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

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 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.
+== Dosing (Plasticizing) ==
+Plasticizing is the process of dosing polymer pellets into the heating cylinder and press it forwards to melt and circulate (knead) the polymer melt inside the heating cylinder. A major part of the heat required to melt the polymer originates from friction energy during this kneading process. Some polymers are very sensitive to heat and thus must be dosed/plasticized gently to avoid decomposition. If samples turn out yellowish or brown it is likely to be caused by too harsh plasticizing settings (or too high heating cylinder temperatures). This problem can thus often be solved by decreasing dosing speed and/or back pressure.
+*'''Dosing speed''': Dosing speed is adjusted by pressing the left (green) ''''='''' sign and entering a value. Alternatively the up/down arrow can be used or the points of the green graph may be edited directly. It is recommended not to increase the speed above ~0,34 m/s. For sensitive polymers it may be necessary to lower the speed.
+*'''Back pressure''': The back pressure is adjusted by pressing the right (red) ''''='''' sign and entering a value, using the arrows or editing the red curve points directly. During dosing the only thing that moves the screw backwards is polymer entering the cylinder and thus pushing the screw backwards. A back pressure is used to "press against" this motion. The higher the back pressure the harder 'kneading' of the polymer. Again, some sensitive polymers might require reduced back pressures to avoid thermal damage (decomposition) of the polymer. It is usually sufficient to keep the back pressure at 120 bars or below, but in some cases (e.g. when using pigments) a higher back pressure can give better mixing of pigment and polymer. But be aware of the risks involved regarding decomposition of the polymer.
+*'''Shot volume''': This is the setpoint for the amount of polymer that is loaded into the heating cylinder. Note that this number '''in no way''' represents the amount of polymer injected into the mold cavity (besides the fact that the amount of injected polymer can of course never exceed the shot volume). The volume of polymer actually injected is defined via parameters on the "Switchover" screen mentioned earlier. As mentioned earlier a shot volume resulting in a cushion of about 1,5 cm<sup>3</sup> is recommended. Loading much more polymer than required for the shot increases the residence time of the polymer in the heating cylinder. This is undesired since it increases the risk of thermal degradation of the polymer.
+*'''Plasticizing delay time''': Adjusting this parameter it is possible to insert a delay between end of holding pressure and plasticizing. This can be beneficial if running processes with long cycle times (usually Variotherm-processes) in which case the polymer is in risk of thermal degradation inside the heating cylinder because it will stay there for a long time. In this case it makes sense to enter a delay so that plasticizing finishes briefly before cooling time has elapsed (a delay which is ~5 seconds less than the cooling time would be a good starting point). For processes with constant mold temperature (where cycle times are usually comparatively low) this value can be left at zero to minimize wasted time.
+*'''Plasticizing time monitoring''': It is recommended to always keep this enabled (it '''must''' remain enabled if leaving the machine running unattended) to make sure the machine stops in case of errors. Usually the maximum plasticizing time is set around 4-8 times the actual plasticizing time (which can be seen in the light blue field just above the setpoint). In this particular screenshot it shows 0,00 sec because no samples has been produced yet after the machine was started. A good starting point for 'Maximum plasticizing time' is around 2-5 times the normal plasticizing time. Then there's amble time for normal fluctuations.
+*'''Decompression''': To avoid problems with stress in the polymer melt it can be a good idea to enable decompression. A setpoint of 1 cm<sup>3</sup> works well for most cases. This means that after plasticizing the screw moves a little further backwards to provide extra volume for the polymer melt to relax. This is particularly beneficial if running at high back pressure.
+== Demolding ==
+Demolding is the process of opening the mold and thereby pulling the sample off the shim. Many of the previously mentioned parameters will influence the demolding process. Generally, the higher the mold temperature, the faster injection speed, the higher switchover/after pressure and the larger the structured area, the more difficult it can be to demold the sample. If the shim thickness is not optimal, polymer may also get in between the shim and the holding plate, making demolding almost impossible. A very critical parameter in easy demolding is designing the structures in a way, that makes them easier to demold (side walls of deep structures should not be completely vertical but have appropriate slip angles to aid in release and roughness of side walls can also severely impact the release properties).
