Quick tips for solving balance problems
By Mark Stephen, editor
The boom in construction of higher cavity tools and demand for better quality parts has put an increased emphasis on proper mold fill balancing. And it makes sense: Many part defects are the direct re...
The boom in construction of higher cavity tools and demand for better quality parts has put an increased emphasis on proper mold fill balancing. And it makes sense: Many part defects are the direct result of flow imbalance, with cavities that fill first typically being heavier and larger than the remaining parts. Flash, sink and short shots are common production headaches that can arise, wasting your time and material.
A well-built hot runner system monitored by a quality temperature controller, however, can help processors to overcome the key issues that cause mold fill imbalance and get good parts every time.
Non-uniform mold temperatures adversely affect how resin flows throughout the mold; added to that, the extra heat introduced by a hot runner can play further havoc with mold fill balancing.
Cooling at the right portion of the mold is crucial to maintaining balance and consistency from cavity to cavity, shot to shot. First, the extra heat in the centre of the tool needs to be addressed with proper cooling techniques; the correct amount of turbulence in the water flow can dramatically improve the efficiency of cooling circuits. Measure and compare the coolant temperature and flow going to each region of the tool.
Second, understand the importance of temperature control in the hot runner system. “The viscosity of the melt is directly related to temperature,” said Mike Brostedt, director of market development with GammaFlux LP. “Many hot runner problems can be solved with better temperature control. Minor balancing issues can be compensated by adjusting the temperature of the tips or other hot runner zones, highlighting the importance of individual heat-isolated control zones.”
Getting a uniform heat profile — and therefore melt temperature — can be made easier with hot runner nozzles that incorporate conductive induction coating technology (CICT), insiders say, which equalize the temperature gradient across the nozzle. Having the latest hot runner temperature control technology can’t hurt, either. Husky Injection Molding Systems’ Altanium Neo2 controllers, introduced last year, are designed to provide two to 24 zones of control and are equipped with 15 amps per zone.
“As molten plastic flows easily through the centre of the hot runner channel, the material along the perimeter is dragged against the channel wall, causing a high rate of shear,” said John Blundy, vice president, business development with Incoe Corporation. “The result can be uneven cavity filling responsible for voids, core shift, inconsistent dimensional properties and other part defects.”
By manipulating the orientation of the melt at flow intersections sections — called melt rotation — proportioned amounts of both sheared and unsheared material can be distributed to all cavities, resulting in a balanced flow. “Melt rotation helps achieve symmetry at intersections, a necessary property of the shear profile so that when split, the shear profiles of the two resulting flows are consistent and evenly distribute proportioned amounts of sheared and unsheared material,” Blundy said.
And don’t forget: Balancing the shear is more difficult in hot runner systems that don’t have a naturally balanced manifold design, because the material won’t flow through identical geom-etry from the machine nozzle to the each of the gates in the first place. The Melt Flipper system from Beaumont Technologies Inc. can help here. Inserted at the intersection of the primary and secondary runners in a standard ‘H’ pattern runner, Melt Flip-per rotates the melt approximately 90 degrees in the secondary runner, rearranging the different melt regions and nullifying the shear-induced variations.
Temperature profile inconsistencies from drop to drop in a multi-drop hot runner system can result in the need to set the temperature set points differently on each drop to achieve uniform gate opening. “It’s not uncommon to see wide variations of set points needed from drop to drop on poorly-designed systems in order to achieve a balanced system with acceptable melt flow from the drop,” said Dave Boxall, president of Ewikon Molding Technologies Inc. “In some cases, this range may be as great as 50F or more.”
Selecting hot runners that have drops designed to deliver a uniform and consistent heat profile, with drop to drop set points at or very near one another over the life of the system, is important. “Ewikon products, for example, are built with the heater pressed into a groove that is machined on the pressure body of the drop,” Boxall said. “This ensures excellent heater-to-pressure contact that delivers a consistent heat profile over the life of the product. Also, the heating profile from drop to drop doesn’t vary, resulting in uniform set points from drop to drop.”
Non-uniform venting might just be the most under-appreciated cause of mold fill imbalance, due to the common misperception that if there’s no visible burning on a part, the venting must be adequate. In fact, too many molds have virtually no uniform or adequate venting. Poor venting can lead to significant back pressure in the cavity and poor fill balance.
According to Terry L. Schwenk, president of Process & Design Technolo Technologies LLC, a good way to check for vent uniformity from cavity to cavity is to perform a pressure test at the parting line. “After clamping a fixture to the parting surface, apply air pressure and then close the valve and measure the time it takes for the air to leak our through the vents,” he said. “The test should tell you if venting is consistent from cavity to cavity.”
Hot runner balance is undermined when valve gates are opening and closing at different times for different cavities.
Invest in a linear, electric actuator for the valve gate system that has an electrically controlled open/close and motion/ position profile.
The Dynamic Feed system from Synventive Molding Solutions, for example, provides separate, closed loop process control at each gate for up to 16 nozzles. “By having a separate throttle and control valve for each nozzle, the processor can set a different packing and filling pressure profile for every cavity,” said Bill Rousseau, Synventive’s chief engineer. “This allows them to eliminate any balance problems inherent to the mold.”
Beaumont Technologies Inc. (Erie, Pa.); www.beaumontinc.com; 814-899-6390
Ewikon Molding Technologies Inc. (Rockford, Ill.); www.ewikonusa.com; 815-874-7270
Incoe Corporation (Troy, Mich.); www.incoe.com; 248-616-0220
Husky Injection Molding Systems (Bolton, Ont.); www.husky.ca; 905-951-5000
GammaFlux LP (Sterling, Va.); www.gammaflux.com; 703-471-5050
Control Solutions Inc. (Brampton, Ont.); 905-458-8382
Process & Design Technologies LLC (Kenosha, Wis.); www.processdesigntech.com; 262-237-2525
Synventive Molding Solutions (Peabody, Mass.); www.synventive.com; 1-800-367-5662
Precision Mold Supplies (Delta, B.C.); 604-943-7702