold, creating the plastic parts. The mold has several ejector pins in it, that extend out to literally eject (push) the new plastic parts tree out of the mold when it opens. If the plastic is still somewhat warm and soft, the faces of the pins will leave a round depression where they came in contact with the part. Most times, kit parts are designed and engineered so that the ejector pins will contact the side of the part that won't be seen when the model is built, but that's not always the case... sometimes the pin marks are on the visible side of the part.

Removing flash and pin marks is one of the very basic steps in building a model.

[my80malibu]

Two part injection molds have knockout pins, to eject the parts when the mold opens.
Flash is excess plastic. It happens when molds wear, but sometimes when the injection pressures are high, and blow the mold open slightly. Often Clamping pressures could be too low. It take's a team of workers to put a mold into action usually. The Guy who starts, isn't necessarily the guy who finishes the setup. There are many adjustments along the way before you finally shoot plastic.

Edited June 3, 2014 by my80malibu

[Ace-Garageguy]

And ejector pin marks are most often the result of slightly incorrect registration of the pins when they're in the retracted position.

Remember, they form part of the mold surface when they're retracted, but they have to move to do their job of pushing parts or trees out of the dies. Because they have to move, they also have to have mechanical linkages to MAKE them move, and if everything isn't absolutely perfectly aligned and clearanced, you'll have pin marks that are either higher or lower than the mold surface.

It's difficult to keep all of this stuff in perfect alignment in a computer model of a mold or die. In the real world with hot, functioning presses and molten plastic, it's almost impossible to hold ejector pin marks to zero. That's why any reasonably competent tooling designer will place the ejector pins on the side of the part what will not be seen.

But this is sometimes impossible...especially where both sides of a part will be seen...like the roof section of a car body...which will usually have pin-marks on the side LEAST LIKELY to be seen, the underside headliner.

Ejector pin marks are pretty much an inherent part of the injection-molding process, and only the most careful tool design and maintenance will minimize them.

The illustration also helps to illustrate how "flash" forms by being squeezed out between less-than-perfectly-fitting mold halves (parting lines), and around the ejector pins themselves.

Another issue with injection molded parts is "sink marks", where a slightly short-shot of molten plastic (caused by a variety of possible factors) will allow for more local shrinkage than is ideal. You'll see slight depressions in the surface, usually close to where the plastic was actually injected into the mold.

Edited June 3, 2014 by Ace-Garageguy

[southpier]

with everything in the market "gluten-free" these days, where does it all go?

and can I order something with extra gluten???

[MrObsessive]

This is flash!

The above photo------that's more of a deep parting line than flash, but you can see flash beginning to form. And yes, as Harry said----you want to spend extra time in getting rid of all those ejector pin marks, parting lines and flash. Very important as it's basic Modeling 101.

If you ever want to enter a contest (or even post pics of your work here on the board), neglect of the above WILL be noticed.

[Ace-Garageguy]

Is that a Johan Chrysler Turbine car? Man, that's REALLY flashy.

[MrObsessive]

Is that a Johan Chrysler Turbine car? Man, that's REALLY flashy.

Yup! Sure is! This is one I did eventually finish years ago, and it's one of the WORST molded ones I've ever seen from Johan! At least the flash wasn't too difficult to get rid of. I practically re-engineered the rest of the kit though to get it the way that it should be.

[Harry P.]

That's a very flashy model, Bill...

[MrObsessive]

I had a Superbird years ago that was just as bad............I guess at some point Johan just didn't have the money to invest in the cleanup of their molds.

[10thumbs]

Another thing about flash is, the reproduced object is probably thicker than the original mold, although minimal depending on the size of the object and the material used.

Anyway, flash is the stuff that's thinner and doesn't belong there.

Michael

[mnwildpunk]

That is just flash that more like a looooon stare lol