technology | Foam moulding
name and earlier this year the company announced it can now offer the technology as an option on its latest MacroPower line of large tonnage machines (which extend from 650 to 1,100 tonnes). The company says the Cellmould system can be
used to save material, reduce frozen-in stress and warpage, eliminate sink marks, ease mould filling and reduce clamping requirements. The system comprises a special screw and barrel drilled for the gas injector and pressure and temperature sensors, a gas regulator, pneumatic needle-type shut off nozzle, compressor, and additional control software. Wittmann Battenfeld CEO Georg Tinschert says the
company refers to the Cellmould technology as a “fine structural process”, distinguishing it from the Mucell microcellular technique.
Below: The
UK-developed retrofit
Streamoulding nozzle and controller (right) doses
water into the melt during injection to
create a foam structure
German injection moulder Schröder Kunststofftech- nik has used the Cellmould technique in production of automotive components for several years, some of which also use two-component moulding methods to combine compact and foamed elements. The company added four more Cellmould equipped machines to its park of 33 moulding units last year, using them to process mostly glass-reinforced PBT and mineral-filled PP. It reports typical density reductions of 7-10%. While most foamed moulding techniques use either
chemical blowing agents blended in with the polymer or direct injection of a gas such as nitrogen into the molten polymer in the barrel, UK-based R&D Factory is marketing an alternative solution using water as a foaming agent. The system is sold as a retrofit kit on an equipment-only basis – there are no on-going licensing or royalty fees. Marketed as Streamoulding, the patented technique
introduces water into the molten polymer using a modified nozzle, where the heat in the melt turns the
water to steam which saturates the molten plastic. R&D Factory director John Heaton says the technique offers many of the performance benefits associated with structural foam or gas-blown techniques such as Mucell, including reduced density, easier mould filling, shorter cycle time, and lower energy usage. The company claims the technique can be used with
existing moulds and requires no machine modifications beyond the installation of the new nozzle and integration of the controller unit. Heaton says corrosion is not an issue and no special mould alloys or surface treatments are required. The Streamoulding nozzle houses two diffusers, which divide the melt flow into numerous mini-melt streams. Water is introduced into the molten polymer in the first diffuser, with the flow controlled by a specially-developed valve. Auxiliary heaters allow the temperature of the melt to be adjusted in the second set of diffuser channels to compensate for the cooling caused by conversion of the water from the liquid to vapour state. The molten polymer is then passed through a static mixer to ensure consist- ency of temperature and foam content. Heaton says very little water is required to create the
foam – it can be as little as a few “pin-head” amounts. The high temperatures and pressures in the nozzle mean the injected water vaporises almost immediately and simultaneously diffuses into the core of each melt stream.
The entire process is electronically regulated. A Siemens controller controls the water injection valve, adjusting the duration and timing of injection and the delay before injection commences as required. It also controls the two heaters in the nozzle – one positioned at the tip and the other around the main body – to maintain the temperature required in the melt to maximise the foaming effect. “At the time of water injection, the temperature of the
melt falls rapidly,” says Heaton. “This reduction is recorded by the PLC and used to compute the required gain to compen-
sate for this heat loss. The gain is then spent on the
subsequent cycle. The controller anticipates the fall in melt tempera-
ture and compensates for it.” Trials carried out on a thick
oval-section kitchen tool handle (with a cross section measuring 15 mm by 25 mm) have shown a 40% weight saving using PS and up to 30% using PP. Mechanical tests carried out by the
Composites Centre at Swansea University in Wales 48 COMPOUNDING WORLD | August 2012
www.compoundingworld.com
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