I got some of the 3M blue masking tape that Vik Olliver recommended as a bed material for PLA. It seems to be available up to 50mm wide, so four strips covers the bed of my machine.
Whereas I could not get PLA to stick reliably to MDF, it sticks easily to the tape.
Why is it black? Well my feed of PLA from the overhead hanging basket snapped. It must have got kinked and bent through too sharp a radius. When I pushed the new end into the top of the extruder to restart it, I must have caught some of the grease from the top bearing. The grease is yellow, but as soon as the stainless steel bearings have run for a while it turns black. That small amount of material was enough to turn the first few layers of my object dark grey.
The object was a complicated shape and came out very hairy: -
It took a lot of cleaning up and has some defects and weak spots where there is a lack of material due to the oozing that occurs on the way there. I have some compensation for this effect, which works well for ABS. Basically I estimate the amount of ooze from the time the extruder is off and then run the extruder for a while to replace it before starting a new thread. I think the constants need to be completely different for PLA.
The object was relatively large, but showed no sign of warping, even when removed from the bed. The base of it is completely flat.
I had successfully removed several small objects but I damaged the tape removing this one. It was easy to replace one strip and reducing the temperature of the first layer from 210°C to 180°C seems to allow large objects to be removed easily.
Another problem I had was the PLA started revolving fairly quickly in the extruder, making the hanging basket spin. Each time it revolves it reduces the amount fed by one thread pitch. If it happens too much the object has material missing. I fixed it by applying some oil to a felt washer that the filament passes through. A good reason for moving to a pinch wheel feed though.
I made a 65mm cube shaped box and it showed no sign of lifting from the bed.
It was easy to remove though and this time did not damage the tape. Even after it was removed the base stayed fairly flat.
Much better than my attempt to make the same box in ABS some time ago.
Not only did it curl after it was removed from the base, it also ripped itself open at the corners while it was being built. There is also a wavy distortion on the left face which I had not encountered before.
I think what happened is that when the cracks opened the edges lifted, causing the nozzle to bear down on the wall it had already built. That meant there was excess plastic for the gap between the surface and the nozzle. Normally when that happens blobs are left on the surface. When the next layer is done the nozzle just plows through the blobs. Because the walls are only 1.5mm thick in this case, and tall, they flexed sideways instead. That caused a ripple and the effect seems to build up layer on layer. I could see the wall flexing as the nozzle passed over it.
So PLA allows bigger objects to be made before a heated bed or chamber becomes necessary.
Even relatively large PLA objects can be made without a raft. That saves a lot of time and material, but you do have to get the z-calibration spot on and the bed perfectly level.
The masking tape makes a good, cheap, reusable bed material and it is quick and easy to replace if you do damage it.