Raft/heated bed questions

Hello

New to RepRap's, have a couple of questions.

Could someone explain to me what a "raft" is and how it relates to using a "heated bed" with a RepRap and what the advantages of a heated bed are and when it is and isn't needed etc.

Thanks
Jim

There are two issues that can occur trying to get the part to stick to the bed. The first problem is that the part does not stick well enough and the lateral forces of the extruder pushes it around before the part finishes. A raft is a large flat area printed under the part to increase the amount of area that is stuck to the bed, making it less likely to come loose. Another problem is that the part sticks too well to the bed to be removed without damaging the part or possibly the bed. The raft is printed with gaps in it so that it will break before the part breaks. The raft also helps to get around bed position issues, such as non-exact homing or un-level beds. I think it is a good idea to use a raft while you are starting out.

The downside to a raft is that it is hard to remove and makes it difficult to get a good smooth bottom on the part. I stopped using a raft a while ago once I had everything else working well.

The heated bed is an attempt to reduce the amount of warping that happens, especially with ABS. When you put a hot layer on a cold layer, they will bend a little as the hot layer cools. The amount of bend is related to the difference in temperatures of the two layers. A heated bed keeps the bottom layers warm so that they do not bend as much. Then, when the part is done all layers are cooled at the same time and the part simply shrinks a minuscule amount instead of warps.

A raft can help reduce warping by providing more contact to the bed near the corners. This provides more holding force and can prevent warping on some parts. Large parts tend to overcome this extra help. There was some experimentation with using a skirt, or printing multiple parts all on the same bed to avoid corners on finished parts that are vulnerable to warping. That seems to work better than a raft alone, but not as well as a heated bed.

---

---

Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

That's very helpful thank you!

You can find a bit more information on warping (with pretty pictures) here and then here.

Its been written with aim of helping those new to repraps, such as your self, get a good head start on warping. So if something isn't clear, or incorrect, then by all means leave a comment.