Introduction: Sugru Overmolding Using 3D Printed Molds

Having different mechanical properties in different regions of a part can be extremely useful, especially when compliance or textures are involved.

As an experiment in improving the grip of the Raptor Hand, produced by a team of designers in the e-NABLE 3D printed prosthetics community, I designed a mold and modified the Raptor distal phalange to allow for Sugru overmolding. Sugru is an air-cured silicone material that is typically packaged in 5gram packets and is quite durable and grippy when cured. After molding and trimming, the Sugru fingertip is secured bonded to the distal phalange and provides some additional friction compared to the original PLA surface.

Step 1: Design and Print Mold and Part

Start by modeling the interface with the overmolded material. Use mechanical joinery features (like dovetails) to ensure a secure bond between the overmold and the base part. After modeling the overmolding and the base part, subtract them from a block, offset all surfaces by 0.25mm (except the XY plane parallel surfaces-- the sides of the phalange) to make for a nice fit with the base part, and mirror the block to make a two part mold. Add some tapered alignment pins (offsetting the negatives by 0.25mm) to make sure the mold registers properly.

Print and double-check the clearances.

Fusion 360 source file here. (Design can be viewed at this link as well as downloaded in a variety of formats, including STEP and STL)

Step 2: Prepare Mold and Sugru

Lubricate the mold surfaces with oil to ensure that the Sugru doesn't stick to anything it isn't supposed to. I used machine oil, but any oil will do. Spread it evenly over all potentially Sugru-interfacing surfaces.

With clean hands, open the packet of Sugru and knead for a minute to soften it up.

Step 3: Apply Sugru to Part

Break of and form a piece of Sugru to approximately match the negative space it is going to fill on the part. Press the Sugru firmly into the dovetail features to ensure a strong mechanical bond with the base part and then fill in to approximate the target Sugru geometry.

Step 4: Insert Part Into Mold and Clamp

Place the part in the mold and apply more Sugru and pressure as necessary to fill the A-side of the mold's cavity. Then apply a bit of excess material to the protruding part and align and press down with the B-side of the mold. Press firmly with your hands or with a C clamp. you can either carefully remove the part after this step, or let it cure for 24 hours in the mold and remove it the next day.

Step 5: Finishing

There will be some seams and excess material after you remove the part from the mold. After the Sugru has cured (18-24hrs), use an exacto blade to trim the Sugru.

This is a pretty basic test, but it produced some promising results. Please feel free to experiment and share your results as well!

Comments

author
Hey Jude (author)2015-08-01

Just found this via SolidSmack. This is pretty darn clever! Nice job!

author
jarikcbol (author)2014-11-05

Of all the things i've seen 3D printing and Sugru used for, I think that this may be the absolute best.

author
gravityisweak (author)2014-10-30

Very interesting idea. I'm working on printing and assembling a raptor hand right now too. Of course, its possible to just print it as normal and press some sugru onto the fingertips. It appears you have changed the model to use sugru to replace a portion of the fingertip. What would be the advantage of doing this as opposed to simply adding sugru onto existing fingertips. Nice work!

author
jarikcbol (author)gravityisweak2014-11-05

As I understand it, Sugru can be peeled off *somewhat* easily. The change he implemented includes dovetails into the fingertip, which the Sugru fills, making it much harder to accidentally tear off. Also, this design maintains the overall shape of the final fingertip, instead of adding bulky 'pads' of Sugru to the tips of the fingers.

author
carlos66ba (author)2014-10-30

Wow! Amazing idea. I had been thinking of trying to hack a dual extruder so that I could put together PLA and Ninjaflex, but your instructable solves most of my issues in an easier way. Sure, it is not as versatile as co-printing, but it when your method works it is very good. Thank you very much!

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Bio: Engineer, designer, and artist who develops and applies novel 3D printing technologies.
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