50 ways to cut your injection molding cycle time.
What molder doesn't want to reduce cycle times? No, not the kind of shortcuts that sacrifice part quality. We're talking about cycle-time improvements that feed your bottom line, not your grinder. "Just about anyone can cut their cycle times. The real trick is doing so without ruining the part," says Donn Seres, president of Injection Molding Industries, a vendor of mold-cooling systems in Lake Orion, Mich.
With that challenge in mind, take a look at the following collection of time-and cost-saving tips. It was put together with the help of technical-service professionals and auxiliary-equipment vendors who spend much of their time coming up with the kind of cycle-time strategies that can help your process fly.
Their recommendations are organized by topic areas: the mold, the press, the material, and the auxiliaries, among others. When looking for savings, keep in mind this rule of thumb: injection molding cycle times are 5% injection, 80% cooling, and 15% ejection.
Much of what the experts offer is common sense, pure and simple. But there's probably a nugget or two of gold here for even the most experienced molders. Anyone can occasionally overlook some of the basics.
Keep in mind that most of these tips work' best as part of a comprehensive strategy. "You can never really separate the contributing factors of cycle time. They are always related," explains Blair Souder, manager of injection molding at GE Plastics' Polymer Process Development Center in Pittsfield, Mass.
In fact, some of the items here don't have much of a direct influence on cycle time - but their indirect influence can be helpful or harmful. To take a commonly cited example, some molders compensate for improperly dried resin by adding excess heat to the process, which in turn extends cycle times. Or consider screw selection: Rich Lair, product support manager at Hoechst Technical Polymers in Summit, N.J., points out that molders may try to overcome poor mixing by increasing barrel heat or backpressure, both of which can needlessly lengthen cycle times. "Many molders stick with the general-purpose screw all the time, but they may need to choose the right screw for a given application," says Lair.
Given the vast differences among molding applications, every tip here won't apply across the board, but the bulk of them can make a difference in just about any process. John Morine, director of engineering for M.A. Hanna Resin Distribution, Lemont, Ill., believes that the biggest and most accessible cycle-time reductions come from improvements in mold cooling, barrel heating, and resin drying, as well as from holding to proper temperature settings. "Making sure you have these things right can solve most of your problems," he advises.
After exhausting the quick fixes, you might still find yourself up against the cycle-time wall. You might then be ready to try more involved and costly measures to achieve savings. These factors come into play when you build a new tool or retrofit an old one. Brand-new or rebuilt molding machines and upgraded auxiliary equipment can also remedy long cycles in many cases.
Once you've identified and eliminated all the big time wasters in your process, there are still incremental gains to be had by focusing on material selection, screw choice, and process optimization. "That last tenth of a percent of improvement will be the toughest," notes Morine.
1 Lower your mold temperature if you can. All the technical-service professionals interviewed tell of instances where they found mold temperatures higher than were needed to provide good physical properties.
2 When processing amorphous resins, experiment with colder molds, says Rich Lair of Hoechst. Users of crystalline resins, on the other hand, need to be aware that they could reduce crystallinity - and part properties - if the mold temperature drops too much. Citing nylon as an example, Lair says, "We recommend a temperature of 150-170 F, but there are cases where molders can get by with a cooler mold if they're not concerned about losing crystallinity."
3 New metallocene poly-olefin plastomers and elastomers run well at colder-than-usual temperatures, according to Wendy Hoenig, a research leader at Dow Plastics, Midland, Mich. She says a cool mold can cut cycle times by 25-30% with these resins. The target mold temperature, she says, is 50 F on both core and cavity side - as opposed to the 80 F you might expect for standard polyethylenes (see PT, April '95, p. 60).
4 Address cooling early in the tool-design process. People too often think of cooling after the fact," warns Seres. You may want to perform a cooling analysis with one of the mold-filling simulation software packages.
5 Make sure you have a high enough coolant flow velocity in your mold. The coefficient of heat transfer can be 10-20 times greater when coolant velocity is high enough to generate turbulent rather than laminar flow, according to data provided by AEC, Inc., Wood Dale, Ill. The required velocity to achieve turbulent flow is a function of channel diameter and coolant viscosity. Since antifreeze raises cooling-water viscosity, it may be counterproductive to run your molds colder at the expense of lower flow rate. Cooling-equipment suppliers can help you determine whether you have sufficient pumping capacity.
6 Make sure you have enough coolant flow and that it is directed to the right place. "Why overcool a thin part of the mold when you have a heavy boss with no water going to it?" asks Donn Seres of Injection Molding Industries, pointing out a common mistake.
