Understand The Impact Of Pressure Loss On Injection Molded Parts

We can simplify the injection molding process by focusing on the "experience" of plastic entering the mold through the machine. This viewpoint can be referred to as "injection molding from the perspective of plastics" or scientific injection molding. It is very useful to consider the behavior or response of plastics when considering process, tool, material, and/or part design changes. There are many parameters that can be adjusted on an injection molding machine, but it is important to recognize that these adjustments will affect one or more physical plastic conditions. The four main plastic conditions are:

Plastic temperature

Plastic flow rate

Plastic pressure

Plastic cooling rate and time

Due to the compressibility of plastic, we will detect pressure loss, pressure gradient, and pressure difference through the runner system, gate, and cavity below. The terms pressure loss, pressure gradient, and pressure difference all refer to the physical compression of plastics to varying degrees. How could such behavior be possible? Plastic is a compressible medium because there is space between polymer molecules, and its volume depends on the applied temperature and pressure. One way to illustrate pressure loss in real life is to consider the use of compressed air. If we measure the pressure of the compressor itself and the pressure at the end of the hose, we will observe a decrease in that pressure. For example, suppose an air compressor produces 80 psi, but at the end of a 100 foot hose, the measured air pressure is 75 psi. Because air can be compressed, we see a pressure loss of 100 psi along a 5-foot hose. For the sake of our discussion, we will ignore the impact of frictional losses, even if they exist. Let's turn our attention back to plastic and focus on the pressure inside the mold cavity detected during the holding stage of the injection molding process. In Figure 1, the pressure sensors inside the mold cavity are located at position A (rear gate) and position B (at the end of the mold cavity filling). Assuming a pressure of 15000 psi is programmed during the holding phase at the front of the screw. From this theoretical example, it can be seen that the pressure loss of the cavity pressure sensor from the front of the screw to A is 6000 psi, and the pressure loss between the sensors located at A and B is 2000 psi. To conceptualize this phenomenon, imagine pushing the sponge during the box receiving stage. Due to the application of pressure, the sponge will undergo compression, and the amount of pressure transmitted along the length of the sponge will decrease.

Source: Umberto Catignani

This pressure loss will result in a higher degree of compression of the plastic at position A after the gate, and a higher degree of compression at position B at the end of filling, which will affect the plastic components, causing more plastic holding pressure to be observed at position A and less holding pressure to be observed at position B. In addition, compared to the shrinkage at position B, the plastic shrinkage rate at position A is lower, and the plastic component size at position A is larger, while the size (such as part width) at position B is smaller. The figure shows an example of plastic components with a high number and severity of shrinkage marks. The change in shrinkage marks is caused by pressure loss in the mold cavity, as well as greater pressure and less overall shrinkage near the gate. In the future, please consider this phenomenon when changing process settings or designing tools, materials, and parts.

Pressure loss of injection molded parts