Adventures in 3D Printing

[daveculp]

     While planning my 1/32 OV-10A project I decided to include a couple B37K bomb racks.  I couldn't find any in the aftermarket, so I started scratch building one, then realized this rack would be a good 3D printing project in itself.  The cost of 3D printers has dropped enough that one can just buy one and jump right in.  After researching the various printer technologies I decided the best one for making model parts would be a DLP printer.  The Anycubic Photon and the Elegoo Mars are both DLP printers costing under 500 dollars, and I decided to try the Elegoo Mars printer for this project.

 

     DLP printers are notable for their ability to print detailed objects with smooth surfaces.  Another feature of DLP technology is the ability to create multiple copies of the same object in the same amount of time it takes to print just one.  My goal was to build the two B37K racks that I needed, then go into production, building several at a time.  Rather than write a tutorial on the subject of 3D printing I figure it would be better to just describe the process from start to finish.  I'm still learning the technology myself, which is a necessity since documentation on the subject is so sparse.  Expect lots of failures!

 

Step 1:  Making the mesh using a CAD program

 

     The first step in the 3D printing process is to build a mesh of your object using a CAD program.  There are plenty of CAD programs available, however since I was already comfortable using AC3D, by Inivis, I went with it.  My prior experience with CAD was all about making airplane models for use in flight simulators.  This type of CAD work can be called "CAD for rendering", or CAD-R.  In this type of CAD your goal is to provide a mesh (along with graphics called "textures") to a computer's GPU (graphics processing unit) so that it can render it on screen.  CAD for printing, which I'll call CAD-P, is similar in that you're creating a 3D mesh, but there are some notable differences.  For one thing in CAD-P you don't have a GPU helping you by smoothing out the surfaces for you.  In CAD-P a surface is just a surface.  The CAD-P surfaces don't have attributes like smoothness, shininess, or color like they do in a CAD-R.  Another notable difference is that in CAD-P it is critical that your mesh has a well defined interior and exterior.  In other words the printer needs to know where the inside is in order to put the resin there, and it needs to know where the outside is so it won't put resin there.  This is not a concern in CAD-R.  In CAD-R you are mostly concerned with which surfaces are in front of which other surfaces (the "z-order").  Here's the B37K rack as built in AC3D:

 

 

 

Step 2:  The slicer

 

     Now that you have a mesh the next step is to save it in binary STL format.  This is the format needed by the slicer.  The slicer is an application that prepares the project for printing and translates it into G-Code, which is the format used by the 3D printer.  The slicer app does several things:

 

1)  orients the object on the build platform

2)  scales the object, if needed

3)  generates the "supports"

4)  defines the printer settings

5)  slices the object into horizontal layers

6)  saves all this into G-Code

 

Orientation of the object has an impact on both the quality of the final print and on the time it takes to print.  You may have to try different orientations before finding the right one for your particular object.  Scaling allows you to set the desired size of the print regardless of what size it was designed at.  It also allows you, if desired, to increase the size of your print by about 2 percent to account for shrinkage during the curing step.  The supports are generated automatically, but you have many options here including manually adding or removing supports.  The printer settings need to be adjusted to match the type of resin you are using, and there can be a lot of trial and error in finding the right settings.  The slicer app then slices the project into horizontal slices.  There can be hundreds of slices.  The final step is to save the project into G-Code, which is the low-level file format understood by the printer.

 

 

     Supports are necessary to the build, since no part of the object can be created unless it is attached to some other previously created part.  Although the slicer automatically generates supports, I've found that you usually need to add more manually.  The down-side of supports is that they leave little "nibs" on your object that will have to be cut or sanded off in the post-printing phase.  The base layer (called a "raft") is designed to allow you to easily pop the printed object off of the build platform.  If you build your object directly on the build platform you run the risk of breaking it during removal.

 

Step 3:  The print

 

     You save your project to a USB stick, then insert the stick into the printer.  Turn the printer on, add resin to the resin tank, select the object you want to print on the touch screen, then take a long break.  Printing time varies from about one hour to ten or twenty hours depending on your printer settings (that you chose in step 2), and on the orientation of the object.  This B37K rack takes about 1.5 hours to build horizontally, and about 8 hours to build if I stand it up on its end.

 

     Apparently you need to wear gloves when handling the resin.  I don't know why, actually.  I don't know if the resin is toxic, or if it is just because the resin is messy.  In any case I'm sure you don't want it in your eyes.  You'll need lots of isopropyl alcohol and paper towels for the next steps.

 

Step 4:  Removal and Curing

 

     When the printer signals complete you remove the build platform and set it down on a paper towel, then carefully pop the object off of the build platform.  Drop the object into a jar of isopropyl alcohol to wash off the excess resin, then remove the supports.  Actually I'm still not sure how much of the support structure should be removed now, and how much should remain attached during the curing step.  The uncured object is still a bit weak, and rough handling could damage it.

