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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.

Polymer injection molding

Polymer injection molding is a high-volume polymer repliction method where a viscous polymer is injected into a custom-made closed mold. After solidification of the polymer, the cavity is opened and the polymer part is ejected.

Description of Machine

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 user manual, user APV, technical information and contact information can be found in LabManager:

Equipment Performance

The polymer injection molding machine previously installed at DTU Nanolab was a hydraulic system with the following capabilities:

Parameter		Value
General	Tools	Flat disc, Disc with Luer connectors, Microscope slide
General	Clamping force	Up to 450 kN
Processing temperatures	Mold temperature	~15°C to 140°C
Processing temperatures	Heating cylinder	Absolute max: 350°C
Molding parameters	Injection speed	Up to 53 cm³/s
	Injection pressure	Up to 2200 bar
	Process types	Constant or variable mold temperature

Polymers

At DTU Nanolab the following polymers were used routinely:

Topas 5013L-10
Topas 8007S-04
PP
PMMA

Process Parameters and Optimization

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 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. Click on each step to get a description of the most important process parameters for each step:

Mold temperature setup

Injection molding processes can be divided into two different types: Constant mold temperature processes and Variable mold temperature (Variotherm) processes.

1. Constant mold temperature processes

In this type of process the mold remains at the same temperature during the entire injection molding cycle. The temperature should not exceed the glass transition (or melting point) temperature of the polymer being used. Otherwise the polymer will never get cold enough to solidify to a degree where it can be demolded without being deformed/damaged. How close one can get to the glass transition temperature depends on several factors like e.g. polymer type, cooling time and whether the polymer has a tendency to stick to the shim (especially an issue when replicating high aspect ratio structures). With Topas 5013L-10 (which has a Tg around 135°C) one can often go up to 100°C and for samples that are easy to demold even higher (110-120°C). For polymers with very low Tg (such as e.g. flexible polyurethanes) one often has to run as cold as possible. With the mold heating switched completely off and mold cooling water fully open, the lowest achievable mold temperature is around 15°C.

Parameter	Benefits	Drawbacks
Low mold temperature	Very fast cycle times due to short cooling times	Problems with replication of small features (incomplete filling). Stress in samples.
High mold temperature	Improved replication	Problems with demolding. Problems with diesel effect. Slightly longer cycle times.

For constant mold temperature processes it is a good idea to manually close the mold cooling water valves e and g. For reproducible results it is also a good idea to make sure that you always run at the same temperature setpoint for the mold casing cooling water controllers (see Chapter 4.11 in the manual). A setpoint around 50-70°C for both sides works well in most cases. If the tool casing is allowed to heat up to 90°C or higher it will often result in problems because the internal parts of the mold will get stuck (especially the ejector pin system).

2. Variable mold temperature (Variotherm) processes

In this type of process, the mold changes temperature during the cycle. There are two principal ways to achieve this:

A: Switch off mold heating during the molding cycle. This case is best suited for situations where a slow and well-controlled cooling is desirable. Since cooling happens quite slowly (heat dissipates out in the tool casing) this method is usually only of practical interest when the mold temperature only has to drop slightly (maybe up to ~10-20°C drop in temperature).

B: Switch off mold heating and enable mold cooling water during the molding cycle. This is basically the same procedure as above except we also send cooling water through the mold. This allows for rapid cooling of the mold making it suitable for situations where large or quick drops in temperature are desired.

A: Variable mold temperature: Switch off heating without insert cooling

To set up the machine for this mode do the following:

Set up the points in your cycle where you want the mold heaters to switch on and switch off (See Chapter 4.12 in the manual)
Close valves e and g as described in Chapter 4.11
Heat up the mold to the setpoint (you have to fulfill the condition set in your program to switch on the mold heating)

There are many possibilities for defining the point where heating should activate (switch on) and deactivate (switch off). Selecting 'Ejector advance' as Activate point and 'Inject' or 'Holding pressure' as Deactivate point usually works well. But there are many other options, including delays and defining the activate/deactivate points directly in the process sequence.

B: Variable mold temperature: Switch off heating with insert cooling

To set up the machine for this mode do the following:

Set up the points in your cycle where you want mold heaters to switch on and switch off (See Chapter 4.12 in the manual)
Set up the points in your cycle where you want insert cooling valves to open and to close. Valves are set up for each half of the mold separately.
Close the magnetic valves f and h by fulfilling the 'Activate' condition set above (you have to fulfill the condition set above to close the valves)
Open valves e and g. Since magnetic valves f and h should be closed the propellers should NOT be spinning. The cooling water flow can be adjusted with valves b and c. For fastest possible cooling rate the valves should be fully open. If better control is preferred the flow can be reduced.
Heat up the mold to the temperature setpoint (you have to fulfill the condition set above to switch on the mold heating)

The machine should now be ready for your process. Please remember that:

Every time you open the gate (door) at the mold (either the front gate or the back gate) mold heating will switch off and the insert cooling water valves (f + h) will open due to a safety interlock. To switch heating back on and close the cooling water vales, the 'Activate' conditions have to be fulfilled again.
Avoid circulation of mold cooling water while the mold heating is switched on. The machine will give an error stating that the mold heating is defective. Mold heating should always be switched off before opening mold cooling water valves.

Injection

Injection parameters are very important in injection molding optimization. The injection speed will of course determine how long time it takes to fill the cavity. It also has a large influence on the rheology of the polymer melt. Generally, viscosity of polymer melt will decrease with increasing injection speed (an effect known as shear thinning). The injection speed also influences how structures are filled, and to what extent the air inside the cavity has time to escape as polymer is injected. It therefore often takes some optimization to achieve all desired sample properties. Injection speed settings are adjusted in the 'Inject' screen:

The injection velocity can be adjusted in several ways. One can tap the '=' sign to enter a constant velocity or alternatively use the up/down arrows to move the curve up and down. For more complicated injection speed profiles it is also possible to tap the curve to enter the graphical curve editor where the number and position of individual data points can be edited. This makes it possible to make custom injection speed profiles.
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.

Volume-dependent switchover: A certain volume of polymer is injected. Note that the volume entered (3,2 cm³ 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.

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 cm³). 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.