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June 20, 2014

3D Printing - Step Three - The Lessons Learnt

You've unpacked your printer, slapped a few prints through and you now want to tweak a few things to get better or more tricky prints out, so, what's first? This post outlines some of the lessons I learnt along my journey that I didn't find out anywhere on the net. More specifically the bits behind why settings are set the way they are, and what you are aiming at.

Bed leveling

One of the first things that happens when you switch your new printer on is that the bed needs 'leveling'. But this is more than just leveling, you're also setting the gap between the nozzle and the bed as well and if you do this wrong you're in a whole world of pain. A few printers now and more coming soon will do a self level and this takes out the guesswork, but if you have a current model with manual setup you're in for a journey. No one really seems to outline what you're aiming for. From hours of fiddling I think the general idea is to get the bed level for the most part, but the bed as close to the nozzle as possible without actually touching it.

The Ultimaker has a drive spindle at the back of the unit so you start here and it's a 2 step process. First you get the nozzle about a millimeter away. I do this with a thin piece of 'regular' A4 paper folded in half 3 times. I also take one piece of paper for the second step in a minute. You basically raise the bed until the paper feels like there is some pressure when sliding it under the nozzle. Pressing the dial moves the print head to the front left. There is a screw here that raises or lowers the bed, try to get the same pressure. Pressing the dial again does the same thing for the front right.

The second step is the crucial step. This is where you use the single piece of paper and need to get a bit of pressure when sliding under the nozzle. If you look closely (which is difficult with a glass bed and the reflection) you should see a tiny gap. If you move the bed one more notch of the dial, you may hear the motor change noise, but the nozzle will touch the bed you want to back it off this one single notch... that's the back done. Next you need to get the same gap at the front with the screws.

If your bed is not level, you'll end up with a very thin first layer at one side, while possibly extruding plastic into think air on the other. If you have the wrong gap set, you will have one of the issues for an uneven bed, but the problems are worse than this. Basically if you don't have an even coating where the layer being put down touches itself on the passes, you will have adhesion problems. Basically this means that the model may move around at some point and you'll end up with spaghetti. There are 2 ways to combat this if your gap is bigger than expected.

  • Pritt-Stick can be used to coat the glass on the Ultimaker, as it's heated, it will give substantial extra grip. Removal of your model may be 'fun'.
  • Blue painters tape can be put down on the glass as well. this has pores in it that the plastic can adhere to better. Some will most likely stick to your model when you remove it.

The (possible) downside to a perfectly set gap is that you end up with a mirror finish on the bottom and a slight lip on the print.

The other alternative is to use a raft, but until Cura releases the fix that make rafts like the makerbot, don't do this. (**UPDATE: Rafts are now a lot less pants after an update)

Nozzle or Print Temperature

The complimentary parameter to the bed height is the temperature the nozzle uses to melt the filament. Incorrectly setting this parameter can have some wide reaching issues and the temperature is dependent on the material, but the easiest way to demonstrate it is with the first layer of your print. So you've set the bed height to the closest setting to the nozzle without touching and the plastic you uses then you use the temperature to control how solid this layer is. The more solid, the more mirror like and the bigger the lip, but also the better adhesion to the bed without any glue or tape. The cooler, the more rounded the filament so the more gap, less adhesion, but a more uniform print (i.e. you can't tell which side is down).

In this image, the gap is set fine, but the temperature is set to the 'default' for PLA, which is a little cool for PLA/PHA. Note the wirey look. I'll go into what all those other wirey bits are in a moment.


Here is a pic demonstrating the mirror finish where you can't see any filament at all, but note the edge... these are sharp.


This, in my opinion is the perfect setting. You still see a rough-ish surface, but its mostly smooth, and there are no sharp edges.


I touched on the difference for PLA and PLA/PHA above. Different materials use different temperatures, actually they have different ranges and this can be a bit confusing. The Default for PLA is about 210 degrees, but that's for pretty pure PLA. If you have a whacky colour it has additives in it which mean you need to bump up the temperature a bit. How much is 'a bit'? Unfortunately, trial and error will tell you.

