Going Against the Grain: Low Cavitation Vs. High Cavitation

When molders need to run large quantities of plastic parts, injection molding continues to be the preferred process to making these parts - the ultimate goal being identical parts. And while high-cavitation molds are being designed to run these parts quickly and less expensively, injection molding expert John Bozzelli paints an entirely different picture concerning the use of these high-cavitation molds and asserts they should stop being built.

Bozzelli, principal of Injection Molding Solutions (Midland, MI) - a consultant and trainer of scientific molding for almost 15 years - makes a strong case for the use of low-cavitation molds instead of high-cavitation molds. In a nutshell, Bozzelli believes that industry trends of greater part complexity to add function or reduce assembly costs, and thinner walls to reduce plastic and cycle times, are adding to the demand for tighter tolerances. And these factors add up to greater demands on a process that already has hundreds of variables. Molds with high cavitation further compound these variables, he feels. His views are detailed below:

The View from Production

First, Bozzelli points out that while it appears that it costs less to run one 16-cavity mold than four four-cavity molds, there are many other factors that need to be taken into consideration. "With the 16-cavity mold, what are the chances of the $10 or $12 an hour processor or setup person making a mistake and inadvertently causing some mold damage to slides or cores in the life of the tool?" he questions. "This probability is extremely high, and the result is shutting down your client. What's the chance of four four-cavity molds simultaneously having a catastrophic failure? This is extremely low. The worst case situation with multiple molds is that the client winds up with a shipment that is low in quantity. He would not be completely shut down. Most likely, the molder will have one as a spare, always ready to go if a tool in production gets 'accidentally' damaged. So his production flexibility is now enhanced, and on-time shipments are critical today."

Four-cavity molds also afford more flexibility with last-minute production changes. "For example, let's say one of your customers calls with an order for 20,000 computer housings," he says. "So you get ready to make 20,000 computer housings. Then, they issue the PO for it and decide they only need 5,000 housings, but 20,000 of the backplate. So a last-minute change must be made. Production forecasting is getting worse for our industry. If you were to ask how many mold changes occur during a day the answer would be more and more. Often a mold is mounted ready to go and the scheduler walks up and says, 'Hurry up and take that one out and put this other one in.' That kills steady-state production. Molds with fewer cavities allow a molder to run one, two or all of the molds to meet the demand. If orders are low then he just runs one mold for long periods - much wiser than running a 32-cavity beast for a few hours. Have you ever started up a 32-cavity hot runner tool with drooling tips? It's a nightmare - it takes as long to start it as to run the production. That's wasted machine time, lost resin and lost money.

"Now, what the molder needs to ask himself is, 'Is this process a thermal process?'" Bozzelli continues. "The answer is an unequivocal yes. Therefore, anytime one interrupts a thermal process he interrupts a steady-state situation. Steady state or continuous process is the best place to be when you want consistent parts. Now, if he has a 32-cavity mold and it's only going to run for 24 hours and then get pulled - who is paying the molder for changing the mold? He's losing money and that's time he can't sell to anybody.

"Then, he has to wait for this mold to come up to steady-state temperature the next time he runs it," Bozzelli adds. "Some people say that they can change a mold in half an hour, and that's good, but how long does it take to get the mold up to steady-state temperature once it starts? There are molds that take eight hours to get up to uniform temperature before they are steady state, or at 'thermal equilibrium.' Does the accountant ever get told of this warm-up time; is it ever on that cost sheet? However, if the molder has a four-cavity mold, he can just mount it and run it steady state for three or four days a week, just like it was intended to be," Bozzelli says. "The industry isn't looking at the issue hard enough," he states. "Have I made my point? Production flexibility is something that needs to be driven home that few are paying attention to. It's not how well you can predict the future, it is how fast you respond to changing needs. Anybody want to bet that things will change at a slower rate?"

Another aspect to production flexibility is lights-out molding. "It's much more simple to mold 24 hours a day on a low-cavitation tool and have several presses going," Bozzelli comments. "If one shuts down at midnight because something went wrong and the molder is running a high-cavitation tool, that will kill him. It's no big deal if he has low-cavitation tools on several presses - the others keep running."

Going with the Flow

Next, Bozzelli explains the importance of understanding how the plastic flows to fill the cavities. Here, he looks to colleague John Beaumont of Beaumont Runner Technologies, Inc. (Erie, PA) for his expertise. Beaumont has studied the effects of molding imbalance and published numerous papers on the subject (see www.meltflipper.com). "Beaumont's papers illustrate how flow is not symmetrical in runner systems - especially after the melt experiences a second flow branch by flowing into a 'tertiary' runner," Bozzelli states. "Thus, most molds with more than four cavities have an inherent unbalance - one that no financial or cost savings by going to a higher number of cavities is going to solve. This unbalance will result in identical cavities producing non-identical parts.

"Isn't the name of the game here high tolerances and consistency?" he asks. Additionally, he estimates that 80 percent of the high-cavitation molds are not being run at full cavitation because of this viscosity issue. "The mold will start out making all 32 cavities, but in an hour or two cavities block and the mold is down to 28 or lower," he contends. "It's very common in the industry, although no one likes to admit it. Rarely do the accountants ever find out or put this on the cost analysis."

Dollars and "Sense"

The third reason counteracts the financial department's mission, Bozzelli maintains. "The financial department will invariably come up with a cost analysis, and on a black and white sheet of paper it shows that it is cheaper to make parts from a 16-cavity tool than it is to make parts out of a four-cavity tool," he says. "However, they are not counting all of the costs of the 16-cavity tool - like startup time, problems with hot runners, clogged tips in hot runners, repair costs, etc. They're not looking at the big picture - the hidden costs in a high-cavitation mold.

"As far as the OEMs are concerned, a four-cavity mold will have a 16-second cycle and a 32-cavity mold will have an 18-second cycle," Bozzelli continues. "If a molder is going to charge so much per hour per machine tonnage, it looks like a cheaper cost for the 32-cavity tool. Usually these companies have little knowledge of plastics and don't consider all of the factors - including startup time, maintenance and downtime. For example, if a hot tip becomes clogged, how many cost analyses take into account that the toolmaker will break 25 to 50 percent of the wiring, etc., of the hot runner just to get to that clogged tip? In the end, it costs less to use low-cavitation molds and part quality is greater."

Expect to Inspect

Quality parts are the direct result of effective QC, Bozzelli notes. "Say you have 16 cavities coming out of a tool, and upon ejection you are trying to sort and check each cavity," he comments. "Do you know how hard it is to catch 16 parts, look at them (humanly or robotic), and figure out what's going on with each of them versus catching four and looking at them? It's cheapest not to make the reject, but if you do you must catch it at the machine, not down the pike when it is in the assembly and packaged. Finding mistakes at the press can be pennies, finding them in the assembly can be hundreds of dollars. How much does it cost if the customer finds it? You have to find parts at the press and you have to know on that shot, not the next shot. It's extremely difficult to do with high cavitation - and much easier to do with low cavitation." Bozzelli adds that mechanical inspection - including a computer vision system for cosmetic issues - is best so there is no chance for human error.

In conclusion, Bozzelli reiterates that a number of factors influence the decision on how many cavities a mold should have and need to be addressed. "These issues must be considered in developing, quoting and running multi-cavity tools," he emphasizes. "Although financial considerations are important and indicate lower part costs for multi-cavity tools, trends of thinner walls and more complex parts in both large and small components, financial justification for large cavitation molds is not necessarily correct. There are times when high cavitation is the correct path but few take the time and spend the money to do it right. Using high-cavitation molds makes it more difficult to process identical parts."