what are mold lines?

[modelbuilder22]

does any one here have a link or can tell me what mold lines are. I did some research on here and and can not find what iam looking for. a picture would help big time. I will try google to see what i can find on their about mold lines.

[Harry P.]

Injection molded plastic kits are made by injecting molten styrene under pressure into two (or more) piece steel molds. The plastic fills in the hollow cavities in the molds. These hollow cavities are the "reverse," or negative shape of the finished parts. The molten plastic fills those voids and that's how the plastic parts are formed. (The molds are in separate parts because they have to be opened up in order to eject the finished plastic parts once the styrene has cooled and hardened).

Wherever two parts of the steel mold meet, there is a seam. The more wear on the molds, the sloppier the fit between the mold pieces becomes, and that joint between the mold pieces might not be quite as tight as when the molds were newly machined. Molten plastic will fill any gap or misalignment between the steel mold pieces... causing those "mold seam lines" that you see on the finished parts.

[modelbuilder22]

thanks harry, iam still looking for a pic to show me what they are, as this is the first time i am ever removing the mold lines.

that did explain what they are. so i know what they are now just gota find them

[sjordan2]

Harry described well how it works. In this image of a 1/12 Mercedes Gullwing, you can see a really bad vertical mold line between the rear wheel and rear bumper, requiring some heavy-duty sanding to remove it. That's the least of this kit's problems.

Edited February 29, 2012 by sjordan2

[Harry P.]

Wherever you see a raised, thin ridge on a part that wouldn't really be there on the real thing (like a raised ridge or line on the body where the real car has no such line)... that would be a mold seam line. Good, basic building technique includes always sanding off any visible mold seam lines before paint and assembly.

[modelbuilder22]

thanks guys really do appericate it, iam building the 69 camaro ss kit from mpc 1/25

[cruz]

I like to highlight mine, it's easier to sand them off when you can see them!

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[cruz]

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[modelbuilder22]

so are they mostly on the engine and chassis i really couldnt find much on the body but did find a lot on the chassis

[sjordan2]

One of the worst things about mold lines is when you find them on chrome parts. You pretty much have to destroy the plating to remove them.

[Jordan White]

They could be on any part really, just depends how the piece was oriented when it was molded. Some lines are obvious, others not so much.

[kingtoy]

Well modeling has come along way in the past few decades. It use to be kit makers would put mold seperation lines in random places. Now, they do a great job at hiding them on belt lines, hood openings, bumper lines and drip rails. The best way to find them is run your finger tip over the body. If you feel a slight scraping feeling or dragging, you found a mold line.

[Harry P.]

so are they mostly on the engine and chassis i really couldnt find much on the body but did find a lot on the chassis

They can be anywhere, on any part. And by the way, that second photo cruz posted is not a mold seam line, but an ejector pin mark. Ejector pins are steel pins that pop out from the surfaces of the steel molds to "pop" the plastic pieces out of the mold. The plastic is still very warm when it's ejected from the mold... that's why the round steel ejector pins will leave an indentation in the part. Ejector pin marks are like mold seam lines... they aren't there on the real thing, and should be sanded off on the model pieces.

[sjordan2]

Well modeling has come along way in the past few decades. It use to be kit makers would put mold seperation lines in random places. Now, they do a great job at hiding them on belt lines, hood openings, bumper lines and drip rails. The best way to find them is run your finger tip over the body. If you feel a slight scraping feeling or dragging, you found a mold line.

Interesting to note that in my Jo-Han 1/25 kit of the Mercedes 500K coupe - tooled in the late 60s - they took extra pains to make the mold lines follow the curvature of the character lines on the fenders, making them virtually unnoticeable. To me, this is one of the best-engineered kits I've ever dealt with, though there are plenty of mold lines on mechanical bits like shock absorbers.

[The70judgeman]

Here is a two part injection mold. They are milled from a solid block of some sort of metal...aluminum or steel, etc. After many uses being bolted together, taken apart, and use, they wear on the edges where the two pieces meet.

[The70judgeman]

This picture is a doll leg with a distinct line down the middle of the plastic. That is a mold line. All you need to do is carefully sand it down to match the profile of the piece.

[Harry P.]

Interesting to note that in my Jo-Han 1/25 kit of the Mercedes 500K coupe - tooled in the late 60s - they took extra pains to make the mold lines follow the curvature of the character lines on the fenders, making them virtually unnoticeable. To me, this is one of the best-engineered kits I've ever dealt with, though there are plenty of mold lines on mechanical bits like shock absorbers.

They spent the time and money where it counted... the body. Those parts not as visible on the finished model didn't get any "special treatment."

[modelbuilder22]

Thanks guys for all the fast response's

[Guest Johnny]

We have those on our window sills right now!

Oops! Wrong kind of mold!

[Guest Dr. Odyssey]

<span style="font-family: comic sans ms,cursive">This picture is a doll leg with a distinct line down the middle of the plastic. That is a mold line. All you need to do is carefully sand it down to match the profile of the piece

Actually that is one is supposed to be there. Makes it easier after applying the fishnet stocking decals to add the line that runs up the back of those stockings...

[sjordan2]

Actually that is one is supposed to be there. Makes it easier after applying the fishnet stocking decals to add the line that runs up the back of those stockings...

Hah! My mother told me that during World War II, when nylon was rationed and unobtainable as stockings, women would take something like mascara (can't remember what exactly) and draw seam lines up the backs of the skin on their legs to make it look like they were wearing stockings.

[Guest Johnny]

Hah! My mother told me that during World War II, when nylon was rationed and unobtainable as stockings, women would take something like mascara (can't remember what exactly) and draw seam lines up the backs of the skin on their legs to make it look like they were wearing stockings.

Heard my aunts talking about that too! I think it was eye liner pencils they used.

[martinfan5]

I am getting better about removing the mold lines, I try and remember, but is usually after I already painted it

[heatride]

I like to highlight mine, it's easier to sand them off when you can see them!

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Thanks for the tip Marcos ! Gonna use that one ..........

[martinfan5]

I now prime the bodys to help me find them, then reprime to paint