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Six scientific steps to optimize the injection molding process

1) Viscosity curve

The purpose of making a viscosity curve is to select a suitable injection speed injection molding process

. When there are slight fluctuations in various parameters, it will not cause a large change in melt viscosity. The fluctuation between each mold should be as small as possible to ensure the repeatability of product quality.

Referring to the viscosity curve in the figure above, it can be seen that when the injection speed is higher than 55mm/s, the viscosity of the melt is basically very stable. Therefore, setting the injection speed to 65mm/s will ensure the consistency of the filling process. Slight fluctuations in the parameters themselves will not cause a large change in the viscosity of the melt injection molding process.

Of course, there are special cases where this optimized speed cannot be used, such as reducing gate halo. In this case, of course, appearance is prioritized, but this optimized speed should be used as a reference for the injection curve, such as starting to pass through the gate at a low speed to reduce gate halo, and then quickly increasing to this optimized speed.

2) Flow balance test

This test is only required for multi-cavity, such as 2 cavities or more. The purpose is to check the maximum deviation percentage between cavities at different filling stages.

Filling imbalance may be acceptable or unacceptable, depending on the product quality requirements. This information is best determined after the appearance molding window (step 4) is completed.

  1. If the product can be fully held and the molding window is large, check whether the product dimensions are within the tolerance. If they are, the filling imbalance is acceptable.
  2. If the molding window is small, and the first cavity to be filled has burrs, while other cavities have short shots or sink marks, find out the cause of the filling imbalance.

There are usually 4 main reasons for filling imbalance:

  1. Different runner size
  2. Different gate size
  3. Different exhaust size
  4. Different cooling, but this reason often has little effect when the machine is just started

Another case is imbalance caused by shear, especially for 8-cavity cold runner molds.

3) Pressure drop test The purpose of the pressure drop test is to evaluate the pressure loss at different stages of filling. This usually includes the machine nozzle, vertical runner, main runner, secondary runner, gate and filling end. The molding process should not use the maximum pressure of the machine. For example, if the maximum pressure of the machine is 180Bar, the maximum pressure required for filling should not reach 180Bar. If this is the case, it means that the screw needs more pressure to achieve the set injection speed, but it cannot reach it due to pressure limitations. This situation is called “pressure limitation”. Generally, the injection process should not exceed 90% of the maximum pressure of the machine. In the pressure drop curve, if it is in “pressure limitation” or exceeds 90% of the machine pressure, find a steeper section in the pressure curve and try to reduce the pressure loss here. For example, in the right figure, the pressure drop of the secondary runner is large, which means that a lot of force is needed to push the plastic flow in this section. Increasing the runner diameter in this section will help reduce the pressure.

4) Appearance molding window

The molding window is a very important test. Usually, this appearance 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 while obtaining a product with acceptable appearance. The ideal situation is to have a relatively large molding window. If the molding window is relatively small, it will be easier to produce quality defects. For example, in the above figure, if the molding window is relatively small, it is easier to have short shots or burrs due to fluctuations in the process itself. A robust process is to have a relatively large molding window to compensate for the fluctuations in the process itself.

The appearance molding window will also provide the upper and lower limits of the material temperature/mold temperature and holding pressure allowed for subsequent mold trials due to size adjustment or DOE tests.

5) Gate freeze test

For cold runner molds or semi-cold runners (hot to cold), to ensure repeatability between injection

molds, the holding pressure must be continued until the gate is completely cooled.

After the curve is generated, select a time after the product weight stabilizes. In the above figure, the product weight no longer increases after 7s, so for safety reasons and to compensate for the fluctuations of the process itself, the holding time should be set to 8s.

It should be noted that the extra 1s in the holding stage does not increase the cycle time, because during this time, the gate should have frozen and only the cold runner will be held briefly, while the product has already begun to cool. Therefore, the extra 1s should be subtracted from the cooling time to ensure the same cycle time.

6) Mold Temperature Spectrum

The purpose of the mold temperature spectrum is to record the distribution of the instantaneous temperature of the mold surface after the product is ejected. It can be used to confirm whether the cooling water circuit is working or whether there are “hot spots”.

In addition, this information can be used to solve problems later – for example, when the product size is inconsistent, it can be used to confirm whether the mold surface temperature is the same as before.

The temperature must be measured with a contact pyrometer.

It should be noted that after the first startup or shutdown, the mold temperature will gradually rise to a stable state. Therefore, it is necessary to measure after the mold temperature stabilizes (at least 10 molds later).

The above figure provides 5 frames, which can be applied to different molds according to the situation. For example, if you are testing a set of 4-cavity molds, you need to record the points measured at the same position in each cavity in the corresponding four corner boxes, and leave the middle box empty. If the test is a single-cavity mold, all 5 need to be used to represent different measurement positions.

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