INJECTION MOLDING: Develop Guidelines—Not Strict Procedures— For a Robust Molding Process

Back in the early 1990s, discussions about the “viscosity curve” did not generate much interest. Believe it or not, the topic was rejected for an SPE ANTEC presentation for couple of years back then.

Fast-forward to December 2015. The viscosity curve is more accepted but most molders use it to find fill time. This is not its primary purpose and can cause problems. So what is the primary point?

First, understand that the single biggest influence on polymer viscosity is shear rate. This is not a number that can be pulled off a machine controller, but fill time is available and correlates with shear rate: The shorter the fill time, the higher the shear rate. The major point is that viscosity changes if you change the fill time/shear rate.
But the mold is three weeks late and everybody is frantic—the pressure is on and somebody wants parts shipped the next day. You do not have the time to do the full viscosity-curve study, or you make the time and do your calculations and yet production is still poor and/or customers are returning parts. This certainly speaks to the need for a new approach. But what?

To make things work, you need to put together five critical components of a successful molding application. This list was developed by well-known consultants  Glenn Beall and Mike Sepe (a fellow Plastics Technology columnist):
 

1.  Part design;
 2.  Resin selection and handling;
 3.  Tool design and construction;
 4.  Processing;
 5.  Testing.

We are not going to cover each here, but suffice it to say that the one you ignore will wind up biting you. Think of it this way: Your car has four tires, but it most likely won’t take you to where you want to go if three of them are filled properly and the fourth is flat. Granted, most molders cannot afford or even need in-house expertise for all five critical components. If that’s the case with you, arrange for a specialist in the area needed. No one knows everything, even molders with 40 years under their belt. Besides, it’s beneficial to have mold and tool drawings double-checked. The time to find problems is at the drawing stage, not after steel is cut. So let’s move on to mold delivery and developing a process.

Most would like, and would even pay for, a step-by-step procedure that, when followed, would magically result in a “production” process. That is the ISO 9000 or TS mentality. Forget it. Fact is, you are not slinging hamburgers here, and no one can write a fool-proof procedure for something as complex as molding. Furthermore, such an approach kills creativity and critical thinking. Every mold/resin combination has its own oddities. Finding them is the challenge, and following a set procedure is not going to get you there.

Guidelines, not strict procedures, are OK with me. You must allow operators to think. You also need trained, proficient operators following these guidelines. They should be instructed to use their talents to interpret results of various experiments and to define any weakness in any of the five critical components. They should be directed to find and define problems, not process around them. Any process workarounds will show up in production, and the competitive molder knows that finding them before production will cost less than doing so afterward.

My guideline experiments include the following:

 •  Cooling,
 •  Viscosity curve with visual part inspection at each velocity,
 •  Velocity-to-pressure response, including momentum,
 •  Pressure-loss analysis,
 •  Short-shot study,
 •  Gate seal,
 •  Melt temperature,
 •  Part temperature via infrared imaging (not expensive nor time-consuming),
 •  Second-stage pressure range,
 •  Screw recovery,
 •  Cycle time. 

There may be others if something odd turns up. I never know beforehand. Note, there is no race to make a part that “looks good.” It takes a series of experiments to define

the critical parameters for the particular mold and resin combination.
These experiments should be aimed at finding and establishing a base process with a list of concerns to be addressed. Do a Design of Experiments (DOE) if necessary.

From my list of experiments above you’ll establish levels of the variables (factors), and determine those variables that are binding. This often speeds the DOE—now you have data to establish levels and factors, not guesses. All of this should take about two hours; anything longer suggests that significant production problems lie in waiting.

Developing a process does not start with shooting plastic into the mold. It starts with ensuring the mold is set up properly in an appropriate machine that works. Each coolant channel is documented as running with turbulent flow (Reynolds number at least 5000) and regulated such that the flow (gallons or liters/min) can be duplicated run to run. Cooling is 90% of your cycle time, and it is the exception that cooling is done right. Most of the time somebody hooks up a half-dozen or more hoses to the mold from a manifold and thinks the job is done. But those who understand coolant flow pay attention to these issues and more. There is a science to cooling, and it’s time we get serious about learning it.

Once you have a base process, start making some parts for testing to center the process at the center of the part specifications. Don’t send or show parts to the customer (or your boss) just yet, even if they “look good.” Instead, hide them or push them under the fork lift. There is still testing to be done.

Testing includes melt-flow rate on granules and parts, thermal cycling, and any other tests that show the part meets the critical requirements. During this testing phase, you will learn how to center the process to the part specifications. Often this requires tool modifications. At that point you will hear, “Can’t be done, no time or money, find a way around the problem.” Don’t do any such thing, because you will wind up killing your company’s profit margin with endless “fixes” and will convince the client you are less than competent.

Once the operator feels the process is stable, and initial part testing shows promise, you must challenge the process. That is, find out whether it is truly a stable process. Are fill time, pressure at transfer, cushion, recovery time, cycle time, and part temperature within an acceptable range as you start up, shut down, and have somebody new start it up?

If the answer is yes to all, the next step is to force a viscosity change. This is critical. It will happen in production, so it is best to know in advance what will occur, so you can be prepared and not have to shut down the press. Force a viscosity change by testing a different lot, or wet resin, 100% regrind, different colors, or whatever you can think of that will mimic the variables in 24-7 production. Do the process outputs reflect these variations? Is first-stage filling consistent as they process through? Can you accommodate these changes by changing the second-stage pressure?

Forcing a viscosity change and seeing if it can be accommodated is perhaps the most critical test for a true production process. But it is rarely done. All the work goes into testing one lot of material, but in actual production you have to process different lots. You cannot blame the resin supplier. As Dr. Deming taught us, all processes have normal variations. Your process has to deal with them. Bottom line: If you have tested only one lot of material you do not have a production-capable process.

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