Die-cut EVA midsoles should die.

The footwear industry should eliminate the use of the highly wasteful die-cut EVA midsole manufacturing process, especially on high-volume styles, and move to manufacturing methods that are less environmentally impactful.

EVA is the most commonly used material for athletic shoe midsoles. Historically, it has proven to be one of the more durable, comfortable, easy to manufacture, and cost effective elastomeric foams. Die-cut EVA produces by far the most waste of all EVA manufacturing methods yet is still commonly used by many major brands and nearly all small ones.

EVA is created when a blowing agent is incorporated into an ethylene-vinyl acetate elastomer, and “blown” to create a cellular foam structure.?

There are four primary methods of manufacturing EVA midsoles:?

• Die-cut (aka cut-and-buff or DCEVA)
• Compression molding (CMEVA)?
• Injection-expansion molding (IMEVA)
• Poured pellets molding (AMEVA)

Die-cut (aka cut-and-buff)

• Sustainability score: ??????

Process: First, a large sheet of EVA foam is molded. This sheet is typically created in a large (~1m x 0.6m) expansion mold. The pre-form is a sheet of un-blown EVA thermoplastic with a heat-activated blowing agent incorporated. This mold is heated, the blowing agent activates, and a sheet of foam is formed. Steel cutting dies are made, typically bent to shape by a skilled worker from steel roll stock, then sharpened. Dies are used to cut midsole 2D shapes from foam sheets. Midsoles are then buffed by hand to create a straight sidewall and to remove any imperfections, and skived to create a thinner forefoot.

• Benefits: You don’t need to create molds to make your midsoles, saving time and cost. Only cutting dies are required to cookie-cut sole shapes from large sheets of foam. Some think the exposed cellular aesthetic looks nice.?
• Downsides: There’s a lot of cutting waste, up to ~1/3rd of total raw material, as there must be cutting margin between parts and midsole shapes can only pack so tightly. Design is mostly restricted to flat vertical sidewalls (though some broad-stroke sculpting is possible via a secondary buffing process).
• A mold is an order of magnitude more expensive (~several thousand USD) than a cutting die (~several hundred USD or less). Considering a full size run, mold cost savings can approach $200k USD. Amortized over small-volume styles, this can become a substantial, or prohibitive, contributor to a shoe’s total cost.?

Compression molding (CMEVA)

• Sustainability score: ?????? to ????????
• Process: First, a near-net-shape “blocker” is formed. The blocker is either injection molded (which requires a first mold) or die-cut-and-buffed from foam sheet stock (which generates lots of cutting waste). A two-part compression mold is machined or cast. The blocker is placed in the compression mold, which is closed, clamped, and heated for a period of time to allow the blocker to set into the final midsole geometry.?
• Benefit: Good detail and design freedom on the midsole sidewall. Can achieve a wide range of good mechanical properties.
• Downsides: Needs a relatively expensive mold, or molds. Requires a near-net-shape “blocker” which is compression molded into the final shape. Molds are energy-intensive to create and come with substantial lead times and costs. Final density, and consequently mechanical properties, varies throughout the midsole due to inconsistent compression ratio.?
• Two-part compression molds are cheaper than many-part injection molds.
• Historically, compression-molded midsoles delivered higher sidewall detail resolution (crispness) than injection molded midsoles.?
• Historically, mechanical properties of compression molded midsole materials were superior to injection molded materials.?
• Neither of the above two statements holds true today.

Injection expansion molding (IMEVA)

• Sustainability score: ??
• Process: IMEVA is not your grandmother’s thermoplastic injection molding. First an injection mold tool is machined. The mold cavity is approximately 1/2 the size of the final midsole. The mold is secured inside of an injection molding machine. EVA with a heat-activated blowing agent is melted and injected into the mold. The mold is heated, and the blowing agent activates. The mold is held shut as the curing reaction (crosslinking) takes place. The mold is then opened quickly, and the midsole “pops” out, expanding to its final size as pressure drops to ambient allowing the gas created by the blowing agent to expand into cells. The runners and sprue are trimmed, and the midsoles are placed on a conveyor belt to be gradually cooled as they pass through cooling chambers.?
• Benefits: A wide range of good material properties, sharp sidewall detail, and broad design freedom (these days). Runners and sprues are the only waste, typically <10% of total material consumption. Consistent mechanical properties throughout the midsole.?
• Downsides: The process requires a mold, which is time-, cost-, and energy-intensive. Injection mold lead time and mold cost are higher than compression molds. Slightly worse dimensional accuracy than CMEVA. Bigger minimum thickness than CMEVA.?
• Oldest of the EVA manufacturing methods. Originally had poor detail resolution and poor mechanical properties, but advances in process and chemistry have flipped the gap.

Poured pellets molding (AMEVA)

• Sustainability score: ??
• Process: An AMEVA mold is similar to an injection mold in quality and manufacturing, but doesn’t have any runners or gates. Instead of melting EVA pellets in an injection mold, pellets are weighed and poured into an open mold. The heated mold is closed, the pellets fully melt and the blowing agent is activated. The mold is held closed long enough for the reaction to adequately take place. When the mold is opened, the midsole “pops” out to its final size similarly to IMEVA.?
• Benefits: Same as IMEVA, but lower waste (no runners or sprues to discard).?
• Downsides: Same as IMEVA, AMEVA still requires a mold.
• Newest of the EVA manufacturing methods.

So what should brands do??

There are some uses for reground EVA, but considering it’s a highly crosslinked thermoset, it can’t be re-melted and re-used for similar products. It’s occasionally chopped up and used as a small-percentage filler in “more sustainable” EVAs or downcycled for other non-footwear products. Sadly, it’s often disposed of as waste, shipped off to destinations unknown for a (likely) dirty-burn disposal.

The cut-and-buff porous cellular midsole aesthetic is clean and cool but is not the only clean and cool option. Move to injection EVA (IMEVA) or poured pellet (AMEVA). Create compelling and interesting sidewall details instead of nostalgically relying on wasteful useless porosity. If you can’t sell enough shoes to adequately amortize new tooling, re-use old molds. Alternatively, IMEVA or AIMEVA the midsole and buff the sidewall to achieve the same aesthetic.?

There are many methods to make footwear less environmentally impactful. Reduction of material consumption is always good (“reduce” comes first in reduce/reuse/recycle for good reason). There are academic advances in de-crosslinking thermoset polymers, but as far as I know they haven’t yet arrived at industrial scale. There are a growing number of downcycled applications for the waste. I suspect many of the big brands are on the way to eliminating die-cut EVA, but it can’t come fast enough. As long as there is a non-zero quantity of dirty-burn die-cut EVA waste, we have a problem on our hands.?

Brands should embrace the concept that throwback shoes do not need to be exactly the same as their OG predecessors. Listen to your customers, but also to your senses. A barely-there, non-impactful product change can have a large impact on sustainability.