Injection Molding in the footwear industry

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What is plastic? 

And now continuing with some chemistry: The correct chemical term for plastic is polymer. The word polymer comes from the Greek prefix polý-, which means „many“, and the suffix -méros, which means „parts“. Polymers = „Many parts“.

Polymers are giant molecules made up of chains or rings of linked repeating subunits called monomers.

Synthetic Polymers

• Polyethylene (PE)
• Polypropylene (PP)
• Polyvinyl chloride (PVC)
• Polystyrene (PS)
• Polyamide (PA)
• Polyetherblockamides (PEBA)
• Thermoplastic styrene elastomers (TPS)
• Thermoplastic Polyurethan (TPU)
• Polyurethane (PU)
• Epoxy resin

Synthetic polymers · Four different groups

First the bad news: Not very surprisingly, the footwear industry’s focus is still on synthetic polymers which are mostly not biodegradable. Good news, due to their melting properties, with the right idea, concept, design, and process some synthetic polymers can be the core material for real material loops.

Please do not worry, natural polymers will be payed attention to in subsequent blogs. So first, let’s have a deeper look into synthetic polymers. The “synthetics” can be grouped into 4 types – relevant for injection molding.

Thermoplastic Elastomers

A fourth hybrid form is Thermoplastic Elastomers.

They are a physical mix of polymers with both thermoplastic and elasto-meric properties but they are fully re-meltable e.g., TPS, TPU, EVA and PEBA

Thermoplastics in injection molding

Now you know everything about the different polymer groups, but which of those are now used in injection molding? The answer comes here. Due to their flexible characteristics the two groups of Thermoplastics are perfectly right.

When thermoplastics are heated, they melt to a liquid which makes them suitable for the injection molding process. These materials can be cooled and heated several times without any change in their chemical structure. Moreover, depending on the desired injection parameter only minor changes in the mechanical properties, e.g., colder molding temperature, are needed.

In essence, Thermoplastic is suitable for mass production of components because of its short circle time and its melting abilities. 

Further, we will give you an overview about one Thermoplastic Polymer

• Polyamides (PA)

 and three Thermoplastics Elastomers

• Thermoplastic Polyurethan – (TPE-U or short: TPU)
• Polyetherblockamides – (TPE-A or short: PEBA)
• Thermoplastic Rubber Compound (TPR) – (TPE-S = TPS = SBS+PS)

All four are commonly used for injection molding in the footwear industry. And in case it was not mentioned yet. All four are recyclable.

One of the (still raising) stars in the footwear industry, is the material Thermoplastic Polyurethan (TPU). TPU was first discovered in 1937 by Otto Bayer and his coworkers at the labs of I.G. Farben in Leverkusen, Germany. A TPU is a so-called „block copolymer“ consisting of alternating sequences of hard and soft segments. It is formed by a poly-addition reaction between methylen-diphenyl-diisocyanates (MDI), one or more short-chain Diols („chain extenders“) as well as long-chain Diols.

Aromatic TPU

As indicated above, TPU materials can be used for almost all categories of footwear applications. The hardness can be adjusted from very soft (e.g., sneaker outsoles) to very hard (e.g., shank applications). Moreover, TPU is UV resistant and can therefore be used for articles which are exposed to sunlight.

Let’s pick some requirements of footwear components which are perfectly fulfilled by TPU.

Side panels need to be UV resistant, have an appropriate strength to resist the pulling of laces and need to be flexible (shore A) bend along the upper. External heel counters need to be both UV resistant and hard/stiff enough in accordance to protect and support the heel. Shanks need to be tough and stiff (Shore D) and UV resistant.

There are actually great examples of existing shoes on the market, which are totally made of TPU materials and can therefore be fully recycled. 

