Printing Molds for Rubber Repairs

This project will attempt to record the production 3D printed molds, in this case for a glider landing-strut boot

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If you do any semi-regular restoration work, you're very likely to understand the frustration that's caused by trying to find obsolete components, especially if its something hard to re-produce. Rubber boots seem to fall well into the category. They have over hanging walls that make printing difficult, poor tensile strength and relatively poor heat tolerance.
My goal only really developed as I saw a series of projects around me requiring odd shaped rubber that hadn't been made for over half a century. Some of these parts could be directly replicated with a flexible filament, a topic of its own, but for rubber pieces that had to work well in tension as well as compression I had many difficulties.
Ultimately my success was in trial and error, but after five different mold designs, I was able to produce a solid silicone boot that should be able to withstand severe conditions for a prolonged duration.

Taking the Edison approach to designing my mold, I ultimately revised and printed five different iterations. As a machinist I'm more of a maker than a calculative designer, so it was super helpful to work out a 3D printed solution for creating a working part. If not for the ease and cost effectiveness of printing, these molds would have been taxing to produce by standard machining convention. I do have some other projects that may utilize this process during their work, and hope to add some future insight to this page.

The Process:

My first attempt to create a 3D printed rubber boot was actually a complete failure. In this instance I was refurbishing the drive train on my 1972 RV and found that the master cylinder boot was completely torn and rotten. Upon later inspection and attempted re-honing of the master cylinder, I found that the entire unit was shot. Luckily I was able to source an entire new cylinder with replacement rubber, but still my attempts to refurbish this unit started me down the path of cast silicone molding.

     My original goal was to directly print a master cylinder boot that could expand and contract with the movement of the pusher shaft, just as the original part does. The original part has many horizontal shelves that allow the rubber to move without tearing, but for a 3D printed part this was a challenge. The plan of attack was to modify my part geometry to emulate a pyramid, and hopefully allow for an accordion style body that as 3D printer friendly shape. This design actually seemed to work except that I couldn't achieve any real durability. Even the best of my boot prints seemed to fail after a few dozen cycles of use. The final nails in the coffin for this attempt was the consideration of heat resistance that should be necessary for a component sitting so close to an engine. Since I ultimately procured a new boot with the new master cylinder, I marked this attempt as a fail, but still kept toying with the idea of how to produce a 3D printed piece of rubber that wouldn't fail so quickly in an industrial environment.

Another Failure?

After finding that I wouldn't need any of the master cylinder boots that I had printed, I decided to try a post process heat treatment to improve the durability. Seeing that all my part failures were due to layer splits, my target was to somehow in essence, anneal the part. That is, bring it up to a temperature where the plastic can reform into a more consistent molecular blob ( real sciencey word ) without the mechanical lateral layer definitions. The well known problem here is that heating up the entire printed part will cause it to warp and deform. Therefore I tried to encase an over extruded part completely within a piece of plaster of paris, then bake. My own attempts at this were failures, but I'm still not fully convinced that this wouldn't work if properly carried out. The biggest problem encountered for me, was the lack of heat generation. Apparently baking plastic inside is not appealing to some, and the portable toaster oven I used didn't have enough juice ( or maybe time) to liquefy my part. I took care to keep half of my boot aside for a control, but the samples I took in my trials still showed clear layer boundaries in the profile. 

After finishing up my master cylinder work, and realizing I had no need to really make a good boot anymore, my interest slowly diminished. It wasn't until later, while working to refurbish a 1978 Lark landing strut ICA -IS-28, that I realized could take another swing at boot building.

    For this latest attempt at boot building I opted to cast a silicone mold. The reasoning being that silicone, while although not available as a filament, has excellent thermal and elastic properties. In this case, since there was no original boot to base geometry on, I also had to play around with different shapes and sizes before I had any success.

 There have been a few...

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  • 1
    Design and Modeling

    The first step to creating any new piece of hardware will naturally be the conceptual design and 3D modeling of the part. For some software suites it will be ideal to first model the part you wish to create, and then use that model to define the geometry of the inner and outer mold pieces. If this is not convenient to your modeling software, then a pencil, paper, and calculator will suffice to calculate your numbers.

    For the purpose of a cylindrical part, such as our boot, a clam shell style mold works well. a full circular profile can be modeled for up to 180 degrees of the part, and the inner core will serve as support and locating fixture to the outer halfs

    The purpose of splitting the inner core into two halves, is two fold. The first is to help reduce the amount of stretching the boot has to endure during its "de-molding" process, and secondly to make the horizontal edges easier to print in the vertical direction. Also being as the cylinder is an isometric part, we only really have two different pieces that are printed twice to produce the full mold, two inner cores, and two outer halves.  The inner cores are produces by taking the geometry from the inside of the boot, and making a swept profile 180 degrees around center. 

    Similarly the outer mold halve is created by using the exterior boot geometry, but rather than a revolved extrusion, a revolved cut is made thru a block large enough to hold the volume needed. The outer mold block only needs to be large enough to allow a reasonable parting line width( maybe 1/4" or so) , and room for fill holes, any more than that and you're just adding time to your mold prints.

    Some other consideration is given to the seat of the two molds. a distance is extended above and below where needed. The lower extension allows for the molds to orientate and seal, while the top extension is used as a vent and trim line indicator. Its better to have your silicone form past where it's needed, rather than risk it being too short if you don't allow any fill.

  • 2
    Printing the molds

    Assuming that the reader has a reasonable understanding of 3D printing, this step will be glanced over. The subject could easily be expanded on, but it would be taxing to undertake that detour in this process.

    It is worth mentioning that the calibration and accuracy of a printer could limit the size of features produced. The "seam" where the mold half's join, commonly called  the "parting line", will determine how accurately the molds fit together. In industrial standards, sanding the parting line is a taboo, any non conformance, miss-match or miss-shaped geometry will show greatly on the finished parts final product. Next time you have a plastic bottle around, try to find the seam where the two halves of the mold closed, chances are that it will only be found it you looked for. It's not always critical to produce industrial standard molds, but take it from someone who's had to spend countless hours polishing things, use the technology to get as close as you can, then B.S. it from there.

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zakqwy wrote 01/31/2018 at 23:23 point

Might be worth some kind of surface finishing on the 3D printed part to improve the roughness of the moulded boot. Maybe acetone polishing?

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classicrockfanatic88 wrote 02/02/2018 at 01:04 point

I used wax, both as a release agent, and a finishing layer. Additive polishing lol

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classicrockfanatic88 wrote 01/30/2018 at 03:00 point

I know a good deal of mold emulation is done with 3D printing, but I found very little documentation on it online. Its just too useful of a process for solid cast parts. Good luck with your epoxy coating and thank you for your feed back

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Joshua Elsdon wrote 01/29/2018 at 17:12 point

I will be casting some epoxy using a very similar technique this week, great minds think alike eh. I have my filling syringes ready to go, and for the same reason, bubbles will ruin the object. I will be trying to over-mould a 3D printed ring with clear epoxy to enclose some electronics, our mould forms are looking suspiciously similar!

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