Quoted from epthegeek:

Any game with single wound coils that 'hold' the flipper with a rapid pulse, rather than using a lower power 'hold' coil like WMS does will heat up if you hold the flippers a lot, won't it?

Quoted from xTheBlackKnightx:

A flipper assembly cannot be effectively cooled by any type of fan. Keep assemblies clean, reduce slop, maintain the electronics, circuitry, and wiring properly. This includes areas such as excess solder. No shortcuts. Friction is the the number one generation source of excess heat and reduced power, not increases in coil resistance due to the heat itself. The other main reason is improper voltage, which if exceeded shortens a coil's lifecycle or causes burns or melting.
Historically, the reason coil sleeves were switched from brass to plastic was due to cost, and ease of replacement. Engineers did consider the thermal properties of components, but also durability. I have games that continue to play wonderfully with 60 year components, that were kept serviced.
Prior to code updates, I upgraded several Stern Whitestar games with WMS flipper components for better performance. The difference was significant. This was a problem with Stern, not historical pinball coil design.
Do this if you must.

Quoted from TheLaw:

Personally I like to go liquid cooled. I take small tuperware containers and use L brackets to attach them to the underside of the PF cradling the coils. I've used water before but it's tough to lift the PF that way so you have to keep a small wet/dry vac handy to suck the water out first. I've been trying a lot of gels that are inside those cooling freezer bags for coolers and such? Not huge success but I'm getting close. I wish I was rich then i would just cut holes in the bottom of the cab and have tiny people stick their heads in and continually blow on the coils to keep them cool. Ha one time they got so hot i had to piss on them to cool them down. Sounds weird i know but it was a real "Red Dawn" moment believe me.

Quoted from Black_Knight:

Or any poorly adjusted EOS physical or logical switch too. I think the LOTR issue was solved by shortening the eos logic? (maybe)
xBKx is right, cleaning and proper maintenance is the #1 solution, however, some coil designs are not conclusive to flipper usage.

Quoted from xTheBlackKnightx:

It is estimated that a clean coil sleeve and polished plunger, and rebuilt parts can increase coil power by an average 25-40% dependent on manufacturer and era. Measured by a force gauge meter after restorations.
I am not here to teach people physics, they have do that on their own. The fundamentals and properties of electromechanical parts have been known for centuries. I already explain heat conversion in coils which also is based on generated resistance of the wire gauge, not exclusive to the electromagnetic field itself, or friction from unoptimal parts condition.
I try to open people's eyes to other major causes of weak flippers not just residual coil heat from electromagnetism. I understand the principle of entropy and the law of conservation of energy.
What people might not know is why pinball machines use "coils" versus other methods of activation. The answer is simply they remain the most efficient method both in speed and electrical energy usage for conversion to mechanical work, except in circumstances where more torque is required, or controlled speed applications.
Dual wound coils were invented to solve some of these heat problems, but Stern decided to cheap out. An owner can actually retrofit these games, but it is beyond the scope of this thread.
If someone wants to make a new type of improved flipper assembly or even a better electrmagnetic coil (with baffling or fins), I welcome the innovation. Unfortunately, a patent has already been granted (for magnet coils, not wire coils but the principle is the same in application).
https://patents.google.com/patent/US20120023969
So, why is it not already included on games by manufacturers?
Extra cost for minimal benefit.
The engineers already know.