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What is Plastic Mold Steel

Plastic mold steel is mainly used to manufacture molds for forming and processing plastic materials. It is classified as “P group steel” in the AISI classification system and as cold work tool steel in the EN/DIN standards. The temperature of plastic molding processes is usually below 200°C. Compared with metal molding, the heat resistance or wear resistance of plastic mold steel is often not critical, but the requirements for surface quality are very high.

Many plastic mold steels are supplied in a prehardened condition (e.g., P20 grades at 32 HRC, DIN 1.2711 at 40 HRC). This eliminates the need for the mold maker to perform final heat treatment, minimizing the risk of distortion and reducing overall manufacturing time and cost. If not prehardened, the steels are typically sold in the annealed condition to facilitate shaping and then hardened by heat treatment.

Specific Steel Grades

Several specific tool steel grades are commonly used for plastic molds, chosen based on the part’s complexity, production volume, and required surface finish:

1. P-Series Steels. The most common grades are P20(1.2311), P20S(1.2312), and P20Ni(1.2738). Low-carbon grades, such as P2, P4, and P6, are typically used for “hubbed” (hobbed) cavities, where a hardened steel master hub is forced into a softer die blank to form the impression.
2. Hot-Work Steels – H13. An excellent mold steel for plastic molding and die casting dies. It exhibits excellent resistance to tempering, less tendency to crack, and minimal distortion with air hardening. When heat-treated to higher hardness (>50 HRC), it offers excellent polishability and responds very well to nitriding, achieving high surface hardness.
3. Martensitic Stainless Steels–420 stal nierdzewna and modifications. Commonly used for injection molds, especially for corrosive plastics or in severe atmospheric conditions, as it provides inherent corrosion resistance without the need for chromium plating. It can achieve a mirror finish and good dimensional stability after heat treatment. However, tempering in a specific range (425-595°C) can cause embrittlement and loss of corrosion resistance.
4. Shock-Resisting Steels – S7. Recommended for molds requiring very high shock resistance.
5. Cold-Work Tool Steels (e.g., A2, D2). Used for applications demanding higher wear resistance, particularly when molding abrasive plastics (e.g., fiber-reinforced plastics).

Właściwości

1. Polishability & Texturability. Molten plastics can reproduce minute details on the mold surface, making a perfect, defect-free surface crucial for the final product’s appearance. High surface hardness of plastic mold steel significantly enhances polishability and minimizes polishing defects like “orange-peel” (localized strain hardening).
2. Hardness and Strength. The high surface hardness and high core strength of plastic mold steel can prevent the cavity from “sinking” under high injection pressures. While the hardness of prehardened plastic molds typically ranges from 30-40 HRC, they can be surface hardened via carburizing or nitriding for higher wear resistance.
3. Wytrzymałość i odporność na wstrząsy. Molds are subjected to mechanical shocks during operation (opening/closing) and thermal shocks from rapid heating/cooling cycles, especially in fast-cycling machines. Good toughness can prevent cracking.
4. Stabilność wymiarowa. Minimizing distortion or dimensional change during heat treatment is critical for precision molds
5. Obróbka skrawaniem. Machining constitutes a significant portion (up to 70%) of the total mold manufacturing cost. Steels that are easier to machine can significantly reduce overall costs.
6. Odporność na korozję. Essential when processing corrosive plastics (e.g., polyvinyl chloride (PVC), which can release HCl when degraded) or when molds are stored in humid environments.
7. Spawalność. Good weldability is beneficial for mold repair and maintenance, which can extend tool life.

Obróbka powierzchni

• Nawęglanie: A process where carbon is diffused into the surface to increase hardness and wear resistance, particularly for low-carbon mold steels like P2 and P6.
• Azotowanie: Diffuses nitrogen into the surface, significantly increasing hardness, corrosion resistance, and reducing friction. H13 steel responds very well to nitriding.
• Chromium/Nickel Plating: Can be applied for wear resistance or corrosion protection, especially against aggressive plastics like PVC. However, the use of corrosion-resistant stainless steels can eliminate the need for plating.
• Flame Hardening: Used for localized hardening of specific areas subject to wear or brinelling.

Challenges and Defects

• Polishing Defects: Can include “pin-holes” from nonmetallic inclusions and “orange-peel” due to localized strain hardening during over-polishing.
• Texturing Issues: Poor surface preparation or microstructural inhomogeneities can lead to uneven etching results.
• Distortion during Heat Treatment: Expansions, contractions, warpage, and shape changes can occur, necessitating precise control and, ideally, the use of prehardened or air-hardening grades.

Aplikacje

Injection Molding of Thermoplastics
Compression Molding of Thermoset Plastics
Blow Molding and Extrusion
Die Casting Dies: Used for low-melting alloys like tin, zinc, and lead alloys.
Glass Product Molds: Some mold steels are also used for glass manufacturing, requiring properties like high wear resistance, resistance to scaling, high-temperature strength, and polishability.

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