Specific Process Knowledge/Back-end processing/Polymer Injection Molder: Difference between revisions
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'''Feedback to this page''': '''[mailto:wetchemistry@nanolab.dtu.dk?Subject=Feed%20back%20from%20page%20http://labadviser.nanolab.dtu.dk/index.php/Specific_Process_Knowledge/Back-end_processing/Polymer_Injection_Molder click here]''' | '''Feedback to this page''': '''[mailto:wetchemistry@nanolab.dtu.dk?Subject=Feed%20back%20from%20page%20http://labadviser.nanolab.dtu.dk/index.php/Specific_Process_Knowledge/Back-end_processing/Polymer_Injection_Molder click here]''' | ||
'''<p style="color:red;">The Polymer Injection Molding machine is no longer available as an open access tool at DTU Nanolab. The machine has been moved to DTU Construct.</p>''' | '''<p style="color:red;">The Polymer Injection Molding machine is no longer available as an open access tool at DTU Nanolab. The machine has been moved to DTU Construct.</p>''' | ||
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[[image:Imm.jpg|200x200px|right|thumb|Injection molding machine in the basement under the cleanroom]] | [[image:Imm.jpg|200x200px|right|thumb|Injection molding machine in the basement under the cleanroom]] | ||
The injection molder at DTU Nanolab was an Engel Victory Tech 80/45 which is a hydraulic machine with single motor. The machine was also equipped with a robot which could pick up finished samples and place them on a conveyor belt. See the table below for key capabilities of this machine. | The injection molder at DTU Nanolab was an Engel Victory Tech 80/45 which is a hydraulic machine with single motor. The machine was also equipped with a robot which could pick up finished samples and place them on a conveyor belt. See the table below for key capabilities of this machine. | ||
==Equipment Performance== | ==Equipment Performance== | ||
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There are no standard injection molding processes because it varies significantly how users prefer to optimize their process and how they prioritize parameters such as polymer type, replication fidelity, dimensional accuracy, optical properties, residual stress and cycle time. However, browsing through the [http://labmanager.dtu.dk/function.php?module=Processlog&view=editlog&machid=278 Process Log] it's usually not a problem to find a program made for the polymer and tool at hand which can be used as a starting point. | There are no standard injection molding processes because it varies significantly how users prefer to optimize their process and how they prioritize parameters such as polymer type, replication fidelity, dimensional accuracy, optical properties, residual stress and cycle time. However, browsing through the [http://labmanager.dtu.dk/function.php?module=Processlog&view=editlog&machid=278 Process Log] it's usually not a problem to find a program made for the polymer and tool at hand which can be used as a starting point. | ||
The injection moulding cycle consists of the steps outlined below | The injection moulding cycle consists of the steps outlined below: | ||
# | #Mold heating setup | ||
# | #Closing mold | ||
# | #Injection | ||
# | #Switch-over type | ||
# | #After (holding) pressure | ||
#Cooling | |||
#Dosing (plastizising) | |||
#Demolding | |||
#Ejection | |||
#Robot sample pickup | |||
#Nozzle settings | |||
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== Process overview == | |||
Once a process is running satisfactory, the machine also has tools that can be used for monitoring a running process. These tools can help to ensure desired sample quality and reproducibility. Press the second button from the top on the right column of buttons to get to the ''''Cycle time analysis'''' screen: | |||
[[File:13-cycleanalysis.png|600px|thumb|left|The 'Cycle time analysis' screen provides an overview of the entire injection molding cycle.]] | |||
*''''Mold Close' to 'Part removal'''': In the left part of the screen the individual phases of the injection molding cycle is listed. Just to the right of these names there's a small box. If it's gray it means this step i currently not active. If it's red it means this step is currently active (so in this example the sample is currently being ejected by the ejector pins and the part (sample) is removed by the robot). | |||
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*'''Progress bars''': A little further to the right (in the center of the screen), progress bars will indicate position and timing of each individual step in the cycle (both last cycle and current cycle). Note that the 'Part removal' step of Variotherm processes often takes quite a long time. This is because the 'Mold pause time' delay (mentioned in section '9. Ejection') used for reheating the mold is included in this step. | |||
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*'''Last and Current columns''': In these columns it's possible to see how long time each phase takes in the currently active cycle. For comparison the corresponding time during the last cycle is also shown. | |||
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*'''Total time''': In the bottom of the screen a progress bar will indicate progress for the currently running (yellow) and previous cycles (green). Please note that the time axis of the progress bars do not autoscale. Min and max time scale values must be entered manually (but since the total cycle time for both the current and last cycle is shown right above, it's very easy to enter a sensible value). | |||
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The injection molding machine also has tools for monitoring process parameters during the injection molding cycle. Usually the most interesting events happen around injection, so data logging is often centered around this event. But there are many and almost endless possibilities. To ensure reproducible results it can be beneficial to monitor parameters that are tightly linked to sample quality/properties which include injection speed and injection pressure. Pressing the second to last button on the left side of the screen will take you to the ''''Micrograph'''' tool: | |||
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[[File:14b-micrograph.png|600px|thumb|left|The Micrograph tool can be used for monitoring process parameters.]] | |||
*'''Setup''': To get started you can press 'Setup' and then 'Manual setup' and select relevant parameters. On the tab 'Measurement duration' a start condition (in this example 'Start injection' was chosen) and a measurement duration (in this example 1,5 sec) can be selected. On the tab 'Curve selection' parameters to be plotted are selected (in this example specific injection pressure, injection speed and shot volume were selected, but there are many other possibilities). Since all selected process parameters are plotted on the same y-axis, this axis will always show the unit per cent and always be scaled 0-100%. The 100% value is then set independently for each plot (in this case 800 bar, 40,0 cm<sup>3</sup> and 15,0 cm<sup>3</sup> was selected for the 100% value of specific injection pressure, injection speed and shot volume respectively. | |||
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*'''Other settings''': There are many other possibilities for setting up monitoring features, logging process parameters to file, monitor sample quality and e.g. discard samples that do not meet set requirements. Consult the original machine manual or ask for details. | |||
== Mold temperature setup == | == Mold temperature setup == | ||
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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. | 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. | ||
*'''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 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. | ||
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*'''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). | *'''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. | *'''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 == | == Cooling time == | ||
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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. | 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. | |||
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*'''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. | |||
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*'''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. | |||
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*'''Retract profile''': These parameters control the ejector pin retraction and is edited in the same way as described for the advance force. | |||
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*'''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. | |||
*'''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. | |||
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*'''Ejector shake position''': This position defines the position the ejector pins move to during shake procedures. | |||
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*'''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). | |||