First of all, let’s understand the characteristics of injection molding plastics, so as to better develop the content of scientific injection molding.
First understand the following definitions:
All plastics are non-Newtonian fluids.
This means that their viscosity does not remain constant over a range of shear rates.
Strictly speaking, the rheological behavior of plastics is a combination of non-Newtonian mechanics and Newtonian mechanics. At lower shear rates, plastics are non-Newtonian. But as the shear rate increases, plastics tend to exhibit Newtonian behavior.
This is because as the shear rate increases, the polymer molecules start to separate from each other and start aligning in the direction of flow. The reference is as follows:
During injection molding, materials are subjected to substantial shear forces during the cavity filling phase.
The shear rate is directly proportional to the injection speed. If the shear rate is in the non-Newtonian region of the curve, then a small change in the shear rate will cause a large shift in viscosity.
This will cause inconsistent mold filling and affect mass-produced parts. Therefore, it is important to look for the Newtonian region of the curve and set the injection rate (and the shear rate) within this region.
Viscosity curves for any mold can be obtained using an injection molding machine. Shear rate has a much greater effect on viscosity than temperature. Therefore, as long as the actual melt temperature is within the recommended range, a similar viscosity profile will be obtained during molding.
When the plastic enters the cavity through the runner, the melt has a certain temperature, pressure and speed. All three variables are time-dependent, which means that the value of each variable will change for a short period of time until the filling is complete.
For example, the melt temperature decreases with time. If the melt is injected when it’s 280°C, after one second, the melt temperature is lower than 280°C. The final size and quality of each injection molded part depends in part on temperature, pressure and speed.
Consider a one-cavity mold:
The melt temperature at the end of fill was 450 degrees Fahrenheit, the plastic pressure was 8,000 psi, and the plastic entered the cavity at a rate of 4.5 inches per minute. Now, if the temperature drops to 400 degrees Fahrenheit, the part shrinks even less, and the resulting part is now larger than it was before. Similarly, if the end of fill pressure and velocity are changed, the size and/or end of the part will change.
Now consider a two-cavity mold where each cavity has the same hole size:
If the two cavities are not filled with similar filling conditions, then from the discussion above we know that the two parts resulting from each cavity will be different.
This is why a cavity balance test is required.
Molding machines have a limited maximum amount of pressure available to drive the screw at a set injection speed. The pressure required to propel the screw at the set injection speed must never exceed the maximum available pressure. In this case, the process becomes stress-limited.
During process development, knowing the pressure loss of each section helps determine the overall pressure loss and pressure drop sections. Modifications can then be made to the mold to reduce this pressure drop and achieve better consistent flow.
Injecting plastic into a cavity can be broken down into two main stages:
(1) Injection phase
During the injection molding stage, the mold cavity is completely filled with molten plastic.
(2) Feeding stage
The feeding phase follows the injection phase. The feeding pressure must be filled into the mold cavity. And it is equivalent to the volume shrinkage that occurs during the cooling of the plastic, because the plastic hits the cold wall of the mold.
In most cases, the feeding and holding phases are not distinguished and are collectively referred to as the pressure holding stage.
The ideal holding pressure is determined by evaluating the process window of the mold.
Processing windows are also known as shaping area charts. This is the area where acceptable parts are molded. The larger the window, the greater the range in which molding fluctuations are allowed as follows:
The process is set in the center of this window so that any changes within the window will produce acceptable parts.
Plastic enters the cavity through the gate. As long as the gate does not freeze, plastic can enter or exit the cavity.
Therefore a holding pressure must be applied until the gate is frozen.
Use a very simple test to determine the holding time:
Weigh the samples with different holding times, as the holding time increases, more and more plastic enters the cavity to increase the weight. However, once the gate freezes, plastic cannot enter the cavity and the part weight remains the same. This is known as gate freeze time or gate closure time. See below:
In the image above, the part weight remains the same after 9 seconds. The holding time is set one second higher than the gate seal time to ensure that the gate is frozen during each pour. In the case of the image below, the time is set to 10 seconds. This will ensure consistency and any small changes will be compensated.
Once the plastic touches the walls of the mold, it begins to cool.
The mold remains closed until the cooling time is over. The mold is then opened and the part is ejected. Before the mold can be opened, the part must reach an acceptable ejection temperature for the plastic.
If the part is ejected before it reaches an acceptable ejection temperature, the part is too soft and will deform during ejection. And excessive cooling time is just wasting machine time and profit.
Determining an appropriate cooling time is complicated.
In thick sections, it is difficult to measure the internal temperature in the center of the thickest section. In some parts of the mold, it is difficult to get enough cooling.
Changes in cooling time can also affect shrinkage.
In the figure above, dimension A (blue) is not affected by the cooing time range test. However, dimension B (red) changes with cooling time. The target value for dimension B is 0.135. So we can set the cooling time at around 17 seconds.
Finally, when setting up the injection molding process, experienced engineers prefer simple, low-fluctuation range of injection molding process parameters, which is a correct and practical direction.
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