Advanced mold performance with hot runnersDocument Transcript
Advanced mold performance with hot runnersBy adminPublished: December 31st, 2005The requirements of the injection molding industry are quickly evolving. What wasdeemed good enough only a year ago wont cut it with todays market. Thiscontinuous change is affecting all aspects of the injection molding process, includingmachine, mold, resin, and hot runners.Recent advancements in hot runner products, technologies, and processes arehelping molders deal with the changes and realize better part quality and reducedoverall part costs. When properly integrated into a system, hot runners can providecycle-time reductions, improved part quality, resin savings, higher outputs anduptime, and increased versatility-all critical requirements of todays injection moldingprocess.Cycle timeCycle time is the most critical performance parameter of an injection mold. It isinfluenced by many factors, including machine, resin, mold design, mold material,and the design of the runner system.The mold can be designed with three runner variations: cold runner, direct gating witha hot runner, or a hybrid solution where a hot runner is gating into a cold runner.Figure 1 provides an overview of the pros and cons of the different options.Careful evaluation is needed to determine which solution delivers the optimumperformance characteristics for the mold. If only cycle time is being considered, thehot runner delivers the shortest cycle times.In all of the simplified molding sequence steps shown in Figure 2, the use of a hotrunner will help save time. Mold open and close will be reduced because of thereduction of the stroke to eject the part. Inject, hold, and recovery will be reducedbecause the amount of injected plastic is reduced.The most significant reduction, however, will be experienced in the cooling time. Theuse of an 8-mm-diameter cold runner, for example, would limit the cooling time toapproximately 60 seconds. A molded part with an approximate wall thickness of 2.5mm would only require a 13-second cooling time. With the elimination of the coldrunner, the cooling time could be reduced by more than two-thirds.
Secondary savings in machine injection pressure and energy, and the elimination ofauxiliary equipment, such as grinders or pickers, reduce cycle time even further.Along with the use of a hot runner, the selected gating method can also help reducecycle time. There are two principal gating methods (Figure 3) for hot runners: thermaland valve gate. The application and part determine which gating method is used.Figure 4 provides some primary selection criteria.Valve gate hot runner nozzles were introduced to the injection molding industryshortly after the first hot runner systems appeared on the market. Valve gatesprovide a mechanical shutoff for the gate area-a valve stem in the nozzle meltchannel opens and closes the gate area. The valve stem is actuated eitherpneumatically or hydraulically. Until recently, valve gates had been chosen primarilyfor applications where thermal gate vestige is unacceptable.However, valve gates offer several additional part quality and processing benefits tothe injection molder, including:t Elimination of drool and gate string. Improved physical properties with lower molded-in stress. Cycle time reduction. The ability to balance family molds and control weld line location with sequentialvalve gating.v Superior molding processes for thin-wall parts.Some resins are limited to only one of the gating methods. Other applications can bedone with either one. Figure 5 shows the cycle time difference using a hot tip and avalve gate in the same application.As Figure 5 shows, the cycle time improvement is 6% for this specific application.And the 76% reduction in injection pressure will have a tremendously positiveinfluence on mold reliability and system uptime.In addition to the cosmetic advantages, valve gate nozzles can reduce cycle time,especially when molding large parts. Hold time can be reduced with a valve gatenozzle, and melt plastication can begin as soon as the valve gate is closed. Also, thevalve gates lower shear rate in the gate area minimizes shear heating of the melt,reducing the parts cooling requirements.Thermal gates close off the resin flow by thermally freezing off the gate. This is amore cost-effective gate shutoff method, but without the same performancecharacteristics as a valve gate hot runner. Thermally gated molds must have asufficiently frozen gate before hold pressure can be released and screw recoverybegins. This difference can add several seconds to the cycle time for parts with largegates.
