Wednesday, August 21, 2013

Round molds for round part

Have you ever tried to mold and reproduce a solid round cylinder? You make the perfect mold and the part still comes out slightly oval... not so good if it need to fit into something or you have to finish it....
So this was an experimental mold for polyurethane casting in our plastics shop. I was looking for a way to improve on a system we developed almost three years ago.
Our current system relies on speed, ease of use, accuracy and mold life.

This mold is used to encase an electric device in a protective polyurethane casing designed to take a beating.

The outside is the "mother mold" its made of acrylic and split down the center and drilled to create alignment marks.

The orange part is the master and the grey part is the sprue and funnel to pour the liquid polyurethane into the mold.

The mother mold was taped together with blue painters tape and becomes the mold frame. It seems to be the best tape to use because the adhesive doesn't react with the platinum cure RTV when the two touch. Maybe the lack of sulfur compounds in the adhesive, I'm not sure....

The mold has only one seam in it and it is designed to leave no mold line on the finished piece.

The blue tube  in the bottom of the mold is a peice of brass tubing wraped with that same blue tape. Its designed to hold the electric component upright in the mold so it doesn't touch the sides of the mold while casting.

Over all this mold seems to preform very well but it missed on two points of our criteria...
1 Speed
assembling the mold to cast is a four part process, our current process has only 2 steps.
2 Ease of use
the top half of the rubber mold is like a condom and is hard to remove from the casting.

We could over come the ease of use if we use a rubber with a lower durometer but we might sacrifice the accuracy if we do.

The next test in this arena will be to add a vertical parting plane... to be continued

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