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.
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!