11 FACTORS FOR EFFICIENT MOLD COOLING
11 FACTORS FOR EFFICIENTMOLD COOLINGBalancing speed and quality to reducecycle time
INTRODUCTIONHow cool is that?In every minute of cycle time, cooling consumes the vast majorityof clock-ticks. That makes it an important factor in determiningthe profitability of a part. Mold and part designers understand theimplications, relying on some combination of expertise, experience,intuition, prototyping and CAE analysis to develop trusted coolingsystem designs.This approach helps practicing engineers maximize productivity.In other words, when you create a cooling system that works welland performs reliably for your application, you can spend more timedesigning high-quality molds as quickly as possible.But pressures to reduce cycle time and increase cost-efficiencyare growing more intense by the day. If you can find new ways toshorten cycle time or minimize piece part cost — without sacrificingyour quality standards — it’s a clear opportunity to build valuablecompetitive advantage.2 11 FACTORS FOR EFFICIENT MOLD COOLING
INTRODUCTIONHow cool is that?Cooling is a critical part of thisdynamic, because even small changesin cooling can have a huge impacton cycle time. This ebook offers afew simple ideas you can use torefresh your thinking about coolingsystems, maximize the efficiency oftraditional cooling systems, producehigher-quality parts more rapidly,and ultimately strengthen yourcompany’s bottom line.Mold closeFill timeEjectionPack & HoldMold openElements of theInjection Molding CycleCooling & RecoveryFigure 1: Mold cooling can account for more than two-thirds of the total cycle time in the production of injection-moldedthermoplastic parts.3 11 FACTORS FOR EFFICIENT MOLD COOLING
TABLE OF CONTENTS01: COOLING OBJECTIVESPages 5 - 10 Uniform temperature Uniform pressure Mold temperature02: BASIC COOLING STRATEGIESPages 11 -16 Channel placement Circuit design Baffles, bubblers and thermal pins03: ADVANCED COOLING STRATEGIESPages 17 - 19 High thermal conductivity inserts Conformal cooling Rapid heating04: COOLING FACTORS IN PRODUCTIONPages 20 - 22 Corrosion and scale Consistent setup05: CONCLUSIONPages 23 The role of simulation Final thoughts Sources4 11 FACTORS FOR EFFICIENT MOLD COOLING
COOLING OBJECTIVESElevating cooling system performance to an elite level hinges on the mold designer’s ability to achieve the ideal balanceof temperature and pressure.5 11 FACTORS FOR EFFICIENT MOLD COOLING
COOLING OBJECTIVESFinding your balanceThink about the perfect injection molding scenario: theram moves forward, plastic flows into a mold at theglass transition temperature of the polymer, the moldcomes up to pressure and fills, and the hot polymercools instantaneously to proper ejection temperature ina uniform manner.This, of course, never happens. Physics gets in the way.There is always some variation in melt temperaturecoming from the barrel of the injection machine.Complex parts feature thinner and thicker sections,asymmetries, and cores, all of which make it difficult6 11 FACTORS FOR EFFICIENT MOLD COOLINGto ensure consistent heat transfer and even cooling.Coolant pressure affects flow rates and turbulence,creating hot spots. The mold material has a profoundimpact on heat exchange. And even if everything workedperfectly six weeks ago, the system may performdifferently due to a new batch of material, a differentchiller, corrosion, scale, or leaks in the mold itself.Efficient cooling systems help overcome theserealities, enabling mold designers to balancetradeoffs of temperature, pressure and time to meetthe ultimate objective: lower cycle time. Here arethree factors to consider in your designs.
COOLING OBJECTIVESUniform temperatureCycle time is most influenced by changes in moldtemperature. This is why maintaining a uniformtemperature is so important to mold designers. Onechallenge in this respect is the temperature differentialbetween the core and the cavity.As the polymer melt cools, it tends to shrink toward thecore side. Because more material is in contact with thecore side, more heat will escape through the core thanthe cavity, which is why the core side requires moreefficient cooling. If the gradient is too high, warpage isthe inevitable result.7 11 FACTORS FOR EFFICIENT MOLD COOLINGMold designers should strive for a differential ofno more than 5ºC between the core and cavity. Thisdepends on many factors, but material selectiontops the list. When the thermal conductivity of themold changes, so does temperature uniformity anddifferential.The two factors engineers must consider are speedand consistency. Shorter cycle times increase partwarpage with less expensive mold materials like H13and grade 420 stainless steel (see Fig. 2). Longer cycletimes reduce warpage for all materials, but add tototal manufacturing costs.
