Specific Process Knowledge/Back-end processing/Polymer Injection Molder: Difference between revisions

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 *The 'Specific injection pressure limit' is an upper safety limit that can be set to protect the mold and/or shim (e.g. if using shims that are soft and thus prone to deformation). Note that '''this setting does not in any way ''set'' the injection pressure'''. It is only an upper safety limit that will not be exceeded.
 *The 'Minimum injection time' is usually set around 0-0,05 sec and 'Max. injection time' around 5-8 times the actual injection time. The actual injection time can be seen in the blueish/gray filed just over the max injection time setpoint (in this case it says 0.00 sec because no sample has been produced yet). Actual injection times are usually around 0,2 to 2 seconds so the 'Max. injection time' can typically be set to 1,0 - 15 seconds.
+== Switch-over Type ==
+The switch-over type is a very important setting! It has a significant impact on the properties of the sample. This setting defines when the injection-phase is done and thus when the machine will continue to the next phase of the injection molding cycle (after pressure / holding pressure). Since this setting defines when we consider injection is done it significantly affects the properties of the produced sample, in particular filling. The switch-over settings are found on the 'Switchover type' screen (a sub-icon of the 'Inject' icon):
+[[File:03-switchover.png|600px|thumb|left|On the 'Switchover type' screen it's possible to select between volume, time, pressure, external and parallel switchover]]
+*'''Volume-dependent switchover''': A certain volume of polymer is injected. Note that the volume entered (3,2 cm<sup>3</sup> in this example) is not the volume injected, but the amount of polymer left after injection! Also note that changes in shot volume (the amount of polymer in the heating cylinder before injection) will also change injected volume unless the switch-over point is adjusted accordingly. Volume dependent switchover is particularly well suited for developing new injection molding processes (so you can gradually find the point where the cavity is almost filled and then work from there) or if you already know the exact volume of polymer required.
+*'''Time-dependent switchover''': After a certain amount of time, injection is considered complete. Not used very often because it offers very little control over both injection volume and pressure.
+*'''Injection pressure-dependent switchover''': Polymer is injected until a certain pressure is reached. Please note that the pressure in question is measured in the injection unit (heating cylinder). The pressure inside the cavity is not known. Special molds with an integrated pressure transducer is required to determine cavity pressures (very interesting, but unfortunately not available at DTU Nanolab). This switchover type is very easy to use and is not influenced by e.g. changes in shot volume. The selected switchover pressure (in this example 800 bar) is critical for replication. Very low switchover pressures can result in bad replication or even incomplete filling while too high pressure can cause the sample to stick so well to the shim, that it gets very difficult to demold.
+*'''External switchover''': Injection continues until a signal is received from an external device (e.g. an external pressure transducer). Can not be used, since no external devices are currently available.
+*'''Parallel switchover''': Injection is defined by a combination of parameters. Has never been used until now.
+== After (holding) pressure ==
+After injection it is generally necessary to maintain the injection pressure for a certain time. The time required will depend on processing parameters (especially mold and melt temperatures), but generally it is desired to maintain a pressure for long enough time for the gate to have solidified. Too short duration or too low after pressure can cause the polymer to flow backwards (out of the cavity) or that the sample gets an uneven surface (sink marks) because the sample shrinks as the polymer cools down. On the other hand too high after pressure or time can cause high levels of stress in the sample (due to excessive packing of polymer) and at some point increasing after pressure and time will cause the sample to get stuck in the mold, masking it very hard to remove.
+The after/holding pressure settings are found on the 'Holding pressure' screen:
+[[File:04-holding.png|600px|thumb|left|Holding/after pressure and time are set on the 'Holding pressure' screen]]
+*'''Holding pressure profile''': Holding pressures can be set as a constant pressure (by pressing the '=' next to the curve and entering a value) or as a pressure that varies with holding time (by pressing the graph areas and editing the points). It is preferable to let the holding pressure gradually taper off as shown here to avoid very abrupt changes in pressure. Just letting go from e.g. 1000 bar to 0 bar causes the screw to fly back violently and should be avoided.
+*'''Holding pressure time''': The holding time is entered here. The time axis in the graph will update automatically when the value is changed.
+*'''Cushion monitoring''': The cushion is the amount of polymer left in the barrel (heating cylinder) after injection. A small amount must be left in order to be able to maintain a holding pressure. It is recommended to keep the cushion around the center of the interval (i.e. ~1,5 cm<sup>3</sup>). If the cushion is too small the shot volume should be increased and if the cushion is too large the shot volume should be decreased accordingly. Please note that the cushion is pressure dependent. Increasing the holding pressure will decrease the cushion (because the higher the holding pressure the higher compression of the polymer).
+*'''Shot volume''': If the cushion is too small or too large, adjustments of the shot volume can be quickly made here, as described above. All other settings relevant for dosing (plasticizing) are found on the '''Plasticizing''' screen.
+== Cooling time ==
+When the holding time has elapsed the machine will switch to cooling time. The optimal cooling time is very process dependent, but the overall purpose is of course to give the sample enough time to cool sufficiently below the polymer's melting point (or glass transition temperature) so that the sample can be removed from the mold without deforming.
+The optimal demolding temperature (temperature of the mold when the mold opens) depends on both the polymer and mold used and the structures being replicated. Generally the lower the demolding temperature the better because this makes the polymer more rigid and in some cases the shrinkage of the polymer as it cools down can also help releasing the sample from the shim. But lower demolding temperatures come with a time penalty. Generally the Luer tool is most tolerant to varying demolding condition because the 12 Luer connectors help in pulling the sample off the shim. The flat disc tool can require a little optimization for reliable demolding while the microscope slide tool can be quite challenging (especially with deep or high aspect ratio structures) because of its geometry. If you are running a variotherm process remember that mold heating and cooling water timing will greatly influence the cooling phase.