Why is CLIP so much faster than SLA?

Question

Stereolithography produces parts by projecting ultraviolet light on the top of a vat of liquid photopolymer, causing it to harden. CLIP produces parts by projecting ultraviolet light through the bottom of a vat of liquid photopolymer, causing it to harden. This seems like a minor difference, yet CLIP is reportedly much faster (I've seen numbers as high as 100x). Why is this?

Answer 1

It's important to understand what specifically is being compared. CLIP is much faster than bottom-up technologies that require a peel step between every layer. For example, the Form1 galvo SLA printer tilts the resin vat to separate the transparent bottom from the print. That is, by far, the slowest part of SLA/DLP printing with most modern light sources. Where the speed comes in is that without a peel, a continuous "movie" can be used to cure the resin rather than a series of alternating images and peels.

Top-down printers can print dramatically faster than bottom-up-and-peel printers. CLIP is not necessarily faster than top-down. For example, the Gizmo 3D line of top-down printers are very similar in print speed to CLIP. (http://www.gizmo3dprinters.com.au/)

Most "consumer" SLA printers these days use bottom-up-and-peel techniques, because this has some practical advantages over top-down printers:

• Way less resin is required to fill the printer when the part is pulled out as it builds rather than being lowered into the tank (along with the Z stage) as it builds. Resin is expensive. This also means bottom up printers can be smaller and have fewer mechanical parts such as leveling devices submerged in resin.
• Standard resins contain an inhibitor chemical that prevents polymerization in the presence of oxygen, which causes the surface layer exposed to air (and low-level stray light) to not cure. So top-down printers must shoot light through a non-curing layer before reaching curable resin. This makes the tuning more sensitive and can somewhat reduce detail compared to a bottom-up printer curing right on the window.
• Replacement vats or windows for bottom-up printers may be seen by manufacturers as a profit-generating consumable, since they have to be replaced somewhat frequently.
• Top-down printers have to worry somewhat more about resin flow rates as the part is lowered. Air bubbles may be pulled into the resin or the fresh resin layer above the part may vary significantly in thickness if the part is submerged too fast for the resin viscosity. (Admittedly, bottom-up printers will experience excessive suction forces and potentially break off bits of the print at high peel speeds.)

CLIP is a bottom-up technique that doesn't require a peel step, because the vat creates an oxygen layer over the window that keeps the resin from curing directly on the surface and sticking. In that way, it arguably performs more like a top-down printer than a bottom-up printer.

Top-down printers that are designed to overcome the above issues and use high-intensity light sources can achieve exceptionally high print speeds. This includes similar "continuous" build techniques used as in CLIP.

Answer 2

The number of 100x could be true in some situations. I wish I could see the part(s) that they printed to measure this 100x, but that is another story.

Looking at their videos they can move the build plate at a maximum speed of 10mm per minute. You will see the Eiffel tower video where they have to change over to standard speed for flat layers. It is because the flat layers stop resin flow and can't be printed using continuous printing.

Prodways have shown that they can move it at 20mm per minute, but again there are things that are not advertised/mentioned. To achieve higher speeds you need to make the resins more reactive. Making resins more reactive means the resins won't last as long in the vat/bottle. So they expire sooner. It also means they could harden under normal light conditions so it makes it difficult to work with. If you look at the Prodways video you will see resin waste on the build plate. That shows that the projector brightness was set too high.

Gizmo can print between 5 and 25 times faster than a leading brand of SLA printers, also depends on the number of parts and complexity on the build plate. The decision was made to advertise the build plate moving speed, e.g. 3mm per minute, rather than the number of times faster than anything else, because that is a value that doesn't change with the number of items on the build plate, but it does change depending on the projection area size.

Imagine you just have a single line going upwards then most SLA printers could do it at the same speed. B9 Creator (bottom up) users have actually shown they can do continuous printing when printing really tiny items with very thin walls, because they don't have suction problems in those situations.

Now when you add many small objects with small features on the build plate DLP printers will display the full layer in one go where laser based SLA machines need to draw out each part like an FDM machine.

Continuous printing does have limitations. Resin needs to flow from the bottom or you will get resin starvation and cause holes so you might not be able to use it for everything. You should see it as another tool in your printing toolbox rather than the be all and end all.

So after all that, the 100x depends on so many things, you shouldn't get stuck at looking at that number other than knowing it is a way of marketing the printers.