My first attempt at a filament splicer (http://www.thingiverse.com/thing:14438) worked after a fashion, but it had some real limitations. It was not very portable and one end of the filament had to be passed right through the whole device to remove it. A more portable splicer which is able to be split away from the filament was the design criteria for Mk 2. (See “Prior Attempt” below for background information.)

Here’s how Splice MK2 was made:
1. A 15-16mm dia cylinder of steel about 40mm long was drilled at 6mm dia through the centre. This cylinder was then welded to the jaws of an old pair of pliers.
2. Next, the cylinder was sawn through the middle parallel with the pivot axis of the pliers. A 40mm length of PTFE tube of 6mm OD 3mm ID was then sliced in half lengthwise with a scalpel blade.
3. The two halves of the PTFE were then ‘stuck’ to the inside of the new ‘jaws’ with contact adhesive and/or superglue.
4. Now the filament heater block (with its usual resistor and thermocouple) needs to be able to move aside after the filament ends are made hot and sweaty. I’ve arranged the heater block mounting to be attached to the top jaw so that it moves away automatically as the jaws of the pliers are closed. This is the design feature that I’m particularly happy about.
5. The two ends of the filament can be immediately pushed together inside the PTFE jaws and clamped together. Moving the filament back and forth inside the PTFE with the jaws closed will give the PLA welded joint the desired shape.

Seemingly now, the only limitation remaining is that it takes three hands to operate, ie one hand for each piece of filament, and one for the pliers.