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3/1/2001 | 10 MINUTE READ

Porous Molds’ Big Draw

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A new “breathable” mold metal is giving thermoformers better surface quality, finer detail, and shorter lead times. This new toolmaking material is proving its worth in both packaging and industrial applications.

Thermoform tooling without vacuum vents? It’s not an idle daydream for processors who are looking for a way to form parts with crisper detail and no telltale dimples from drilled vent holes. The latest innovation in thermoform tooling is a micro-porous, air-permeable metal composite that draws a uniform vacuum across the entire tool surface.

The tool is made from 65-90% aluminum powder and 10-35% epoxy resin binder. A proprietary process creates a block of metal that contains 15% empty space in the form of tiny interconnecting channels. On the surface, those channels terminate in pores of 15-16 microns diameter, about 131 of them per square inch. The epoxy reduces the density of the metal by 30%.

This tooling material has been produced since 1995 by Portec Ltd. in Switzerland. Portec sells the porous tooling composite in machinable blocks called Metapor and in a castable form dubbed Espor. It has been offered in the United States for the past four years through sales agents, including Portec-North America, International Mold Steel, and Edward D. Segen & Co.

Processors are using the porous metal to make prototype and production molds, as well as cavity inserts. The tooling was initially used mainly for prototyping, but is now entering commercial applications as users gain familiarity.

This porous metal provides design, processing, and part-quality benefits that growing numbers of thermoformers of both roll-fed and cut-sheet products are using to process PS, PP, PE, ABS, PVC, PET, TPO, acrylics, and other materials. The tools are finding their way into thin-gauge consumer and industrial packaging, as well as highly cosmetic heavy-gauge applications such as refrigerator door liners and automotive dashboards.


A spectrum of benefits

Users say tooling lead times shrink by weeks because drilling of vacuum holes is eliminated. “A cast aluminum tool that takes eight to nine weeks to deliver can be produced in Espor air-permeable metal in as little as two weeks, due to faster machining time and the elimination of vacuum holes. Time can be saved with machined Metapor tooling, as well,” says Scott Hopkins, sales engineer at International Mold Steel.

“Computer-aided design time is shortened too, since there is no need to program tool paths for machining vacuum vents,” says Kurt Nofz, president of Atek Thermoforming Inc. in Belleville, Mich., a major user of Metapor.

Processors say the porous metal can also shorten forming cycles, due to the occurrence of vacuum draw over the entire mold surface, and the resulting faster draw than with conventional venting. More mold-surface vacuum enables forming and cooling to begin sooner. “Heating and cooling times can be shortened, because air is removed so quickly,” says Hopkins. The pull of the vacuum may allow formers in certain situations to forego the use of plug assist. “With fast cycles, the benefit isn’t much. But with longer cycles there is a time savings. However, the greatest benefit of faster forming is that the plastic remains within a tighter forming-temperature window,” says Michael Tsenter, v.p. of sales and marketing at Portec–North America, the U.S. marketing partner for the Swiss firm.

Since vacuum is drawn throughout all vertical and horizontal surfaces of the permeable mold, the porous metal can help pull vacuum in the belly of deep-draw molds, where it can be difficult to install traditional vacuum lines. Metapor tools are being used today on jobs requiring draws as deep as 18-20 in.

Processors also benefit from the unusual self-cooling effect of porous molds. The vacuum pulls air in through the sides of the mold to carry away heat from the plastic. This air cooling within the block of metal may not be sufficient for all applications, but it provides all the cooling needed for 12 tools used in heavy-gauge forming by Altrista Thermoformed Products (formerly Triangle Plastics) in Independence, Iowa. The parts are highly cosmetic dash-panel inserts, 0.04-in. thick, made in four to eight cavities. “The material is so porous it dissipates the heat at a rate comparable to an aluminum tool with cooling lines,” says Jeremy James, project engineer. These molds have run nearly 20,000 cycles each.

Another firm typically forms hundreds of prototype parts from a Metapor tool without using any cooling devices. “We’ve done a few hundred tools with Metapor over the last two years. We typically use it as a prototype tool,” says Todd Williams, die-shop manager at Display Pack Inc. in Grand Rapids, Mich., a maker of packaging products that designs and builds its own tools. “Roughly 60% of the prototyping jobs we do now use Metapor molds rather than wood, epoxy, or aluminum. We get a tighter draw in complex shapes without using plug assist. Short tooling-up time means we can get a sample part into a customer’s hands fast, says Williams.”


When looks count

Decorative parts that require high detail or optical quality can be formed with porous tooling without marring the surface. The tool can be polished to a 600-grit finish. Although a standard aluminum tool can be polished to a higher finish, Hopkins argues that there is little practical advantage in doing so. “A straight aluminum tool can achieve a mirror finish, but if it’s too polished, the parts will be difficult to demold. A 600-grit finish works well for a thermoforming tool,” he says.

The large number of tiny vacuum pores reportedly produces large, extremely flat products that are free of blemishes, vent marks, or other surface imperfections.On the other hand, with conventional vents air trapped between the sheet and the mold creates a lake or wave effect on the sheet surface. “That results in a shiny spot on the part,” says thermoforming consultant Jim Throne, president of Sherwood Technologies.

“This tool material can be advantageous when working with olefins, particularly polypropylene,” says Throne. “That material’s surface is smooth when heated, so it doesn’t stay fixed on the mold’s surface, sometimes producing a sliding effect on the sheet. Metapor cuts the sliding effect. Users of transparent or high-appearance polyolefins can use high-gloss sheet without getting the scuffing that can occur due to sliding in the mold.”

