In fiercely competitive manufacturing industry, lifespan, production efficiency, and product quality of injection molds directly impact a company's core competitiveness. A precision mold is incredibly valuable, so how can we keep it "strong and healthy" even after years of use? The key lies in mold surface treatment technology. It acts like "functional armor" for mold, significantly improving its wear resistance, corrosion resistance, adhesion resistance, and fatigue strength. This fundamentally addresses premature mold failure and is the key to extending mold life. This article systematically reviews mainstream surface treatment technologies to provide a reference for mold selection.

 

During injection mold service, cavity surface is subjected to extreme operating conditions. Surface treatment targets these pain points:
Wear and Strain: Engineering plastics containing glass fiber (GF) and minerals (such as talc) (e.g., PA66 + 30% GF, PBT + 20% GF) act like "high-speed abrasives" during injection molding process, scouring mold cavity surface. This can cause dimensional deviations, surface roughness, and ultimately, scratches and dimensional defects in molded part.
Corrosion and Rust: PVC, fluoroplastics (such as PTFE), and flame-retardant plastics (containing bromides) are susceptible to decomposition at high temperatures of injection molding (180-300℃), releasing corrosive gases. These gases can penetrate mold steel surface, destroying matrix structure, exacerbating surface pitting and flaking.
Adhesion and seizure: Highly adhesive plastics such as TPE, PP, and PE tend to physically adhere to mold cavity surface while molten, making demolding difficult. This can lead to manual demolding (reducing production efficiency) or, in severe cases, strain on part and mold cavity (directly damaging mold).
Fatigue cracking: "Heating-cooling" cycles during injection molding process (temperature differences can reach over 200℃) generate alternating thermal stresses on mold surface. Over time, these stresses can easily form microscopic thermal fatigue cracks (commonly known as "crazing"). Crack propagation can lead to mold cavity collapse, resulting in direct mold failure.
Core value of surface treatment technology lies in its ability to modify chemical composition, microstructure, or physical properties of mold surface to specifically mitigate these failure risks, ensuring stable mold performance under demanding operating conditions.

Based on technical principles, injection mold surface treatment can be categorized into four main categories: chemical heat treatment, surface coating/film coating, surface quenching, and electrospark hardening. Each process has significantly different application scenarios and performance characteristics, requiring selection based on specific needs.

This technology uses high temperatures to infiltrate elements such as carbon, nitrogen, and boron into mold surface, forming a high-hardness compound layer. Its advantage is a strong bond between infiltrated layer and substrate, making it resistant to peeling.
Nitriding/Nitrocarburizing (Soft Nitriding): The most widely used general-purpose method. At low temperatures of 500-570℃, active nitrogen atoms are infiltrated into surface, forming a nitride layer with a hardness of 1000-1200 HV. Its advantages include minimal mold deformation, balanced wear resistance, and corrosion resistance. It is suitable for precision injection molds and die-casting molds, especially where low deformation is required.
Boriding (TD Treatment): "King" of ultra-high wear resistance. In a high-temperature molten borax salt, boron atoms bond with carbon in mold steel, forming a boride layer with a hardness exceeding 3000 HV. This layer offers excellent wear resistance and adhesion resistance, making it suitable for molds used to inject high-fiberglass plastics. However, it requires a matrix carbon content greater than 0.4%.
Chromizing and vanadiumizing: These methods offer both wear resistance and corrosion resistance. Similar in principle to boronizing, they form a high-hardness carbide layer. Chromizing offers improved corrosion resistance (suitable for molds containing fluorine-containing plastics), while vanadiumizing offers improved toughness (suitable for molds for thin-walled plastic parts).

