Impact of Bio-Based and Recycled Resins on Molds

Plastic waste is a widely debated topic globally. While plastic products offer unique advantages and a lower carbon footprint compared to alternatives, addressing their full life cycle and waste management poses challenges. Innovative technologies, including advanced recycling methods, are emerging as potential solutions to transform plastic waste into valuable resources.

However, aside from the recycling perspective of resins, biodegradable resins are also expected to experience significant growth, representing a sustainable shift in manufacturing. They require modified tooling parameters to accommodate variable material properties and processing temperatures while maintaining product quality.

Bioresins: Bioplastics, Biopolymers, Biomaterials

Before discussing the challenges faced in moldmaking, it is essential to understand the various types of resins and their distinct characteristics. Each resin acts as a substitute and is derived from different processes (see Chart 1).

Bioresins are produced from sustainable, naturally occurring materials (corn/maize, tapioca, potato, vegetable oil, sugarcane, wood pulp, castor beans), chemically altered, fermented or processed as fillers.

Not all bioresins are biodegradable or compostable, requiring different reduction methods listed below:

• Biodegradable: Material that decomposes naturally over time. 
• Compostable: Material that decomposes within three to six months in a composting facility, leaving no toxic residues.
• Sustainable: This term refers to the “feedstock” used for the resin. While some resins can be sustainable if derived from renewable feedstock, they may not be biodegradable or compostable. In such cases, recycling these materials contributes to circularity.
• Circularity: The practice of keeping resources in use for as long as possible through recycling or finding alternative uses for the material.

Moldmaking and Processing Challenges

Tooling, handling and processing methods for bioresins differ from those of existing polymers. They are not direct replacements for existing polymers. Although they are listed for interchangeable use, they cannot be directly replaced. Here are some tooling challenges and considerations when processing bioresins.

The growing demand for bioresins in the U.S. and Europe is reshaping the plastics industry, but their high viscosity and heat sensitivity create new challenges in tooling and processing.

Viscosity

High viscosity in many materials leads to increased injection pressures. To prevent damage to equipment, high-strength and stable steels are essential. Proper tolerancing of injection molds and other processing equipment is crucial to prevent leakage. Additionally, enhancing the wear resistance of tools can help address these issues.

The inserts can be categorized into several types, including matrix grades such as hot work grades (H13 types), corrosion-resistant grades (420, 440C, Bohler M390 Microclean) and higher alloyed cold work or powder metallurgy grades (D2, PMM4, 10V, K294 Microclean and more). Each of these grade types has distinct strength and wear properties, making them suitable for different types of resin processing.

Both tensile and compressive strength can be enhanced by increasing hardness, volume and types of carbides present, as well as alloying elements such as carbon (C), chromium (Cr), molybdenum (Mo), vanadium (V), cobalt (Co) and niobium (Nb).

For example, H13 and other matrix-type grades have minimal carbide-forming elements, with their strength primarily derived from increased hardness achieved through heat treatment. In contrast, corrosion-resistant 400 series and higher alloy cold work and powder metallurgy (PM) grades typically attain greater strength primarily through hardness. However, these grades often possess a higher volume and a variety of carbide types in their microstructure, contributing to their superior strength compared to matrix grades.

One drawback of using steel grades with a higher carbide volume is that toughness tends to decrease as carbide content increases. This reduction in toughness can lead to cracking issues in certain applications. Additionally, incorporating cobalt (Co) into the steel's chemistry can enhance strength without increasing the carbide content. Overall, these factors contribute to minimizing deformation and wear on the molding surface, reducing leakage.

Enhancing the abrasive wear resistance of the molding surface can be achieved by applying the same factors mentioned previously (see Figure 1). When dealing with highly abrasive resins, there is a risk that the steel matrix may erode while the carbides remain intact. 

In particular situations, significant improvements can be achieved through the use of PVD coatings. There is a range of PVD/DLC coatings that can enhance surface hardness well beyond 70 HRC. These coatings also improve lubricity, remain stable at high process temperatures and prevent erosion of the underlying material, as demonstrated above.

