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Six Aspects to Understand Scientific Injection Molding Theory
2025-11-07
Injection Molding viscosity
First, let's understand some definitions:
Newtonian fluid: A fluid whose viscosity is unaffected by the shear rate applied to it. The viscosity remains constant as the shear rate changes.
Non-Newtonian fluid: A fluid whose viscosity varies with shear rate. The viscosity does not remain constant when shear rate changes.
Rheology: The study of the flow of non-Newtonian fluids.
All plastics are non-Newtonian fluids. This means that their viscosity does not remain constant within a certain range of shear rates. Strictly speaking, the rheological behavior of plastics is a combination of non-Newtonian and Newtonian mechanics. At lower shear rates, plastics are non-Newtonian, but as the shear rate increases, they tend to exhibit Newtonian behavior. This is because, with increasing shear rate, polymer molecules begin to separate from each other and begin to align along the flow direction. See below for reference:
Importance of viscosity profiles to injection molding processes
During injection molding, the material is subjected to significant shear forces during the cavity filling stage. The shear rate is directly proportional to the injection speed. If the shear rate is in the non-Newtonian region of the curve, even a small change in the shear rate will cause a large displacement of viscosity. This will lead to inconsistent mold filling, thus affecting mass-produced products.
Therefore, it is important to find the Newtonian region of the curve within this area and set the injection speed (and thus the shear rate). Viscosity profiles for any mold can be obtained using an injection molding machine. The 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 the molding process.
Cavity balance
As the plastic flows through the runner into the mold cavity, the melt exhibits specific temperature, pressure, and velocity. All three variables are time-dependent, meaning that the value of each variable will change over a short period of time until the filling is complete.
For example, melt temperature decreases over time. If the melt temperature is 280°C during injection, it will drop below 280°C after one second. The final dimensions and quality of each injection-molded product depend in part on temperature, pressure, and speed.
I. Consider a one-cavity mold
At the end of the filling process, the melt temperature was 450 degrees Fahrenheit, the plastic pressure was 8000 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 will shrink less, and the resulting part will be larger than before. Similarly, if the ending filling pressure and speed are changed, the part size and/or the ending temperature will change.
II. Now consider a two-cavity mold, where each cavity has the same acupoint size:
If the two cavities are not filled with similar filling conditions, then based on the discussion above, we know that the two parts produced from each cavity will be different. This is why cavity balancing tests are necessary.
Pressure drop
As plastic flows through different parts of the machine and mold, pressure is lost at the flow front due to resistance and friction. Furthermore, as the plastic impacts the mold walls, it begins to cool, increasing its viscosity and requiring additional pressure to propel it. The plastic surface layer that forms on the walls reduces the cross-sectional area of the plastic flow, which also contributes to pressure drop.
A molding machine has a limited maximum available pressure to drive the screw at a set injection speed. The pressure required to drive the screw at the set injection speed must never exceed the maximum available pressure. In this case, the process becomes pressure-limited.
During process development, understanding the pressure loss in each section helps determine the overall pressure loss and the areas of pressure drop. The mold can then be modified to reduce this pressure drop and achieve better, more consistent flow.
Pressure holding and process window
The process of injecting plastic into the cavity can be divided into two main stages.
The first stage is the injection stage. During the injection stage, the mold cavity is completely filled with molten plastic.
The second stage is the shrinkage compensation stage. This stage follows the injection stage. The shrinkage compensation pressure must be filled into the mold cavity, and the plastic undergoes volume shrinkage during cooling as it impacts the cold walls of the mold.
In most cases, the feeding and holding stages are not distinguished and are collectively referred to as the holding pressure stage. The ideal holding pressure is determined by evaluating the mold's process window. The process window is also known as the molding area map. This is the area where the acceptable part is molded. The larger the window, the greater the allowable range of molding fluctuations. See below:
This process is set in the center of this window so that any changes within the window will produce an acceptable result.
Gate sealing
Plastic enters the cavity through the gate. As long as the gate does not freeze, the plastic can enter or leave the cavity. Therefore, holding pressure must be applied until the gate freezes.
Perform a very simple test to determine the holding time:
Samples were weighed for different holding times. As the holding time increased, more and more plastic entered the cavity, increasing the weight. However, once the gate freezes, the plastic can no longer enter the cavity, and the part weight remains constant. This is called the gate freezing time or gate closing time. See the figure below:
In the image above, the part weight remains constant after 9 seconds. The holding time is set one second longer than the gate sealing time to ensure the gate is frozen during each pour. In the case shown below, the time is set to 10 seconds. This ensures consistency, and any small variations will be compensated for.
Injection molding cooling
Once the plastic contacts the mold walls, it begins to cool. The mold remains closed until the cooling time is complete. Then the mold opens, and the part is ejected. Before the mold opens, the part must reach an acceptable ejection temperature for the plastic. If the part is ejected before reaching this acceptable temperature, it will be too soft and deform during ejection. Excessive cooling time simply wastes machine time and profits.
Determining the appropriate cooling time is complex. In thick sections, it's difficult to measure the internal temperature at the center of the thickest part. In some areas of the mold, sufficient cooling is difficult to achieve. Variations in cooling time also affect shrinkage.
In the graph above, dimension A (blue) is unaffected by the cooling time range test. However, dimension B (red) changes with the cooling time. The target value for dimension B is 0.135. Therefore, we can set the cooling time to around 17 seconds.