Rule 1: Mold tolerances become more critical.
As parts get smaller, any miscalculations have a significantly greater impact. When requesting a micro mold, aim for 10 percent part tolerances, as opposed to the 25 to 50 percent level commonly seen in macro molds,

Rule 2: Form affects maximum wall thickness.
On a macro design, a 0.030 in. wall thickness allows some flexibility in form, Tully notes. But when the wall thickness falls below 0.005 in., as is often the case for a micro part, then overall size and shape becomes an important factor.

  Rule 3: Gating becomes more critical.
Gating rules are somewhat material-specific. Even still, most materials passing through micro gates sizes of 0.002-0.005 in. behave differently than when passing through macro gates of 0.020 or 0.030 in.

   If you are a macro molder and you have trouble getting material to flow, you can just crank up the pressure, temperature or fill speed,But that will not always be an option through a small gate, which induces high shear rates that can change the viscosity of the material. Nor can you just heat the material to a higher temperature to lower viscosity to help it flow. Either case can destroy the properties of the material. In some cases it is best to error on the safe side by running at 75 percent of the wall thickness for the gate size.

  Rule 4: Stand ready to relocate the parting line.
“When dealing with micro molds, the parting line cannot always be placed in the ‘ideal’ location from a design standpoint,” Tully cautions.

  A common mismatch allowance for a macro part can range between 0.003-0.005 in. whereas that wide a margin on a micro part might mean missing the whole other side of the mold. The mold must interlock properly to support the critical mismatch requirements of the parting line and improve “registration” of the two halves.

  Rule 5: Material specs cannot always be called out from published data.
While still open to many engineering plastics like resorbable polymers, Ultem, PEEK, PPS, LCP, shape memory polymers and polycarbonates, micro molding 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, according to Tully. In such cases, the manufacturer must rely on the molder for empirical data.

  Rule 6: Micro molds require high fill pressures.
According to Tully, at thicknesses less than 0.01, 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.

  Rule 7: Take into account ejecting.
After cooling, the ability to cleanly eject the part from the mold is often affected by its design. Tully explains that once the plastic is injected into the tool, the finished part has to be removed, which can be difficult. There’s a popular misconception that because the parts are so small, they don’t need draft; but that is definitely a misconception,They do 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.

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