Rule 5: Material specs cannot always be called out from published data

Micromolding analysis requires special consideration since the lowest published data regarding thickness cuts off at about 0.040 in., which completely ignores the needs of micro product manufacturers. In such cases, the manufacturer must rely on the molder for empirical data. The molder should have a large database that grows with every completed molding project. This database should have information on how far each material might flow at micro thicknesses and what kind of pressure it takes to move it in the right direction, all of which helps characterize each material for possible new applications. Our test plaque molds are used to gauge materials from 0.002 to 0.009-in. thick to give project engineers an appreciation of the relative properties of their first-choice material at a given thickness.

Rule 6: Micromolds require high fill pressures

At thicknesses less than 0.01 in., plastic cools extremely quickly, so liquid plastic must be shot into the mold cavity at fast speeds and extremely high pressures up to 40,000 psi. However, such conditions risk altering the material properties. Our mold-flow analysis maps changes in the material caused by the intense pressure and shear heat in the micro process to ensure that problems didn't occur.

Rule 7: Account for eject

After cooling, the ability to cleanly eject the part from the mold is often affected by its design. When a part goes into the human body, the last thing desired is unwanted jagged edges. A popular misconception is that because the parts are so small, they don't need draft; but that is definitely a misconception. Draft is needed because the walls are thin and the relative forces of the very small pins that you use for ejecting still can cause a problem if you don't have draft that allows the part to release easily.

In regards to ejecting, and most every aspect of mold design, accurate tooling is by far the most significant contributing factor to success.

BOSTON SCIENTIFIC'S MICRO PERSPECTIVE

“With my knowledge of the medical industry, I'm already aware of all the biologic restrictions, but when you get to the specifics of micro injection molding, a knowledgeable partner is needed,” opines Isaac Ostrovsky, an engineer with Boston Scientific Corp (BSC), Natick, MA. For Ostrovsky's most recent project, a 0.04-0.05 in.-diam. element that gets inserted through the urethra, BSC teamed with Miniature Tool & Die, Charlton, MA, which claims to be the country's only vertically integrated mold maker and molder that exclusively specializes in micromolds.

“Our part has four holes, and the thickness between these and the outside wall is only 0.005 in., so it takes very high pressures to make the plastic flow into such tight areas,” says Ostrovsky. “MTD created a mold-flow analysis to map changes in the material caused by the intense pressure and shear heat in the micro process to ensure that problems didn't occur.

“For us, everything turned out well and within budget by working with a micromolder,” says Ostrovsky. “It only took two weeks for MTD to make a working model of our design.