3D Printing. A "How-To" pattern making from PLA

[thor]

If you've been reading my build thread on my 1/32nd Russian WWII Shch Fleet boat you have seen the project that I have been messing around with since 1991. This subject has been an obsession with me over the last 29 years and the project is in the homestretch. The patterns are complete and all ready to mold up now, but I do not have enough material on fabricate a set of molds for such a large hull (73"). I have the material on order, but due to the current industrial shutdowns nationwide I will be waiting for months before all of the material is available. So, I have turned to a smaller project in the mean time for which I do have enough material on hand (barely) to complete the hull mold and get the boat ready for the Carmel Fun Run in 2021. I have decided to take the 3D CAD work that has been worked up for my 1/32nd PIKE and reduce it to .6667 to produce a 1/48th scale version of the same boat. Instead of machining the hull out of wood on my CNC Router, I have decided to demonstrate how to produce a super accurate pattern made from readily available and very inexpensive PLA plastic for my 3D printers. PLA (poly lactic Acid) plastic is a very nice material to work with for model building. While it is extremely easy to work with it does have a few drawbacks. For an application such as fabricating a functional hull, PLA is really not the best choice. ABS or ASA are much better choices as they stand up to the UV radiation and exterior heat much better than PLA. I happen to have about 30 rolls of PLA on hand so I am going to print the hull out in sections, assemble them on a rack, and then prepare the hull pattern in the very same manner that I would if I had machined the hull from locust or yellow pine.

Here is a semi-rendered image of the boat as I intend to finish her.

[Sub-standard]

cool, I'm super looking forward to it.

I do have one question on 3D and PLA printing for model sub fabrication and that is:

Would 3D PLA printing a (negative) mold work?
If a model sub can be drawn in CAD, then why couldn't the same drawing be used derive a CAD negative mold from that?
3D print the negative mold, in pieces if need be, glue them together and then cast a glass positive model sub from that? ... and you are done.

Hope I'm not hijacking, delete or ignore if you wish.

[thor]

That is an excellent question! Yes, you can do that! I prefer to use the old fashioned method of making a positive hull pattern so I can test fit the associated tid bits and detail parts onto the hull pattern to make sure everything fits as expected. Another reason is that with smaller patterns like this getting into the interior curves of the mold to create a nice finish can be very problematic depending, of course, on how you break your mold up. Remember, even with the tightest resolution, you will have to address the print lines and the section joints. If you are not extremely careful you will sand "bulges" into your mold without knowing it until you pull the fiberglass parts out of your molds. That can really ruin your day....

[eckloss]

You mentioned ASA. I've decided to halt my printing of a 1/72 Astute in PLA and begin fresh with ASA. After reading up on it and seeing some very good and comprehensive videos of various filament types, I'm convinced ASA is the way to go for high heat and light exposure. I'll be posting a thread later on down the road as printing nears completion and construction actually begins. It took a few tries to get my printer dialed in for ASA. It's kind of picky like ABS is. But I think I've got it figured out enough to allow me to work with it.

[thor]

Erich,

If you're intending to use the printed hull, then that is the right move. ASA can be really finicky and the print head temperature is critical.

Do you have a shroud for your machine to keep cold air off of it while printing?

[eckloss]

Yes, I will be using the printed hull itself. My printer is fully enclosed by design so ambient temperature is not really an issue. Luckily my print head can reach 250 if needed. Right now I've got it dialed in at 230 and the tests so far have been promising.

[thor]

The first step of printing a hull master is determining how you want the final print to be arranged. In this case, I like to split my smaller patterns at the waterline. With this in mind the 3D CAD pattern is cut horizontally using a cutting plane to separate the upper hull from the lower hull.



After the two sections are fully separated the open surfaces on the bottom of the upper half, and the top of the lower half must be sealed by creating a planar surface that defines the hard separation. This is to provide a sealed "volume" instead of an non-closed volume which a G-Code generator will lose its mind trying to figure out how it will print an open volume. Be sure to check all of your volume edges to make 100% certain they are closed and sealed!


The upper half has been fully separated from the lower half via the horizontal cutting plagne:



Here the lower hull half, which will be printed first, is turned over on its top to create the separation planes which will begin the process of breaking the hull pattern 3D model up for printing:

[thor]

The process of "breaking up" the model has begun with the first cutting planes installed:



After you have successfully broken up your pattern into sections for printing it is time to move them, one at a time, to X,Y,Z=0,0,0 such as seen here:



Most all 3D printers utilize .STL files for processing by the G-Code generator. Most all 3D printers come with a G-Code generator that you download via a link to the manufacturers website. Most of them are very easy to operate, but not all are intuitive in nature. Read the instructions very carefully and practice with a test print before committing to printing a large, complex part.

