Scientific Molding, Part II: Cooling

This is the second in a three-part series on scientific molding. THe first part, which appeared int October 1997 issue of IMM, discussed the filling stage of molding and included some background and history on the scientfic molding process. This month, we focus on the cooling stage of the process, and how to optimize it to help presses and parts run more efficiently.

Sidebars:

As much attention as one might like to give the filling stage of the molding cycle, the fact is that it is only a fraction of the overall process. Most of the cycle, usually about 90 percent, is spent packing, holding, and cooling the part or parts. Optimizing cooling provides parts of consistent quality and consistent weight.

The tendency, says John Bozzelli, one of the molding industry's leading advocates of scientific molding, is to overfill the part during injection. "If the part is filled on first stage (boost) you waste energy, cannot control velocity, even on closed loop machines, and will produce flashed and short parts as lots change," he says. This leads to molded-in stress and other structural defects. Bozzelli, the principal of IM Solutions, his Midland, MI-based consulting business, often works with Ashland Chemical Co.'s General Polymers Div. The two are among several in the industry trying to make molders more efficient and competitive by using scientific molding, also known as systematic molding or Decoupled molding.

As discussed last month in IMM, the goal during the filling stage using scientific molding is to leave the part about 1 percent short (99 percent full). This gives the second stage the "wiggle" room it needs to properly fill out a part. As with the filling stage, the goal with cooling is to derive a set of parameters specific to the mold and material that can be transferred to any other capable machine. These "universal" parameters for cooling include pack and hold time, pack and hold melt pressure, and mold temperature.

Note: The process of leaving a part 99 percent full involves optimization of injection velocity, injection pressure, and transfer position. Please see the October issue for details.

Cases in Point Revisited

Last month we examined the filling stage optimization of two presses at two different molders. The first molder is in South Carolina, molding a 4-by-4-inch gun shot primer tray in an eight-cavity hot runner mold. The tray has 100 evenly spaced holes, each about .25 inch deep. The material is a precolored black polypropylene. Intrusion had been used on this gun shot primer tray during the cooling stage to eliminate a sink that developed at the center of the part. Bozzelli and McDonnell turned off intrusion, increased pack and hold presure, and ran a agate seal test.

When we last left the tray, Bozzelli and General Polymers (GP) technical service representative John McDonnell had built an on-machine melt rheology curve for the material, selected a fill time based on the curve, and determined the transfer position to provide a part about 99 percent full. Cooling then begins on second stage.

This particular part had been using intrusion during the cooling stage to eliminate a sink that developed at the center of the part, a sink exacerbated by the suction cups on the part takeout robot. To start, Bozzelli and McDonnell turned off intrusion and increased pack and hold pressure in 200-psi increments until the part looked full. Next, they performed a gate seal test to determine the optimum pack and hold time.

The gate seal test tells at what point the plastic in the gate cools enough to provide a seal against plastic backflowing out of the part and into the runner or sprue. If parts are run without the gate freezing, the process is less robust. Plastic is compressible and acts as a compressed spring to push plastic back into the runner if the gate is not frozen when second stage pressure ends or drops off. Says Bozzelli, "Parts may have inconsistent dimensions, quality, and/ or weight. Many molders are running multicavity tools with some parts seeing gate seal and others not. Then folks wonder why identical steel cavities produce nonidentical parts."

Sealing the gate also means that melt in the mold will not back into the gate before it has hardened; such an action can produce parts of inconsistent weight and quality if other processes are not tightly controlled. While some molders may wish to mold with gate seal, there are instances when it is not preferable. Either way, a molder should at the very least know at what point the gates seal for each mold for each material used. Although the gun shot primer tray mold uses hot runners that theoretically never actually seal, the melt will reach a point where it is cold enough to prevent the material from backing out.

To do the gate seal test, Bozzelli left the pack and hold pressure constant and incrementally reduced the pack and hold time during a series of cycles, careful to make sure cycle time remained consistent throughout. He started at 6 seconds and reduced the time in 1-second increments to a 1-second pack and hold time. All eight parts from the mold were collected after each cycle and weighed together. Results are presented here.