Valve gate nozzles can be especially useful for thin-wall molding. Rapid fill rates,high pressures, and fast cooling characterize thin-wall applications. Rapid fill rates, inthe range of .5 second or less, are necessary to fill the cavity before the frozen layerthickens and prevents further cavity filling. Valve gates are ideally suited to meetthese requirements. The large gate diameters with no flow restrictions allow fastfilling, while minimizing pressure drop and shear heating. In many thin-wallapplications, rapid part cooling permits the valve stem to close immediately aftercavity fill. A thermal gate would require cool time for proper solidification.OutputDecreasing cycle time can be one alternative in increasing the output of a mold.However, cycle time can be restricted by part design, especially by part thicknessand part weight. This can influence the cooling behavior of the part and the timerequired to solidify the plastic to a condition that allows stable ejection.If the cycle time cant be further reduced because of the limitations of the part, butmore output is required, a stack mold can be an alternative. A stack mold can beconsidered as two single-face molds mounted back-to-back. One core half ismounted to the injection side machine platen, while the other is mounted to the clampside machine platen. The resin is transferred from the machine nozzle to the manifoldon the center platen by a sprue bar.Figure 6 (opposite) shows the concept of a stack hot runner. Stack molding offers arange of benefits. The output doubles compared to a single-face mold while seeinglittle increase in clamp force. Fewer machines are needed to produce the requiredoutput, which leads to reduced utility and maintenance costs. Overall, the capitalinvestment, and ultimately the part cost, will be reduced.Stack molding can increase competitiveness, if done properly. But not every part canbe stack-molded. It is important that there is synergy between the machine, mold, hotrunner, and part design. The hot runner used in stack molding is differentiated in theway the resin is delivered into the cavity. There are three basic resin deliveryconcepts in stack molding: center entry sprue bar, offset center sprue bar, and stackplaten concept.The center entry sprue bar (Figure 7, opposite) is the most commonly used. Themold concept must be done in this way so that the sprue bar in the center does notinterfere with part fall or any take-out device.The offset center sprue bar (Figure 8, opposite) is used in mold designs whenabsolutely no interfering geometry can be allowed in the center of the mold. The meltdelivery is offset to either the bottom or the non-operator side of the mold.The stack platen concept (Figure 9) is similar in design to the offset center entrysprue bar. The difference is within the center section of the hot runner, which allows
two independent single-face molds to be used in a stack configuration. This solutiondelivers production flexibility as molds for different parts can be used within certainlimitations, molds can be interchanged, and one side can even be shutdown.The decision to use a single-face system, a stack-mold solution, or a stack-platensolution depends on volume requirements. Figure 10 shows how each solution hasvarying impacts on part price.UptimeUptime is the amount of time that a system is producing acceptable parts. A hotrunner increases the ability of a tool to produce good parts by providing tightercontrol on the volume and by improving the quality of resin being delivered to thecavity. As well, a hot runner reduces the wear and tear on a mold by distributing theinjection forces over multiple cavities as opposed to direct injection from the machineto the mold using a cold runner. Reducing mold wear allows a mold to run for longerperiods of time between scheduled mold maintenance.While cold runners traditionally have provided high uptime thanks to a relativelysimple design, its harder for them to meet the requirements of todays advancedmolding operations. Thats where hot runners make a difference-they can meet thecycle time and gate quality requirements of a highly competitive global moldingmarket.In order to select a hot runner system for maximum uptime, ask for the following fromyour hot runner supplier:y Proven, robust hot runner design. A design optimized specifically for your application in terms of pressure drop,shear rate, color change, etc.s Ease of start up (especially with valve gates) and optimal balance. Ease of maintenance-easy replacement of wear components, as well as heatersand thermocouples.a Global 24-hour technical service and spare parts availability.Part qualityThe quality of a manufactured part depends on uniform properties of the materialwithin the part. In plastics processing, this uniformity results from melt homogeneityduring the shaping phase of the process.Melt homogeneity is obtained by mixing. There are two types of mixing: distributivemixing and dispersive mixing. Mixing can also be classified in two categories:dynamic and static.A dynamic mixer is usually a mechanism with rotating components (i.e., machinebarrel and screw); a static mixer uses flow pressure to perform the mixing. Most
static mixers used in polymer processing to improve melt homogeneity aredistributive mixers. Homogenous melt improves part quality by providing uniformshrinkage, reduced warpage, and improved optical qualities.An added benefit of a mixer is improved color distribution. This can be achieved byinstalling a mixer in the nozzle tip (Figure 11, p. 57). This provides the followingadvantages:a Reduced color change time (60 to 80 shots is typical for PP). Improved melt mixing and color dispersion. Flowline-free metallic and pearlescent parts. Improved melt homogeneity and color dispersion. Uniform filler/reinforcement orientation.No preferential flowColor change can have a significant impact on cost. Figure 12 shows a comparisonbetween the number of shots required changing a 1.5g PP closure from black toyellow in a two-drop system using a triverted tip and a Husky UltraFlow tip.The use of an UltraFlow tip from Husky, for example, eliminates the flow/weld linescreated by the hot runner. In Figure 13, the flow line created by the valve stem for aperlescent application has been completely removed.SummaryFaster cycle times, higher output using stack molds, increased uptime, and improvedpart quality are the direct benefits of a properly designed hot runner system.Long-term future investments will also be reduced by using smaller machines andless auxiliary equipment. And although hot runners can represent a large capitalinvestment when building a mold, the return can be realized soon after running thetool in production.Martin Baumann, sales and marketing manager, Husky Injection Molding Systemsbaumann@husky.caContact informationHusky Injection Molding Systems+1 905-951-5000www.husky.ca