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Modeling in General: Advice on...
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MartinJK
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British Columbia, Canada
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Posted: Tuesday, February 13, 2007 - 10:58 AM UTC
While cleaning up spure gates from parts, you can never truley get rid of the mark that is left on the part. For example, this is my method of removing spure gates. Using side cutters, I cut as far from the part as possible ensuring no to damage the part. Using a flat metal file, I gently file the remaining stub down to a safe level, then using fine sandpaper, carefully eleminate the stub. Thing is, no matter how much you sand and file, what once looks like a perfectly flat surface, when buffed, and looked at a certain angle torward the light, there is ALWAYS a little bump or imperfection left over. Like I said, this bump only appears when you polish the part and look at it at a certain angle.
Like I stated, this is not due to the part being damaged. I give up, how do you car modellers or plane builders get such a smooth finish?
Any advice or am I being too picky?

~Martin
MeCOwhy
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Posted: Tuesday, February 13, 2007 - 03:13 PM UTC
I always had the same problem building planes, and unfortunatelly haven't found a real solution yet. I once overdone the sanding trying to get rid of a nasty bump and it all ended with applying a thin layer of putty on that spot and some more polishing. The end result was satisfying but it took me quite a lot of time to get the part really smooth after fixing it with the putty.

It would be cool if someone shared some good ideas on the topic before i get to painting my HMMVW, cause I have a really ugly spot left where the part was connected to sprue, although it looks smooth at first sight.
slodder
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Posted: Tuesday, February 13, 2007 - 04:28 PM UTC
I' ve seen and experienced what you're talking about.
The way I get rid of the sprue bump is to snip the part away from the sprue at a reasonable distance.
I then use a sharp #11 blade and cut off the remaining piece as close to the part as possible. I then sand down the remaining mark.
A good base coat of paint will add one last evening layer.
Jamesite
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Posted: Tuesday, February 13, 2007 - 05:20 PM UTC
My theory is that as the plastic was poured through that point it somehow differs from the surrounding plastic in its texture, leaving us with this problem. I can't prove this but its my best explanation.
I often notice the plastic at this point can differ in its colour slightly which could back up my idea. Any thoughts?

Anybody who knows about plastic moulding is free to disprove/prove what i'm going to call James's Spruce Theorum :-) :-) !

James
drumthumper
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Posted: Tuesday, February 13, 2007 - 05:22 PM UTC
I agree with Scott's remedy of the dilemma. I believe the problem is created when we use nippers or cutters to remove the sprue too close, it distorts the part - therein leaving the often faint, but trying blemish. I'll usually cut the gate from the sprue tree, then use a single-edged blade or fresh No. 11 to remove the remaining tab from the part. A quick swipe with sandpaper typically finishes off the area.

Mike
drabslab
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Posted: Tuesday, February 13, 2007 - 06:50 PM UTC
I use a nailclipper. The cutting edge is slightly bend which helps a lot to follow the surface
Gunfighter
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Posted: Tuesday, February 13, 2007 - 06:53 PM UTC
I too have noticed this and do what Scott and Mike do. As for why it acts differently, maybe the plastic cools differently in this area?

One thing I have noticed in some cases, such as using a duller #11 or too fine sandpaper, you start to polish the area rather than sand it down.

James - "Spruce" Theorem? I didn't realize evergreens had an impact!

I like that though, the Sprue Theorum of Relativity.

- Frank
MTDriver
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Posted: Tuesday, February 13, 2007 - 10:17 PM UTC
Yeah,I've noticed this problem too,my thoughts are is it something to do with compression of the plastic when using side cutters?Just lately I've taken to cutting finer and the more exposed parts off the sprue with a very fine etched saw blade.If the parts surface doesn't need to be absolutely perfect,I use the cutters,still taking care though to achieve a good a finish as possible.Since using the saw the problem is not as great,but it can be a little tricky at times.
Dave.
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Posted: Tuesday, February 13, 2007 - 11:33 PM UTC
I use a pair of despruing tweezers I bought at Micro Mark a few years ago. Just a swipe or two with a needle file or snading stick and done.

cfbush2000
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Posted: Wednesday, February 14, 2007 - 05:21 AM UTC
Thanks for the tip Rob. I just went to thier site and ordered a set.

Chuck
MartinJK
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Posted: Wednesday, February 14, 2007 - 08:20 AM UTC
Gents.
Thank you for contributing into this post.
For many of us, myself included, modelling is about the building park of our hobby. There is noting more I enjoy than a perfectly put together model. I have been noticing this problem on my builds and for many years now I have been trying to eleminate this. I would have to agree with James on that this problem is created in the moulding process. As the plastic gets injected into the mould from the spure attachment points, it changes the grain of the plastic in comparison to the rest of the part. It is not from careless cutting or sanding as I always ensure to cut away as far as possible, then gently file the nub down to a safe level. As mentioned, the next step is to gently sand the remaining plastic with sandpaper 300 - 600 grit. I always ensure this is done properly as cuttin too close to the park will leavy ugly scars in the plastic that are nearly impossible to get rid off.
Keep them good tips coming so we can finally put an end to this conspiracy

~Martin
jw73
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Posted: Saturday, February 17, 2007 - 08:09 PM UTC
I use these tools and I think it is the best. Look here www.xuron.com

kevinb120
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Posted: Sunday, February 18, 2007 - 01:25 AM UTC
The best way is to trim it 1/16" or more from the part, then shave it with a very sharp razor. If you cut it too close it will leave a noticeable scar. even with sharp snips, the injection tab does have plastic that is flowing perpendicular to the part as it cools. The slight crush/squash before the snipper blades actually pierce the plastic will tear it slightly leaving that rough area or even a hole.

The razor blade cleanly cuts through this without tearing plastic at the joint. If it is a considerable nub, you should shave it down in a few passes for the same reason, rather then trying to force/chop it in one swoop. If you are using considerable pressure to cut it, its probably taking too much off at a time and will leave a pockmark scar. You will learn this very quickly building ship models, at 1/350th, a sprue scar is huge! Rotor blades or AA guns in this scale(by the hundreds in a build) will eat up a lot of razor blades. Even the most complex armor model is much more forgiving then a plastic ship.

You should also plan your snipping to not stress a part, like a grab rail. Sometimes clipping the middle nub first will break the part. Even with sharp snippers, the blade gets very wide quickly after the cut and can bend parts. Figuring a nub that is 1/16" thick, you will spread the joint that wide with snippers after the cut is made. Some are better using a razor to trim. You can also support the joint with a piece of scrap balsa wood under the sprue so you don't over stress the part .

I use straight razors as they are 5 to 1 cheaper then #11's, they are sharper, and have 3 times the useable cutting area. Nor to the tips break off, stab, or flex. Exacto blades all have special shapes for special needs, #11's are not designed for bulk cutting of parts, but for detailed trimming in certain situations. I keep the exactos for corrective work. Plus you can carefully 'shave' parts much faster when you can grip the entire blade.
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