Polyamides (PA) – The stiff ones

Another classical material of injection molding in the footwear industry are Polyamides (PA). Chemically speaking, Polyamides are long-chained molecules in which the repeating units in the molecular chain are linked together by amide groups. Amide groups (CO-NH) are commonly formed by a poly-condensation reaction between a carboxyl group (COOH) and an amine group (NH2).

PAs occur both naturally (e.g., proteins, such as wool and silk) and synthetically. The footwear industry uses synthetical PA, such as well-known Nylon.

PAs commercially exist in different Polyamide types, such as PA6, PA6.6, PA4.6, PA6.10, PA11, PA12. Depending on the number of carbon atoms in between the amide groups physical properties such as density or liquid absorption vary on a wide scale.

Compared to TPUs Polyamides are considerable stiffer. Therefore, they are mainly used for applications where a certain reinforcement is required, e.g., a Shank system, Strobel insole board, eyelets.

But PA has its limitations. In comparison to PA11 and PA12 especially PA4.6, PA6, PA6.6, PA6.10 absorb more water & humidity. Moreover, low temperature harms the flexibility of PA, which disqualifies PA for outdoor activities. Compared to TPU, the price of PA is relatively high. You can tell as a rule that the longer the PA chain, the more expensive the PA becomes. A PA12, for instance, has almost double the price of a PA6.

Due to the stated limitations and except for a few PA6 grades, mainly PA11 and PA12 grades have been successfully established in the footwear industry, usually in high performance applications.

A PA12 soccer outsole needs to be light weight, reinforced, UV resistant and flexible even under low temperatures. A PA12 soccer outsole in connection to TPU ether studs works perfectly, because of the bonding ability of PA with TPU (TPU has a better traction behaviour and is more comfortable to walk on). 

Polyether Block Amide (PEBA) – The luxury one

Polyether Block Amide (PEBA) count as the luxury, high performer under the materials for the footwear industry.

What is PEBA, chemically speaking? PEBA materials are block copolymers made up of rigid polyamide blocks and soft polyether blocks. By manipulating these blocks and their relative ratio a large property spectrum from very hard and rigid to very soft and flexible can be achieved.

PEBA grades combine the benefitting properties of Polyamides (e.g., toughness, low density) with the high flexibility/elasticity of Polyether’s:

• Low temperature flexibility and impact resistance (at temperatures lower than -40 °C)
• Relatively low density (1.01 g/cm3) compared to TPUs (1.19g/cm3)
• Proper energy return
• Fatigue resistance
• Good chemical resistance
• the lightest of all thermoplastics

Similar to TPU,  the hardness of PEBA can be adjusted from very soft (e.g., external heel counters) to very hard (e.g., insole boards, sprint plates, or outsoles). The option of usage are almost endless. Especially in the sport footwear industry PEBA grades are well established, but due to their relatively high price (approximately as expensive as PA12) they are mainly used for high-performance applications. The above stated superior properties PEBAs are predestined for winter sports (e.g., ski and snowboard boots) and other competitive sports where low weight, weather resistance, energy return, and fatigue resistance are required (e.g, ultra trail running).

And as if all these positive skills of PEBA are not enough: Designed and applied in an clever way, PEBA is also sustainable! There are actually great examples of existing footwear on the market, which are totally made of PEBA materials and can therefore be fully recycled.

Styrene-Butadiene-Styrene (TPS-SBS) – The biggest group

The most occurring thermoplastic elastomer (TPE) is the styrene based thermoplastic styrene elastomer (TPS).

The easiest, best performing, and cheapest TPS is a mix of styrene and butadiene, Styrene-butadiene-styrene (SBS). As illustrated above, the butadiene is the elastomeric (soft) linking segment between the polystyrene blocks. Due to its synthetic nature, SBS is logically not biodegradable. In its not hydrated form, it is almost only used to modify other materials. Common applications are in the area of street construction and baby diapers. SBS can also be used in the footwear industry. However, due to its poor UV resistance it is more suitable for footwear components which are protected from the sun (e.g., heel counters).

