Six scientific injection molding process optimization steps

1）Viscosity curve

The viscosity curve is made to select a suitable injection speed that will not cause large changes in melt viscosity when there are small fluctuations in each parameter. The fluctuation between each mold should be as small as possible to ensure the repeatability of product quality.



Referring to the viscosity curve above, it can be seen that the melt viscosity is basically very smooth when the injection speed is higher than 55mm/s. Therefore, an injection speed of 65 mm/s will ensure a consistent process during the filling phase. Small fluctuations in the parameters themselves do not cause large changes in melt viscosity.

There are, of course, special cases where this optimized speed cannot be used, such as reducing the gate halo. In this case, the appearance is the priority, but the optimized speed should be used as a reference for the injection profile, e.g. start with a low speed through the gate to reduce the gate halo, then increase rapidly to the optimized speed.
 

2) Flow balance test

This test is only necessary in case of multiple cavities, e.g. 2 or more cavities. The purpose is to check the maximum percentage of deviation between the cavities at different filling stages.



The filling imbalance may or may not be accepted, depending on the product quality requirements. This information is best determined after the appearance of the molding window (step 4) is completed.

1. If the product can be fully pressurized and the forming window is large, check that the product dimensions are all within tolerance and if they are, the filling imbalance is acceptable.

2. If the forming window is small and the first cavity to be filled has burrs, while the other cavities have short shots or shrinkage marks, find the cause of the filling imbalance.

There are four main reasons for filling imbalance.

1. different runner sizes

2. different gate sizes

3. different exhaust size

4. different cooling, however, this cause often has little effect when the machine is first started

There is also a case of imbalance caused by shear, especially for 8-cavity cold runner molds.
 

3) Pressure drop test

The purpose of doing a pressure drop test is to evaluate the loss of pressure at different stages of filling. This usually includes the machine nozzle, the vertical runner, the main runner, the secondary runner, the gate, and the filling end.



The molding process should not use the maximum pressure of the machine, for example, if the maximum pressure of the machine is 180 Bar, then the maximum pressure needed to finish filling should not reach 180 Bar. If this is the case, it means that the screw needs more pressure to reach the set injection rate, but is not able to do so due to pressure limitations. This situation is called "pressure limitation".

Normally, the injection process should not exceed 90% of the maximum pressure of the machine. If the pressure drop curve is "pressure limited" or exceeds 90% of the machine pressure, find the steeper part of the pressure curve and try to reduce the pressure loss there. For example, in the diagram on the right, the pressure drop in the secondary runner is very high, meaning that it takes a lot of force to push the plastic through this section. Increasing the runner diameter in this section will help to reduce the pressure.
 

4) Appearance of the molding window

The molding window is a very important test. Usually, the appearance of the molding window is composed of holding pressure and material temperature (amorphous material), holding pressure, and mold temperature (crystalline material).

The appearance molding window will tell how much room there is to adjust the process and still get an acceptable-looking product. The ideal situation is to have a relatively large molding window. If the molding window is small, it will be easier to create quality defects. For example, in the picture above, if the molding window is small, it is easier to have short shots or burrs due to fluctuations in the process itself. A robust process is one that has a large molding window to compensate for the process fluctuations.

The appearance of the molding window also provides the upper and lower limits of the material temperature/mold temperature and holding pressure allowed for subsequent resizing or DOE testing.
 

5) Gate Freeze Test

For cold runner molds or semi-cold runners (hot to cold), to ensure repeatability from mold to mold, the holding pressure must be maintained until the gate is completely cooled.

After the curve has been generated, choose a time after the product weight has stabilized. In the figure above, the product weight stops increasing after 7s, so for safety reasons and to compensate for the fluctuations in the process itself, the holding time should be set to 8s.

It is important to note that this extra 1s in the holding phase do not increase the cycle time, because during this time the gate should have frozen and only a short time will be spent holding the cold runner while the product is already cooling. Therefore these extra 1s should be subtracted from the cooling time to ensure the same cycle time.
 

6) Mold temperature mapping

The purpose of mold temperature mapping is to record the immediate temperature distribution of the mold surface after the product is ejected. It can be used to confirm if the cooling water line is working or if there are "hot spots".

In addition, this information can be used subsequently to solve problems - for example, to confirm that the mold surface temperature is the same as before when there are inconsistencies in product dimensions.

The temperature must be measured with a contact pyrometer.

It is important to note that after the first start-up or shutdown, the mold temperature will gradually rise to a stable state. Therefore it is important to measure after the mold temperature has stabilized (minimum 10 molds).

The chart above provides 5 frames, this can be applied to different molds depending on the situation. For example, if you are testing a 4-cavity mold, you need to measure the same points in each cavity in the corresponding four corner boxes, leaving one box in the middle empty. If the test is a single-cavity mold, all five are needed, representing different measurement positions.