3D Print: When to use Rafts, Brims, or Skirts


When is comes to the success of your 3D print, the first layer and continued adhesion are the most important factors. Without that good foundation, the print is doomed to fail somewhere in the printing process.

I have had so many failed prints due to poor first layer adhesion that I’ve lost track. While glue sticks are sometimes a good option, it’s not always the most appealing to smear glue all over your print bed.

To avoid using annoying adhesive alternatives, you can turn to using adhesive structures. These structures are called skirts, rafts, and brims.

Jump to a specific section:

Skirt

What is a skirt? 

To put it simply, a skirt is a 2 to 3 layer thick outline around your print. The outline evens out the flow of filament before the first layer starts printing.

A skirt may seem like an unnecessary addition before a print, but it can actually make a huge difference in your first layer. Without one, your first layer might start out rough or under extruded, making it more prone to warping or detachment.

One way to observe the effects of a skirt is to watch your nozzle closely as the printer begins extruding. If this is your first print after warming up the printer, you may notice that the filament doesn’t even come out right away.

Less noticeably, the filament might have an inconsistent flow at first. This will be evident by small gaps in the first few lines of the layer.

When to use a skirt

It’s completely a matter of preference, but I use a skirt on my prints 100% of the time. A skirt is such a tiny part of a print and only costs a very minimal amount of filament.

Some people have never needed to use skirts before, which is great. Personally they have saved my first layer from looking bad too many times for me to leave it off.

Skirts also never touch the print in any way, so it will never damage the part. They also peel off the bed really easily after the print is done.

You may be using another structure on this list such as a raft or a brim, and in that case a skirt is unnecessary. In fact, Simplify3D categorizes skirts and brims under the same settings. The only difference between the two in that slicer is the offset from the part.

Skirt settings

Simpify3D skirt settings

Skirts are the simplest of these structures, so there aren’t many settings to cover. Simplify3D has the most settings for this, so I included a screenshot of the settings I usually use.

Extruder

The first setting on the list is the extruder type. Since the amount of layers is so minimal, and since a skirt never touches the print, you can safely leave it on your primary extruder.

If you do have multiple extruders, you can still use a different one if you wish. There’s no downside to using either one. It’s just about your preference.

Most printers don’t even have a second extruder, so you should set it to primary and not worry about changing it.

Layers

Just like the setting implies, the Skirt Layers option will allow you to increase how high the skirt is by adding multiple layers.

There really isn’t any reason to be making a skirt multiple layers tall, so this setting mostly applies to brims. By the time the second layer starts printing, you will definitely know whether the print is going well or not.

Skirts only benefit the very first layer of the print, so having outlines on multiple layers is a waste of time.

Offset from Part

The skirt offset from the part is the setting that determines if the structure is a skirt or a brim. If the offset isn’t touching the print at all, it’s a skirt. These structures are always separate from the print itself.

You can make these as far away or as close as you want to. You should be wary of it being too close though, as your print’s first layer might be wider than expected. This can cause the skirt to fuse with the print.

I personally like to keep this setting at a 5.00mm. offset, as this allows enough expansion room without being too far away.

Outline

The outline setting determines the number of concentric outlines printed around your part. I recommend setting your prints to 2 outlines, with a maximum of 3.

You may notice that certain filaments take longer to start extruding than others. If that’s the case, then you might want to consider adding more outlines.

There might be another issue such as too low of a temperature, so I would make sure those settings are correct.

Raft

What is a raft?

A raft is a thick, flat adhesive structure that is printed below the first layer of your print. Rafts are designed to keep your print secure during the printing process, and to peel away from the print without damage.

As far as adhesive structures go, rafts are the ones I use the most. They are extremely versatile and apply to a lot of different prints.

The only downsides are that they add on extra filament and printing time to your prints, as well as having the potential to damage your bottom layer. With a raft, the bottom layer is bound to be rougher than printing on straight glass anyway.

