Engineering Plastic Locomotive Frames ?

[jrbarney]

The thread on powering static models piqued my curiosity. I'm certainly no expert on plastics, but what would preclude making model steam locomotive frames from an engineering plastic such as Delrin or Nylon rather than styrene ? Delrin and Nylon seem to be inherently lubricious. Would there be creep or stretching over time ? Would seem that detail could be cast in or machined. Weight might be a problem to overcome and a compatible paint or dye would be needed, but what other problems might there be, such as cost ?
Bob
NMRA Life 0543

[orsonroy]

with the flexible properties of plastics, I would imagine that binding issues would kill this idea pretty fast. I've HEARD of modelers who have scratchbuilt steam engines out of mostly styrene, including the frame, but I've never seen one of these engines run, not heard of how LONG or WELL they ran.

Now Delrin bearing inserts in a metal frame might be a solution...

Of course, it's almost impossible to paint or glue Delrin, so it's have to be a one-piece cast frame. Repairs would be impossible for the average modeler.

[nfmisso]

Hi Bob;

Delrin® (Acetal) or Nylon (PA) are not suitable for the job; Acetal because of creep and PA because of dimensional stability with respect to humidity.

Ultem®(PEI) from General Electric with some additives would be a good possibility, but relatively expensive. PBT with fillers would be good. LNP offers a series of polymers filled with tungsten powder for ecological replacement of lead in many applications down to fishing lures.

The issues with these materials is that require more capable molding machines than typically used for model railroad itmes. More tonnage, higher temperatures and longer cycle times further increase the costs.

It is very hard to beat die cast zinc alloys for frames. The material is much cheaper than any plastic, the sprue material is thrown back in the pot, tooling is cheap. It is very forgiving with respect to process parameters.

[DSchmitt]

Another factor is weight. Higher weight means more traction and usually a better operating loco. That's one reason many lower end model loco producers, who used to use plastic frames, have gone to metal frames when the decided to up-scale their lines.

[jrbarney]

Ray, Nigel and DSchmitt,
Thank you all for your responses. I thought it was an educational and interesting discussion. Just trying to keep everyone's creative juices flowing.
Die cast zinc is okay so long as they keep out the disintegration inducing impurities that I encountered in some Mantua kits.
Bob
NMRA Life 0543

[orsonroy]

Impurities? Ah, zamac rot! Yeah, I've had a couple of my Mantua engines with this disease too. It seemed to affect everyone but Bowser; wonder why?