The Case for Low-Cavitation Molds & A Case Against High-Cavitation Molds

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When there is a need for large quantities of complex geometry plastic parts, injection molding continues to be the preferred process. The ultimate goal is producing identical parts, and emphasis is on “identical.” While high-cavitation molds are being designed to run these parts quickly and less expensively there is an entirely different perspective to using these high-cavitation molds. Unless flow balance technology is used and all the costs accounted for, they should stop being built.

There is a strong case for the use of low-cavitation molds instead of high-cavitation molds. Briefly, some industry trends are:

Greater part complexity to add function or reduce assembly costs
Thinner walls to reduce plastic and cycle times
Demand for tighter tolerances
Fast response and low inventories

These trends tend to make processing, which already has hundreds of variables, significantly more complex. These factors add up to greater demands on a process. Molds with high cavitation further compound these variables. Some, unfortunately not all, details follow.

The View from Production
First, while it appears that it costs less to run one 16-cavity mold than 4 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 set up person making a mistake and inadvertently causing some mold damage to slides or cores in the life of the tool? In addition, what happens if the press breakdown? This probability is extremely high for both, and the result is shutting down your client. This is extremely low. What is the chance of 4 four-cavity molds simultaneously having a catastrophic failure? (By the way, you will not need 4 four-cavity molds. Previous history has shown that 2 to 3 four-cavity molds will out produce one 16-cavity tool) 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 is "accidentally" damaged. So, his production flexibility is now enhanced. On time shipments are critical today.

Four-cavity molds also afford more flexibility with last-minute production changes. For example, let us say one of your customer’s calls with an order for 200,000 parts. So you make 200,000. However, at shipment time they need more, and in a hurry. So a last-minute change must be made. Production forecasting is getting worse for our industry. Ask around, how many mold changes occur during a day? The answer: more. Often a mold is mounted ready to go and the scheduler walks up and says, "Hurry up 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 the molds to meet the demand. If orders are low then he just runs one mold for long periods, much wiser than running a 16-cavity beast for a few hours. Ever start up a high cavitation hot-runner tool with drooling tips? A nightmare, it takes as long to start it as to run the production. That’s wasted machine time lost resin and lost $$$. What happens if a high cavitation mold plugs a gate or two or three? Yea, I know this never happens in your shop. The whole production is shut down for repair. Again as above the other molds in low cavitation are running.

What the molder needs to ask himself is, ‘Is this process a thermal process?’ The answer is an unequivocal yes. Therefore, anytime one interrupts a thermal process he interrupts steady-state situation. Steady state or continuous processing is the best place to be when you want consistent parts. Now, if he has a 32-cavity mold and it is only going to run for 94 hours and then pulled – who is paying the molder for changing the mold, and warm-up time? He is losing money because it is machine time and resin he cannot sell.

Then, he has to wait for this mold to come up to steady-state temperature the next time he runs it. Some people say that they can change a mold in half an hour, and that is 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? What about the wasted resin in startup and warm-up?

However, if the molder has a four-cavity mold, he can just mount it and run it steady state for long periods of time, just like it was intended to be. The industry is not looking at this issue hard enough. Do you get the point, that there are two sides to the low inventory equation? Production flexibility is something that needs to be driven home that few are paying attention to. It is 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 is much simpler to mold 24 hours a day on a low-cavitation tool and have several presses going. If one shuts down at midnight because something went wrong and the molder is running a high-cavitation tool, that will kill him. It is no big deal if he has low-cavitation tools on several presses, if one goes down it can stay down until 1st shift – the others keep running.

Going with the Flow
Next, one must understand how the plastic flows to fill the cavities. Look 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. Beaumont’s papers illustrate how flow is not symmetrical in supposedly “balanced” runner systems. This is especially true after a ‘tertiary’ branch, that is the split as you go from a four- to eight-cavity balanced runner mold filling analysis.

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. Most molders have seen the inside cavities of an 8-cavity tool be different from the outside cavities. The problem stays in that location of the mold, it does not move with the cavity if the cavity is moved to a different location. For reference, see the figure below. This figure shows the typical imbalance of plastic flow corrected with the melt flipper technology. This flow imbalance can also influence 2 and 4 cavity molds depending on layout. This flow imbalance must be corrected if you want to have identical parts. Few pay the extra dollars to obtain this balance. Mold filling analysis does not address this problem.

Isn’t the name of the game for injection molding, high tolerances and consistency? Additionally, it is estimated 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 contends. It is 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. 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 32-cavity tool than it is to make parts out of four-cavity tools. However, they are not counting all of the costs of the 32-cavity tool – like startup time, problems with hot runners, clogged tips in hot runners, repair costs, etc. They are not looking at the big picture – the hidden costs in a high-cavitation mold.

As far as the OEM's are concerned, a four-cavity mold will have a 16-second cycle and a 32-cavity mold will have an 18-second cycle. 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 do not consider all of the factors – including startup time, maintenance and downtime. For example if you clog a hot tip how many cost analyses take into account that during tool repair someone will break 50% or more of the wiring etc of the hot runner just to get to that clogged tip. Ask what the cost is to repair a mold where the hot-runner flashed! In the end, it costs less to use low-cavitation molds and part quality is higher.

Expect to Inspect
Quality parts are the direct result of effective QC. Say you have 32 cavities coming out of a tool, upon ejection; you are trying to sort and check each cavity. Do you know how hard it is to catch 32 parts, look at them (humanly or robotic) and figure out what is going on with each of them versus catching four and looking at them? It is 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 at the press can be pennies, finding it 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 is extremely difficult to do with high cavitation – much easier to do with low cavitation. In addition, inspection must be by computer or robot NOT by a human.

In conclusion, a number of factors influence the decision on how many cavities a mold should have. These issues must be considered in developing, quoting and running multi-cavity tools. Although financial considerations are important and indicate lower part costs for multi-cavity tools, trends of thinner walls and more complex, tight tolerance parts add complexity to the molding process. If you want six sigma, you must design it into the part. Has that been done? Financial justification for large cavitation molds is necessary but it MUST account for all the costs involved. Most do not. 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. Take CD's and DVD's for example, sorry each one is made in a single cavity tooling. You do not hear the stories of all the attempts to make multicavity CD molds.

There is no way to prove all the above unless somebody makes both the 32 cavity beast and the four cavity molds and does the comparison. I am not saying that 4-cavity molds will be easy to run they will have issues but they, in my opinion, are manageable. Process with constant plastic variables not to the same machine set points.

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