Why do some kits have fit issues?

[steelpillow]

There are three basic reasons for poor fit:

 

1. Imperfect tooling. This may be no more than failing to allow for shrinkage due to inexperience, or it may just be bad workmanship or laziness. Once a tool has been made, it is hard to put missing material back on! Sometimes, even trimming a bit away would thin the material in the wrong place and weaken it too much. Not everybody has the tooling to capture a prototype part exactly, reverse-calculate the distortions it will undergo when being moulded and then automatically form the tooling. Then again, some tooling in use today is over half a century old and is so worn that each moulding shot is a gamble.

 

2. Imperfect manufacture. Issues include The cleanliness, closing pressure and temperature of the mould, the consistency (e.g. air bubbles) and temperature of the material, the injection pressure and the curing time before release all affect the finished part.

 

3. Poor assembly. Sometimes it is easy to misalign parts and then have trouble fitting the next one.

 

All in all, it's a wonder that any kit ever goes together well. Hats off to those who do take the trouble to number-crunch the distortions in cooling and allow for them.

[treker_ed]

5 minutes ago, Phantome said:

 

That's not how making the mold works. Once the mold is done, it's done. If a piece is too wide, how do you shrink a steel mold to correct it? You can't. Some (very few) manufacturers may correct a kit by re-molding a certain piece but that's it. If an easy to replace part is wrong, the aftermarket community steps in and hopefully produces a resin replacement, not the manufacturer. Major dimensional errors are never corrected.

 

Also who is this mystery customer who buys the first kit produced? No company sells just one kit and then sits around expecting feedback...

The customer in question is the customer who ordered the mould, and the kit is the test pressing from the initial run from the first batch. Many magazines will have a build review based on a "test kit" - with caveats stating that the production kit may differ based on feedback from the test build. The "customer" may be Airfix, Revell, Tamiya, Eduard etc. They will also build copies of the test pressing and feed back to the mould manufacturer if there are any faults, and if any corrections need to be made.

[hendie]

1 hour ago, Phantome said:

 

That's not how making the mold works. Once the mold is done, it's done. If a piece is too wide, how do you shrink a steel mold to correct it? You can't. Some (very few) manufacturers may correct a kit by re-molding a certain piece but that's it. If an easy to replace part is wrong, the aftermarket community steps in and hopefully produces a resin replacement, not the manufacturer. Major dimensional errors are never corrected.

 

Also who is this mystery customer who buys the first kit produced? No company sells just one kit and then sits around expecting feedback...

 

Sorry but I beg to differ.

I've had roughly 20 molds designed and built in the last 12 months alone. From small parts less than 25mm^3 to larger parts around 380 x 265 x 50 mm.

Without exception, every mold has been cut steel safe because no-one can accurately predict shrinkage.  Try and hit dimensions first time around and if you are too large, it's a lot more difficult to add steel back to the mold than it is to remove it.  Plastic want to shrink an enormous amount "freely" - second stage prevents that although the parts will still shrink to some extent. Exactly how, depends on how the part is gated, what the process is, the material, plus a few  other factors. It will shrink differently in the X, Y & Z directions.

Once the mold is sampled the plastic part undergoes metrology (first article) and based on the steel dimensions vs part dimensions, plus an educated guess on shrinkage (based on first time around), the tool is adjusted, and goes through the process again.

I've had molds where we hit dimensions second time around - and I've had molds where it's taken 3 or 4 recuts to get into spec.  That is built into the price I pay the tool builder.

There's no mystery about the customer - read the post - the customer is the person/company who buys the mold.... airfix, hasegawa, me... whoever.

 

 

[DaveCromie]

On a related note to this topic I have always wondered why sprue gates always seem to be at the most awkward place imaginable e.g. example leading and trailing edges of wings, prop blades etc. Is it not possible for the molten material to be injected from the underside/inside of fuselages, wing and tail halves thus eliminating gates on leading/trailing edges?

 

I have no knowledge of injection moulding other than the finished product and equally I have no real complaints about the process, I just like to question existing practices to see if they can be improved.

 

DC

[steelpillow]

2 hours ago, DaveCromie said:

On a related note to this topic I have always wondered why sprue gates always seem to be at the most awkward place imaginable e.g. example leading and trailing edges of wings, prop blades etc. Is it not possible for the molten material to be injected from the underside/inside of fuselages, wing and tail halves thus eliminating gates on leading/trailing edges?

 

Several reasons. One is simply geometry: the mould has to pull apart without breaking up the sprue all over the place, so the opportunity for undercutting is at best limited. Second is cost - it's easiest to run the sprue along the same face of the tool as the part, to save making a ridge in the other half of the tool. The other is the need to distribute the material evenly and not leave unfilled gaps where a part is incomplete on the one hand, while not exerting excess pressure elsewhere. Finally, a part may fit only one way next to the others without making a wasteful and expensive mould, and that might just be an unlucky way round from the point of view of the runners.

[Vanroon]

7 hours ago, DaveCromie said:

On a related note to this topic I have always wondered why sprue gates always seem to be at the most awkward place imaginable e.g. example leading and trailing edges of wings, prop blades etc. Is it not possible for the molten material to be injected from the underside/inside of fuselages, wing and tail halves thus eliminating gates on leading/trailing edges?

