Matrix has the ability to perform melt flow testing in our inspection lab. We use this to confirm that an incoming resin's melt properties are within the range specified for that grade.

Melt flow index (MFI) is a measure of how easily the melt of a thermoplastic polymer flows. More specifically, it is the mass of polymer (in grams) that flows in 60 seconds through a tube of specified diameter and length by application of prescribed pressures at prescribed temperatures. The specific method for this testing is described in ASTM standard D1238 and ISO standard 1133. Melt flow rate is inversely proportional to viscosity of the melt under the conditions of the test. Ratios between two melt flow rate values for one material at different pressures are often used as a measure of the broadness of the polymer's molecular weight distribution. Melt flow rate is very commonly used for polyolefins, with polyethylene being measured at 190°C and polypropylene at 230°C.

Something to consider in selecting the proper material grade is to choose one with a melt flow index high enough to easily form the part, yet low enough to provide sufficient mechanical strength for the part's intended use.

DOE or design of experiments (sometimes called experimental design) can be a powerful tool for any molder to have in his or her arsenal.  We live and mold in a demanding era.  We must mold with tighter tolerances, less scrap, and quicker cycles than ever before.

I was brought up by my mentors to change only one variable or parameter at a time, then measure the part or observe the outcome of that change. Curing a defect or establishing a robust process was often a matter of days, weeks or more.

DOE can cut the time for defect remedy, process establishment, and process validation to a fraction of what the old “trial and error” method took.

DOE may sound complicated to many Molders, but where once DOE was the territory of statisticians and engineers, new software developments have simplified the process and interpretation of the resulting data.

At Matrix Tooling/Matrix Plastic products, we use a software package designed for injection molders.  It supports up to Taguchi Level 8 experiments.  We can focus on, say, three inputs or factors in an attempt to achieve one or more desired responses or outputs, also called outcomes.  Factors could include: mold temperature, melt temperature, injection speed, and pack pressure among others.  The response could be anything from warp, flashing, a change in physical properties, or certain dimensions. Choosing inputs and responses requires knowledge of and experience with the injection molding process. This is much more important than being a statistician.

Taguchi L8 experiments require eight runs, and each run will have changes to multiple inputs. Results are measured, noted, and entered into the software which then maps the results on various graphs and charts for analysis, including: response surface graphs, scatter plots, main effects plots, Pareto Diagrams, ANOVA and other high powered statistical tools. In short one can see graphically what parameters or combination of parameters affect the desired outcome. You may not necessarily cure the problem during the first DOE if it is a hunt for a defect cure, but you will likely be pointed in the right direction.

Aside from troubleshooting, DOE is a recognized tool for process evaluation and validation, especially for FDA requirements for the medical device industry. There are a number of methods and tools recognized for FDA evaluation: SPC control charts, capability studies, Failure Modes and Effects Analysis (FMEA), error proofing, and DOE.  Many nonconformities are the result of excessive variation.  DOE can be a great tool to reduce and control variation. Different types of designed experiments are used here to identify key input variables and one kind of Taguchi experiment actually emulates the variation that could be found in a process over time through small but structured parameter changes.

A Molder must use all the tools at his or her disposal to quickly identify key process influences and arrive at a robust process that is defect free.  DOE is a powerful tool, and astute molders should know how and when to use it.

Here at Matrix we often manufacture complex plastic components used in surgical devices and other medical applications. These parts can vary greatly in terms of size, material, and design but they all share several characteristics that can make them difficult to inspect using traditional techniques.

Performing first article inspections with these methods can be particularly time consuming and labor intensive. In addition to creating fixtures for each setup, the parts often need to be “sectioned” (sawed, cleaved, ground down) in order to inspect internal dimensions that are not naturally accessible via a touch probe or optical scope.

The associated tasks may require an inspector with a high skill level and/or experience performing similar procedures. It also opens up additional steps where operator bias and other errors can be introduced. Were all cavities saw cut and treated the same? Do different inspectors reproduce the exact same setups?

Above that, the sectioning process itself is inherently flawed. Sawing a plastic part to access a cross section will almost certainly introduce its own level of error, and this error can often exceed the tolerances of the dimension and distort inspection results. Warp, burrs, rolled edges, inaccurate trimming, inaccurate positioning of the section line and melting are all possible byproducts of manual sectioning methods.

And after all is said and done, you end up with first article data that is historically limited to the original points in your inspection layout. If you want to go back later on and inspect any additional dimensions, the setup will have to be recreated with the original parts.

To sum it up, performing a first article inspection (FAI) on complex parts often comes with the following issues:

• Time consuming & labor intensive
• Require highly-skilled technicians
• May introduce operator bias
• Allows for subjectivity in results depending on operator
• Historical reference data is limited to inspection points taken from original sample parts, making any future inspections from those samples very time consuming and possibly inaccurate.
• Requires inherently flawed sectioning process which can introduce error that exceeds dimensional tolerances (warp, burrs, rolled edges, trim marks, melting)

This is where cross-sectional scanning (CSS) comes into play. CSS is a unique process developed by CGI of Eden Prairie, MN. It offers an automated alternative to traditional first article inspection techniques that provides consistent and objective results. Using reverse engineering principles, CSS begins with an actual part and rapidly deconstructs it, cross-section by cross-section, to create a comprehensive set of measurement data capturing every dimension on every surface of the part, both external and internal. This video demonstration of the Cross-sectional scanning process will help clarify the process.

In the end the CSS capability reduces the amount of time and labor required for inspections, nearly eliminates operator bias and human subjectivity from the process, minimizes the dimensional stresses caused by manual sectioning, and leaves you with easily retrievable, electronic historical data that can be interrogated at any point in the future.

Posted by:

Gary Johansson
V.P. of Quality / Regulatory