I predominantly use ColorFabb PLA/PHA now and the box comes with the temperature range on it, but I use it as a guide. For example the Lila says 195-220, I use 220. The Leaf Green says 190-210, I use 220... These are the settings I found work for me... chances are they will change in the winter.

Bed Temperature

For those printers lucky enough to have a heated bed, this can significantly help with adhesion and thereby reduce the need for rafts or brims or any support requirements to increase the surface area that touches the bed... although without these, you actual print may lift at the corners, more on these later.

The Ultimaker also comes with a glass bed so you have an excellent platform. The temperature keeps the plastic a little soft and rubbery and the glass provides a good sticking platform. When the bed cools after the print completes, the model lifts off without any pressure at all, it is truly wonderful. Obviously if you use the glue stick you're in for a serious job removing a large print involving lots of cold water to make the glue a little brittle and the plastic print shrink, and then lots of warm water to clean the bottom of your print.

I have found the default of 70 degrees works fine for PLA and PLA/PHA. If it were hotter the bottom layer could liquefy a little and make your print wonky, as well as sharpen the edges a lot. Too cool and you lose the adhesion properties.

Supports

Supports are required where parts of your print are either started in mid-air, or where and edge leans further than a particular angle. Here is an example:


You can see under the chin there is a pharaoh style beard, the particular model did not have one, this is the support structure. Once the print is complete, you can break off these and clean up the join points:


Supports are quite a tricky thing. You need a gap so they don't fully adhere to the model, but not to much of a gap so the print has a loose underside. That's not a euphemism. I found a blog that outlined the makerbot implementation. they reckon a gap of 0.25mm. You'll need to adjust this setting in Cura in the expert settings panel. It's called "Distance Z(mm)" under the support section.

There is also another setting "Distance X/Y" and this defines the space required around the support structure. This is really just so it doesn't form part of the layer and needs to have a decent distance to allow for error and to get something in behind to wedge it off later.

I've not tweaked any of the other Cura defaults for Supports.

Rafts

I love the makerbot rafts, they work so well. Cura is currently pants at them, but there is an update in the works.

Basically a raft sticks to the print bed by having a big solid bottom layer and then allows for the print to 'sit' on this. You then peel the raft off the print and it has a nice consistent feel so you can't tell which side was down when printed. This is important if you have a model that doesn't have a defined "down". A good example of this is the makerbolt.

Given the stickyness of the Ultimaker supports and the ease of the makerbot rafts I'm not sure if Cura will fix this well, actually I guess it's more of a printer implementation issue rather, but I did not play with makerbot supports so cannot compare. (**UPDATE: Rafts are now a lot less pants after an update)

The downside of rafts is that you use more plastic... you'll need to decide whether this is worth it.

Brims

Brims are an attempt to reduce the amount of material you use to improve adhesion. Basically the outer edge of your print is expanded out like a hat brim. This has the additional upside that edges of your model doesn't lift either. For example, see the funny dent at the top of the purple puzzle piece:


When you don't use brims, this is a common problem. Here is a nicely printed brim:


The downside with this is that the cleanup can take a while longer. If you have my perfect bed height and temperature setting as shown above, the brim can pull away nicely. Also above I mentioned I would explain what all the wirey bits were. They are a brim having been printed with an inappropriate temperature and height set. The filaments in the same layer don't fuse properly so it isn't a layer, more a laying down of plastic wire.

Shell Thickness and Infill

Your 3D model in Blender (or whatever you use) is not a solid model, it is really just a surface and you need to decide what you want to do with it when you get to printing it. First of all the surface is a shell and you need to give it a thickness. Best if this is a factor of the nozzle diameter. In the case of the Ultimaker, it has a 0.4mm nozzle so a good shell is the default 0.8mm. This will run round the outside of the shape twice. Once will be faster, but twice just gives you a bit more resilience and strength.