 

I have no knowledge of injection moulding other than the finished product and equally I have no real complaints about the process, I just like to question existing practices to see if they can be improved.

 

DC

 

I'm a big fan of having the injection gates onto the mating surfaces. 

Hendie , thanks for your explanation of the process. 

 

Grant

[dancho]

I believe the instructions are often the source of "poor fit." The fit on even ancient Airfix or Revell kits is reasonably good, but it's the way the instructions tell you to assemble the kit that leads to trouble. Unfortunately, "better" instructions would be too difficult for a beginner (child) to follow and too difficult to write in a simple "do this, then do that" format. The real source of our trouble is that we learn that following instructions is always the "right" thing to do and then we grow into old guys who habitually follow instructions... leading to the use of plenty of filler. Usually, you can find a way to assemble the parts that does not cause a gap, but you must first free your mind to enable you to cut off the locating pins and then re-think the whole thing from scratch. Not a simple thing after a lifetime of following those blasted instructions...

 

But once your mind is free, you'll find plenty of locating pins to cut off, all around you-- and you'll need a lot less filler.

[DaveCromie]

Steelpillow and Hendie

 

thanks for the explanations, makes it a lot easier for me to visualise and understand.

 

Cheers

 

DC

[Stonar]

1 hour ago, dancho said:

The real source of our trouble is that we learn that following instructions is always the "right" thing to do and then we grow into old guys who habitually follow instructions... leading to the use of plenty of filler.

I think that's a valid point, but how many experienced modellers slavishly follow instructions? I certainly don't. I do read through them, and they are obviously useful for finding parts (not an issue in some resin kits I've built with a low parts count), but in the end I treat them as a rough guide, just another person's opinion of how the kit goes together

Cheers

Steve

[LaurieS]

I used to just throw parts together thinking they must all fit. Ha ha.

 

Now I dry fit as much as possible and it makes a difference. Plus as has been said the instructions are a guide as to how the pieces fit not how to proceed in putting the model together.

 

There is a plus to a poor fit. I have gained numerous skills in putting things right. In most cases i have enjoyed all of that.

 

To date only a Revell Apache 1/48 have I dumped. That is now a wreck on a diorama i am slowly putting together.

 

Laurie

[Beardie]

I have also learned (the hard way) that logic is a handy tool to have in your bag when considering how something goes together on a model. I recently put together the innards of a 1/32 Roden Fokker Dr.1, my first attempt at this kit and, If I had only realised that it was unlikely that the internal framing would slope inwards towards the top and that was why the cross members for the top were too long to fit things would have been a whole lot better. From now on I will use my head a bit more during construction.

[Eric Mc]

15 hours ago, dancho said:

I believe the instructions are often the source of "poor fit." The fit on even ancient Airfix or Revell kits is reasonably good, but it's the way the instructions tell you to assemble the kit that leads to trouble. Unfortunately, "better" instructions would be too difficult for a beginner (child) to follow and too difficult to write in a simple "do this, then do that" format. The real source of our trouble is that we learn that following instructions is always the "right" thing to do and then we grow into old guys who habitually follow instructions... leading to the use of plenty of filler. Usually, you can find a way to assemble the parts that does not cause a gap, but you must first free your mind to enable you to cut off the locating pins and then re-think the whole thing from scratch. Not a simple thing after a lifetime of following those blasted instructions...

 

But once your mind is free, you'll find plenty of locating pins to cut off, all around you-- and you'll need a lot less filler.

I picked up a 1960s issue of the Airfix Hampden at the recent Farnborough IPMS Modelfest. It has the original old style verbal instructions. I am amazed that I was able to follow these as a child almost 50 years ago. You certainly needed a good command of English to follow them.

[Graham Boak]

They also taught you about what you were making.  That funny bit was an oil cooler.  Those things on the front were propellers.  I think it a loss that we don't have such detail now.  I don't recall having any problem with following them, and neither did a large proportion of my class in primary school, not all of which were particularly gifted in English.  Just normal kids.

[JohnT]

4 minutes ago, Graham Boak said:

They also taught you about what you were making.  That funny bit was an oil cooler.  Those things on the front were propellers.  I think it a loss that we don't have such detail now.  I don't recall having any problem with following them, and neither did a large proportion of my class in primary school, not all of which were particularly gifted in English.  Just normal kids.

 

 

One like is not good enough Graham.  Thats the best thinking non build post I have seen this year.  How true.  I learned a lot about what was on an aircraft etc by reading those things otherwise part 62 was just a blob of plastic.  But a venturi tube - what was that??  Mass balances???  So off I went to find out.  Same with oil coolers, as you say, and a kid like me reckoned they must be important to make the thing work/ fly.  

 

Perhaps a plea to the manufacturers to start putting those back in and stop dumbing everything down to a universal diagram?  heck - if a Heller kit was in French I'd still manage to learn something - even if it was that I didn't know French!! 

