Recommended minimum drain hole and vent hole sizes (by material)

I was curious to see what some of your recommendations are for the minimum drain hole and vent hole diameters(particularly for casting and clear grades)? I am printing a hollowed out part for investment casting. The smaller th holes, the better the casted part. I am new to printing hollow parts so I was wondering what hole sizes you have been using? Also, I ask based on material because the higher the viscosity of the material (such as durable), the larger the hole must be to be effective. If the formlabs team has info on this and I missed it, I apologize, if this is gained through experience, please share yours.

This was addressed in the new white paper for casting. I just found this recently. You’ll have to provide them contact info in order to download. I can’t find it on my computer…I’ll have to sign up and download again.

The white paper says something about being able to fill the inside with investment, but also the small amount of investment that fills the hole being able to support the weight of the “core” and hold it in place.

What exactly are you casting? Jewelry, models, other? What is the thickness of the piece (not size, but thickness of the shell?). Is there a possibility of casting in two pieces and soldering/welding/gluing/joining the two halves?

I recently modeled a rubber chicken and put 2mm holes to drain the resin and be able to clean the uncured resin from the inside. However, my intention was to fill the holes to make a plaster mold which could be used to make a hollow rubber chicken with “slush” rubber.

Thanks for the reply. I am an engineer working for an OEM that manufactures industrial machinery. I am looking to investment cast relatively small parts in stainless and aluminum. These parts would not require a shell interior as is common practice in jewelery, the shell process would be used just to minimize the amount of resin used and to ease burn out. I believe the foundry has the capability of patching holes with wax which I will rely on, but to help them, I wanted to minimize the number and size of the holes. The wall thickness of my shell is .078". I currently have 2 drain holes and 2 vent holes set to .060". I am hoping this is enough, but I suspect that cleaning out uncured resin from inside may be a bit of a challenge. Since the internal shape has no importance, I hope extra port cure time will take care of that as well.

If printing hollow just to save resin and to prevent it from expanding when burning out, then you can make the hole fairly large…
Simply PRINT the Plug you remove to create the hole. you can create the hole as a part that has keys to ensure it fits correctly in the hole, and print it in the same build as the rest of the part.

But if its an entirely enclosed hollow- be aware that you need TWO holes… one on top and one on the bottom so that air can come in one as the resin flows out of the other.

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You really DON’T NEED 2 holes, you only need one hole per hollow, the top vent hole. The amount of resin that gets trapped at the bottom of the print when the final layers close the cavity is minimal, worst case, the amount that will fill the cavity up to the level of the resin in the tank.

That resin usually ends up being diluted and drained along with the IPA when you wash your model.

I use a 2mm hole for the larger models, and a 1.5mm hole for the smaller ones. As for plugging the hole, once the mode is washed and dried, I use a toothpick to get some resin and fill the hole, and a cheap blue/violet 405nm laser to cure it. Once it’s cured, I use a small file to sand it flush.

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dang- you’re right… I forgot that the bottom remains open until the layer sealing it is printed.
My thinking is a holdover from 20 years of submersion SLA printing.

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With the orientation of my part, two 1/16" dia drain holes and two vent holes of the same size (one of each per section of my part that had potential to hold fluid and act as a cup) worked well. I found one vent and one drain hole per hollow worked great to act as a drain not just for the last bit of resin to escape but for the IPA as well. Without at least one vent hole, IPA would be trapped inside as air replacing the draining fluid would have to enter and exit at the same hole. I would say two holes is requires for cleaning purposes… I might have gotten away with a smaller hole then 1/16", but so far 1/16" (1.5mm) hole is proving to be decent with the casting resin.

Air can exit through te same hole as IPA enters just fine. Take the model and submerge it in IPA, do you see air bubbles escaping, that’ shows you IPA is going in, displacing the air inside. The IPA will dilute whatever small amounts of resin is still inside the model and simply drain out once you remove the model from the IPA bath and leave it out to dry.

Since the vent hole is at the top of the model as it’s being printed, when it’s removed from the build platform and you stand the model up, still on it’s supports, the hole is now at the bottom, and it will drain just fine.

I’ve printed at least 300-400 parts that were hollow and I’ve never had any issues with trapped resin.

And even if a tiny little bit does end up trapped inside, once you seal the hole, it won’t leak out, nor will it affect the stability of your print.