Advanced mold performance with hot runners

The requirements of the injection molding industry are quickly evolving. What was deemed good enough only a year ago won''t cut it with today''s market. This continuous change is affecting all aspects of the injection molding process, including machine, mold, resin, and hot runners.

Recent advancements in hot runner products, technologies, and processes are helping molders deal with the changes and realize better part quality and reduced overall part costs. When properly integrated into a system, hot runners can provide cycle-time reductions, improved part quality, resin savings, higher outputs and uptime, and increased versatility-all critical requirements of today''s injection molding process.

Cycle time

Cycle time is the most critical performance parameter of an injection mold. It is influenced 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 with a 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 optimum performance characteristics for the mold. If only cycle time is being considered, the hot runner delivers the shortest cycle times.

In all of the simplified molding sequence steps shown in Figure 2, the use of a hot runner will help save time. Mold open and close will be reduced because of the reduction of the stroke to eject the part. Inject, hold, and recovery will be reduced because the amount of injected plastic is reduced.

The most significant reduction, however, will be experienced in the cooling time. The use of an 8-mm-diameter cold runner, for example, would limit the cooling time to approximately 60 seconds. A molded part with an approximate wall thickness of 2.5 mm would only require a 13-second cooling time. With the elimination of the cold runner, the cooling time could be reduced by more than two-thirds.

Secondary savings in machine injection pressure and energy, and the elimination of auxiliary 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 reduce cycle time. There are two principal gating methods (Figure 3) for hot runners: thermal and 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 industry shortly after the first hot runner systems appeared on the market. Valve gates provide a mechanical shutoff for the gate area-a valve stem in the nozzle melt channel opens and closes the gate area. The valve stem is actuated either pneumatically or hydraulically. Until recently, valve gates had been chosen primarily for applications where thermal gate vestige is unacceptable.

However, valve gates offer several additional part quality and processing benefits to the injection molder, including:

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 sequential valve gating. Superior molding processes for thin-wall parts.Some resins are limited to only one of the gating methods. Other applications can be done with either one. Figure 5 shows the cycle time difference using a hot tip and a valve 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 positive influence 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 gate nozzle, and melt plastication can begin as soon as the valve gate is closed. Also, the valve gate''s lower shear rate in the gate area minimizes shear heating of the melt, reducing the part''s cooling requirements.Thermal gates close off the resin flow by thermally freezing off the gate. This is a more cost-effective gate shutoff method, but without the same performance characteristics as a valve gate hot runner. Thermally gated molds must have a sufficiently frozen gate before hold pressure can be released and screw recovery begins. This difference can add several seconds to the cycle time for parts with large gates.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, in the range of .5 second or less, are necessary to fill the cavity before the frozen layer thickens and prevents further cavity filling. Valve gates are ideally suited to meet these requirements. The large gate diameters with no flow restrictions allow fast filling, while minimizing pressure drop and shear heating. In many thin-wall applications, rapid part cooling permits the valve stem to close immediately after cavity 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 thickness and part weight. This can influence the cooling behavior of the part and the time required to solidify the plastic to a condition that allows stable ejection.If the cycle time can''t be further reduced because of the limitations of the part, but more output is required, a stack mold can be an alternative. A stack mold can be considered as two single-face molds mounted back-to-back. One core half is mounted to the injection side machine platen, while the other is mounted to the clamp side machine platen. The resin is transferred from the machine nozzle to the manifold on the center platen by a sprue bar.Figure 6 (opposite) shows the concept of a stack hot runner. Stack molding offers a range of benefits. The output doubles compared to a single-face mold while seeing little increase in clamp force. Fewer machines are needed to produce the required output, which leads to reduced utility and maintenance costs. Overall, the capital investment, and ultimately the part cost, will be reduced.Stack molding can increase competitiveness, if done properly. But not every part can be stack-molded. It is important that there is synergy between the machine, mold, hot runner, and part design. The hot runner used in stack molding is differentiated in the way the resin is delivered into the cavity. There are three basic resin delivery concepts in stack molding: center entry sprue bar, offset center sprue bar, and stack platen concept.The center entry sprue bar (Figure 7, opposite) is the most commonly used. The mold concept must be done in this way so that the sprue bar in the center does not interfere with part fall or any take-out device.The offset center sprue bar (Figure 8, opposite) is used in mold designs when absolutely no interfering geometry can be allowed in the center of the mold. The melt delivery 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 entry sprue 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 solution delivers production flexibility as molds for different parts can be used within certain limitations, 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-platen solution depends on volume requirements. Figure 10 shows how each solution has varying impacts on part price.UptimeUptime is the amount of time that a system is producing acceptable parts. A hot runner increases the ability of a tool to produce good parts by providing tighter control on the volume and by improving the quality of resin being delivered to the cavity. As well, a hot runner reduces the wear and tear on a mold by distributing the injection forces over multiple cavities as opposed to direct injection from the machine to the mold using a cold runner. Reducing mold wear allows a mold to run for longer periods of time between scheduled mold maintenance.While cold runners traditionally have provided high uptime thanks to a relatively simple design, it''s harder for them to meet the requirements of today''s advanced molding operations. That''s where hot runners make a difference-they can meet the cycle time and gate quality requirements of a highly competitive global molding market.In order to select a hot runner system for maximum uptime, ask for the following from your hot runner supplier: Proven, robust hot runner design.A design optimized specifically for your application in terms of pressure drop, shear rate, color change, etc.Ease of start up (especially with valve gates) and optimal balance.Ease of maintenance-easy replacement of wear components, as well as heaters and thermocouples.Global 24-hour technical service and spare parts availability.Part qualityThe quality of a manufactured part depends on uniform properties of the material within the part. In plastics processing, this uniformity results from melt homogeneity during the shaping phase of the process.Melt homogeneity is obtained by mixing. There are two types of mixing: distributive mixing 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., machine barrel and screw); a static mixer uses flow pressure to perform the mixing. Most static mixers used in polymer processing to improve melt homogeneity are distributive mixers. Homogenous melt improves part quality by providing uniform shrinkage, reduced warpage, and improved optical qualities.An added benefit of a mixer is improved color distribution. This can be achieved by installing a mixer in the nozzle tip (Figure 11, p. 57). This provides the following advantages: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 comparison between the number of shots required changing a 1.5g PP closure from black to yellow 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 lines created by the hot runner. In Figure 13, the flow line created by the valve stem for a perlescent application has been completely removed.SummaryFaster cycle times, higher output using stack molds, increased uptime, and improved part quality are the direct benefits of a properly designed hot runner system.Long-term future investments will also be reduced by using smaller machines and less auxiliary equipment. And although hot runners can represent a large capital investment when building a mold, the return can be realized soon after running the tool in production.Martin Baumann, sales and marketing manager, Husky Injection Molding Systems [email protected]Contact informationHusky Injection Molding Systems+1 905-951-5000www.husky.ca