7 Don't forget to bring water to slides, lifters, or any area of the mold that comes in contact with the melt. "You're missing the boat if you leave any uncooled areas in the path of the melt," says Seres.
8 Big baffle holes are best. "People have a tendency to make their baffle holes and flow holes the same size," says Seres. That arrangement will result in a pressure loss and diminished cooling capacity. Seres says a good rule of thumb is to make the baffle hole 40% larger than the flow hole in order to get the same flow rates throughout the system.
9 Treat your cooling water to prevent scale formation in molds and cooling systems. Just 0.006-in.-thick scale can decrease heat-transfer efficiency by 30%, says an AEC source (see PT, Oct. '93, p. 54).
10 Beware of a low pressure differential between supply and return in the cooling system. "Supplemental devices," says Seres, "can mess up the return pressure." He says this flow-inhibiting condition typically occurs when beside-the-press mold chillers or heaters are temporarily turned off but still connected: They can dump high-pressure water back into the water circuit. Seres has seen extreme cases where "the water was almost flowing backwards." A good rule of thumb, he adds, is at least 5 psi differential pressure, though he recommends 20-30 psi or more.
11 Mold dehumidification is one way to get to a lower mold temperature without suffering moisture condensation that can mar your parts. Dehumidification systems pave the way to cycle-time cuts of as much as 30%, according to Mario Ranieri of Cargocaire, a maker of desiccant dehumidifiers in Amesbury, Mass. (see PT, July '96, p. 100).
12 Consider pulsed cooling, which can rapidly switch between heating and cooling cycles. Seres and others argue that it gives you the best of both worlds as far as cycle-time goes: Faster heat extraction reportedly lets you run molds hotter so they fill faster (see PT, Oct. '95, p. 49).
13 For better cooling, replace tool steel with a more thermally conductive alloy. Now that wrought, machinable beryllium copper comes in 20 and 30 HRC, it offers the same hardness as P-20 tool steel while providing five times the thermal conductivity. For example, the cycle time for a minivan radiator end tank fell from 50 to 35 sec when P-20 mold cores were replaced with Moldmax BeCu from Brush Wellman Inc., Cleveland. That time savings occurred even though the mold temperature was raised by 40 [degrees] F, reports Brush Wellman product manager Scott Smyers.
14 Make sure your water lines are as close as possible to the areas of greatest heat concentration. Smyers cites a cup as a simple example: "At the interface between the base and side wall (tip and core), the thermal energy is three times more concentrated than in other sections of the cup," he says.
15 High-tech cooling systems and highly conductive alloys can provide even more benefit when used together. "Combine pulsed cooling with beryllium-copper inserts, and you have an animal of a tool," says Smyers.
16 Beware of thick sprues. "What controls cycle time? Often 50% of it is the sprue," notes GE's Souder. "You end up waiting for the sprue to cool." Switching to hot runners is one way to avoid the problem. Another is to select a higher-flow resin or optimize sprue design beforehand with a mold-filling analysis.
SAVINGS AT THE PRESS
17 Don't run your screw recovery faster than you have to. Rick Shaffer, product manager at Van Dorn Demag Corp. in Strongsville, Ohio, has seen cases where injection molders rev up their screws to speed recovery, but that adds enough shear heat to extend cooling times. He recalls an example where the molder had a 10-sec cooling time and a 3.5-sec recovery time. "That put too much heat into the resin more quickly than was needed for a good-quality melt," Shaffer says. "By bringing recovery time in line with the cooling time, you can reduce the overall cycle slightly," he advises.
18 Pick the right screw for your molding job. "You want a screw that will give you the melt quality you need at the lowest temperature. If you don't put in heat in the first place, you won't have to take it out later," says Schaffer. For polyolefins, he says, a long screw (25:1 L/D) with a generous transition zone usually works best. Crystalline resins call for a screw with about a 4:1 compression zone, according to Hoechst's Lair.
19 Pay close attention to temperature setpoints on the feed throat and adjacent barrel zone. Shaffer has seen molders forced to extend cycle times just to compensate for small temperature fluctuations in the rear barrel zones.
20 Get your machines to do two things at once. AC servo screw drives are one way, says Shaffer. "It makes it easier for the machine to plasticate while it's injecting. That can eliminate a chunk of the recovery time." An upgraded hydraulic package on older machines can accomplish the same thing, adds Steve Schroeder, president of Epco in Toledo, Ohio. "Older machines can gain the ability to perform parallel functions with an extra pump and larger motor," he says.
