In the field of plastic processing, bio-based and recycled resins are increasingly used, and their properties (such as melt flow, shrinkage, impurity content, etc.) may have a significant impact on service life of mold, maintenance frequency, and molding accuracy.

 

Plastic waste is a widely discussed topic around the world. Although plastic products have unique advantages over alternatives and a lower carbon footprint, there are still challenges in solving their full life cycle and waste management issues. Innovative technologies, including advanced recycling methods, are becoming a potential solution to convert plastic waste into valuable resources.
However, in addition to recycling perspective of resins, biodegradable resins are also expected to achieve significant growth, representing a shift in manufacturing towards sustainability. Such resins require mold parameters to adjust to variable material properties and processing temperatures while ensuring product quality.

Before discussing challenges of mold manufacturing, it is important to understand types of resins and their unique characteristics. Each resin exists as an alternative and comes from a different production process (see figure below).
Bioresins are made from sustainable, natural materials (corn, cassava, potato, vegetable oils, sugar cane, wood pulp, castor) that are chemically modified, fermented, or processed as fillers.
Not all bioresins are biodegradable or compostable and require different treatments:
Biodegradable: Materials that break down naturally over time.
Compostable: Materials that break down in a composting facility within 3 to 6 months without toxic residues.
Sustainable: This term refers to “raw material” from which the resin is made. Some resins are sustainable if they are derived from renewable raw materials, but they may not be biodegradable or compostable, in which case recycling these materials promotes recycling.
Recycling: Practice of keeping resources in use for as long as possible by recycling or finding alternative uses for materials.

 

Mold design, handling, and processing methods for bioresins are different from those of existing polymers, and they are not direct replacements for existing polymers. Although often listed as interchangeable materials, they are not direct substitutes. Here are mold challenges and considerations when processing bio-resins:
Growing demand for bio-resins in European and American markets is reshaping plastics industry, but their high viscosity and heat sensitivity bring new challenges to molds and processing.
Viscosity issues
High viscosity of many bio-resin materials leads to increased injection pressure. To avoid equipment damage, high-strength and stable steel must be used.
Tolerance control of injection molds and other processing equipment is critical to prevent leakage. In addition, improving wear resistance of mold can help solve such problems.
Classification of mold insert materials
Inserts can be divided into following types, including base steel grades (such as hot work die steel H13 type), corrosion-resistant steel grades (420, 440C, Bailu M390 Microclean) and high-alloy cold work die steel or powder metallurgy steel grades (D2, PMM4, 10V, K294 Microclean, etc.). Each steel grade has unique strength and wear resistance characteristics and is suitable for different types of resin processing.
Strength Enhancement Mechanism
Tensile and Compressive Strength
Can be enhanced by increasing hardness, carbide content and type, and adding alloying elements (such as carbon C, chromium Cr, molybdenum Mo, vanadium V, cobalt Co, niobium Nb).
Case Comparison
Base steels such as H13 have very few carbide-forming elements, and their strength is mainly achieved by increasing hardness through heat treatment.
Corrosion-resistant 400 series steels, high-alloy cold-working die steels, and powder metallurgy (PM) steels usually achieve higher strength mainly through hardness. However, these steels often contain more and more diverse carbide types in their microstructure, making them stronger than base steels.
Material Property Trade-offs
Disadvantages of High Carbide Content: Increasing carbide content in steels leads to reduced toughness and may cause cracking problems in some applications.
Role of Cobalt (Co): Adding cobalt to steel can increase strength without increasing carbide content, thereby reducing deformation and wear on mold surface and reducing risk of leakage.
Mold surface wear resistance treatment
Mold surface resistance to abrasive wear can be improved by same factors mentioned above (see figure below).
When processing highly abrasive resins, there may be a risk of "steel substrate erosion and carbide residues".

 

In certain cases, use of physical vapor deposition (PVD) coatings can significantly improve mold performance. A range of PVD/DLC (diamond-like carbon) coatings can increase surface hardness to more than 70 HRC while enhancing lubricity, maintaining stability in high-temperature processes, and preventing substrate erosion, as shown in above case.
Thermal sensitivity and shear sensitivity
Bio-resins exhibit a variety of thermal sensitivity properties: some are thermally stable, while others may degrade or soften at high temperatures. Similar to petroleum-based resins, effect of heat on bio-resins depends on many factors, including substrate type, curing process, additives and chemical structure.
Recommended selection of thermally conductive materials
To avoid degradation of mechanical and product properties due to overheating and shearing, it is recommended to use high thermal conductivity steel for injection molds, screws and extrusion dies.
Preferred materials with high thermal conductivity
Low alloy matrix steels are commonly used due to their excellent thermal conductivity, such as L6, S7 and H13 steels. These materials are often selected for long core parts, especially because they have excellent toughness and strength.
Limitations of L6 and S7 steels: Sensitive to temperature and lack secondary hardening ability. When exposed to high temperatures during molding process or PVD coating process, they may lose hardness and strength due to softening, and even deform.
Characteristics of H13 steel: It has secondary hardening ability, but there is a hardening threshold limit, and it may become brittle at high hardness.
Alternatives: W360 steel also has secondary hardening characteristics, can maintain high hardness, strength and toughness at high temperatures, and its optimized chemical composition significantly improves its thermal conductivity (see figure below).

