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In high cavitation molding, achieving consistent part quality from cavity-to-cavity can be a challenge if the system is not optimized to produce consistent parts shot-after-shot. Dealing with the many variables in the molding process remains one of the biggest challenges a molder faces every day. Yet producing consistent parts is exactly what a plastic injection molding system should do. The ultimate goal is to have a tool that is not only repeatable, but one that can economically produce identical parts across all cavities for the life of the system.

Clare Goldsberry

November 8, 2013

4 Min Read
Hot runner technology for optimal part-to-part consistency

Part-to-part inconsistency can manifest itself in a variety of ways, from short shots to flash, to dimensional variation, to sinks and warp, to mechanical or optical differences. These inconsistencies can limit the process window, lengthen the mold qualification time, or increase costs associated with poor quality, including scrap rate, inspections, warranty, and more.

To optimize part quality and avoid increased production costs, a focus on minimizing variability is key. Reducing variability in the molding system results in a more capable tool and improved production process. In turn, producing more parts within specification and enabling higher cavitation molding for reduced part price improves customer satisfaction and profitability.

First, let's look at some sources of variability in the molding process. Variability in a plastic injection molding system can come from the process, machine, mold, hot runner, auxiliaries, as well as the resin and its preparation, i.e. was it dried properly.  Since each component of the system can introduce its own variables, it is important to review and address the entire system.

The hot runner is a key component of the melt delivery system, which means to maximize productivity it is important for manufacturers to work with hot runner suppliers who can minimize sources of variability and offer products that reduce risk and deliver consistent melt to each cavity, shot-after-shot.

A well-designed hot runner is 100% geometrically balanced with symmetrical flow lengths and diameters for equal pressure loss to each cavity. Channel sizes and layout are engineered to balance hot runner pressure drop, shear rates, and residence time for the specific application.  In a Husky-designed and built hot runner system, the shear history is addressed by changing melt channel levels within the manifold. This is done simply and effectively through rotating the melt to avoid any preferential flow to certain cavities. Proper hot runner design that includes geometric balance provides shear management and opens the molding process window so the tool can run successfully.

Complementing geometric balance, Husky's recent improvements include thermal uniformity of the hot runner, precise temperature control and synchronized valve gate open and close. The goal of these improvements is to eliminate hot runner sources of variability within the molding system to achieve the most consistent mold filling resulting in more conforming parts. Prior to shipment Husky ensures hot runner thermal uniformity with thermal finite element analysis (FEA), precise manifold heater position and installation with automated tools and 100% thermal and mechanical verification of the assembled system.

Thermally uniform hardware is only as good as the temperature controller, which must be considered when reviewing hot runner options. Poor temperature control can drive thermal variability in the system, affecting part consistency. Husky's family of Altanium temperature controllers provides precise temperature control to deliver accurate temperature measurement, optimized control algorithm, and rapid reaction times.

A control algorithm is only as effective as the accuracy of the data its calculations are based on. Altanium controllers utilize isolated thermocouple inputs to eliminate the effects of electrical noise and ensure a true temperature signal. The control algorithm is essentially the brains of the operation and contains the instructions for the amount of power to apply based on feedback from thermocouples. Altanium controllers automatically optimize the control of each heater to a specific operating environment so that each zone receives exactly the amount of power needed to maintain a uniform set point.

Finally, reaction time is important. The longer it takes to process a command, the farther out of tolerance of the temperature will be once the calculation from the algorithm is executed. Altanium controllers use distributed control architecture to minimize the distance signals must travel, so that commands are carried out in the shortest time possible.

In addition to minimizing geometric and thermal sources of hot runner variability, Husky has also eliminated variability of gate open and close with UltraSync valve gate plate actuation technology. Standard individual pneumatic valve gates provide good gate control, but are still susceptible to variability in stem movement from drop-to-drop. To eliminate this risk and provide full control of the valve stems, Husky UltraSync-E technology connects all valve stems to a single plate driven by an electric servo motor. This provides 100% synchronization and allows other advanced processing techniques such as stem motion profiling, which can reduce wear and increase gate life. Full valve stem synchronization eliminates valve stem motion variability, improving part quality and consistency.

Reducing variability and optimizing for consistently high part quality and productivity in a high-cavitation mold is the goal of molders. The hot runner system is a key element in the melt delivery system to achieve this goal. Working with a supplier that has made investments that systematically minimize or eliminate hot runner sources of variability, and provides the support required to ensure molders meet part consistency and quality requirements shot-after-shot, is key to success and profitability.

Nick Tanner, Product Manager, Hot Runner Applications, Husky Injection Molding Systems, also contributed to this article.

About the Author(s)

Clare Goldsberry

Until she retired in September 2021, Clare Goldsberry reported on the plastics industry for more than 30 years. In addition to the 10,000+ articles she has written, by her own estimation, she is the author of several books, including The Business of Injection Molding: How to succeed as a custom molder and Purchasing Injection Molds: A buyers guide. Goldsberry is a member of the Plastics Pioneers Association. She reflected on her long career in "Time to Say Good-Bye."

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