Functional films with a thickness of 5-20μm are deposited on mold surface through physical or chemical methods, allowing for flexible adaptation to different working conditions.
PVD (Physical Vapor Deposition): Choice for low-temperature precision. In a vacuum environment, target atoms such as titanium and chromium are vaporized and combined with nitrogen and acetylene to form coatings such as TiN (golden), CrN (bright silver), and TiAlN (purple-black). Process temperature is 200-500℃, with minimal impact on mold substrate. Advantages include high coating hardness and a smooth surface, making them suitable for key components such as cores, cavities, and ejector pins, offering corrosion and adhesion resistance.
CVD (Chemical Vapor Deposition): A high-bonding, heavy-duty solution. At temperatures of 800-1000℃, TiC and TiN coatings are formed through a vapor phase reaction. These coatings offer strong bonding and excellent wraparound properties (capable of covering complex cavities). However, high temperatures can easily cause mold deformation, requiring subsequent reheat treatment. They are primarily used for molds requiring high wear resistance and low precision requirements.

This process alters surface microstructure through "localized heating and rapid cooling," eliminating need for additional elements and making it suitable for targeted strengthening of wear-prone areas.
Laser Surface Quenching/Cladding: A powerful tool for precision repair. A high-energy laser beam is rapidly scanned across mold surface to achieve localized quenching (forming a martensitic structure with a hardness of 55-62 HRC) or to deposit an alloy layer. Advantages include concentrated energy, minimal deformation, and ability to repair worn cavity edges, making it suitable for localized mold strengthening and repair.

 

Flame quenching: A low-cost solution for large molds. Using a high-temperature flame to heat surface followed by quenching, it requires simple equipment and is suitable for on-site processing of large molds. However, its accuracy and uniformity are poor, making it unsuitable for precision molds.

Pulsed discharges are used between electrode and mold surface to melt electrode material (such as carbide) and coat mold surface. Advantages include portability, flexible operation, and absence of mold disassembly. It is suitable for repairing micro-damage such as chipping and scratches, making it a cost-effective online maintenance method.

Selecting a surface treatment process should adhere to the "best fit" principle, comprehensively considering following four factors:
Mold Steel: Different steels have different compatibility. For example, boronizing requires a carbon content greater than 0.4%. For hardened precision molds, low-temperature processes (nitriding, PVD) should be used to avoid softening the substrate.
Plastic Materials: For materials containing corrosive components like PVC, nitriding or PVD-CrN are preferred. For materials containing wear-resistant fillers like high-fiberglass, boronizing or PVD-TiAlN are preferred. For sticky plastics (TPE, PP), nitriding or PVD coating is preferred.
Mold Structure and Precision: For precision molds, choose low-temperature, low-deformation nitriding or PVD. For large molds, flame quenching or spark quenching can be considered. For molds requiring repair, spark quenching is preferred for minor damage, while laser cladding is preferred for moderate wear.
Cost and Delivery Time: For low-cost, short-delivery options, choose electrospark quenching or flame hardening; for medium-cost, medium-delivery options, choose nitriding or PVD; for high-cost, long-delivery options, choose boronizing, CVD, or laser cladding. It's crucial to balance budget and performance requirements.
Trend: Combined Processing Achieves "1+1>2"
A single process cannot meet multiple requirements, so combined processes are becoming increasingly popular. For example, "nitriding + PVD" first forms a support layer, followed by PVD to create an ultra-hard film, enhancing adhesion and load-bearing capacity. "Laser quenching + nitriding" first strengthens key areas, followed by overall nitriding, achieving gradient hardening and extending service life by 2-3 times compared to a single process.

Pretreatment is fundamental: Mold must be finely polished (even to a mirror finish) to ensure a surface free of oil, dirt, and defects. A rough surface will magnify post-treatment problems.
Process control is crucial: Parameters such as temperature, time, and atmosphere must be precisely controlled. For example, in nitriding, ammonia decomposition rate must be controlled, and in PVD, vacuum level must be controlled. Otherwise, coating peeling and uneven hardness can occur.
Post-processing is essential: some processes require secondary polishing to reduce roughness, or stress-relief annealing (such as after CVD and laser cladding) to prevent mold cracking. Post-processing testing, including hardness and bonding strength, is required to ensure compliance.
Final Note: Mold surface treatment has evolved from an "optional process" to a "must-have" for improving mold performance, ensuring stable production, and reducing costs. Correct selection and application of surface treatment technology can extend life of a mold and maintain its competitiveness in the market. The key lies in precisely matching mold to working conditions, avoiding blindly pursuing high-end performance or compromising on cost, ensuring that each mold is optimally equipped.