Technology Hot runners clear the way for transparent part production

Editor: Eric Culp

High demands on processing clear thermoplastics can leave toolmakers searching for solutions. One hot runner supplier says its systems can help.

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Rising demands on automobile headlights have given mould makers even more specifications for their products.
Rising demands on automobile headlights have given mould makers even more specifications for their products.
(Source: HRS Flow)

HRSFlow has announced the introduction of a line of hot runners for polycarbonate applications that demand high aesthetic quality, such as front headlamps and large injection-compression transparent surfaces for the plastic glazing market. According to the supplier, the Diamond Lux hot runner system is suitable for any type of moulding of front headlamp components for the automotive industry, including housings, bezels and, especially, polycarbonate and polycarbonate high-heat moulded lenses.

Glass replacement spurs demand for systems

The supplier said the line also suits the plastics glazing sector. Here, quarter windows and panorama roofs – formerly manufactured in silica glass – are now being moulded in polycarbonate by using multishot technology, which allows for the integration of locating and mounting features.

See: Milacron to buy Mold-Masters

Improving thermal properties of system proves key

The line features hot runner thermal optimisation, which focuses on insulating the mould plates using spacers with low thermal conductivity and paying particular attention to power loss in the thermal bridges. All contacts toward the mould have been geometrically optimised to reduce thermal power dissipation and are made of materials with low thermal conductivity, the company said. It noted that heaters are symmetrically positioned at the top and bottom of the manifolds to reduce cold zones and high residence time. The screwed-in nozzles do not allow for any contact with the mould except for at the tip area.

The company said it has improved the thermal control of nozzles with special brass sleeve heaters. Each heater is dedicated to the thermal control of the gate zone, a step said to ensure more precise and steady process control. The high thermal conductivity of brass also helps standardise the thermal profile, the company noted.

These technical solutions are said to provide a more constant temperature, with a tolerance in the region of 10 C across the entire system. Users receive thermal analysis of the manifolds for each system to demonstrate that temperature uniformity is reliable.

The nozzle is protected by a stainless steel tube that ensures consistent temperature distribution by reducing thermal dissipation. The tube also protects the thermocouple wires. The nozzle design has been modified to minimise cut-out pocket space in the injection area.

All internal hot runner channels have been engineered to avoid stagnation spots that could cause unacceptable defects in crystal clear lenses, the supplier explained. Special shaped plugs have also been installed to further avoid any stagnation close to the valve gate pin.

Attention to detail provides prowess

Tip and end ring geometries, and the material used to produce them, have been selected to optimise gate area temperature, the supplier noted. Special attention has also been paid to the quality of the injection points in the moulded part. Furthermore, the units have been designed to provide optimal performance even for high injection pressure, according to the manufacturer.

The supplier added that the entire hot runner is made of a special steel and coating which prevents polycarbonate degradation and, as a result, the risk of unaesthetic black spots on crystal clear applications such as lenses and window panels. The result is fewer rejects and higher productivity, the company said.