Gate seal is achieved when the total weight stops increasing - that is, no unmelted material is backflowing through the gate. In this case, gate seal occurred at 5 seconds. Bozzelli and GP like to set the pack and hold time a little bit longer than the gate seal time - about 10 percent longer (or more) in most cases - to accommodate any variables that might cause the gate to seal later than usual. In this case, they chose 6 seconds. This pack and hold time also eliminated the sink at the center of the parts.

Because these parts must comply with quality control standards, Bozzelli and McDonnell then produced parts with the pack and hold time set at 6 seconds, with lower and higher pack and hold pressures. These samples were marked and sent to the quality control department in the mold shop to determine which pressure setting produced the most acceptable parts.

The goal of Bozzelli's analysis of the mold and part was to reduce the cycle from the original 21.3 seconds to the quoted cycle of 19 seconds. By optimizing the filling and cooling, he reduced the cycle to 18.3 seconds, saving the molder about $25,000 in machine time, given a 4500 hour/year schedule and a $40/hour machine recharge rate. Says Bozzelli in the end, "Now, we've taken some time (about 2 hours) and wasted a little material, but it will more than make up for itself in the time and energy saved."

Thermal Consistency

At the molder in Wisconsin, Bozzelli and GP senior technical service representative Alan Larsen demonstrated how providing thermal consistency to the material can save money. Like many molders, this shop was dominated by manual labor. On one press molding a pump housing with six manually placed inserts, Bozzelli noticed that the mold closed as soon as the operator situated the inserts and closed the safety gate.

One of the variables that affects material viscosity is residence time. The way the job was configured, the next cycle didn't start until the gate was closed. If the operator took more or less time than usual placing the inserts, material viscosity would change, causing the press to produce parts of varying quality. The rule is simple: the higher the viscosity, the slower the material moves; the slower it moves, the higher the viscosity gets. If you can stabilize viscosity you can stabilize part quality

Bozzelli and Larsen set the mold open time to just higher than average for the operator. Then, after the operator placed the inserts and closed the gate, the clamp did not close until the set time was reached, about 2 seconds after the gate was shut. Although it flustered the operator at first to close the gate and not see the mold close, Bozzelli's adjustment brought consistency to the cycle, providing the same residence time for each shot. "Total part count will probably decrease because of what I did, which annoys the bean counters," says Bozzelli, "But consistent viscosity of material produces better parts and reduces the scrap rate. In the long run, they save money. It's like making toast to the same brownness - consistent time is critical."

Mold Cooling

One other check commonly performed by Bozzelli and GP is of water flow through molds. Checking water lines is simple and sometimes wet, but again often ignored by many molders. All it requires is a flow meter. At the Wisconsin molder, water flow in most lines was good - more than 2 gal/minute - but Bozzelli and Larsen did encounter one water line that was completely blocked. Actually, the channel in the mold to which the blocked line ran was an old one, like a dead end road. The line was blocked because it went nowhere. This wasn't detected until Bozzelli checked it with the flow meter.

Other cooling water characteristics to watch are pressure loss between inlet and outlet, temperature differential, and water quality. Generally you need to see a pressure differential between inlet and outlet of the tool of 30 to 35 psi to drive the right amount of water through the channel. Lots of water must flow through the channel to obtain turbulent flow, which will optimize heat removal. "Temperature is not as critical as driving the maximum volume through the channel. Slow flow due to a plugged line can kill a process," Bozzelli notes. Plugging can be caused by Teflon tape, rust, sand, or scale buildup.

Bozzelli reports that scale buildup of as little as .0625 inch can reduce cooling efficiency by as much as 40 percent. Make sure your cooling water is properly filtered to remove rust, dirt, and other buildup. The temperature differential between inlet and outlet water should be at or below 4 deg F for best cooling.

All of these factors, if not properly maintained, can lead to inefficient and insufficient cooling of parts. One good way to monitor mold temperature is by use of an infrared camera. It picks up a lot of the subtle temperature variations from cavity to cavity and core to core to highlight hot spots in the mold. This is a favorite Bozzelli tool, and will be the topic of next month's final installment on scientific molding.

