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Styrene Kit Defects & Fixes


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The easiest defect to notice is flash on the parts and/or the sprues. Flash happens then the two halves of the mold don't join perfectly. Then, when the styrene is injected, some of it oozes into the gap between mold halves. A small amount of flash isn't a big problem, but if you see a lot (as in this image) then you might expect there to be some fit problems when you assemble the parts. Since there's enough room in the mold for flash to form, then there's likely some extra plastic on the parts too. If you find "flashy" parts, mark them so that you can pay extra attention to their fit during assembly.

(from the "Area S4 UFO" kit)


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Sinks are another common defect in styrene kits. You'll typically find these on thicker areas of kit parts. The plastic contracts as it cools in the mold, leaving a small sunken pit in the part. A lack of sinks is an earmark of quality in a kit.
Later, you'll fill in the sinks with filler putty. For now, just note where they are.

(from AMT/ERTL's Episode 1 Droid & STAP)

Ejector Pin Marks

After a sprue has been molded, small pins embedded in the mold pop the piece out. In most case, the pin marks will be on inside surfaces of the parts or someplace where it won't be seen after construction.
The pin mark on the right won't be hidden, so it needs to be filled - although it is clean enough that it could be left as "detail."

(from AMT/ERTL's Episode 1 Droid & STAP)

Incomplete Molding

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It's rare that you'll find a defect like this - not enough plastic made it into the mold, resulting in incompletely-formed parts. Notice the "whiskers" of instrument panels along the edge of this cockpit tub.

(from Hasegawa's 1/72 F-4 Phantom)

Draft Angle Errors

These aren't so much molding defects as they are a manufacturing shortcut. Notice how the marked edges of these panels slope - there's no crisp edge. In order to be able to get the parts out of a two-piece mold, the edge slopes. If it were a crisp edge, it would catch on the mold when popped out. A better manufacturer would have used a 3-piece mold on these parts, or made more parts for the kit. This can sometimes be fixed by filing a crisp edge along the panel.

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(from AMT/ERTL's STMP & ST6 Klingon Cruiser) You'll see similar problems with the secondary hull aft deflector grid lines on Monogram's USS Voyager kit, except these can't be fixed by filing.

This is a particularly-glaring example of draft error. The back of Robby's leg (bottom) has a sharp edge, but this edge is lost on the front part of the leg.

So, what does a good molding job looking like? Well, try Revell's 1/48 Mercury / Gemini kit.

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  • Crisp detail - all facial features on the astronaut can be made out. The wiring bundles are distinct and completely formed. the corrugated surface of the capsule shells are distinct.
  • Minimal draft angle errors in the capsule shell, because it's molded in three parts.
  • Parts are connected to sprues with tiny runs of plastic. Tiny runs are easy to cut and don't leave a large stub on the piece.



The Model Citizen's "Sprue U" pages are �2000-2004 Roger Sorensen
last updated 04 December, 2007