Driving Success Through Process Validation and Living Prints

Tolerances specified in initial prints may not always align with manufacturing capabilities or actual product requirements. Robust product testing can help refine print specifications based on real-world manufacturing capabilities. Although often viewed as time-consuming and expensive, thorough process development and validation are critical for long-term project success. These steps help identify the key factors controlling product quality and enable faster troubleshooting when issues arise.

Mold Sampling and Process Development

Through a best practices approach to new product development, project scope has been defined and understood, a DFM study has appropriately enhanced part design, mold design has been finalized and approved and the shop has built the required components/plates/bases (along with any additional tooling technologies like in-mold close, auto-unscrew and multiple material applications). Also, a hot runner system, if required, has been specified, ordered and delivered.

After mold assembly, the mold is transferred to a suitably sized injection molding machine for a functional sample run. This is the first time the new mold will be heated, have molten plastic injected into it (generally between 6,000 and 18,000 psi or much higher) and be clamped under tons of pressure.

A comprehensive performance evaluation includes:

• Mechanics (mold open-and-close; any steel actions)
• Water flow verification
• Balance of fill and injection pressure requirements
• Hot runner system performance
• Ejection system (parts eject without issue/deformation)

Although the sampling event verifies that the injection mold is functioning without issue, it does not demonstrate that the mold can produce parts to the acceptance criteria. To ensure compliance with these criteria, it is best to use the press that will run production with that mold at the production facility.

It is through the collaborative problem-solving efforts of the designers, moldmakers, process engineers and metrologists that our industry continues to evolve and progress.

In many cases, the mold shop that built the mold can offer valuable assistance to the production molder or contract manufacturer by conducting a preliminary scientific sampling phase in-house. However, this requires the mold builder to have access to a variety of appropriately sized and capable molding machines for the sampling activities.

While the mold shop’s press will not be used for production, a similar-sized press and a comparable screw and barrel configuration will provide foundational data. This setup can also be used to determine whether the mold can produce parts to spec, if the mold builder has the required metrology equipment and personnel to support quality activities.

Cp/Repeatability

A simple definition of Cp is a measurement of how much of the part tolerance is being used up based on the dimensional results. The more tolerance used, the lower the Cp. If too much of the tolerance range is consumed, the part or process will have low repeatability and will not be considered capable. Using numbers as an example: if you have a part where the nominal is 5, the lower spec is 3, and the upper spec is 7, and your actual measurements range from 3 to 7, the process will not be repeatable.

With too much range used up, sooner or later, a part will move out of the acceptable quality range. For example:

• If your dimensions range/move from 4 to 6, you are repeatable, and because it is on target, you have good capability/Cpk.
• If your nominal is 5 and your parts measure between 6 and 7, you have good Cp. You have poor Cpk because the mold is not properly targeted, but with steel adjustments, you can resize and ultimately achieve good Cp and Cpk.

Cpk/Capability

Cpk is the mold's ability to repeat within the tolerance target to nominal. The key point is that, until you establish Cp and repeatability, you cannot achieve good Cpk and capability.

Voice of the Process

The dimensions most affected by changes in the injection molding process typically have a large range and a low Cp. These are the larger dimensions of the part as they are most impacted by process variation. They serve as the "voice" of process variation because they react/change most notably.

Therefore, conducting these studies can be highly beneficial, as they enable a higher number of parts-measuring activities with fewer dimensions, reducing costs and lead times.

The concept is that a high quantity of parts, along with a prolonged run that exposes the mold to time variations, can be beneficial, even when there are fewer proven process-sensitive dimensions. Once the system is dialed in and demonstrating an acceptable Cp, a full First Article Inspection (FAI) can be justified, as it indicates that your system is repeatable.

When transitioning a capable process to production, there needs to be a period of time during which part verifications and in-press maintenance of the mold are closely monitored.

Pilot mold

In some cases, a well-planned and properly implemented pilot mold can provide a road map for the process and a metrology plan for the higher cavitation production mold.  When well-executed, it can streamline production mold validation by mapping out process-sensitive dimensions and, if confirmed, request the counts used in the study. 

Gage R&R

This protocol verifies that the measurement of the part is consistent and does not exceed the tolerance, based on variations in the results of measuring the same set of parts multiple times. These are well-defined events with rules.  If you do not access the health/Cp/Cpk of your measurement system, you risk making critical steel adjustment decisions based on a system with variation. 

