Injection molding is a precise manufacturing process, and ensuring consistent part quality requires testing various machine and process parameters. Three critical tests—Load Sensitivity Test, Injection Speed Linearity Test, and Dynamic Check Ring Test—help molders identify machine performance characteristics, optimize processing conditions, and maintain part quality. Below, we’ll explain what each test is for, how to perform it, and what insights it provides.

1. Load Sensitivity Test  

Purpose:  

The Load Sensitivity Test helps evaluate how an injection molding machine reacts to changes in material viscosity during the molding cycle. It ensures that the machine maintains consistent performance when the material or process conditions fluctuate.

How to Perform the Test:  

1. Run the molding machine at a set injection speed while maintaining a consistent material and mold temperature.
2. To simulate a change in load that would naturally occur with a change in material, we can use a purge plate or by using decompression before rotation to the shot size. These two methods will create an air shot, drastically reducing the load on the molding machine.
3. Measure how much the machine’s injection pressure and fill rate change in response to these variations. To calculate the result, perform a percent change calculation on the fill time. Also, perform a percent change calculation on the fill pressure. Here is the formula: (FT1 – FT2) ÷ FT1) ÷ (PK1 – PK2) ÷ 1000) × 100 = % error/1000 psi. When calculating for electric machines use 10,000 as the denominator instead of 1,000.

What It Tells You:  

• If the machine responds predictably, it is robust and maintains consistent part quality under varying conditions. Ideally, we would want the speed to remain consistent.
• If the machine’s speed fluctuates significantly, it may indicate poor control of hydraulic or electric systems, leading to inconsistent parts.
• This test helps determine whether a machine is suitable for running tight-tolerance parts or if adjustments are needed to improve repeatability.

2. Injection Speed Linearity Test  

Purpose:   

This test checks whether an injection molding machine can maintain a smooth and predictable injection speed across different settings. This is crucial for controlling shear rate, material flow, and avoiding defects like burning or hesitation marks.

How to Perform the Test:  

1. Set up the molding machine to inject at different speeds (e.g., slow, medium, and fast).
2. Record the fill time on the molding machine.
3. Since Rate = Distance / Time, we can calculate how fast we expect the machine to fill the mold versus how fast it actually fills the mold. We can take the distance the screw traveled during the filling phase and divide it by the actual fill time. This will give us the average fill rate in in/s or mm/s. Compare this to the set point on the machine.  
4. Plot the results on a graph to observe how closely the actual speed follows a linear pattern.

What It Tells You:  

• If the actual injection speed matches the programmed speed, the machine has good linearity and provides consistent part filling.
• If the speed varies unexpectedly (e.g., jumps too fast or slows down randomly), it may indicate control system issues, valve problems, or mechanical inconsistencies.
• A machine with poor linearity may struggle with flash or short shots, leading to variations in part weight and dimensions.

3. Dynamic Check Ring Test

Purpose:  

This test evaluates the performance and sealing reliability of the check ring during the injection phase. A faulty or inconsistent check ring can allow back flow of material during injection, leading to short shots, inconsistent part weights, or dimensional variability.

How to Perform the Test:

1. Perform several consecutive short-shot trials at identical machine settings.
2. Measure the part weight for each cycle.
3. Analyze the variation between shots—high variability may indicate the check ring is leaking or not sealing consistently. To perform this calculation, perform a percent change calculation between the heaviest and lightest parts in the sample.

What It Tells You:

• Low variation in shot size/weight confirms that the check ring is sealing properly and consistently.
• High variation points to wear, contamination, or mechanical issues with the check ring, which could compromise process repeatability.
• This test helps identify mechanical wear early, before it leads to scrap or rework.

Why These Tests Matter for Part Quality  

These tests help molders reduce scrap, improve process stability, and ensure that machines are running optimally. By understanding machine behavior under different conditions, molders can:

• Minimize dimensional variation in parts
• Reduce inconsistencies in fill and pack pressure
• Identify machine issues before they lead to defects
• Ensure repeatable and reliable production

Next Steps: Schedule an RJG Gap Assessment  

If your injection molding process isn’t delivering the part quality you need, an RJG Gap Assessment can help pinpoint the root causes of variation and inefficiencies. Our experts can evaluate machine performance, process stability, and mold design to help you achieve consistent, high-quality parts.

Schedule a Gap Assessment today.