Flying with a Prusa Mendel 3d printer

There shouldn't be an issue with electronic components, electric motors, fans, metal and plastic parts. Just be sure you don't have the standard prohibited stuff like gels and liquids, flammable and chemicals, screwdrivers? and knives.

http://www.heathrow-airport-guide.co.uk/security.html

References: I flew within the US and trans-Atlantic with items such as a projector and fully packed backpack for overnight hiking including gas stove as carry on.

In the "post 911" environment, security staff have become VERY conscious of possible (even if unlikely) exploits. Travelling with anything valuable in carry-on which "pushes their limits" is risky. Customs and security personnel can make almost any decision they wish and often err on the side of safety.

If "metal parts" look ANYTHING like a knife they may be confiscated - and how much they take is up to them. You may be lucky with your gas-stove, strips of metal (eg daypack bracing spines) or other perceived weapon substitute - but, you may not. At the end I describe my own experiences - I lost an inkjet refiller on the 5th security check - it having previously passed 3 international checks and one Chinese internal check. Four security teams passed the item as harmless. The 5th confiscated it. A good set of fingernails would have been more dangerous.

Despite common sense suggesting that a DIY printer kitset does not resemble a weapons delivery system close enough for anyone to care, you definitely run the risk of customs confiscating your printer so that you lose it. If this happens you have no 'comeback', & your insurance may not cover actions taken by security officials.

When you enter a customs area you are effectively in international "space" and outside the country which is just on the other side of the gate that you passed through. You have no right to go back or send anything back. In some cases people are allowed to do so - but I have heard of numerous cases where they are not. To allow people to return after entering a customs controlled area is to open the possibility of eg clever drug smuggling exploits, and authorities are aware of this and actively seek to prevent it.

So, if customs do decide to take exception to anything that you are carrying you have no guarantee of having any means of "undoing" the situation. ie as well as not letting YOU go back, it is not usual for customs or security to provide means for you to return the goods concerned to the country you have left and forward them in some other manner.

SO - if I was in your situation I would pack the printer and/or component parts adequately well - not an overly hard task, and carry it as checked baggage.

My experiences:

In the "post 911" environment customs officials have been known to take exception to the carriage of the most trivial items that they deem to pose a security threat. These include nail scissors and even (I'm told) nail clippers.

(1) I had an ink-jet printer refilling system confiscated in China. About the size of a coffee mug. It contained inside an inaccessible container three ink tanks and three needles long enough to puncture inkjet cartridges - probably each about 2 cm long. They could not have been used to prick or injure anyone without first smashing open the refilling system.
Broken open and used as a "weapon" they would have been laughable - you could probably do more damage with a well grown set of fingernails. The "needles" were no more "hypodermic needles" than you can say that strips of steel are knives. Even nail scissors get taken.

I had carried the refiller in my carry on baggage from NZ-Brunei-Hong Kong - China border - internal China airtrip to Guilin. ie 3 international border crossings and one security checked internal flight. Despite this I was not allowed to carry the refiller from Guilin back to Guangzhou by air and it was confiscated. It had never been used :-). I was adequately annoyed that, after trying polite and reasonable to no avail, I became very noisy and assertive - not something I've done before or since and not something I'd recommend with foreign security deep inside an Asian (or other) country. They were a lot politer that I was - but they still confiscated it.

(2) I have had the Chinese leave me until last in the customs queue (Qingdao during the Olympics when they were especially edgy) and then spend an hour or so going through every single item in what was essentially a comprehensive portable electronic workshop. BUT they did not confiscate anything :-).