List of TOPAS® COC grades

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3. Molding machines

3.1 Machine selection

For molding TOPAS® COC, commercially-available molding machines are generally used, so special molding machines are not needed. When a molding machine is selected, the size of the molding machine should be determined from the one-shot injection volume and projected area of molded products. In other words, make a selection using the following two points as a guide.

Selection of a plasticizing unit:

One-shot injection volume =injection capacity × 20 to 80% (30-50%, if possible)

Selection of mold clamping force:

low-pressure molded products like lenses:

  Mold clamping force > Projected area of molded products × 49 to 69 MPa

high-pressure molded products like light guide plates:

  Mold clamping force > Projected area of molded products × 343 MPa

3.2 Plasticizing mechanism

For molding TOPAS, a general-purpose screw can be used, but an optical screw is recommended in consideration of mixing of carbides and stability. If some molten resin comes in close contact with the screw, it will be oxidized and mixed in molded products in carbide form. Therefore, what is important in designing the screw is how to decrease close contact with resin.

Since TOPAS is generally low in viscosity, the functioning of the backflow prevention valve is important. This is to prevent molten resin from flowing back to the hopper side and to apply sufficient pressure to the cavity during the pressure-keeping process. It should be noted that inadequate backflow prevention will cause sink marks, bubbles (voids), dimensional variation, strength deterioration, etc. Select a securely functioning valve that mechanically closes the passage by rotating reversely to metering, or that closes the passage by using resin pressure.

As for the compression ratio of the screw, select approximately 2.0 to 2.5. If the selected compression ratio is relatively high, shear stress goes up during plasticization with the result that the resin may be discolored or resin carbides may be mixed.

Since appropriate design of the screw varies depending on the heater position, etc., contact your molding machine manufacturer.

3.3 Nozzle construction

Either an open type or a shutoff type can be used; however, an open type with less residence of molten resin is recommended.

Since TOPAS is low in viscosity, an open-type nozzle sometimes causes drooling and stringing, but these can be reduced by using a nozzle having a small diameter or a long-type nozzle. Also, since the occurrence of problems is greatly reduced by heater position and nozzle tip design, careful consideration of the advice above regarding nozzle selection is recommended when selecting a molding machine.