COOLING OBJECTIVESUniform temperatureIn applications with higher tolerances, mold designers can safely specify more affordable materials, knowing that thetemperature differential will likely exceed 5ºC and a predictable amount of warpage will occur. Applications with tightertolerances or higher overall volume will need extended cycle time or a more expensive material, such as copper alloy, tostay within the 5ºC limit.2.0KEYPart warpage (mm)1.6H13 (Tool Steel)420SS (Stainless Steel)1.2C17200 (Beryllium Copper)0.8C18000 (Copper Chromium Nickel Silicon)C17510 (Beryllium Copper)0.40.067891011Cycle time (sec)Figure 2: Core materials have a significant effect on part warpage in the first 10 seconds of cooling time.8 11 FACTORS FOR EFFICIENT MOLD COOLING12
COOLING OBJECTIVESUniform pressureLike temperature, pressure within cooling lines shouldbe kept as uniform as possible. Ideally, mold designersshould aim for a maximum pressure drop of 5 psi acrossthe mold. With uniform pressure in all branches ofthe cooling system, coolant will flow with sufficientturbulence and blockages will be easier to detect. Ifpressure drops more than 5 psi, the Reynolds numberwill fall below the desired level, creating hot spots thatcompromise thermal balance.Turbulent flow is critical for efficient cooling.Unlike laminar flow, which transfers heat onlythrough conduction, turbulent flow transfers heatby both conduction and convection, which increasesefficiency dramatically.The critical point to reach is a Reynolds number greaterthan 10,000. Once this level of turbulence is achieved,the increase of heat transfer will diminish as coolantflow becomes greater, so there is no need to expend9 11 FACTORS FOR EFFICIENT MOLD COOLINGany energy to exceed it. In other words, any smallimprovement in heat transfer will be offset by ahigher pressure drop across the cooling channels,along with more pumping expense.Achieving a Reynolds number greater than 10,000requires a flow rate of 2.35 GPM for 3/8" NPT pipe.Note that for a 50% glycol mix, this flow rate needsto be doubled. Of course, the required flow rate in anyapplication will depend on the coolant temperatureand pipe size.Inlets should be sized consistently with cooling linesto maintain uniform pressure. Within complex partgeometries, baffles and bubblers may be used toachieve uniform temperature in the mold. These alsoneed to have the same diameter as cooling lines, asalternative cooling devices that are too small willcause pressure drops and affect flow rates.
COOLING OBJECTIVESMold temperatureThe temperature differential between hot polymer filling the mold and the mold itself is another factor to considerwhen designing an efficient cooling system.Ideally, mold designers should plan for a difference of no more than 25%. The closer the temperature of the mold isto the melt, the better the finish quality you can expect in the part—and a gap greater than 25% will typically result inunacceptable finish quality.This range, however, does give mold designers another lever to pull when designing a mold for efficient cooling. Ifthe application does not require the best possible surface finish quality, starting with a slightly cooler mold may helpaccelerate the entire cooling process.10 11 FACTORS FOR EFFICIENT MOLD COOLING
BASIC COOLING STRATEGIESEfficient cooling systems start with a few fundamentals of mold design.11 11 FACTORS FOR EFFICIENT MOLD COOLING
Starting strongBASIC COOLING STRATEGIESAchieving uniform temperatures andpressures starts with the fundamentals.The following three points will befamiliar to many engineers, but theyare essential for optimizing coolingperformance in a way that delivers theright combination of speed, quality andcost for your application.The goal when locating cooling channels in the mold is uniform mold surfacetemperature, which is determined by cooling channel depth and pitch. Considerthe following examples of cooling channel configurations in P20 mold steelwith a water line diameter (D) of 11.1mm (7/16"), coolant temperature of 30ºCand cycle time of 17 seconds (see Fig. 3).Channel placementWith a depth of 1.0D and a pitch of 2.5D, the mold surface temperature is fairlyuniform, with a temperature difference of about 1ºC. The average temperatureis slightly less than 40ºC, or 10ºC higher than the coolant temperature. Whenthe pitch is increased dramatically (to 10D) but the depth stays the same, themold surface temperature difference increases to 25ºC with an average of 56ºC.Channels that are further apart cool less efficiently.With depth and pitch spacing equivalent at 2.5D, the mold surface temperaturedifference is nearly uniform, but the difference between coolant temperatureand the average mold surface temperature increases to just over 20ºC. Deeperchannels, even if spaced correctly, affect efficiency as well.12 11 FACTORS FOR EFFICIENT MOLD COOLING