21 Watch out for oil-wasting secondary functions and hydraulic-system inefficiencies. "The more efficient your hydraulic system, the more usable horsepower you can extract from your machine," says Pierre Pinet, service manager for Husky Injection Molding Systems Ltd., Bolton, Ont. He explains that secondary hydraulic functions - like core pulls - often rely on relief valves to accommodate different system pressures. He recalls an industrial container application where cycle-time reductions were thwarted by very large mold cores that were stealing oil from the system. A better strategy, says Ian Crookston of Husky's control group, is to run all auxiliary functions at system pressure, rather than at lower pressure with higher flow volume.
22 Control retrofits, such as motor-speed controllers, can cut cycle times while saving energy, too. When custom molder Summit Plastic Solutions, Inc., added motor-speed controllers to its presses in Florence, Mass., cycle times on some large older presses dropped by as much as 5%. Energy costs also dropped 45-50% (see PT, Jan. '95, p. 15).
23 Don't inject too slowly. This is one of the most common ways in which molders rob themselves of productivity, says Denes Hunkar, CEO of Hunkar Laboratories Inc., Cincinnati. Most of the time, he says, the first 3040% of the injection stroke can be accelerated to the limit of the machine's hydraulics. Fast injection reduces the melt viscosity and keeps the melt channels wide open. The overall speed benefit could be 20%, Hunkar says. Just make sure your controls allow you to back off the speed in the last 10-15% of stroke so as to prevent flashing.
24 Make sure your hold time isn't too long. Hunkar says this is the number-one reason for excessive cycle times that he encounters in the field. Most molders don't know how long to set their hold time, Hunkar says, but a cavity-pressure sensor can remedy that problem. The sensor will show you the drop-off in cavity pressure that occurs when the gate has frozen off. If your hold time extends past the gate-freeze point, "you'll be packing out your sprue, not the part," cautions GE's Souder.
25 Eliminate unneeded ejector strokes. One way to do this is to use machine vision to make sure the parts have left the mold. These devices can detect whether all parts are ejected in a fraction of a second - faster than the parts can fall clear of the mold (see PT, Feb. '96, p. 50).
26 Invest in mold-filling and cooling simulations. They can predict the speeds of filling and cooling, among other elements of tool design. "Simulation helps you evaluate the effects on cycle time of reduced wall thickness, optimized cooling-channel layout, or hot-runner conversion," says Jim Spann, marketing manager of software supplier C-Mold in Louisville, Ky. In a recent case, C-Mold customer James Hardie Irrigation of Laguna Niguel, Calif., used simulation to revise the tool design for an existing medical part. The new design helped it shave 15% off the original cycle time, Spann reports.
27 If you're using hot runners, think about using dedicated gate cooling, where cooling channels are built into the gate region. "These systems can be significantly faster," says Dario Vettor of hot-runner supplier Mold-Masters Ltd., Georgetown, Ont.
28 On high-cavitation tools, don't run gate cooling in a series-type circuit or the gates at the end of the circuit will run hotter than the gates at the beginning. Such a temperature differential could lengthen your cycles by 1-2%.
29 Use hot-runner valve gates whenever possible. This gating style cuts cycle times because the machine can start to plasticate once the gate is mechanically shut, explains Vettor.
30 Avoid common gating mistakes. Gates that are too small make it harder to fill the cavities and add excess shear heat to the melt. That, in turn, retards cooling and can degrade the plastic.
31 Avoid mold designs that leave a large "slug" of material between subgates and short ejector pins. "That slug may be twice the thickness of the part wall and it takes time to cool," says Seres of this common gating error. Shorter cycles will result if you bring the ejector all the way to the part. "Create a wall section closer to the ejector pin," he advises.
32 Using multiple gates to fill a cavity can reduce the overall cycle substantially, says Hanna's Morine.
33 When using multiple gates to fill a long part, try sequential valve gating (or "cascade" molding) for further cycle-time reductions (see PT, May '95. p. 17).
34 Add some gas to core out thick sections. Gas-assist molding isn't just for weight reduction or stiffness enhancement - it can be used solely as a way to cut cycle time by eliminating a slow-cooling thick section of a part. One appliance molder recently retooled an existing part for gas-assist solely to boost productivity, according to C-Mold's Spann. The part cycled 30% faster (PT, March '95, p. 49).
35 Molders of nylon and polyester should not short-change themselves with improper drying. It's common to compensate for incomplete drying with longer cycle times, says Rich Lair of Hoechst Technical Polymers. But it's cheaper and better in the long run to do the drying job right in the first place.
