Engineering Insights: Monitoring Pressure Loss in Injection Molding
The thermoplastics fabrication world is a high-pressure environment. Plastic, water, hydraulics, air and clamping pressure must work cohesively with every production cycle.
For this reason, monitoring pressure loss is critical to maintaining stable production; however, most injection molders only run pressure loss studies when setting up a new mold. Nick Rose, Technical Development Engineer at M. Holland, explains how pressure loss studies can be a great tool to detect and troubleshoot pressure loss changes through various points of fill within the mold and press.
Pressure Loss Causes During Injection Molding
“Plastics are non-hydrostatic, meaning they are compressible” Nick explained. “The pressure applied during the injection molding process is not equally/evenly distributed on all surfaces of a part and actually decreases with increased distance from the source of pressure. Any changes that occur during the molding process — from the front of the screw to the end of the cavity filling — can cause additional pressure loss.”
Pressure loss could be caused by changes in the material, mold or press. Material changes, such as a change in melt flow rate or viscosity of the polymer, could change pressure drop. Wear and tear on a mold could be another cause, but the most common reason for pressure loss according to Nick, is centered on changes to the injection molding equipment. Changing the melt temperature or swapping out machine components such as the nozzle, nozzle tip or filters can cause an unexpected change in pressure.
“It’s not always ideal, but occasionally, a mold may need to move to a new press due to capacity constraints or maintenance,” Nick said. “The move results in short shots or sink marks near the end of fill. Why? Because no injection molding press is the same, even if machines are from the same manufacturer. A different diameter barrel, type of screw, added melt filters, different nozzles or tips — the possible changes are nearly endless, and all can contribute to pressure loss.”
Pressure Loss Monitoring Benefits
Tracking pressure loss regularly improves consistency, according to Nick. “In injection molding, the goal is to mold the same part thousands of times with very little change to the part. Being aware of pressure loss throughout the process will help you better predict the quality of the resulting part.”
As plastic is non-hydrostatic, any change in cavity pressure can alter the part. If you are experiencing a change in pressure loss, you might see dimensional changes to the part or cosmetic issues such as a change in gloss level on a textured surface. Pressure loss isn’t the only issue that might cause defects, such as short shots, sink marks or warping. Nick recommends monitoring pressure loss to help prevent quality issues.
Best Practices for Monitoring Pressure Loss
There are a lot of ways that pressure loss can happen, Nick warns. He recommends using a pressure loss study, which is a test that helps identify pressure changes at specific points of the filling stage. This can help identify issues and ensure consistent part quality over time.
“Most shops only test pressure loss during tool qualification,” Nick said. “The results from this initial test can be used as a baseline to compare future results and identify changes in pressure throughout the process. In this way, any subsequent pressure loss tests can be used as a troubleshooting tool to determine where pressure is being lost and focus efforts on that area to find the root cause.”
There are several opportunities where Nick recommends running a new pressure loss study:
- Whenever you move a mold to a new press.
- If injection pressure at transfer changes outside the given tolerance.
- If peak injection pressure changes outside the given tolerance.
- Whenever you encounter a part defect.
Running a Pressure Loss Study
Pressure loss studies can be run in-house by a process engineer or technician to measure pressure drop throughout the progression of filling the mold. The following four steps comprise a pressure loss study and assume a determined fill rate:
- Start by running the injection molding process with an air shot through the nozzle and record the peak plastic pressure. This chart assumes 10:1 intensification ratio, this needs to be confirmed on given press.
- Repeat this through the nozzle, sprue and runner, just before the gate, and through the nozzle, sprue, runner, and just after the gate, and a fill-only part. The recorded plastic pressure of the air shot is the pressure loss from the nozzle.
- Subtract the nozzle pressure loss from the pressure recorded through the nozzle, sprue and runner to determine the pressure loss.
- Repeat the process for the last two stages as seen in the example table below.
| Hydraulic PSI | Plastic PPSI | PPSI Drop | Max PPSI Drop | |
|---|---|---|---|---|
| 98% filled part PSI | 1100 | 11000 | 3200 | |
| Nozzle | 200 | 2000 | 2000 | 4500 |
| Nozzle, Sprue, Runner | 450 | 4500 | 2500 | 6000 |
| Nozzle, Sprue, Runner and Gate | 780 | 7800 | 3300 | 5000 |
As a best practice, air shot through the nozzle should not be more than 4500 Ppsi. Pressure loss from the sprue to just before the gate should not exceed 6000 Ppsi. Through the gate should not exceed 5000 Ppsi. During your initial pressure loss, if you see more than the max amount in any area, Nick recommends immediately determining the cause for the excessive pressure loss and correcting.
“Keep in mind that you will want an abundance of available fill pressure when you’re working with plastic injection molding because viscosity can change from lot to lot or from normal variation in the process,” Nick said. “Additional available pressure provides a more stable process despite any viscosity variation that might occur. For instance if your peak fill plastic pressure is 5000 Ppsi you will want to have a max injection plastic pressure setting approximately 20-25% above that.”
Running a pressure loss study at the beginning of a tool’s life or when it is moved to another machine provides a baseline pressure loss for future comparison. For example, if your baseline nozzle air shot was 2000 Ppsi at the start, but a year later, it’s 4000 Ppsi, then there may have been a change to the process, material, or machine. This increase in pressure loss could be showing up as short shots or sink marks. Or, if after moving to a new press you’re experiencing issues, the pressure loss baseline results from the original machine can help determine how the new machine should be adjusted for better results.
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Legal Disclaimer: Please note that results cannot be guaranteed. Outcomes may vary based on individual circumstances, and M. Holland cannot ensure a specific result. M. Holland is not responsible for any discrepancies between expected and actual outcomes. For a comprehensive review and assistance from our experts, contact your account manager.
Nick delivers technical plastics expertise in material selection, engineering, process improvements, part/mold design and more. His 18-year career spans the automotive, packaging, injection molding and extrusion markets, covering process engineering, quality and continuous improvement. Nick earned a B.S. degree in Plastics Engineering Technology from Ferris State University, along with Master Molder I and Master Molder II certifications.