Selective Foaming Offers Cost, Process Options

December 31, 1996

Coralfoam, a patented technology recently developed in England for selective foaming of moulded plastic parts, greatly increased its visibility and accelerated its licensing activities at the recent Interplas exhibition. Combining an endothermic blowing agent with specific mould design and process control technology, Coralfoam allows the foaming of selected areas of a part, thus changing the shape of the moulded part during the moulding cycle. Key benefits of the process include reduced part weight coupled with increased rigidity, faster cycle times, and thermal insulation, plus the ability to design parts not previously possible.

The beverage cups shown in the photo above were produced during the Interplas show on a Mannesmann-Demag Ergotech ET 35-120 Compact machine using Elf Atochem's Appryl High Clarity polypropylene. The tool was an existing one modified for the Coralfoam process. The foamed version is 6 to 7 percent lighter and three times stiffer than the nonfoamed one, and it is moulded in a shorter cycle time. Polished windows have been added to the tool's original matte surface to show the level of clarity possible.

Peter Clarke, the developer of the process and technical director of Pentex Sales Ltd., which is handling the marketing, noted that while modifying a current mould will work, designing a mould from scratch will yield even more dramatic results. For example, a 50 percent reduction in weight is possible while still increasing rigidity. Cycle times are fast since they are dependent on the thin-wall sections rather than the thick, foamed areas. Foaming actually occurs in fractions of a second after the mould is opened.

Standard Machinery

At another Interplas stand, a 4-inch flower pot was being moulded using Coralfoam technology on an 80-ton Netstal machine. Cycle time was 3.7 seconds for a part with a .5-mm wall section and a 6-mm foamed lip. According to Clarke, a quality standard injection machine with a precision NC controller is all that's needed, plus a good dosing unit for the foaming agent. The injection machine needs no modification. There is also the added cost for the endothermic blowing agent, which is Safoam from U.S.-based Reedy International, but that cost is more than offset by reduction in overall material costs.

Pentex/Coralfoam has been actively working with container manufacturers in the U.K., but sees other markets and products that could benefit from the process. Pallets could be a major market, particularly since European food processors want to palletize within their clean area and can't do so with wood. Then there are crates and boxes, coat hangers, razor handles, safety helmets, closures, cutlery handles, toilet seats, and more. Car bumpers made of one material instead of the usual skin plus core could greatly simplify recycling. Interior auto posts could have a thickened sealing bead. Pentex has already moulded against another material to create a thin skin with selectively placed strengthening ribs.

From the design point of view, Coralfoam offers the ability to place the foam where you want it and to have foamed sections with a high gloss surface. You can place foam selectively in transparent parts for functional or aesthetic reasons. You can also have 10-mm undercuts with no need for split cavities, or 10-mm thick ribs along with a 6-second cycle time and no worry about sink marks. The temperature control can be extended to multicavity moulds.

Gas-assist Alternative

Since Coralfoam can place either foam bubbles or voids in the selected areas, it can be seen as a competitor to gas-assisted moulding for creating certain types of lighter, stiffer parts. Pentex notes the lack of the nozzles, gas-pressure generators, and controls required for gas-assisted moulding. As a relatively low-pressure process, Coralfoam can also save cost on mould construction and permit use of a lesser tonnage clamp. For example, a production mould for a 2-liter ice cream container was modified for Coralfoam. The shot weight went from 56 to 46g and cycle time from 6.3 to 4.9 seconds. Before modification, the mould needed more than 250 tons of clamp; with Coralfoam it moved to a 175-ton press.

Production costs, says Clarke, for a mould developed from scratch will be relatively the same as if it were for normal compact material moulding. The tool modifications mentioned above cost about oe5,000 (US$ 8,400) per tool. In terms of materials applicability, Clarke says that virtually all polyolefins have been tried with good results and that the process is equally applicable to engineering materials. He is eagerly looking at applications where one can replace styrene with PP. That gives the obvious advantage of lower raw material cost per pound, plus a lower specific gravity for more parts per ton, and all while increasing rigidity. A styrene coat hanger converted to PP Coralfoam processing weighs 30 percent less and has double the stiffness.


Coralfoam and Pentex have been busily marketing the new technology, thus far primarily within the U.K. Their partnering relationships, which are aimed at providing broad support to licensees, include Netstal and Mannesmann Demag for machinery; Reedy, Montell, and ATO in materials; plus toolmakers and product designers.

Originally, production sites were to be licensed, but now licensing of processors for the production of specific product groups - for example, coat hangers - in a specific geographic area will be sought. Negotiable options are also offered that could extend up to a year to allow development. Cost of the option would vary from roughly oe30,000 to oe200,000 (US$ 50,000 to US$ 336,000), depending on the size of the market being considered. The cost of the license itself will also vary according to the market, but will consist of an initial purchase price plus royalties on production.

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