Here is the part we are going to print:



This section is #L4, or the 6th section from the aft perpendicular of the lower hull half.

If you examine the vertical face at the hull separation plane on each and every hull part, both upper & lower, you will notice two holes that penetrate into the part 3/8". These are indexing pin holes that will align each of the hull sections together perfectly on a track that will be shown later. This is one of the little secrets on how to assemble the real hull 3D parts in the real world to make certain they are perfectly straight.

To proceed to printing; highlight the hull section you intend to print that is located at 0,0,0 and export it as an .STL file. Next, you will open your G-Code generator and import or load the newly created .STL file for processing.

One very important item to note is that you MUST size your parts to fit the print volume of your 3D printer. The cutting planes above are placed exactly 5.5" apart to match the Z-Axis travel of my print bed, which is 5.5". The total print volume of my unit is 5.5" x 5.5" x 5.5". You can see here the image of #L4 loaded into the G-Code generator. It is pushing the boundaries of the volume capability of this printer:

[salmon]

Matt,
This is an excellent thread. Thank you for doing this!
Peace,
Tom

[thor]

Here is a sneak peak at the 1/48th Scale Russian WWII Shch "Pike" conning tower; semi finished in preparation for molding. It is 3D printed using PLA at my Flash Forge Finder 3D printer. The forward gun deck is printed using a DLP resin printer.

[thor]

Here is the finished 3D print of lower hull section #4. It took 31 hrs 26 minutes to print out on "Hyper" resolution. When printed at the tightest resolution the print lines only need to be lightly sanded with 400 grit wet paper. Two coats of sanding primer on top of the light sanding and it is all very nice and smooth.

[thor]

All of the sections of the lower & upper hull have been sorted out, converted to .STL files and printed out. Here is the set of 3D hull sections awaiting being sent to the printer for processing:



All of the printed hull sections are awaiting alignment and gluing on the CNC machine bed. The reason I use the CNC machine bed to align the parts for gluing it that is lies in a 30" long flat plane that is accurate within .003" over its length.



If you look closely in the upper right corner of the CNC machine bed, you will notice 3 sections of the aft lower hull being glued together. I will demonstrate how the sections are all perfectly aligned on the track board as soon as these sections are cured.

[scott t]

Holy sub prints Matt man! Looking amazing.

[salmon]

Holy sub prints Matt man! Looking amazing.

Scott I laughed reading this.....very creative!
Matt, I love your work.

[thor]

Each single piece is printed on end and has a pair of 'alignment' pin holes as shown here:



I like to use K&S 1/8" brass pins to align my parts. They are inserted into the printed part as shown:



Do not glue the pins in place. The pins should take quite a bit of effort to insert into the holes, so friction will be plenty to keep them secured. They will be removed after the alignment & gluing process is complete.

I have had excellent luck aligning my 3D printed hull sections onto a flat section of melamine shelf board. I run the board through my table saw which creates a nice groove that will allow the pieces to stack up neatly with each other. The melamine board shown here had its groove machined in with my mill.



To begin the gluing process mix up a small amount of 2 hour epoxy (or longer cure) thickened with cabo-sil (fumed silica) or micro balloons. The consistency needs to be that of toothpaste. A filled epoxy is an absolute must here! Plain, unthickened epoxy is too elastic which will lead to the parts sagging a small amount when removed from the board. The cabo-sil or micro-balloons provide stability and rigidity to the joint that will not allow it to move. Do not try and use a short term cure epoxy such as 5 minute or 15 minute. These glues are NOT stable enough to provide a reliable, rigid joint.



After you mix up your thickened resin spread it over the entire surface of ONE of the parts to be glued, not both!



Next, place the 2 or 3(no more than three) 3D printed hull parts to be together in the track. Firmly slide them together. NOTE: coat the melamine track board with a good release wax before gluing the sections together to prevent them from self-adhering to the board.



Gently, but firmly press the 3D parts together to get the excess adhesive to flow out of the joint. Wipe away any excess with a cloth, then clean the 3D part joint with isopropyl alcohol to remove any surface epoxy to make your cleanup of the model much easier after the gluing process is complete.