BASIC COOLING STRATEGIESIf the depth is increased to 5D and the pitch to 10D, the mold surface temperature is uniform within 2ºC, but the averagetemperature is 46ºC hotter than the coolant. Again, uniformity is the goal, but the target is a uniform temperature that isnot significantly higher than the coolant temperature.807570Mold surfaceDepthTemperature ( tion on partDepth 5D, Pitch 10DDepth 1D, Pitch 10DDepth 2.5D, Pitch 2.5DDepth 1D, Pitch 2.5DFigure 3: Size and placement of cooling channels have distinct and predictable effects on the difference between mold and coolant temperature.13 11 FACTORS FOR EFFICIENT MOLD COOLING
BASIC COOLING STRATEGIESCircuit designMold designers have two choices for circuit design:parallel or series. Parallel cooling channels are drilledstraight through from a supply manifold to a collectionmanifold. Flow rates vary in parallel channels becauseeach one will have a slightly different flow resistance.This variation causes similar differences in heat transferefficiency, creating hot spots and making cooling uneven.Series cooling channels, which are connected in a singleloop from the coolant inlet to the outlet, are much morecommonly used for this reason. By design, a singlecooling channel that is uniform in size maintains thepreferred flow rate throughout its entire length.Series circuitParallel circuitFigure 4: Flow rates and Reynolds numbers differ widely in series and parallel circuits.14 11 FACTORS FOR EFFICIENT MOLD COOLING
BASIC COOLING STRATEGIESBaffles, bubblers, and thermal pinsCooling the core side can be especially challenging dueto protrusions, extensions, cores and other features thatare difficult or impossible to reach with conventionalcooling lines. Baffles, bubblers and thermal pins are allcooling devices that can help in this scenario.A baffle is a cooling channel drilled perpendicular to amain cooling line with a blade separating the passageinto two semicircular channels. Coolant flows in oneside of the blade from the main cooling line, turnsaround the tip and flows back. The best baffle designshave a slightly larger diameter than the main channel tomake sure the blade completely blocks the channel.15 11 FACTORS FOR EFFICIENT MOLD COOLINGA bubbler is similar to a baffle except that the bladeis replaced with a small tube. Coolant flows into thebottom of the tube and “bubbles” out of the top,like a fountain, then flows down around the outsideof the tube. Bubblers are particularly effective forcooling slender cores and are also useful for coolingflat mold sections that can’t be equipped with drilledor milled channels.
BASIC COOLING STRATEGIESA thermal pin is a third alternative, which allowsmold designers to cool cores and other featureswithout affecting coolant pressure. The pin isa sealed cylinder filled with a fluid. The fluidvaporizes as it draws heat from the mold and thencondenses as it releases the heat to the coolant(see Fig. 5). The heat transfer efficiency of a thermalpin is nearly 10 times greater than a copper tube.For good heat conduction, avoid an air gap betweenthe thermal pin and the mold or fill it with a highlyconductive sealant.Bearing Length2 x Dia. of PinFigure 5: Thermal pins offer a simple, straightforward way to cool cored-outfeatures without affecting coolant pressure.16 11 FACTORS FOR EFFICIENT MOLD COOLING
ADVANCED COOLINGSTRATEGIESOnce the fundamentals are in place, these strategies can help reduce cooling time even more.17 11 FACTORS FOR EFFICIENT MOLD COOLING
ADVANCED COOLING STRATEGIESReaching the next levelIf you’ve already mastered the basics, how can you reduce cooling time even more? By taking a fresh look at some of thenuances in mold design and mold processing. The following factors offer interesting design alternatives. Keep in mind thatwhile they tend to increase the cost of the mold, they may actually lower total operating costs, depending on your application.CONFORMAL COOLINGConformal cooling uses cooling channels that curveto follow the geometry of the part, or “conform” toits unique shape (see Fig. 6). While the concept ofconformal cooling has been understood for years,creating curved channels has only recently becomeeconomically viable due the emergence of moldmanufacturing techniques such as laser sintering,among others. Conformal cooling requires a tooledsteel mold that is less conductive than copper oraluminum, but the results are impressive. Molddesigners can expect cycle time reductions from 10%to 40% with this approach.Traditional coolingConformal coolingFigure 6: Conformal cooling channels closely follow the geometry of the part,eliminating the need to use alternative devices for cores and other protruding features.18 11 FACTORS FOR EFFICIENT MOLD COOLING