Run-Time Duration

One pitfall, once the process is dialed in, is how long the injection mold will need to run to find any underlying issues.  For example, vent performance can change quickly, resulting in a shift in mold performance. 

Another consideration during validation is the potential impact on the budget. How much resin and how many run hours can the project allocate for parts that will ultimately be discarded?  The cost of scrap can quickly become considerable when dealing with large parts, multiple material components, purchased inserts and expensive performance resins/additives.

When transitioning a capable process to production, there needs to be a period of time during which part verifications and in-press maintenance of the mold are closely monitored. This is done to ensure the effects of run-time on the mold are identified and appropriately validated. 

Transducer Option

Implementing transducers offers clear advantages, provided the program's budget permits their use. They help to quickly map the process from development to production, ensuring alignment between the two. If any changes occur, they can be easily identified by evaluating the pressure curve results. The people who serve this portion of the industry can provide valuable insights into how to effectively lay this process out. This approach is a powerful tool.

Prints As a Dynamic Document

The part print should act as the design-for-manufacturing archive, documenting the history of productivity evolution, highlighting the pros and cons.

• Phase 1 - Set scope and deliverables
• Phase 2 - Based on early “parts” challenge the product to accept the widest range possible
• Phase 3 - Set the stage for long-term validation
• Phase 4 - Based on demonstrated performance, adjust for high ROI monitoring

Regardless of how tolerances are established, it is in the best interest of all parties to work and groom the part print from development to final validation.  The part print can become the learning record document if it is approached this way. 

Case in Point

Using an actual experience as an example, an OEM challenged itself as to why it demanded such tight tolerances on a globally distributed high-volume product. The OEM manufactured the same product in three regions of the world with the expectation of interchangeability. Tight tolerances were fully understandable until they identified and evaluated the cost implications. The tooling cost was higher due to the tolerance range, the component cycle times were longer and the cavitation was limited by the tolerancing. 

Collaborating with their tooling and molding partners, they embarked on a unique development protocol. The pilot tools were built with multiple cavity stacks to truly test the required part dimensions for a successful assembly. Maximum and minimum stacks were processed to intentionally make parts that had different physical dimensions. These physically different parts were measured, assembled and tested.

The OEM found that, in this assembly, they could more than double their traditional tolerance structure. This in-depth analysis, through the support of the mold builder and the contract manufacturer, ultimately gave them data and faith to jump from traditional eight-cavity molds to 64 cavities. 

Final Observations and Best Practices

Not every project will receive funding and resources, as illustrated in the example above, to execute that kind of optimized production development study. However, the primary objective should be to establish a comprehensive yet efficiently repeatable process that serves as a template for best practices.

For practical purposes, these are key practices for consideration.

• If the print dimensions were created in the abstract, find a way to challenge them.  Working together, find a way to make parts with dimensional changes to better understand the true needs of the product. 
• Critical to function vs. critical to monitoring process stability.  CTQs are, by their identification, what makes a product work. That does not necessarily mean they will help identify when the production process has become unstable or shifted.  If you can find your process-sensitive dimensions and monitor these during production, the CTQs typically behave as well. This does require a correlation study, but it is worth the effort and cost. 
• Print notes and the change archive should include the lessons learned library for future developments and product alterations. 
• If, during production, one day the parts change, a robust validation history will facilitate identifying the change.  Unless the mold was damaged, the steel did not change in a day.  Components like the press, resin, ancillary systems, manifolds, and water flow all play a role in the process.  The validation parameters can be checked and the source of change can be identified. 
• Injection molds do age and seat over time.  For example, vents tend to get affected and can become restricted.  These details, confirmed at validation, should be checked to gauge their shifts.

The insights and best practices in our New Product Development series reflect the experiences, lessons and viewpoints of the Dynamic Tool Corp. team. Injection molding presents us with constant opportunities to learn from our past successes and failures, and be inspired to improve. The scope of injection molded components ranges from fractions of a gram to hundreds of pounds. Part geometries present a wide variety of shapes, sizes and complexities. These variables create a complicated and challenging manufacturing landscape. It is through the collaborative problem-solving efforts of the designers, moldmakers, process engineers and metrologists that our industry continues to evolve and progress.