From the First Plastic Soles to Current Technological and Environmental Advances

by Marco Arezio

The production of plastic soles has a fascinating history that reflects the evolution of material technology and production techniques.

From the mid-20th century, when the first plastic soles began to replace traditional leather soles, to the present day, the industry has made tremendous progress.

This article tells the story of plastic sole production, the various steps in the production process, the machines used, and the raw materials involved, with a particular focus on recycled materials.

History of Plastic Soles

In the 1950s, with the advent of new synthetic materials, plastic soles began to emerge as an economical and versatile alternative to leather soles.

Polyurethane (PU) and polyvinyl chloride (PVC) were among the first materials used. In the 1970s and 1980s, the introduction of ethylene-vinyl acetate (EVA) further revolutionized the sector thanks to its cushioning properties and lightness.

Over the years, production technology has become increasingly sophisticated, allowing the creation of soles with specific properties for different types of footwear, from sports to elegant shoes.

Today, sustainability is at the heart of innovation in the sector, with an increasing use of recycled materials and bioplastics to reduce environmental impact.

Raw Materials

The raw materials used for the production of plastic soles are varied, each offering specific physical and mechanical properties. Among the most common are:

Polyurethane (PU): Known for its lightness and abrasion resistance. It is often used for comfortable and flexible soles.

Ethylene Vinyl Acetate (EVA): A lightweight and cushioning material, predominantly used for sports soles.

Polyvinyl Chloride (PVC): Used for its cost-effectiveness and versatility, although it is less performant in terms of comfort compared to PU and EVA.

Thermoplastic Rubber (TPR): Combines the properties of plastic and rubber, making it ideal for durable and flexible soles.

Nylon and High-Density Polyethylene (HDPE): Used for specific applications requiring high strength and durability.

In addition to these traditional raw materials, the industry is increasingly adopting recycled materials to reduce environmental impact. Among the most commonly used recycled materials are:

Recycled Polyurethane (rPU): Derived from production scraps or end-of-life products, rPU retains many properties of virgin polyurethane, offering a sustainable alternative.

Recycled Ethylene Vinyl Acetate (rEVA): EVA scraps and discarded products can be regenerated and reused in new productions, helping to reduce waste.

Recycled PVC (rPVC): PVC can be recycled many times without significant quality loss, making it an interesting option for sustainable production.

Recycled Rubber (rRubber): Derived from used tires and other rubber products, this recycled raw material is particularly durable and suitable for long-lasting soles.

Bio-based Polymers: Made from renewable sources like corn starch or castor oil, these polymers offer a significant reduction in CO2 emissions compared to petroleum-derived materials.

Production Details

The production process of plastic soles involves several stages, each requiring the use of specific machinery and precise techniques.

1. Design and Model Development

Before starting actual production, it is essential to develop a model of the sole. This process involves designers and engineers using CAD (Computer-Aided Design) software to create a detailed three-dimensional model.

2. Preparation of Raw Materials

The raw materials are selected and prepared for the production process. The raw plastic material, in the form of granules or powders, is mixed with additives such as foaming agents, colorants, and plasticizers to enhance the final product's physical and mechanical properties.

3. Injection Molding

The most common method for producing plastic soles is injection molding. This process involves several stages:

Heating: Plastic granules are heated in an injection molding machine until they reach a molten state.

Injection: The molten plastic is injected into a mold that shapes the sole. The mold is maintained at a constant temperature to ensure uniform material distribution.

Cooling: Once the plastic is injected into the mold, it cools and solidifies.

Extraction: The formed sole is extracted from the mold and undergoes quality control to check for any defects.

This method allows for rapid and precise production, with the ability to create complex soles with intricate details.

4. Compression Molding

Another technique used is compression molding, particularly suitable for materials like PU:

Positioning: The plastic material is placed in a mold cavity.

Compression: The molds are closed and compressed under high pressure, distributing the material evenly.

Heating and Cooling: The material is heated to allow fusion and then cooled to solidify.

The main advantage of compression molding is the ability to produce soles with superior mechanical properties and controlled density levels, ideal for applications requiring greater strength and durability.

5. Extrusion

For specific applications, the extrusion process can be used:

Heating: The plastic material is heated and melted.

Extrusion: The molten plastic is pushed through a die to create a continuous shape that is then cut to the desired length.

This method is particularly useful for producing simple, continuous-shaped soles, such as those used for work or casual footwear. Extrusions can be further processed to add textures or specific cuts.

Finishing

Once the soles are formed, they must be finished. This may include:

Cutting and Shaping: Soles are cut and shaped to achieve the desired final form.

Surface Treatments: Application of anti-slip coatings, decorative prints, or textures.

Assembly: If necessary, soles can be assembled with other shoe components.

Quality Control

Every stage of the production process includes rigorous quality controls to ensure that the soles meet performance and durability standards. This can include tests for abrasion resistance, flexibility, grip, and longevity.

Machinery Used

The production of plastic soles requires the use of various specialized machinery. Here are the main ones with technical details and selection advice:

Injection Molding Machines: These machines are essential for melting and injecting plastic into molds. Injection molding machines can vary in size and capacity.

Injection Capacity: Measured in grams or cubic centimeters, it indicates the amount of plastic the machine can inject per cycle.

Clamping Force: Measured in tons, it is the pressure needed to keep the mold closed during injection. Machines with higher clamping forces are suitable for larger and more complex soles.

Cycle Speed: The speed at which the machine can complete an injection and cooling cycle. High-speed machines improve productivity.

Selection Advice: Choose a machine with injection capacity and clamping force appropriate for the size and complexity of the soles to be produced. Also, consider cycle speed to optimize productivity.

Compression Molds: Used mainly for materials like PU, these molds must be robust and capable of distributing pressure evenly.

Mold Material: Molds are generally made of hardened steel to withstand pressure and heat.

Heating System: Essential to ensure the material melts evenly. It can be integrated into the mold or the press.

Cooling System: Helps solidify the sole quickly after compression, improving production efficiency.

Selection Advice: Opt for molds with an effective heating and cooling system and high-quality material to ensure durability and precision.

Extruders: Machines that melt and extrude plastic to form a continuous strip. Important for specific productions.

Screw Diameter: Determines the amount of material that can be extruded. Larger diameter screws are suitable for high volumes.

Screw Length: Affects the material's residence time in the screw, important for uniform melting.

Cooling System: Necessary to maintain the ideal temperature during extrusion.

Selection Advice: Choose an extruder with a screw diameter and length suitable for the type of sole to be produced. An effective cooling system is crucial to maintain the final product's quality.

Environmental Considerations

The plastic industry is often criticized for its environmental impact. However, various efforts are underway to make the production of plastic soles more sustainable:

Recycling: The use of recycled materials is increasing. For example, EVA can be recycled and reused in the production of new soles.

Bioplastics: Materials derived from renewable sources, such as corn starch, are being developed, offering a reduction in environmental impact.

Energy Efficiency: Advanced technologies reduce energy consumption during the production process.

Conclusion

The production of plastic soles is a sophisticated process involving various technical stages and advanced machinery. With increasing attention to sustainability, the industry is evolving to integrate recycled materials and eco-friendly technologies, improving environmental impact without compromising the final product's quality.

The history of plastic soles is a testament to technological evolution and the industry's ability to adapt to new challenges, increasingly aiming towards a sustainable future.