All the major slicers have settings for rafts, so they aren’t hard to implement on your print. Before enabling this setting, you should take the time to examine whether you will need one for you print or not.

A lot of people personally hate using rafts, as they can be sometimes difficult to remove. I print with a glass bed, and sometimes they can be the difference between a failed print and an amazing one.

Whether you love them or hate them, they sure beat cleaning glue or tape off your bed.

When to use a raft

Here’s a quick overview of some applications for rafts:

  • Many support structures
  • Flimsy base
  • Prints keep falling off
  • Glass bed providing less adhesion
  • Tall prints with supports
  • Long prints

Support structure adhesion

The first time I used a raft was when a print failed because a support fell over. I must have forgotten to put glue on a certain section, but it was still annoying.

I hated having to put glue on every single section that the print will be, so I turned to the raft. It made a world on difference in my print. Not only did all the supports stay upright, but also the print came off the raft without a hitch.

I can’t say the same for other times though, as sometimes ripping the raft off caused the first layer to come with it. This is a result of the settings I used, which I will be going into more detail about later.

Lithophanes

If your print is very tall or flimsy, you should probably consider adding a raft. The first few lithophanes I printed failed as a result of not enough adhesion.

Now I always add a raft to a lithophane as an extra precaution. The curvature of the lithophane was also a factor, but the raft helped with the adhesion immensely.

Avoiding glue

Sometimes the glass bed is just annoying with its general lack of adhesion. In that case, I throw a raft on there and don’t worry about anything.

As long as the extruder is printing properly, the raft will be the strongest hold you can get other than glue.

I have combined a raft with glue before, and it was a terrible idea. It was almost as if I applied super glue to the bottom of my print, so I don’t recommend it.

When to not use a raft

These are some situations that you do not need a raft:

  • Quick prints
  • Prints already adhere well
  • Prints that have large, flat bases
  • Prints that need a smooth base
  • Very wide prints
  • Prints with delicate features

Quick prints

For a print that’ll only take you a few minutes (such as a calibration cube), you really can skip the raft. Even if it does come off, it won’t be much of a loss to deal with.

If the print has supports that keep coming off, you can add some extra base layers to the support structures. This ensures the supports have a strong hold while keeping the rest of your print smooth.

If your first layer settings are dialed in correctly, smaller prints can be printed without any adhesive help at all. I’ve made phone cases without even the slightest bit of adhesive assistance, and the bottom layer turned out beautifully shiny and smooth.

Prints bases

For an item that needs a smooth base, rafts will completely negate any smoothing from your bed. It’ll look just like your top layer (if not worse).

I like to print shiny phone cases and other items, and in that case I turn rafts off. Try your best to make sure that none of your support structures are connected to your bed, as these will come off easily.

Prints with large bases also probably don’t need a raft. A little bit of glue combined with the grip from the base will ensure that it has a super strong hold.

While still on the topic of big prints, models with a super wide first layer are probably not good for a raft either. Since rafts add a bunch of extra material and layers underneath the entire print (and wider than your print), it’ll add a bunch of extra time to complete the print.

Delicate features

Prints that have delicate features on it will most likely be ruined by a raft. These small parts can get stuck onto the raft, and torn off upon removal.

If the print is small enough, just print them on a the bed with a small amount of glue stick applied to the bottom. Then just pop them off with a putty knife.

Raft settings

Simpify3D raft settings

The settings you apply to your raft will make the biggest difference of how easily it comes off. There are a lot of settings, but luckily they are pretty simple to understand.

Extruder

Rafts are actually best when combined with a secondary extruder. This is especially the case with a water-soluble filament such as PVA.

Rafts are known as being difficult to remove, but PVA completely eliminates that by allowing you to dissolve the tough material away.

If your printer supports these filaments, you absolutely need to use this for your rafts. Dissolving your raft plus support material is perhaps one of the most helpful 3D printing features yet.

Top and base layers

This setting is pretty self explanatory. I recommend you using at least two layers both on the top of the raft and on the bottom.