SBS is rarely used in an unmixed form because it has an amorphous structure and therefore melts very slowly when exposed to heat. To improve its properties, it is often mixed with thermoplastics. For recycling, and the injection molding process of heel counters, pure (Nonylphenol free) SBS is necessary.

The only element to be added, which improves the properties of SBS without harming the ability of thermo-recycling, is Polystyrene. Polystyrene, as mentioned above, makes SBS stiffer and is already an element of SBS.

Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. It occurs solid or foam and is not biodegradable, cannot be decomposed by bacterias. Conventional polystyrene is clear, hard, and brittle. It was first discovered in 1839 by the apothecary Eduard Simon in Berlin. PS is a very cost-efficient material. It is also one of the most widely used plastics. PS can be the material for protective packaging, containers, lids, bottles, trays, tumblers, or disposable cutlery.

In the footwear industry High Impact Polystyrene (HIPS) is used, which is less brittle and impact resistant even at low temperatures. High availability and low prices on the secondary market make PS suitable for recycled components e.g. our heel counters, even though the properties limit the application.

To sum up, an overview of the prices of all materials we introduced in this post and their hardness range.

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What is a heel counter? 

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Strobel vs. AGO

Depending on the manufacturing process, there are two types of heel counters. Experts on the field, call heel counters either “AGO” or “Strobel”. At first glance, AGO and Strobel heel counters seem to be quite the same. However, a tiny construction element makes the difference. 

The decisive criteria which differentiates an AGO from another type of heel counter is that the AGO has a flange. The flange is a small extending element on the bottom of the heel counter. In collaboration with the 3D shape of a heel counter, it provides extra stability, and most importantly it ensures the perfect fit on the heel during the manufacturing process.

The name AGO comes from the manufacturing method AGO (Another Great Opportunity), invented by the Milanese tanner Francesco Rampichini in 1911, in which this type of heel counter is used. To construct the shoe in the AGO method, first the heel counter is glued into an opened upper element (without insole). Subsequently, the upper is mounted on a shoe last, which is already prepared with an insole. The sealing is then made by gluing or pinching the lasted upper to the bottom of the insole. During the whole mounting and sealing process, the AGO heel counter’s flange prevents it from moving away from its perfect heel position.

Heel counters without a flange are called Strobel. They are suitable for e.g., a Strobel construction, which is the most common manufacturing method in the sport shoes industry. The Strobel method requires specialized machines. The name Strobel has its origin from the company with the same name, which invented this type of footwear manufacturing. Differing from the AGO method, the stabilization and fit of the heel counter only comes from the perfect adaptation of the 3D heel counter to the shoe last.

In this method, the upper is already prepared with a glued (or sometimes even stitched) heel counter inside. Before even mounting it to the last, the upper is already connected and sealed to the so-called “Strobel board”. A Strobel board is much narrower and thinner compared to insoles usually used in the AGO method. The stitch sealing of upper, Strobel board, and heel counter can be imaged with the look of a sock.

Because extending heel counter flanges would collide with the stitches of the Strobel connection, a Strobel heel counter does not have any flanges.

In many footwear categories the Ago and Strobel methods are merging. Some shoes are often manufactured with the Strobel method on the heel part of the foot (remember: stitches) and with the AGO method on the front foot. Logically, the method used for the heel part holds sway over the choice of heel counter type.

AGO

Heel Counter vs. Thermo-Sheets

Both heel counters and thermo-sheets are functional elements located on the heel of a shoe. As the names already tell, heel counters and thermo-sheets are not the same. What is the difference again? Don’t heel counters and thermo-sheets have the same purposes of protection, stability, and fit? Yes, both are aiming for all three criteria, but when viewed from the functional perspective, heel counters perform better:

Thermo-Sheets are die-cut 2D counters which are leveled to the last shape via heat and pressure.

