To understand the quality control system of plastic mold frame suppliers, you can take the following steps:

1. Factory inspection: Personally or through a third-party visit to the supplier's production site to understand their production process, quality inspection process, and working environment. This can help you visually evaluate the quality control capabilities of suppliers.

2. Evaluate mold quality: Evaluate the quality of the mold according to national or industry standards, such as GB/T 12554-2006 Technical Conditions for Plastic Injection Molds. These standards will cover requirements for product quality, mold structure, injection molding processes, and other aspects.

3. Understand machining management: Inquire with suppliers about the machining scope of the fitter team and the management methods for collaborative machining. Understand the inspection process after processing and methods to ensure processing quality.

4. Review design capability: Evaluate whether the supplier has the ability to use 3D software for design and whether they have mold flow analysis capability to predict and solve potential deformation problems.

5. Document management: Understanding whether the supplier has a complete document management system is crucial for ensuring the accuracy of design and consistency of production.

6. Certification and Standards: Check if the supplier has obtained relevant system certifications, such as ISO certification, and their understanding of international standard components.

7. Performance indicators: Inquire about the supplier's delivery failure rate, as well as the handling and corrective measures for non-conforming products. Understand whether the supplier has a continuous improvement system and written recognition from the customer.

8. Transparent communication: Establish an open and transparent communication mechanism with suppliers to ensure timely access to quality control related information during the cooperation process.

9. Reference customer feedback: If possible, obtain feedback from previous customers of the supplier to understand their evaluation of the supplier's quality control system.

10. Long term cooperation observation: In the initial stage of cooperation, closely observe the quality control performance of suppliers to ensure that they meet your quality requirements in a stable and long-term manner.

By following the above steps, you can gain a more comprehensive understanding of the supplier's quality control system and make more informed cooperation decisions.