1. Introduction
P20 (3Cr2Mo) is the most widely used pre-hardened plastic mold steel in the mold industry. Unlike hot-work steel, P20 is delivered with pre-set hardness of 28–36 HRC, which allows direct machining without secondary heat treatment. It is widely used for daily plastic products, household appliances, injection molds and blow molds.
Many mold factories believe P20 is “simple and stable”. However, in actual production, P20 molds frequently face problems such as poor polishing effect, uneven etching texture, fast surface wear and welding repair failure. Most of these issues are not caused by material quality, but by incorrect machining, polishing and daily maintenance methods. Standardized operation can significantly improve P20 mold performance and service life.
Figure 1: High-quality P20 mold steel block for plastic injection mold
2. Common P20 Mold Failure Problems & Causes
2.1 Orange Peel Surface and Poor Polishing Quality
Many P20 molds cannot achieve high-gloss finish after polishing, showing foggy surface or orange peel texture.
Main causes: Residual stress from rough machining, uneven internal hardness of large steel blocks, and unreasonable polishing grain sequence. Stress concentration makes the mold surface unable to form uniform mirror effect.
2.2 Fast Cavity Wear in Mass Production
P20 molds wear quickly when producing abrasive plastics, resulting in product burrs and short mold life.
Main causes: P20 original hardness is relatively low; without surface strengthening treatment, the surface is easy to scratch and wear under long-term injection friction.
2.3 Uneven Texture and Etching Defects
When making mold texture and etching patterns, the surface appears uneven, mottled or distorted.
Main causes: Material component segregation, unclean surface residue, and unrelieved machining stress leading to inconsistent corrosion resistance.
2.4 Welding Repair Area Easy to Crack and Wear
Welded positions often produce different color, poor gloss and repeated damage.
Main causes: Mismatched welding wire, no preheating before welding, and missing stress relief after repair.
Figure 2: Precision CNC machining process for P20 injection mold
3. Key Machining & Polishing Solutions
3.1 Reasonable Machining Process
P20 molds should avoid heavy cutting in one single pass. After rough milling, stress relief annealing is strongly recommended to eliminate internal cutting stress, which is the key to prevent later polishing deformation and orange peel defects. Finish machining needs uniform and small allowance to ensure stable surface quality.
3.2 Standard Polishing Steps
Polishing must follow progressive abrasive grain order. Never jump coarse sand directly to ultra fine sand. Completely remove tool marks and stress layers before fine polishing. Only stress free surface can achieve perfect mirror gloss and uniform texture etching effect.
3.3 Welding Repair Standards
Use special P20 matching welding wire. Preheat the mold before welding to avoid cold crack. After welding, perform stress relief tempering and re polish the heat affected area to restore original surface performance.
Figure 3: Professional surface inspection and fine polishing of P20 mold
4. Effective Daily Maintenance Methods
1. Clean mold cavity residues after each production batch to avoid plastic corrosion and surface scratches.
2. Regularly check mold surface for tiny scratches and wear; repair and polish in time to prevent defect expansion.
3. For long-term idle molds, clean and coat anti rust oil to protect surface accuracy.
4. For high-volume production molds, adopt nitriding treatment to improve surface hardness and wear resistance, greatly extending mold life.
5. P20 vs 718 Material Selection Guide
P20: Cost-effective, easy to machine, suitable for conventional plastic products with ordinary appearance requirements.
718 (1.2738): Higher purity, more uniform hardness, better polishing and etching performance, suitable for high gloss and high appearance precision molds.
6. Conclusion
P20 is a cost-effective and versatile plastic mold steel. Most P20 mold failures are avoidable through standardized machining, scientific polishing skills and daily maintenance. Reasonable process control can significantly improve mold surface quality, stabilize product appearance and extend overall mold service life.
FAQ
Q1: Why do P20 molds easily produce orange peel during polishing?
A1: It is mainly caused by unrelieved machining residual stress. Stress relief before polishing can greatly improve surface gloss.
Q2: Does P20 mold need heat treatment?
A2: Standard P20 is pre-hardened and can be directly machined. Only high-precision molds need additional stress relief treatment.
Q3: How to improve P20 mold service life?
A3: Optimize machining stress, standardize polishing, maintain daily cleaning, and properly use nitriding strengthening for mass production molds.