If you review the molds you’ve built and sampled over the years along with the new molds you may be sampling today, you will probably have a list of recurring problems (flash, non-fills, unable to pack the parts, dimensional variations, warp, broken core pins, poor cosmetics, etc.).

And if you ask someone which cavity or cavities are causing a specific problem, you will probably hear “It’s random. Sometimes the problems are in cavity 6, other times in cavity 3, and other times in cavity 7.”

But are those defects and mold problems really random? Why do the same problems keep showing up in nearly every mold you build? How much time and money are you spending fixing the same problems over and over? And what can you do to diagnose and fix the root causes or prevent the problems from happening in the first place, thus improving delivery and quality while saving you and your customer time and money?

The Missing Link
To find the answer, we need to look beyond the steel itself. Not all answers lie within what we can see and measure. And after analyzing the mold and the data, you will probably find that many of the part quality defects and mold maintenance issues for a given mold have a common, but often overlooked, link. And that link is the plastic’s fundamental flow characteristics.

Unfortunately, and ironically, there is a general lack of proper training and understanding of plastic flow in the industry, and in particular within the mold manufacturing sector. And understandably so. After all, moldmakers are usually more concerned with the steel characteristics versus plastic flow characteristics. However, it is just as important for moldmakers to understand plastic flow as it is to understand CNC programming.

The science of plastic flow helps separate out all the variables and noise during the mold debug and troubleshooting phases and ongoing mold maintenance programs. This allows you to quickly and accurately identify the root causes of your mold’s problems. And in turn, proper long-term solutions can be implemented to provide a higher quality mold, and systematic mold diagnostic procedures can be put in place to let you qualify the mold faster and easier. And when done properly, all of this can be done at a lower cost and with a shorter leadtime.

Flow Grouping
Separating the root causes of mold filling and part quality variations should be done using a Flow Grouping methodology. This process will allow you to scientifically track and troubleshoot a mold’s filling and part quality characteristics by identifying groups of cavities that have the same material history.

To better understand this, consider that all filling and part quality variations are typically a result of a pressure drop difference as the material flows to and/or within the cavities. Through Flow Grouping, we can determine that any filling or part quality variation within a given Flow Group is typically caused by steel variations—such as core/cavity steel dimensions, gate size/gate land variations, and venting, in addition to outside variables such as molding machine inconsistencies, heater band functionality, and so on.

Any variation between Flow Groups has been proven to be a result of different material viscosity characteristics flowing to each of the Flow Groups. The different material properties between the Flow Groups are caused by non-uniform shear history of the material as it travels through the melt delivery system.