Acrylic Problem

[mtgrizzly52]

I put together a Wall Street II last evening with a navy blue acrylic blank I got at Woodcraft last week. I've used this same blank several times with no problems. I turned it with a skew and it turned perfectly, nice and smooth, no ripples and really needed very little sanding to smooth it out.

I hit is with 220/340/400 regular sandpaper, wet, and then through the MM 1500 to 12000 again wet sanding with each grit. I always clean my blanks between grits with denatured alcohol which has always worked very well for me.

Speed of sanding from 220 through 8000 was 1800 rpm, again, always use that speed with no problem, and then boosted the speed to about 2400 for the 12000 grit, again no problem.

Once I got through all the grits, I usually hit it with a bit of plastic polish and then a final coat of renaissance wax, speed at 2400 rpm.

Turned the lathe off, started to remove the mandrel and noticed what looked like a flaw in the acrylic blank that wasn't there a few seconds earlier. On the end of the acrylic nearest to the tailstock, it look like a wrinkle in the plastic. Running my fingers of the area confirmed what my eyes were seeing. It looks like the acrylic got really hot and melted, but how, when, why when a few seconds earlier that blank was a smooth as a baby's butt when I applied the wax?

This by the way definitely is not my first acrylic pen. I've done well over 50 of these, in several different types of pen kits with absolutely no problem before. Heck I've never even had a blowup with the acrylic, but I have with wood. This problem has really got me scratching my balding head.

Rick (mtgrizzly52)

 

[NewLondon88]

You can generate a LOT of heat sanding, and it won't all stay at the
surface, although that's where it starts.

Some plastic materials are more susceptible to heat deformation than
others, since they are made to different tolerances for different applications.
Very few resins are made with penturning in mind. :tongue:

If you want to get some idea of just how much heat can be generated,
try sanding a metal blank. It can get so hot that you can't touch the
mandrel, let alone the blank.

Granted, plastics can generally disperse this heat better than most metals,
but that heat is still being generated to start with.

It sounds like the surface may be cooling off but much of the heat is
still inside the blank and once the lathe stops spinning, this heat isn't
being taken away by the moving air. It comes back to the surface and
warps the plastic.

It would be helpful to find out just what type of resin was used, and what
the deformation temp rating is for it. Some turners have even changed
resins because the deformation temp is too low. (one example I've seen
is pens left in a warm car .. if the deformation temp is too low, it can
warp or melt the pen.)

.. just a guess..

 

[MattDaddy]

I've had the same thing happen with some acrylic at higher RPM's. It does help you get a great shine, but I have had a couple melt on me at the higher speed. I'm not sure, but I think some colors/patterns have a lower softening point.

 

[PTownSubbie]

Someone posted in a forum once that they don't create any heat while sanding and polishing. I think this is because of the facts that NewLondon88 above discussed.

I try to minimize any heat buildup when making acrylics now. Especially after what I have learned from casting.

No heat buildup when working with acrylics......

 

[David S]

Had the same problem with wood!

What kind of Glue did you use?

 

[mtgrizzly52]

Someone posted in a forum once that they don't create any heat while sanding and polishing. I think this is because of the facts that NewLondon88 above discussed.

I try to minimize any heat buildup when making acrylics now. Especially after what I have learned from casting.

No heat buildup when working with acrylics......

Guess who the idiot was that posted that little pearl of wisdom? I never thought about the heat transfer to the brass and probably to the mandrel steel, and then it coming back out through the acrylic. Also, I just got the new Delta 46-460 lathe, and this pen was the first thing I've turned on it. Looks like I need to get to know this lathe a bit better.

DavidS I use thick CA for all of my pen/tube glueups. Again, never had a problem before. The only other variable that was different, other than the lathe was, I bought some of the black and white tubes from Woodcraft on Friday, and used one of the black ones in place of the normal brass one. This was based on the brass was a bit visible on the previous pens I've done with this same blank. I don't know if these tubes are more susceptible to build heatup or not???

Thanks for all of your replies and advice. It all makes perfectly good sense.

Rick (mtgrizzly52)

 

[maxwell_smart007]

I wonder if somehow the DNA is affecting that type of acrylic? There are a LOT of plastics out there, and I don't claim to understand any of them....

Could that be, perhaps, a DNA soluble acrylic? (just thinking out loud)

 

[DurocShark]

WET sand.
WET sand.
WET sand.

Also, I've stopped going past 1800 MM (the green one). I buff now. Way faster, easier, and IMHO looks better.