ADVANCED COOLING STRATEGIESReaching the next level19HIGH THERMAL CONDUCTIVITY INSERTSRAPID HEATINGTo cool slender cored-out features (5mm in diameter),mold designers can use inserts made from a materialwith high thermal conductivity. These inserts aretypically press-fitted into the core. They extendinto the mold base and have a cooling channel thatpasses through or touches the insert. For large corediameters (40mm and up), mold designers need toensure a positive transport of coolant with insertsthat allow the coolant to reach the tip of the corethrough a central bore then flow through a spiral toits circumference, then between the core and inserthelically to the outlet.Rapid heating seeks to quickly pre-heat the moldafter each part is ejected, bringing it closer to thepolymer’s glass transition temperature or higher.This technique helps achieve a high-gloss surfacefinish as well as stronger weld lines, fewer aestheticsurface defects and lower injection pressures.Depending on the method you choose, rapidheating can create opportunities to reduce cycletime. Heater cartridges, which are placed in themold block and heat the entire mold, are the moststraightforward and least expensive option—but alsothe slowest.The two most popular choices for inserts are copperalloy and aluminum. The tradeoff has to do with costs.Copper is more durable and will last longer, but ismore expensive initially. Aluminum costs less thancopper alloy and offers similar thermal conductivity,but it is much softer, more susceptible to wear andgenerally has a shorter life span.Injecting pressurized high-temperature steam into thecooling channels, then removing it with compressedair before the coolant flows through, is more efficient.Induction heating, which heats only the surface of themold instead of the entire tool, is a third option. Withthis technique, heating is achieved by applying a highfrequency alternating current through an inductioncoil, which generates a magnetic field that induceseddy currents on surrounding metal objects. Theseelectrical currents flow in a circular path, resulting inJoule heating, or the generation of heat from a currentflowing through a conductor, typically a very thin( 1mm) layer of metal on the surface of the mold. 11 FACTORS FOR EFFICIENT MOLD COOLING
COOLING FACTORS INPRODUCTIONThe realities of the shop floor can compromise cooling in unexpected ways. Mold designers need to account for severalproduction-specific factors to achieve optimal cooling.20 11 FACTORS FOR EFFICIENT MOLD COOLING
COOLING FACTORS IN PRODUCTIONRunning into realityEven if your mold design is perfectly suited to theapplication, delivering the optimal cooling timefor part quality, surface finish and total cost, it’s adifferent story when the injection molding machinestarts running. Keep these final three factors in mindto ensure your design fulfills its potential.Cooling efficiencey loss over timeScale 0.6 - 62.30.30.10.2Cooling time vs. Lime deposits4030No lime1.0mm Lime2.0mm Lime33.52010013.528.816.68.5Thermal ConductivityW/mK10.1Cooling Time, sec.Figure 7: Even a small amount of mineral deposits in cooling lines can have asignificant effect on cycle time.21 11 FACTORS FOR EFFICIENT MOLD COOLINGCorrosion and scaleMachine operators know that small changes in injectionmolding equipment can have a significant cumulative effecton performance. Corrosion and scale, or mineral deposits,are two factors that mold designers need to be aware ofwhen it comes to setting expectations for cooling systems.Corrosion of cooling lines occurs naturally over time as thematerial interacts with its environment and oxidation occurs.Mineral deposits are similarly common, developing as coolinglines are exposed to trace minerals in the water supply.The presence of corrosion or scale in the cooling systemcan severely compromise thermal diffusivity and radicallyreduce efficiency, leading to longer than intended cycletime (see Fig. 7). To avoid this situation, make sure tospecify non-corroding materials for all circuits, such ascopper, Type 420 stainless steel or an electroless nickelplated aluminum.Mold designers can also work with machine operators tomake sure appropriate steps are taken to treat (filter) thecooling water and clean cooling lines regularly.
COOLING FACTORS IN PRODUCTIONConsistent setupNot every mold has a finite run. Many complete oneproduction run, get shelved, then re-installed weeks ormonths later for another run. Even if this gap is one day,the setup process needs to stay consistent from run torun for cooling time to meet expectations.Placement of inlets and outlets, for example, shouldstay the same. This is especially important in morecomplex cooling systems with more than a dozen ports.Changing the placement of inlets and outlets can causeslight deviations in pressure that affect the uniformityof mold temperature. Other factors the mold designersdon’t control, such as the particular pump or chiller22 11 FACTORS FOR EFFICIENT MOLD COOLINGused in a given run, can affect coolant temperature aswell. Ideally, the temperature of the coolant shouldnot rise more than 5ºC from the time it enters themold to the time it leaves.A best practice for ensuring consistency is mappingthe surface temperature during setup. Using asurface temperature gauge, me
The goal when locating cooling channels in the mold is uniform mold surface temperature, which is determined by cooling channel depth and pitch. Consider the following examples of cooling channel configurations in P20 mold steel with a water line diameter (D) of 11.1mm (7/16"), coolant tempe
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