Selective foaming for strength and speed



Coralfoam, a patented technology recently developed in England for selective foaming of molded plastic parts, greatly increased its visibility and accelerated its licensing activities at the recent Interplas exhibition held in Birmingham. Combining an endothermic blowing agent with specific mold design and process control technology, Coralfoam allows the foaming of selected areas of a part, thus changing the shape of the molded part during the mold 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.
Polypropylene cups are only one of many applications that might benefit from the increased rigidity, reduced part weight, and faster cycle times allowed by the Coralfoam process.

The beverage cups shown in the accompanying photos 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. Samples of the nonfoamed cup were available for comparison. The foamed version is 6 to 7 percent lighter and three times stiffer than the previous version, and it is molded 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 mold will work, designing a mold 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 mold is opened, and temperature differential between mold halves is an important factor.

Normal Molding Machinery
At another Interplas stand, a 4-inch flower pot was being molded using Coralfoam technology on an 80-ton Netstal machine. Cycle time was 3.7 seconds for a part with a wall section of .5 mm 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 is more than offset by the reduction in overall material costs.

Pentex/Coralfoam has been actively working with container manufacturers in the U.K., but sees lots of other markets and products that could benefit from the process. Pallets could be a major market. 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 simplifying recycling. Interior auto posts could have a thickened sealing bead. Pentex has already molded 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 esthetic reasons, or 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 multi-cavity molds.

Alternative to Gas Assist
Since Coralfoam can place either foam bubbles or voids in the selected areas, it can be seen as a competitor to gas-assisted molding for creating certain types of lighter, stiffer parts. Pentex notes the lack of the nozzles, gas pressure generators, and controls required for gas-assisted molding. Being a relatively low-pressure process, Coralfoam can also save costs on mold construction and permit use of a lesser tonnage clamp. For example, a production mold 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 mold needed more than 250 tons of clamp; with Coralfoam it moved to a 175-ton press.

Selective foaming creates both insulation value and increased rigidity, without extending cycle time.

Production costs, says Clarke, for a mold developed from scratch will be relatively the same as if it were for normal compact material molding. The tool modifications mentioned above cost about $8400 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. The company is eagerly looking at applications where PP can replace styrene. That gives the obvious advantage of lower raw material cost per pound, plus a lower specific gravity for more parts per ton, all while increasing rigidity. A styrene coat hanger converted to PP Coralfoam processing weighs 30 percent less and has double the stiffness.

Marketing and Licensing
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. They have conducted numerous demonstrations and seminars, generated a lot of interest, initiated a number of developments, and will continue giving seminars, both group and private, to potential Coralfoam molders and end-users. Details of their entry into the U.S. market will be decided in early 1997. In general, they are expanding in line with their ability to give full support to licensees.

Originally, they were considering licensing production sites for the process but have changed the strategy. They are now seeking to license processors for the production of specific product groups, for example coat hangers, in a specific geographic area. They are also offering negotiable options that could extend up to a year to allow development. Cost of the option would vary from roughly $50,000 to $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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