36 Perform regular dryer maintenance. Refresh your desiccant beds. "Just because the dryer is blowing hot air doesn't mean that it's doing its job. The dewpoint could be all wrong," says Hanna's Morine. The same goes for changing dryer filters. "If you get fines and dust into the desiccant bed you can destroy its performance," says Dave Cosner, v.p. of Universal Dynamics Inc. in Woodbridge, Va.
37 Consistent materials save cycle time, resin suppliers agree. You should have uniform, consistent melt flow throughout the entire lot in order to eliminate time-wasting process adjustments, says Gene Sokolowski, senior processing specialist at Bayer Corp.'s Polymer Div., Pittsburgh.
38 If possible, switch to a higher-flow resin. "Better flow lets you cycle faster because you can run at a lower temperature and reach your 'eject' temperature faster," says GE's Souder.
39 An endothermic blowing agent can cut cycle times - even in unfoamed parts. The pressurized C[O.sub.2] gas dissolves in the melt and acts as a viscosity reducer, permitting longer flow lengths and lower melt temperatures, says Michael Reedy, president of Reedy International in Keyport, N.J. As the part shrinks, the gas also absorbs heat, cooling the part from the inside. Some of Reedy's molding customers use endothermic blowing agents to cut cooling times by as much as 5-10% (PT, March '97, p. 13).
ROBOTICS LEND A HAND
40 Conversion from semi-automatic to fully automatic production can pay off in big time savings. According to a study by Yushin America, Inc., Cranston, R.I., robot users' cycle-time reductions range from at least 15% to more than 50%, with a 30% reduction being typical. Human workers' "coffee breaks, nose scratchings, and bathroom runs" can really add up, says Yushin president John Mallon.
41 Robots shorten cooling time. Many thin-wall parts and others that damage easily when hot have extra cooling time built into the cycle. Mallon estimates that 2-3 sec can be picked up whenever a robot can ensure safe handling of hot, fragile parts.
42 Eliminate double or triple ejector strokes with a sprue picker. You could save between 0.5 and 1 sec.
43 If manual secondary operations are holding up your press cycles, a robot can help. Mallon has seen 515% shorter cycle times result when molders stop holding back their presses to pace a single operator that is performing post-mold operations.
44 Robots can beat free-drop ejection. Some of the latest high-speed side-entry robots and specialized parts-handling devices like Husky's swing chutes can now beat part-drop systems. It may be a small advantage - about 0.2 sec, says Mallon - but it could be meaningful when considered along with automation's other benefits.
SPEED BY DESIGN
45 Eliminate heavy sections from parts. Instead, use ribs or core out excess material, says Bayer's Sokolowski. Don't overdesign your fibs, either. There are well-known design tricks for keeping them lean but strong (see illustration, p. 40).
46 Thin out those walls. Thin-wall molding may have its own set of challenges, but reducing wall thickness can have a huge effect on cycle time. GE Plastics researchers report that cycle times in thin-wall molding may be only one-third of those for comparable conventional parts. GE estimates that a 3-mm-thick part with a cycle of 40 sec can be trimmed to under 20 sec at a 1-mm wall thickness.
47 Don't skimp on draft angles. "I'm always amazed at how many tools don't have enough draft," says Hanna's Morine. A good rule of thumb, according to Rich Lair at Hoechst, is to use at least 0.5 [degrees] per side of the part, though 1.5 to 3 [degrees] is preferable. "Parallel draft" can often avoid any thickening of the wall sections.
AND DON'T FORGET...
48 Even the simplest maintenance lapses can affect cycle time. Hanna's Morine has seen numerous cases of haywire thermocouples causing excessive heating and, ultimately, longer cycles. "The simplest maintenance issues will affect your cycle," he warns. "Just making sure everything on the machine and the auxiliaries works and is properly maintained will save cycle time."
49 Maintain tight process control. "Process instability leads to longer cycles," notes Van Dorn Demag's Schaffer. It takes time to overcome fluctuations in any process variable.
50 Look for new technologies. GE's Souder sees promise in research on "conformal" cooling channels that shadow the contours of the part. Other R&D at GE and elsewhere focuses on understanding little-known contributors to wasted cycle-time - such as heat-transfer problems caused by the air gap between tool and part as the plastic cools and shrinks. So just wait, the fastest cycles are yet to come.
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|Article Type:||Cover Story|
|Date:||Apr 1, 1997|
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