 

     Place the object in UV light to cure it.  Below is a picture of a curing box I built consisting of two 20 watt UV lights, a rotating display table, and power switch.  While the curing is going on (about 20 minutes, I figure, but who knows) you pour the unused resin from the resin tank back into the bottle (through a filter to keep any floating bits out), then clean up the build platform and resin tank using isopropyl alcohol.  It's messy stuff!

 

 

 

Step 5:  Throw away the failed print

 

     Unfortunately there are many ways the print can fail - bad mesh, bad orientation, bad printer settings, old resin, temperature too cold, platform not level, failed supports, etc.  Here's a sample of the failed prints I've made of the B37K rack.  Worst case is when the print never adheres to the build platform in the first place, and instead adheres to the bottom of the resin tank.  Sometimes you can't get the failed print off of the tank bottom (also called the FEP film) without breaking the film.  In that case you have to replace the FEP film - a very laborious process.

 

 

 

    So far the score in this battle is about Printer 12, Dave 0.  I'm hoping I'll eventually get a satisfactory print.

 

Edited June 19, 2019 by daveculp
typo

[Out2gtcha]

Good efforts here!  I'm always up for any new available accessories for the OV-10.    Was the B37K rack used for other things besides the practice bombs?

[daveculp]

42 minutes ago, Out2gtcha said:

Good efforts here!  I'm always up for any new available accessories for the OV-10.    Was the B37K rack used for other things besides the practice bombs?

 

Yes, it could also carry "logs" or flares. 

[SCRATCH BUILDER]

Well without knowing your settings its hard to help you, I have 2 Photons and a Photon S arrived today, I have had great success with my prints.

Just from looking at your print in the slicer you need to angle it no less then 45 degrees, you want to try and have the least amount of surface area on the FEP as possible when you able to, i have also found the ball tip to be useless.

 

The resin is toxic and can get you sick if left on you skin over long periods of time, ALWAYS WHERE GLOVES !!!!!!!!!!!!!!!!!!

 

TIP: Before you cure and after you clean I put my print in warm to hot water and the cut off the supports, the water makes the supports soft to cut instead of cutting them when they are brittle after curing. You may have to re dip in the water because it will cool, one caution you print will also be more flexible in water so make sure not to skew or twist it.

 

If you need any help let me know.

[daveculp]

Thanks, SCRATCH BUILDER, it's good to know the technology actually works.

 

I'm presently using the following settings (with the "grey" resin provided by Elegoo):

layer ht:  .03

bottom layer count:  6

exposure:  15

bottom exposure:  150

light-off delay:  0

bottom light-off delay:  0

 

I'm clearing space in my garage to try printing there, where it's much warmer than in the air-conditioned house.  I've noticed some resins require warmer temperatures, and although I've seen no temperature specified for the grey resin I'll assume warmer=better in general.

 

I've noticed printing with the object sitting on it's tail went better than the prints with the object horizontal.  I guess the rule is to accept the much longer build time in exchange for a more optimal orientation.

 

Thanks also for the hot water tip! 

 

Dave

[SCRATCH BUILDER]

Hey Dave,

I will have to do some research on your printer because I'm not familiar with it, but here is a rule of thumb. the smaller the layer ht the less exposure, 15 is high for.03, your bottom layers look ok but the bottom exposure seems way high,your cooking it to the FEP and not the build plate, try more around 80, looks to me like you are over exposing.

Here are setting i use on my AnyCubic and i use Phrozon  ABS like resin Grey.

 

Layer height-.05 (you don't see much difference between .025 and .05)

Exposure - 12 or 13 ( rule is to start at .05 and add 25% exposure if you go to say .10 and minus 25% when you go down to .025, so if i went to .025 my exposure would be roughly 2.5 - 12 =9.5, you can work on fine tuning those for your printer) note: If you use a clear resin(any color) you can lower your exposure even more because the light pass's though the resin better. when you start printing things with little holes you will see what i mean,to much exposure and the resin in the holes will get cured also and the result is no hole.

Bottom layer count  6-8

Bottom exposure  80 - 100

 

I don't have a heater installed but i live in Florida so no big deal, but 75 degrees +/- is ok, you can always let the bottle sit in some warm water also before you shake it to print...Remember to always shake!!!!

 

Orientation- yes at an angle, you want to have the least amount of material trying to peel from the FEP, you don't have to go full vertical, 45 degrees is a good starting point and yes it may have to print all day.

 

Another tip: after you have cleaned your tray after a print you can use a micro fiber towel to make it clear again because it does fog up from the resin and alcohol after each use.

 

You may have to do some searching but there should be plenty of info on the web for your unit.

 

If you need anything else let me know.

 

Almost forgot, looking at your sliced file you could use some more supports on the corners, don't be afraid to add more then you need until you get more familiar with your printer.

 

 

Edited June 20, 2019 by SCRATCH BUILDER