Heel counters are injection molded and have a 3D shape. The fact that heel counters can be designed in perfect accordance with the shape of the shoe last (based on which a shoe is manufactured) ensures a superior fit over 2D thermo-sheets.

Still, there are advantages of thermo-sheets, making them attractive: cost effectiveness and flexibility. The construction of a thermo-sheet tooling (cutting die) costs less than a heel counter mold, and is herewith more flexible when it comes to design changes. Therefore, thermo-sheets usually have a lower unit price.

Even though price competitiveness, which is one of the most important criteria of the footwear industry, is unchanged, surprisingly an increasing quantity of thermo-sheets has recently been replaced by heel counters.

Here is why:
On the one hand, sustainability aspects are gaining importance, and the production of thermo-sheets has an unavoidable flaw: the process of die-cutting and skiving accrues waste. Opposed to that heel counters are injection molded and therefore produce less waste. Moreover, can consist of up to 100% recycled material, whereas most thermo-sheets only contain a relatively small share of recycled ingredients.

On the other hand, new design and manufacturing concepts of heel counters enable a competitive price compared to thermo-sheets.

Generally speaking, thermo-sheets are preferably used for casual footwear, where the component price is usually a more important criterion. In high performance footwear categories, where high resistant protection, strong heel support, good fit, and advanced wearing comfort is required heel counters are in use.

Material Components

Styrene-Butadiene-Styrene  Soft Component

Reprocessed single-source post-industrial waste from the sanitary products industry (foil extrusion).

This source ensures stable mechanical properties comparable to virgin SBS grades and enables our fully sustainable compounds to be used even for performance applications.

High Impact PolyStyrene  Hard Component

Reprocessed post-consumer and post-industrial  waste (consumer electronics and food packaging) from constant sources in Japan.

Since the replacement of the virgin polystyrene in 2013, the weight of recycled HIPS has reached up more than 8,500 tons until today.

Sustainability

Every year around hundreds of millions of shoes are produced. Since heel counters can be found in almost all of them, their impact on the environment is considerable high.

framas group is currently producing more than 100 million heel counters per year which requires around 2,500 tons of raw material.

Considering that framas heel counters are made of 100% recycled raw materials, the material-related carbon footprint was reduced by more than 90% (> 7,500 tons of carbon per year) compared to the usage of virgin materials.

For next generation compounds framas is currently investigating bio-based and bio-degradable options and also target to reduce the carbon missions caused by the logistics (trucking, shipping) in the entire supply chain for heel counter materials.

History framaprene

Having expertise in last-making since 1948, framas produced the first complementary heel counters in the late 1970s. Since then, more than 90 heel counter series have been developed, some of them being produced for more than 40 years.

In 2009, framas founded an R&D project in cooperation with the University of Applied Sciences in Kaiserslautern, Germany targeting to develop a well performing heel counter compound partly based on recycled ingredients.
As a result, the framaprene ECO grades were introduced, containing more than 50% recycled Polystyrene. After intensive product testing and material evaluation, those grades were officially approved and launched in mass-production in 2013, reducing the usage of virgin materials in specific products by more than 1,000 tons per year.

In 2017, the first R&D trials with the recycled SBS (soft component) were initiated with the target to finally provide a fully sustainable heel counter compound.​

The first 100% recycled framaprene grade was introduced in 2019 but it took another 18 months to establish a solid vendor base to guarantee a safe R-SBS supply of sufficient quantity and reliable quality.

From the middle of 2021, after almost 12 years of continuous progress, framas Group can cover an annual production quantity of more than 100 million pairs exclusively based on recycled ingredients, reducing material related carbon emissions by more than 90% compared to the usage of virgin raw materials. A GRS certification is currently under process and will be finalized until spring 2023.

If you liked this article, make sure to follow our Social Media channels. You can contact us anytime via LinkedIn, Instagram & Facebook. We are happy to receive any feedback and tell us what other topics are of interest to you. We will try to address them in the near future.

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