Heat and Shear Sensitivity

Bioresins exhibit a variety of heat sensitivities; some are thermally stable, while others may degrade or soften when exposed to higher temperatures. Similar to petroleum-based resins, the effects of heat on bioresins depend on several factors, including the types of base materials, curing processes, additives and chemical structures used.

To prevent degradation of mechanical properties and performance due to excessive heat and shear, high thermal conductivity steels are recommended for manufacturing injection molds, feed screws and extrusion dies.

When it comes to high heat transfer and high thermal conductivity, lower alloy matrix grades are commonly preferred. Grades such as L6, S7 and H13 are often used, especially for long cores, due to their exceptional toughness and strength. However, a significant issue with L6 and S7 is their sensitivity to temperature, as these grades lack secondary hardening. When exposed to elevated temperatures — either from the molding process or during PVD coating — these grades can lose hardness and strength or may even distort due to softening.

In contrast, H13 does experience secondary hardening, but it has its drawbacks; it can be challenging to harden above a certain threshold and may become brittle at high hardness levels. An alternative to consider is W360 steel, which also undergoes secondary hardening and maintains high hardness, strength and toughness at elevated temperatures. Additionally, its improved chemical composition results in a high thermal conductivity value (see Chart 2).

420 stainless steel is a popular choice for mold inserts. While standard 420 grades generally provide satisfactory performance in terms of corrosion resistance, strength and toughness, their lower thermal conductivity can pose challenges for cycle time and thermal management of the mold. M333 refines the chemistry of 420, resulting in thermal conductivity values that are more comparable to those of H13-type grades. Below is a comparison of thermal conductivity values for M333, 420, and H13.

Corrosion

The chemical composition of bioresins plays a crucial role in their corrosive behavior. When subjected to high processing temperatures and pressures, the corrosive properties of bioresins may be affected, potentially leading to compatibility issues with tooling materials. Additionally, environmental factors can also influence the corrosive characteristics of certain bioresins.

Stainless steels are recommended where feasible, and nonreactive coatings should be applied to non-stainless steel. Corrosiveness increases with heat degradation.

When discussing martensitic stainless grades, specifically the 420 and 440 types, it's important to understand how carbide content and heat treatment affect the performance of the steel and its susceptibility to corrosion, including both pitting and oxidative forms. There are two primary strategies to minimize these issues: selecting a steel with lower carbide volume and tailoring the heat treatment according to the type of resin being used.

In the 400 series stainless grades, such as 420 and 440, two tempering options are available: “high” and “low.”  Low-temperature tempering (approximately 400-550°F) typically offers the best corrosion resistance because a lower number of carbides precipitate during this heat treatment. In contrast, high-temperature tempering (around 940-1000°F) results in a greater amount of carbide precipitation, which compromises corrosion resistance.

The main carbide-forming elements in 400 series stainless steel are carbon (C) and chromium (Cr). As carbides precipitate out of the solution, they deplete the surrounding areas of chromium. When the chromium concentration in these regions falls below 12%, they become “non-stainless,” making them prone to pitting and other forms of corrosion on the molding surface (see Chart 3)

To address these concerns, M333 was developed to improve upon the typical 420 chemistry by reducing the overall carbide volume. This adjustment not only increases thermal conductivity but also significantly enhances corrosion resistance. The images below illustrate the difference in carbide precipitation between M333 and 420 ESR, highlighting the impact on corrosion resistance (see Figure 2).

Exposure to moisture can degrade bioresins and damage machinery and tooling. To mitigate this issue, properly drying materials to remove absorbed and adsorbed moisture is critical for processing.

Summary

The bioresin market is growing rapidly, especially in the U.S. and Europe. However, using these materials poses challenges in tooling and processing due to their high viscosity, heat sensitivity and the need for stronger, wear-resistant tooling.  Addressing these challenges through advancements in processing techniques and a focus on sustainability is essential for integrating bioresins. This will support a move toward a circular economy, reducing plastic waste and promoting a sustainable future.