 

Application of 420 stainless steel in mold inserts: 420 stainless steel is a common material for mold inserts. Although standard 420 steel grades generally perform well in terms of corrosion resistance, strength and toughness, their low thermal conductivity can pose challenges to mold cycle and thermal management. M333 steel optimizes chemical composition of 420 steel to make its thermal conductivity closer to H13 steel grades. Following is a comparison of thermal conductivity of M333, 420 and H13 steels:

Chemical composition of bioresins is a key factor in determining their corrosion characteristics. Under high temperature and high pressure processing conditions, corrosiveness of bioresins may change, leading to compatibility issues with mold materials. In addition, environmental factors can also affect corrosion characteristics of some bioresins.
Protection recommendations
Use stainless steel materials as much as possible, and non-stainless steel molds need to be coated with inert coatings;
Thermal degradation of resins will increase corrosion, and processing temperature needs to be controlled.
Corrosion Mechanisms of Martensitic Stainless Steels
Using 420 and 440 series stainless steels as an example, it is necessary to understand how carbide content and heat treatment affect properties of steel and its ability to resist pitting and oxidation corrosion. Main strategies to reduce risk of corrosion include: selecting steel with low carbide content and tailoring heat treatment process according to resin type.
Comparison of tempering processes for 400 series stainless steels:
Low-temperature tempering (about 400-550°F): Usually provides the best corrosion resistance because less carbide precipitation occurs during this heat treatment;
High-temperature tempering (about 940-1000°F): Causes more carbide precipitation, weakening corrosion resistance.
Corrosion Inducement Analysis: Main carbide-forming elements in 400 series stainless steels are carbon (C) and chromium (Cr). When carbides precipitate from solid solution, they consume chromium in surrounding area. When chromium concentration in these areas drops below 12%, "stainless" properties are lost, making mold surface susceptible to corrosion phenomena such as pitting (see figure below).

 

To address above issues, M333 steel has modified chemical composition of traditional 420 steel by reducing the overall carbide content. This adjustment not only improves thermal conductivity, but also significantly enhances corrosion resistance. Following images show difference in carbide precipitation in M333 and 420 ESR (electroslag remelted) steels, highlighting its impact on corrosion resistance (see figure below).
Core of technical improvement of M333 steel
Carbide content control: Traditional 420 steel is depleted in chromium due to precipitation of carbides (such as Cr₂₃C₆). When volume fraction of carbides exceeds 5%, local chromium content may be lower than passivation threshold of 12%;
M333 steel controls volume fraction of carbides below 3% by reducing carbon content (C≤0.35%) and optimizing alloy ratio (such as increasing Ni content to 1.5-2.0%) to avoid grain boundary chromium depletion.
Mechanism of improving thermal conductivity: Carbides are poor thermal conductors (thermal conductivity is about 10 W/(m・K)), and their reduction increases proportion of matrix ferrite (thermal conductivity is about 45 W/(m・K));
Thermal conductivity of M333 steel reaches 30-35 W/(m・K), which is about 30% higher than that of 420 steel (22-26 W/(m・K)), close to level of H13 steel, and can shorten mold cooling time by 15-20%.

 

Bioresins will degrade when exposed to moisture, and may also damage machinery and molds. To alleviate this problem, it is important to properly dry material before processing to remove adsorbed and absorbed moisture.
Key risks caused by moisture
Resin degradation mechanism: Ester bonds of bio-based polyester resins (such as PLA and PBAT) are susceptible to water-catalyzed hydrolysis, resulting in a decrease in molecular weight (for example, when moisture content of PLA is greater than 0.02%, tensile strength decreases by 15%);
Starch-based resins (such as corn starch blends) will swell after absorbing water, resulting in melt fluidity fluctuations and injection molded parts weight deviation exceeding ±3%.
Damage to equipment and molds: Water vaporizes at high temperatures to form bubbles, resulting in high-pressure cavitation in mold (especially in gate area), accelerating mold surface wear;
Water vapor combines with resin thermal degradation products (such as lactic acid) to form an acidic solution, which intensifies electrochemical corrosion of mold steel (such as 420 stainless steel).

 

Bio-resin market is growing rapidly, especially in United States and Europe. However, due to high viscosity, thermal sensitivity, and demand for more wear-resistant molds of this type of material, its application faces many challenges in mold design and processing. Addressing these challenges through processing technology innovation and focusing on sustainable development is the key to achieving large-scale application of bio-resins. This will help promote a circular economy, reduce plastic waste and promote a sustainable future.