The uniform drawing ability of the porous metal reportedly imparts fine surface details, engravings, or textures such as simulated leather grain. “We can create detail such as fine lettering and texture that were previously impossible to achieve with thermoform tooling,” says Al Samson, president of Edward D. Segen, a distributor of Metapor. “With conventional tooling, small vent holes must be put into every letter of an engraving, with three or four pinholes for each letter. The more intricate the engraving or decoration, the more time-consuming the drilling operation. With Metapor, the detail can be engraved in the tool, and you’re ready to go,” he says.

Permeable tooling also lends itself to unusual geometric shapes like a pointed cone or other special features that are impossible to create in conventional tooling because of the difficulty of locating vents where needed, says Samson.

Hopkins of International Mold Steel presented a paper early this month at the Society of Automotive Engineers (SAE) annual conference in Detroit. He compared the capability of Espor with nickel-shell or cast-aluminum tooling in producing highly detailed parts from textured molds.


Do’s and don’ts

Air-permeable molds are not a panacea for all mold troubles. One concern is consistent vacuum level. “There can be a pressure-drop problem,” says consultant Throne. “The small diameter of the porous channels creates a lot of resistance for the air to flow through. It’s like comparing a garden hose to a soda straw. The air-flow rate and pressure drop won’t be the same as with drilled vent holes, even though the same amount of vacuum is used,” he explains. According to Samson of Edward D. Segen, vacuum tends to be effective with porous mold plates up to 4 in. thick. Thicker plates can also have a problem of too large a temperature gradient between the mold’s cooling plate and sheet-contact surface, Samson adds. For very deep molds, Portec suggests building the mold in the form of a hollow box by joining thin slabs of the material with epoxy adhesive.

Despite the fact that vacuum pores are distributed across the entire face of a Metapor mold, some attention is required in order to obtain uniform vacuum draw. Typically a grid pattern of primary vacuum distribution channels is created on either the back of the Metapor tooling or the metal cooling plate on the back side of the mold. Segen’s Samson says the location of the vacuum piping is important. It is best mounted in the center of the rear of the mold. If it is mounted more to one side, the vacuum draw will be stronger on the side closer to the pipe. Portec suggests that a 12 x 16 x 1.5 in. mold use vacuum-distribution channels that are 0.600 in. wide, 0.200 in. deep, and 2 in. apart.

Metapor molds can accept most conventional engraving and milling procedures. Conventional CNC equipment with carbide bits or high-speed steel bits can be used. However, the epoxy in the metal composite limits the utility of EDM engraving on the tool. That is because EDM works by conducting electricity, and epoxy is a poor conductor, notes Samson.

Polishing the surface of a Metapor mold can have a detrimental effect on the pores. “Polishing slightly closes the pores, reducing total porosity,” says Hopkins. But he says the effect is not so great as to prevent sufficient vacuum for processing.

Little maintenance is needed to keep the mold surface pores unclogged by foreign matter. Because the holes are so small, material has a very hard time becoming lodged. And air blown back through the vents tends to expel contaminants, says Hopkins.

The porous metal composite lends itself to easy tool repairs. Conical-shaped slices of the tooling material can be inserted where needed with spots of adhesive.

Because the epoxy binder in porous tools makes the material softer than standard aluminum, processors often ask about the life of porous-metal molds. “Some molds are in high-volume applications, generating 1.5 million to 5 million parts and still going,” answers Hopkins.

Examples of such high-volume use of air-permeable tooling are plentiful at Atek Thermoforming. It has many commercial applications, ranging from 1 x 1 in. closures to a tailgate for a pickup truck. “We have a six-up dunnage-tray tool for DaimlerChrysler that has run over a million cycles,” Kurt Nofz says. The firm has completed hundreds of projects using Metapor molds, including a 30 x 36 in. dashboard of PVC with foam backing for Visteon Corp., Dearborn, Mich. Porous tooling eliminated problems of grain washout during forming. Approximately 90% of Atek’s usage of Metapor is forming preprinted sheet to be used as decorative inserts for parts like cell-phone housings. Traditional vacuum vents would mar the glossy surface of the 0.5-mil sheet.


What it costs

Porous mold materials are fairly costly. Produced in slabs of 500 x 500 mm in thicknesses from 10 to 400 mm, Metapor can cost up to twice as much as a similarly sized piece of 6061 aluminum. “Processors looking for a 6 x 4 x 2 in. mold would have to pay for a 20 x 20 in. slab, which can cost up $750,” says Samson. Segen recently introduced a service whereby it will custom cut pieces from a Metapor slab as a means to reduce tool cost. Processors say that using Metapor just for tool inserts rather than the entire mold is another way to lower the price.

Processors and suppliers say the cost of the tooling material is offset by time savings through faster machining and elimination of vacuum-vent drilling. With the Espor castable version, the savings are even greater, since complex tool textures are reproduced during casting, rather than applied afterwards, says Hopkins. Espor addresses those applications that are difficult or not cost-effective to machine, such as textured, deep-draw, or highly detailed molds. Espor molds are currently in production in automotive and custom applications, says Hopkins.

Permeable tooling may not be cost-effective for every application. “Drilling vents isn’t that expensive. But if the part is complex, and many vents must be drilled, and the mold has multiple cavities, then it may be a job for an air-permeable material,” says Throne.

Metapor comes in three grades suitable for thermoforming molds. The standard BF 100 AL grade has a density of 1.80 g/cc and flexural strength of 8000 psi. It withstands a maximum temperature of 226 F. (The epoxy in the formulation could degrade if exposed to higher heat.) A high-density version, HD 100 AL, is recommended for forming transparent parts. It has a density of 1.90 g/cc and flexural strength of 6200 psi. Portec recently rolled out a new high-temperature grade, BF 210 AL, with a more durable epoxy component, to handle forming at 410 F.