Adding more than three layers is kind of a waste of time, as rafts are a throwaway part. It just doesn’t add enough to the structure of your print to be worth it.

Offset

The offset, like offset in a skirt, is how far the edges of the raft will extend past your model. The main purpose of this is creating a surface that is able to be gripped either by your hands or another tool.

Be sure not to make your offset too low, as then it’ll be extremely difficult to remove the raft. Leverage to pull it off will make a massive difference.

Separation distance

The separation distance is how far large the air gap is between the raft and the part.

Simplify3D recommends that you use a separation distance of at least .1mm, but I like to be safe and make it .14mm. The extra .04mm helps it come off even easier.

Top infill

Top infill is also pretty self explanatory. It is the infill percentage of your top raft layers.

In the image I have it set to 100%, but that percentage isn’t really necessary. The higher the percentage, the more grip it will have on the part.

Usually I keep it at around 70%, which saves a minute amount of filament. It will also be another measure making sure it separates easily from the finished print.

Above raft speed

This setting changes how quickly the first layer above the raft is printed. This is important as it determines how well the print grips the raft.

I usually keep my setting at 30%, and the rafts I use come off without any issues.

Brim

What is a brim? 

A brim is extra layers around the base of the print that extend outward. These layers give a stronger hold, and help to prevent warping.

They use much less filament and time than a raft, are are often easier to remove. Brims are essentially skirts attached to the model.

When to use a brim

Common brim uses:

  • Avoiding warping
  • Extra layer hold
  • Printing without adhesive aid

Avoiding warping

The most common reason to use a brim is to avoid warping from the hot end. This is especially useful for tricky filaments such as ABS.

These extra layers keep a nice hold on the edges of your print so that the heat can’t curl them. If you get a lot of warping, the best and easiest solution is to add a brim.

When not to use a brim

  • A lot of supports (personal preference)
  • Large prints

A lot of supports

In my experience, supports and brims don’t work very well together. This is applicable for Simplify3D supports, but I’m unsure of support structures in other slicers.

I have used brims paired with supports and sometimes they still come off. When I have this issue, I add a raft. This immediately fixes any problems with supports falling of due to poor adhesion.

Large and prints

Obviously the bigger and flatter the print is, the better hold it will have on the bed. You are also much less likely to have warping on a large print.

For large prints, you probably won’t need either a brim or a raft. A small amount of glue stick will make sure that the print isn’t going anywhere.

Brim Settings

Simpify3D brim settings

As I stated earlier, the only thing separating a skirt from a brim in Simplify3D are the settings. Other slicers have the settings separated.

Extruder

Like the setting implies, you can change the extruder that prints the brim to whatever you’d like. Obviously your printer will need to be equipped with multiple extruders first.

Brims actually benefit very well from using water soluble filaments. This isn’t as necessary as it is for a raft, but it’s definitely easier and cleaner than peeling the brim off afterwards.

Layers

The layers setting determines how many layers high the brim will print. For this setting, 2 to 3 layers high will work very well.

You don’t really need any higher than that, unless you really want to be extra careful about warping.

Offset

Simplify3D recommends that your offset be at least .1mm. If your offset is set to 0, the brim will be so attached to the print that it will be difficult to remove.

The offset setting being too high will turn your brim into a skirt, as it will only outline the print instead of being attached.

Outlines

The more outlines you have, the more outward grip you’ll have around your print. My preference is to have at least five outlines, but some people like to add many more.

Conclusion

Rafts and brims are great to have, but they aren’t always completely necessary. I always keep a skirt on to ensure smooth bottom layers, but you should do some testing to see if you need to do this as well.

Always make sure you use the right structure for the specific print you’re using. If you don’t, you could end up with a giant spaghetti-like mess of filament on your build surface.

Jay Simmons

Jay Simmons is the main writer for The 3D Bros. He has several years experience working with 3D printers, and is the the co-owner of Cubold Manufacturing, LLC.

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