Plastic Injection Pilot Runs: How To Use them To Mitigate Quality Issues

In plastic injection molding, pilot runs are a standard practice employed by manufacturers to evaluate the feasibility and quality of a design before transitioning to full-scale production. However, these initial pilot runs can sometimes reveal inconsistencies or quality issues. For importers, minimizing such problems is paramount to ensure a smooth transition to mass production and reduce the risk of receiving parts that deviate from their specifications.

This article explores key strategies to mitigate quality issues during plastic injection molding pilot runs.

Trial the Mold for Functionality

Ensuring the mold performs as expected is paramount during the pilot run. This critical process, known as “trialing the mold,” involves closely monitoring various aspects of its performance throughout the entire molding cycle. By closely observing these factors, we can identify and address potential issues that might affect part quality. Here’s a breakdown of the key areas to focus on during mold trailing:

Temperature Control of the Injection Mold

1. Dialing in the Numbers: Verify that both the mold cavity and the cooling channels reach and maintain the pre-determined temperature setpoints. These temperatures significantly impact factors like:
   1. Material Viscosity: The melt’s viscosity is highly dependent on temperature. Deviations can lead to issues like incomplete filling (short shots) or surface defects.
   2. Solidification: Proper cooling temperatures ensure controlled solidification of the polymer, influencing part strength, dimensional stability, and warpage.
2. Monitor for Deviations: Implement a monitoring system to track and record mold temperatures throughout the cycle. Investigate any significant deviations and adjust settings or troubleshoot potential malfunctions in the heating/cooling systems.

Mold Cooling Efficiency

1. Evaluate the mold’s cooling channels: Inspect the mold for signs of uneven cooling, which can manifest as:
   1. Warpage: Uneven cooling can cause the part to warp or twist as different sections cool at varying rates.
   2. Sink Marks: Rapid cooling in specific areas can lead to localized shrinkage, resulting in sink marks on the part surface.
2. Optimizing Cooling Channels: Analyze the cooling channel layout and flow rates. Consider adjustments like:
   1. Channel Placement: Strategically position channels closer to thicker sections or areas prone to sink marks.
   2. Flow Rate Adjustments: Fine-tune the flow rate of the coolant to achieve more uniform cooling across the mold.

Ejection Smoothness

1. Observe the Release Process: Carefully observe the ejection sequence to ensure parts are released cleanly and efficiently from the mold cavity. Look for signs of:
   1. Sticking: If parts stick to the mold during ejection, it can damage the part surface or cause mold wear. This could indicate insufficient ejection force, temperature issues, or surface texture problems.
   2. Dragging: Dragging of parts during ejection can cause scratches or gouges. This might be due to improper mold design, undersized ejector pins, or material sticking.
2. Refine the Ejection System: Based on observations, refine the ejection system by:
   1. Adjusting Ejection Force: Increase or decrease ejection force based on the observed sticking or dragging behavior.
   2. Optimizing Ejector Pin Placement: Ensure ejector pins are positioned effectively to engage with the part at the designated points.
   3. Mold Surface Polishing: If necessary, consider polishing the mold surfaces to reduce friction and improve part release.

Gate Seal and Material Flow

1. Inspect for Leakage: Closely examine the gate area for signs of material leakage, which indicates:
   1. Improper Gate Design: The gate design might not be sealing effectively against the sprue, allowing molten plastic to escape prematurely.
   2. Insufficient Injection Pressure: Low injection pressure could be causing the gate to not fully seal, leading to leakage.
2. Evaluate Flow Patterns: Assess the material flow within the mold cavity. Look for signs of:
   1. Short Shots: Incomplete filling of the cavity could indicate insufficient injection pressure or gate design issues.
   2. Air Traps: Entrapped air can cause voids or surface imperfections. Analyze the filling pattern and consider adjustments to venting or injection parameters.
3. Refine Gate Design and Injection Parameters: Based on your observations, refine the gate design or injection parameters as needed:
   1. Gate Optimization: Consider modifying the gate size, shape, or location to achieve a clean seal and proper flow.
   2. Injection Pressure Adjustments: Calibrate the injection pressure to ensure complete filling while avoiding excessive flash formation.

Injection Mold Press Optimization During the Pilot Run

Plastic injection pilot runs serve as a perfect opportunity not only to assess the mold functionality but also to optimize the settings of the injection molding press itself.

Here’s a breakdown of key press parameters to focus on during this phase:

1. Injection Pressure: This parameter directly influences how well the mold cavity fills with molten plastic. During the pilot run, you can:
   1. Start with conservative settings: Begin with a slightly lower pressure and gradually increase it until the cavity fills completely.
   2. Monitor for flash: Excessive pressure can lead to “flash,” which is thin plastic that overflows beyond the mold parting line. Aim for a pressure that minimizes flash while ensuring complete filling.
2. Melt Temperature: The melt temperature significantly impacts the material’s flow properties and influences factors like:
   1. Viscosity: Higher temperatures result in lower viscosity, allowing for easier flow but potentially leading to surface defects.
   2. Degradation: Excessive temperatures can degrade the polymer, compromising its mechanical properties.
   3. Cycle Time: Melt temperature also affects cycle time. Aim for a temperature that balances good flow with efficient cycle times.
3. Cycle Time: The cycle time encompasses the entire process, including mold filling, holding, cooling, and part ejection. During the pilot run, you can:
   1. Analyze cycle breakdown: Break down the cycle time into its individual stages (injection, holding, cooling, ejection) to identify areas for potential optimization.
   2. Balance efficiency and quality: Strive for a cycle time that is efficient (minimizing production time) but also allows for adequate cooling to achieve the desired part quality.
4. Clamping Force: This parameter ensures the mold remains securely closed during the high-pressure injection phase. The pilot run allows you to:
   1. Verify sufficient clamping: Ensure the clamping force is sufficient to prevent the mold from flashing open during injection.
   2. Avoid excessive force: Excessive clamping force can put unnecessary strain on the machine and potentially damage the mold.

Summary of Why Plastic Injection Pilot Runs are Essential for Flawless Injection Molding

Once the pre-production samples are approved and the green light is given for mass production, maintaining consistency is paramount. This involves meticulous documentation of all process parameters, using the same machines and the exact polymer formulation, and ensuring proper material drying. These practices are essential for replicating the success achieved during the pilot run on a larger scale.

But why are plastic injection pilot runs so crucial in the first place?

They’re the bridge between design and reality, a controlled environment where potential problems can be identified and addressed before full-scale production kicks in. By proactively testing the mold, materials, and machine settings, you gain invaluable insights that can prevent costly delays and quality issues down the line.

Think of a pilot run as an investment in peace of mind. It allows you to:

• Catch Issues Early: Identify and rectify problems with the mold design, material selection, or machine settings before they disrupt mass production.
• Optimize for Efficiency: Fine-tune process parameters to achieve optimal cycle times, minimize material waste, and ensure efficient operation.
• Establish a Baseline: Set a benchmark for quality and performance that can be consistently replicated during mass production.

In conclusion, a well-executed pilot run is not just about getting pre-production samples. It’s a proactive strategy that paves the way for a successful and efficient mass-production process, ultimately delivering high-quality plastic parts that meet your exact specifications.

Keep Learning about Plastic Injection Molding

We wrote answers to the top questions our customers usually ask us about plastic injection molding in this guide:

For more information on the Sofeast Group’s plastic injection molding services, take a look at our Plastic Injection Molding Parts In China service (at our business unit, Agilian Plastic & Molds in South China), or you can contact us as normal for a discussion about your needs.