The Universal Setup CardIn his effort to make molding a process that is machine independent and less subject to injection machine variables, John Bozzelli has constructed the universal setup card, a list of data and parameters that focus on the material and the mold, not the press. These are parameters that can be derived using the scientific molding method (also known as systematic or decoupled molding). They allow you to transfer the mold and material to another machine, without loss of part quality. They focus on plastic variables, not machine variables. The universal data include:Mold number, number of shots to date, part name, customer, date, molder's name, and any other information your plant may require.Fill time for a part 95 to 99 percent full.Weight and picture of part 95 to 99 percent full.Transfer volume, transfer position, or cavity pressure (time and hydraulic pressure transfer modes are not recommended).Nozzle melt pressure range for different lots at transfer volume, position, or cavity pressure.First stage set melt pressure (nozzle); this is first stage set pressure times the intensification ratio. Cycle time.Quoted cycle time(s).Gate seal time.Pack and hold time.Pack and hold melt pressure.Shot size in volume.Mold temperature, cooling channel map.Water flow diagram, with gallons/minute of each channel, temperature of water in and out, and water pressure in and out.Screw run time (average).Mold open and closed time, cure time, or cooling times.Melt temperature via hot probe.Nozzle tip length, diameter, land length, radius, and type.Hydraulic pressure vs. time response curve.Cavity pressure integral at the gate and end of fill.The Load Compensation TestThere is an easy test you can perform on your machine to see just how repeatable it is, and how well it controls pressure in the mold. The theory behind the load compensation test says that once you set a fill time, the machine should be able to control itself to meet that time whether shooting into a mold or shooting into the air (no resistance). By John Bozzelli's standards, that time shouldn't vary by more than .04 second.At the Wisconsin molder, Bozzelli demonstrated the load compensation test on a late-1960s model 125-ton press molding a spring housing from ABS. After optimization of the fill stage, a standard shot was made into the mold, with a fill time of 2.19 seconds. Then, the mold was opened and a shot was performed into the air. This one took 2.12 seconds. Although the .07 differential was significant by Bozzelli's standards, he says "the difference is good for a machine of this age."At the South Carolina molder, a shot into the mold took 1.03 seconds at 1420 psi of hydraulic pressure. Into the air it took .99 second at 474 psi. If you record the hydraulic pressure each time, there's a formula (developed by RJG's Rod Groleau) you can use with this data that gives you the actual load differential per 1000 psi of hydraulic pressure.Bozzelli says he likes the load differential to be less than 5 percent/1000 psi. Anything between 5 and 10 percent can be lived with but will have to be addressed eventually. Anything above 10 percent, he says, "you must do something to fix it right away."Top 10 questions that send Bozzelli over the edgeLast month we described in some detail the passion, exuberance, and enthusiasm with which John Bozzelli carries the message of scientific molding. Never one to embrace "shortcuts" or "quick fixes" in a molding shop, Bozzelli is easily agitated, antagonized, and driven to animation by questions that suggest a molding machine should or could be operated in a manner that is less than efficient or even logical.Kendall Healthcare Products, based in Ocala, FL, is a molder that has benefited from Bozzelli's expertise; it is also a molder that has apparently caught glimpses of Bozzelli's impatience with inefficiency, impracticality, and sloth. Matt Lofgren is a molding supervisor at Kendall and wrote this list of 10 questions likely to make John Bozzelli blow.10. When our Brody Ring wears out, is it possible to grind a groove in it to fit a smaller Brody Ring, or should we just replace it?9. How many flights on our screw can be broken off before we should consider trading it out for a screw with fewer missing flights?8. How fast should we turn the screw to gain shear heat to compensate for bad heater bands?7. Most of our molds need negative pressure units to stop all of the leaks in the cores/cavities, but we still have some that leak badly. Is it possible to tie two negative pressure units together to solve this problem?6. What's the highest injection pressure to which we can safely go to compensate for a clogged filter nozzle?5.When running a mold in a press that has too little clamp tonnage, what is the best kind of knife to use to trim the flash?4. Our presses will only allow nine eject strokes. Would it be better to put a full-time operator on the press, or call the factory to have them retrofit our machine to add another digit for ejection?3. Is kitty litter a better alternative to "floor dry" around our machines? When sucking up all the oil from the floor and sump to put back in our machines, the "floor dry" tends to float on top of the reservoir, whereas the kitty litter tends to sink. Which one is better?2. Our machine has one broken tiebar. How do we estimate how many and what thickness of shims to install under the corner of the mold base to keep it from flashing? Or, how much should we loosen the opposite tiebar to compensate?1. When molding with the scientific or decoupled method, is it better to use balsa wood and super glue when plugging off cavities, or just solder and super glue?