 

au revoir

[Beardie]

I have to say that is one of the things I like very much about the Wingnut Wings kits. Even if you don't build them the booklets tell you what the different bits actually are on the real aircraft plus all the historical information that is packed in there.

[LaurieS]

31 minutes ago, Beardie said:

I have to say that is one of the things I like very much about the Wingnut Wings kits. Even if you don't build them the booklets tell you what the different bits actually are on the real aircraft plus all the historical information that is packed in there.

 

That is a brilliant idea. It is difficult to find what you are sticking onto a model at times. No mention of the name just n illustration.

 

Laurie

[Beardie]

They even include a lot of photos, where available from surviving aircraft and archive photographs of engine, cockpit detail etc. so you know what you are trying to replicate

[walkerccw]

Wow, there is a lot of great information and comments here.  I am back into modelling after 40 years and the comments by @dancho and @LaurieS made me laugh as I am still doing the following things

 

Dry fit? Heck no.  I slap the parts together, they must fit.

Cut off alignment pins off to make the parts fit better? Heck no.  The pins must be there for a reason.

Use the directions as a guide and use your brain for certain steps. Heck no.  My father was in the military and you follow the directions to the letter.

 

I need to unlearn these things and start to use my brain and eyes more!

Edited September 29, 2016 by walkerccw

[Raven Morpheus]

Bad fitting kits is why I love Academy's recent 1/72 F-4J Phantom II and Bandai's Star Wars kits.  A lot of companies could learn from those two.  Time will tell though how well the moulds hold up and if problems do start to creep into the kits due to ageing and use of the moulds.

Edited September 29, 2016 by Raven Morpheus

[LaurieS]

18 minutes ago, walkerccw said:

Wow, there is a lot of great information and comments here.  I am back into modelling after 40 years and the comments by @dancho and @LaurieS made me laugh as I am still doing the following things

 

Dry fit? Heck no.  I slap the parts together, they must fit.

Cut off alignment pins off to make the parts fit better? Heck no.  The pins must be there for a reason.

Use the directions as a guide and use your brain for certain steps. Heck no.  My father was in the military and you follow the directions to the letter.

 

I need to unlearn these things and start to use my brain and eyes more!

 

Ouch

[dambuster]

On 28 September 2016 at 3:33 PM, Vanroon said:

 

I'm a big fan of having the injection gates onto the mating surfaces. 

Hendie , thanks for your explanation of the process. 

 

Grant

 

Hmm, IMHO injection gates on mating surfaces are a guarantee for poor fit......

 

Peter

[Phantome]

On 9/27/2016 at 10:52 PM, hendie said:

 

Sorry but I beg to differ.

I've had roughly 20 molds designed and built in the last 12 months alone. From small parts less than 25mm^3 to larger parts around 380 x 265 x 50 mm.

Without exception, every mold has been cut steel safe because no-one can accurately predict shrinkage.  Try and hit dimensions first time around and if you are too large, it's a lot more difficult to add steel back to the mold than it is to remove it.  Plastic want to shrink an enormous amount "freely" - second stage prevents that although the parts will still shrink to some extent. Exactly how, depends on how the part is gated, what the process is, the material, plus a few  other factors. It will shrink differently in the X, Y & Z directions.

Once the mold is sampled the plastic part undergoes metrology (first article) and based on the steel dimensions vs part dimensions, plus an educated guess on shrinkage (based on first time around), the tool is adjusted, and goes through the process again.

I've had molds where we hit dimensions second time around - and I've had molds where it's taken 3 or 4 recuts to get into spec.  That is built into the price I pay the tool builder.

There's no mystery about the customer - read the post - the customer is the person/company who buys the mold.... airfix, hasegawa, me... whoever.

 

 

 

Ah, I get what you were saying by that. I assumed we were talking about the bigger companies which I would have assumed do their own moldings? (perhaps I'm wrong on this too!)

 

But in any case, if the customer doesn't care about accuracy doesn't really matter, it's gonna come out sloppy :/

[Graham Boak]

If you count Airfix as a bigger company then no, they subcontract their tooling in China.  As do most.

 

If you want well-fitting kits then Tamiya has the best reputation of the larger companies.  I have always found Hasegawa very good indeed but some people have trouble with the breakdown for multiple variants.  Preparation and dry fitting helps with every company.  Be particularly wary of detailed internal parts that end up too wide for the fuselage: this was almost a trademark of Eastern European kits but the recent Airfix Spitfire also suffers from it.

[LaurieS]

Depends on which Airfix. The latest mouldings are superb to work with. The plastic is i suspect first grade plastic rather than second or third grade.

Built most of the Herrick range that Airfix produce and with a very few minor exceptions they have gone together well. The instructions on the new

models are superb.

 

Agree on Hasegawa but not as good as the above. Their instructions are not good.

 

Tamiya built two. 1/48 Harrier awful fit and poor detailing especially the large air inlets to the engine.

Swordfish superb fit and very nice detailing.

 

Revell ----------

 

Laurie

 

 

[Don McIntyre]

If you like really detailed instructions find some of the Accurate Miniatures kits. Now those guys knew how to do instructions. And I found when building their TBF and SBDs that it was a good idea to follow said instructions...