When people unfamiliar with plastics processing think of injection molding, they most likely imagine hot, liquid plastic simply being forced by a machine into a mold. The plastic cools and solidifies in the shape of the mold cavity and it’s done. But in many applications, a device is used to mediate the plastic between the injection-mold machine and the mold itself. That’s the hot runner. They are precisely heated and end in nozzles that carefully control the delivery of the material into the chambers of the mold. The hot runner system ensures that the plastic is delivered evenly throughout the mold at a temperature and rate that will result in a defect-free part.
Hot runners also minimize waste of what can be very expensive plastics. With precision nozzle control, a hot runner delivers no more plastic than necessary into the mold, and the plastic that remains in its manifold is still liquid, ready for further use.
One supplier of hot runners is Milacron/Mold-Masters. “We work with mold-makers to optimize the delivery of the plastic,” Sudheer Thrissileri, director of Global Hot Runner Systems for the company, says. “We do preliminary CAE analysis of pressure drop, thermal profile, gate location—all of which are critical to make sure the thermal profile suits the application. Without changing thermal characteristics, if it is a multi-cavity or multi-location mold, we make sure that it fills evenly, without losing temperature, and no degradation, discoloration or any other abnormality occurs while filling.” Each hot runner is customized to work with a specific mold.
As new resins are developed, some clever automotive engineer will eventually redesign a part to maximize the advantage the new material provides. That can mean the design of a more complex mold—requiring, in turn, more complex processing technology, including the hot-runner system.
Mold-Masters has developed a new hot runner series, the DURA+, which was formally introduced in late 2017. The system, which is used in the production of automotive lighting fixtures, is a prime example of how changes in materials can result in technical innovation in the equipment that processes it.
“LED lamps have only been in the industry for 10 years or so, but the style has taken over completely,” Thrissileri notes. “They are very appealing to the driver. But for the molder, the style is extremely complex. The headlamp must pass stringent quality requirements for function and appearance—no discoloration or marks. It’s a complex design for the moldmaker. It requires a pristine mold and high-quality hot runner system to support it. There is a lot of technology that is behind that finished product.”
Mold-Masters has been providing hot runners for several years that help mold clear, flawless, strong lenses. But when customers made the move from an older thermoplastic to polycarbonate (PC) for headlamp lenses and other lighting fixtures, they had to develop a new system. “We had to re-engineer the DURA to meet their changing needs,” he said.
The reason: PC is remarkably low in weight and has high impact resistance and high heat resistance. A customer doing nothing other than switching to PC would be able to create a superior—lighter and stronger—product. But why stop there? Because PC is stronger, using it enabled automakers to redesign parts such as taillight covers to have thinner walls for even further lightweighting. Thinner walls call for higher injection pressure to be kept under control.
You can’t use a garden hose nozzle to control firehose pressure.
Another challenge: PC is more abrasive than its predecessor plastics in this kind of application. If that wasn’t enough, it’s also corrosive. It can react with steel, causing imperfections that appear as black specks on the molded part.
So as advantageous as the material is to the producer of the lenses, this required the development of highly polished stainless-steel nozzles and manifold for the hot runner system to handle it.
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