D3D Universal, Pro, Pro 2, and Pro 3 use an insulated heated bed with a nichrome wire (or kanthal wire) heat element. You can see Nichrome Calculator for how to calculate the desired power. The design of the heater consists of an inner core of fiberglass, around which the nichrome wire is wrapped to reduce the overall length of the heater element to make it workable - otherwise the heater element is difficult to handle if it's too long. Because we typically use around 10'-16' or so feet of nichrome wire - after wrapping it around a core - the length is reduced to 2'-4'. This shorter length is useful because the inner core with nichrome wrapping is then encased in an outer core of fiberglass sleeve. The shorter length of the inner core means that it's easier to weave inside the outer core.
The nichrome wire must be connected mechanically - as opposed to soldered - because the high possible temperature of the nichrome could easily melt solder. In our design, we use a piece of 4" interim wire - steel electrical fence wire - and we wrap the nichrome around a loop on the end. No solder is used at the nichrome-steel wire connection. Because the resitivity of the steel wire is lower than nichrome, it doesn't heat up like nichrome. Thus, we can solder to the end of the steel wire away from the nichrome end. As such, we can connect the high temperature nichrome wire to the rest of our circuit, without overheating or corroding copper wires.
See how the D3D Universal 2 heater is built. D3D Pro versions are identical, except for the length of the inner and outer cores. The outer sleeve should be 12" longer than the inner core, so that the inner core is insulated completely. The outer core is 6 mm fiberglass sleeve (1000F rating) and the inner core is 3 mm or 6 mm fiberglass sleeve (1000F rating). Note: fiberglass sleeve can fray. To prevent fraying, our technique is to wrap a piece of electrical tape around the sleeve where it will be cut so that fraying does not occur. Before wrapping the electrical tape around the sleeve, spread the fiberglass sleeve so that its inner aperture remains wide open (as in picture #11 below).
Heatbed for D3D Universal
See detail here.
Build Detail of Heater Element
This is just about right. Now the connection of copper directly to nichrome doesn't work, it burns out. So instead of going directly from nichrome to copper - use a piece of steel wire, and wrap the nichrome around the steel wire. No solder - solder would melt.
18" Bed, 2020
We added solid fence wire as an intermediate between the nichrome wire and the 200C wire. Direct contact of nichrome to copper burns out the copper. Nichrome simply wraps around the fence wire for a secure connection.