+*'''Opening profile''': A constant speed can be set using the '=' button or the arrows. Alternatively individual points of the graph can be edited. If samples tend to break during demolding it might help lowering the opening speed. In general the speed should be low towards the end position (fully open position) to avoid violently slamming the mold against the end stop.
+<br>
+*'''Mold stroke position''': This value defines how far the mold opens. It should generally be avoided to open the mold all the way to the end position, since this causes the mold to bang violently into the end stop. If this happens (you hear a bang every time the mold reaches the open position) decrease the mold stroke position by a few millimeter (e.g. change the stroke from 235 mm to 232 mm). Remember that changing the opening stroke will also change sample take-off position, so remember to adjust the take off position (under robot settings) accordingly.
+<br>
+*'''Sequence settings''': These settings change how the robot is allowed to move around the mold. These settings should generally not be changed (be very careful if you do) to avoid crashing the robot into the mold.
+== Ejection ==
+Once the mold has opened, the sample must be ejected from the mold to enable the robot to pick up the sample. This is done by pushing out ejector pins from within the mold, which in turn will push out the sample. Once samples are picked up reliably by the robot it is usually not necessary to adjust these settings. Parameters for the ejector pins can be found on the 'Ejector' screen:
+*'''Advance profile''': These settings control how the ejector pins advance. The advance speed (green curve) and advance force (red curve) can be edited using the '=' or arrow buttons or by editing individual points on the graph. Generally it is not advisable to increase the advance speed above a few mm/s. And importantly: '''Under no circumstances should the advance force be increased above 8 kN!''' Otherwise the ejector pins may be damaged (bend or even break)! It is recommended to keep the advance force at 6 kN or below and only increase it to 8 kN if samples cannot be demolded reliably.
+<br>
+*'''Retract profile''': These parameters control the ejector pin retraction and is edited in the same way as described for the advance force.
+<br>
+*'''Ejector stroke position''': These settings control the positions of the ejector pins in the retracted and advanced position. It is recommended to leave the 'Ejector retracted position' around 25,6 mm. The 'Ejector pin advanced position' can be fine tuned if you have problems with unreliable robot pickup, but 56,0 mm usually works well. If you make changes to the 'Ejector advanced position' make sure to adjust the robot take-off position as well. Otherwise you risk missing the sample or that the sample is pushed hard into the robot arm and thus risk damaging both the robot and your samples. As mentioned in the 'Closing mold' chapter, remember that the the 'Opening stroke' parameter also influences the position at which the robot must pick up the sample.
+<br><br><br><br><br><br><br><br>
+Scrolling down will reveal a few more important settings regarding part removal. The most important are:
+[[File:07b-eject.png|600px|thumb|left|Scrolling down on the 'Ejector' screen reveals part removal settings.]]
+<br><br><br><br>
+*'''Ejector shake counter''': If samples are difficult to release from the moving part of the mold, this setting makes it possible to move ejector pins in and out several times to help releasing the sample in order for the robot to be able to grab the sample. Usually leaving it at 1 works fine (meaning the ejector pins will move out once and the robot will immediately attempt to pick up the sample), but in some cases it may help to increase the shake counter to 2 or 3.
+<br>
+*'''Ejector shake position''': This position defines the position the ejector pins move to during shake procedures.
+<br>
+*'''Mold pause time''': This setting is often used when running variable mold temperature ("Variotherm") processes where a delay is required in order to give the mold time to heat back up to the desired mold temperature before the next cycle can be started. The required time will depend on how cold the mold is at this point in the cycle and the mold temperature setpoint. If the mold has cooled down to 40°C and needs to heat up to 140°C it will usually take several minutes to do so (usually 3-4 minutes). The 'Mold pause time' is an easy way of achieving this, since this a delay at the end of the cycle. When running constant mold temperature processes, the mold pause time can be set to zero, since no delay is required (the mold temperature is always at the correct temperature).