MATERIALS | FOAMED SHEET


Accurate metering Lewa says that its Ecofoam system offers accurate metering of blowing agents for making foam products. The system – which is available for rental – offers users an opportunity to see the reliable quality of the extruder system and end products without obligation, says Lewa.


Depending on the intended use and desired


properties of the end product, different blowing agents are used in foam extrusion. These include carbon dioxide, propane, butane, pentane and halogenated hydrocarbons such as Freon 152a. Although their discharge pressures and tempera- tures differ, the metering of the blowing agent into the plastic melt must be precise in order to achieve a homogeneous end product. To ensure constant foam quality, the quantity of blowing agent in the Ecofoam is adjusted to the rotation speed of the extruder. Smart control technology, developed by Lewa, continuously compares the signal from the flow meter with the guide signal and regulates the rotation speed of the drive motor. Due to the pump’s pressure-stiff characteristic curve, metering remains constant even for fluctuating extruder pressure. At its core, the hermetically tight, low-mainte- nance system consists of a Lewa Ecoflow dia- phragm metering pump, which delivers the blowing agent at a pressure of 50-350 bar. Flow rate depends on the set pressure and the com- pressibility of the medium. For example, it can be 13 kg/h CO2, 8 kg/h i-butane or 20 kg/h H2O at 250 bar. Since the testing system was designed for


Right: Lewa’s Ecofoam system offers accurate metering of blowing agents for making foam products


all known blowing agents, it is already explosion- proof for flammable media. To ensure that the system can be easily trans-


ported for testing purposes, all components are mounted on a common mount base. It can be rented for up to six weeks without obligation, though longer periods are also possible. This gives users the chance to test the reliability of the system and the consistently precise metering of different blowing agents in a real application environment.


Optimum solution SML says its PP sheet lines are an optimum solution for making foamed sheet from PP in higher volumes. “In direct response to customer requirements, we have optimised and upgraded specific compo- nents of our PP sheet lines for manufacturing high volumes of foamed sheet,” said Martin Kastner, R&D engineer at SML. “We did this without com- promising the lines’ high efficiency and quality.” The total density of PP foamed sheet that can be achieved is as low as 0.55 g/cm³, referring to a sheet made from standard polymers with an A/B/A structure, consisting of 20% conventional PP in the outer A-layers and 80% foamed PP in the inner core layer (B layer). Compared with regular PP sheet, this corresponds to a reduction of 40%. SML’s sheet line has a coextrusion system with a 180/33 single-screw extruder as the main extruder on which the foamed B-layer can be manufactured. It achieves a plasticising capacity of up to 1,250 kg/h with a mixture of virgin PP and up to 80 % regrind. An HSE 75/37V semi-high-speed extruder serves as the co-extruder (for the A-layers). The complete extrusion system of the line delivers a maximum plasticising capacity of up to 1,850 kg/h. For production of foamed PP sheet, the screw of the main extruder was given a special design. In addition, the extrusion system is fitted with a foaming technology package customised for PP in the melt pipes of the main extruder. Roll stack is also a key element of the line – and


here, SML’s technology is adapted to the process- ing of conventional and foamed PP. For instance, its Smart Parallel Gap (SPG) roller technology adjusts thickness profiles and allows Quick changes of sheet thickness with minimal die adjustment. “The addition of small quantities of inert gases such as CO2 or N2 and a nucleating agent into the melt enables the creation of huge quantities of fine microcells in the core layer (B-layer) of the A/B/A sheet,” said Kastner. Thermoforming companies are starting to discover the potential of foamed sheet for packaging, says SML.


� 22 FILM & SHEET EXTRUSION | November/December 2024 www.filmandsheet.com


IMAGE: LEWA


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