Guide to Dimensional Inspection in Plastic Injection Molding

It's essential to confirm the dimensional accuracy of molded parts produced using plastic injection molding. A thorough dimensional inspection process ensures that molded parts meet specifications, thus avoiding post-mold issues that can be costly and time-consuming to rectify.

How Can Dimensionally Inaccurate Parts Affect Post-molding Operations?

Dimensionally inaccurate plastic molded parts can have several negative impacts on post-molding operations, including:

• Assembly Fit Issues: Parts that do not conform to dimensional specifications may not fit together as intended, leading to assembly problems.

• Seal Integrity: If sealing is critical, dimensional variations can compromise the effectiveness of seals and gaskets, causing leaks or failures.

• Visual Appearance: Inconsistencies in dimensions can affect the visual appeal of assembled products, making them less marketable.

• Increased Rejection Rates: A robust quality management system will reject dimensionally inaccurate parts, leading to higher rejection rates and wasted materials.

• Costly Corrections: Addressing dimensional issues during assembly or post-production is time-consuming and expensive.

• Modified QA Processes: Dimensional issues may necessitate changes in testing and quality assurance processes, adding further costs and delays.

• Customer Dissatisfaction: Ultimately, products that fail to meet dimensional accuracy can affect product quality and usability. This can lead to customer dissatisfaction and potential returns.

Best Practices For Conducting Dimensional Inspection of Molded Parts in Plastic Injection Molding

Examine the Design Specifications: Before carrying out a dimensional inspection, refer to product drawings, CAD models, and relevant documentation to develop an understanding of the design specifications and tolerances.  

Utilize Advanced Measurement Technologies: Traditional measurement tools like height gauges, calipers, and micrometers are being increasingly supplemented or replaced by advanced 3D and CT scanning technologies. These methods offer higher precision and the ability to capture intricate details, providing a more comprehensive analysis of the part's dimensions. ‍

• 3D Scanning: 3D scanning is a non-contact measurement method that uses structured light or lasers to capture the geometry of a part in high detail. It is useful for complex shapes and surfaces that are difficult to measure with traditional tools.
• ‍CT Scanning: Computed tomography (CT) scanning goes further by providing internal and external measurements. It uses X-rays to create cross-sectional images of the part, which are then compiled into a 3D model. This method is invaluable for detecting internal defects and ensuring that internal features meet dimensional requirements.

Ensure a Controlled Environment: Dimensional inspections should be performed in controlled environments to minimize external influences and establish measurement accuracy. The inspection area should be clean and well-lit. Take measures to maintain temperature and humidity levels.

Measure Critical Dimensions First: Start an inspection by focusing on the critical dimensions (dimensions that are expected to significantly impact functionality and product performance) as per the design specifications. Ensuring these dimensions are within tolerance is essential for the part to perform as intended in its final application.

Implement Statistical Process Control: Statistical process control tools allow you to collect and analyze data in real-time and use it to monitor and control the injection molding process. Implementing SPC is a great way to ensure that dimensional variations are within acceptable limits.

Leverage Cavity Pressure Data: Consistent cavity pressure is crucial for producing dimensionally accurate parts. By monitoring and controlling cavity pressure during the injection molding process, manufacturers can ensure a higher degree of consistency in part dimensions.

Conduct First Article Inspection: A first article inspection (FAI) should be performed on the initial sample from the first batch of molded parts. This ensures that the parts meet dimensional requirements before full-scale production begins. Discrepancies should be addressed promptly to avoid downstream issues.

Document Inspection Results: Thoroughly document inspection results and create detailed inspection reports. Analyzing these reports can help identify root causes of deviations and provide insights for continuous improvement. Regularly review and adjust inspection processes based on these findings to maintain high standards of quality.

By adopting these best practices and leveraging advanced 3D and CT scanning technologies, manufacturers can ensure the dimensional accuracy of molded parts, leading to improved product quality, reduced rejection rates, and enhanced customer satisfaction.

Nel PreTech is a certified 3D and CT scanning laboratory. Our advanced scanning technologies capture every detail of your parts, enabling better decision-making and quality control. Have questions about our 3D scanning services? Call 708-429-4887.