thin wall moulding | Innovations Right: IVG
nozzles from MHS can be arranged
back-to-back with no offset
in stack moulds to save space
micro-cellular foaming technology for more than 20 years. MuCell involves the introduction of supercritical nitrogen or carbon dioxide into the melt during processing. It reduces injection pressure requirements, making it possible to reduce clamp tonnage, and can also achieve significant part weight reductions. As an example, Trexel cites containers made
Centre:
Rheo-Slide articulated
hotrunners can simplify filling of large thin wall mouldings with moving cores
by Coveris Rigid (previously part of Autobar) for Unilever using its SLIM ( Super Light Injection Mould- ing). Trexel says the goal was to reduce package weight by 12-15 % while using the company’s existing 300 tonne injection moulding machines and incorporating the thinnest IML labels available on the market. With the help of MuCell, it was possible to reduce wall thickness of the base and sidewalls to 0.35 mm, cut weight by 13%, and reduce clamp tonnage by 35%. The fact that packing force is provided by the foam expansion in the Mucell process also helps fill the thicker seal surface. The parts are made on 4+4 stack moulds that would otherwise have had to be mounted on machines with more than 450 tonne of clamp force.
Melt management Meanwhile, Canadian melt delivery systems supplier Mold Hotrunner Solutions (MHS) is claiming what it says are “unique and revolu- tionary” solutions in two areas of thin-wall moulding. The first relates to production of thin-wall packaging with walls of less than 0.6mm, for which the company has developed an internal valve gate nozzle. Engineer-
Right: Synven- tive’s nuGate pneumatic
technology can improve gate quality and improve process stability
ing manager Kay Thielen says that in stack moulds, valve gate nozzles typically have to be offset
within the mould because of
the actuator located on the opposite site of the manifold. So if a part is gated in the centre, the projected area inevitably increases and this may mean having to run a mould on a larger machine than it really needs. With the new internal valve gate (IVG) noz- zles, no offset is necessary since the actuator is
located in the nozzle body, and the cavities can be set exactly back to back. “With the actuator in the body of the nozzle, rather than behind it, there is free space to put a second nozzle on exactly the opposite side of the manifold,”he says. Thielen adds that an important benefit of using valve gates on stack moulds for moulding PP, for example, is that the gate size can be increased and (unlike with hot tips) it is not necessary to wait for the material to freeze before opening the mould to avoid drool. “With valve gates, the process window opens up in an interesting fashion,” he says. “Instead of having a melt temperature of, say, 240°C for a thin-wall food container it can drop down to 210°C, which is still within the processing window of the resin, and this obviously
translates into reduced cooling time.” So, for a PP part with a 0.6mm wall thickness,
instead of a typical cooling time of 1.0s in a total cycle time of around 4.0s or less it can be 0.9s. And in the field of thin wall packaging, it is not every second that counts but every tenth of a second, Thielen explains. Alternatively, the cooling time can be kept the same but shrinkage and warp behaviour can be improved. Thielen also claims that the reduced heating and
cooling can result in an energy saving in the region of 7.5 kWh per kg of resin processed. The second MHS solution relates to production of
very large thin-wall parts. The company has developed a jointed moveable manifold that makes it possible to place nozzles not only on the bottom of the A side of the mould, but also on the walls. The Rheo-Pro Slide Manifold features rotating melt transfer joints that allow linked manifold segments to move freely between
38 INJECTION WORLD | January/February 2015
www.injectionworld.com
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