Once you've done the shell the rest of the model is basically empty in this technology. In Laser Sintering and Stereolithography this means there will be untreated material in there. Not a real biggie but it will increase the cost and weight. For Filament printing, in order to add this weight and strength you need to add a grid internally. The amount of grid in the model is the Infill percentage. 0% is completely empty and 100% is solid. Choosing the amount of this requires you understanding a couple of other factors.

First of all, does your model have a flat top anywhere? if so, is there a wide gap? This is called bridging. If you have a big gap there is a chance your printer will not be able to lay a filament strand accurately and without sagging. If you use some infill, you are effectively building internal supports. tweaking the nozzle temp will affect your printers ability to perform this bridging - the hotter, the more sagging.

Here is an example showing the infill grid:


Bottom/Top Thickness

The shell thickness is best as a function of the nozzle thickness. In the Z axis, because of the control over layer thickness yo don't necessarily want the same thickness. You definitely want a couple of layers to ensure your top and bottom have a few layers as this adds strength, but too thick and you end up with a much slower print - infill is much faster if it's less than 100%.

Initial Layer Thickness

The first layer of your print is quite crucial in that its the layer that sticks to the bed. Generally you want a pretty thick first layer and generally regardless of the actual print resolution. the default of 0.2mm is pretty good.

Layer Height

The print layer thickness is basically the resolution of your final print. 0.2mm is a chunky but fast print, 0.1mm is a standard but slower (twice as many layers) print, and 0.06mm is a high quality print on the Ultimaker. The Ultimaker says it will go to 20 microns (0.02mm) From my testing any additional resolution past 0.06mm does not increase the value of the print given the time the print takes to arrive.

One other side effect of layer height is the slicer can make differences between layers a little more pronounced. For example:


These are the exact same files, the one of the left is printed at 0.2mm and you can see a surface texture as the layer takes shortcuts as it averages out the layers. The print on the right is printed at 0.1mm and the texturing is appreciably less.

Print Speed

There are various speeds than can be controlled. Slowing things down has some good advantages, basically the print is more accurate as the motors are less likely to skip steps, also any drag from the filament at higher elevations is lowered so you can generally get taller prints without the need for additional adhesion types. The other upside is that each layer has additional time to cool before you put hot plastic on top again. This makes the print a lot stronger and it warps a lot less. On the Ultimaker you can also specify a minimum layer time - if the layer will finish quicker than this, the print is slowed for the reasons mentioned above.

Look at the top of these prints, you can see the effects of the non-cooling that occurs when you print to fast:


Travel speed you generally want quite high, as the filament is retracted a little bit which cuts it off from the print, then the head moves to the new location and starts printing again, the less time the filament is not being heated the less likely you are to get a jam or dodgy join point.

Material

This is by far the biggest variable in the printing equation. It will probably be the shortest section here as well. Every material type has its own characteristics and issues and you'll need to experiment with them. Even different colours of the same basic material can have different properties that require you to tweak speeds, temp, layer thickness...

I started with PLA, and this was a hard plastic that produced nice prints for the most part. I then moved to PLA/PHA which was a lot more flexible but because of this, the makerbolt no longer worked as well since the rubbing of the parts would not wear the surface enough for it to loosen satisfactorily. Are you stating to see the mine field this area has?

I have not tried ABS, and probably won't in the short term because of the amount of PLA/PHA I use and you need to go though a nozzle purge which scares me at the moment. I ordered a reel of wood fill, but again, I'm scared to dissassemble the nozzle when it blocks. I am looking forward to getting some LayBrick and playing with that. Other stuff like the ninjaflex doesn't hold much draw for me right now, but I think it may for better fitting lids for things and a lot more flexibility.

Summary

Well, that's the key settings I had to fiddle with to get decent prints out. There is no one set of settings that will work in all cases and each print takes a few goes to get right until you get the hang of what your slicer is telling you. Careful planning while creating your model to ensure parts are always at least a factor of your nozzle width will help a lot. Basically though, you will have to trial and error your way though with this type of printing.

Coming up in the next episode...

Next, my thoughts on the future.