Detailed Scheme for Mold Steel Selection in Injection Molding of MDPE Material

MDPE (Medium Density Polyethylene) is a polyolefin material with medium crystallinity, moderate melt viscosity and good flowability, with a wide molding temperature range and no obvious corrosiveness. Its requirements for wear resistance and corrosion resistance of mold steel are much lower than those of materials such as PVC and glass fiber-filled POM. However, for long-term mass production, it is still necessary to take into account polishability, wear resistance, rust resistance and processing economy. Combined with the molding characteristics of MDPE, mold service life and product appearance requirements, a graded steel selection scheme can be formulated based on three core dimensions: product appearance grade, production volume and mold cavity structure. Meanwhile, a complete and implementable material selection system can be formed by matching steel for mold base, core, insert, as well as venting and cooling systems.

1. Core Requirements of MDPE Molding Characteristics for Mold Steel

The molding temperature of MDPE is generally 180–220℃, and a mold temperature of 30–60℃ is sufficient to ensure crystallization and appearance quality. The melt causes minimal abrasion to the mold cavity and produces no acidic corrosive decomposition products, so high corrosion resistance is not required for the steel. The core demands focus on good polishing performance to ensure smooth surface of molded parts, moderate hardness to meet wear resistance in small and medium batches, easy machinability to reduce mold making costs, and thermal stability suitable for conventional injection temperatures. High-hardness and high-alloy mold steels should be avoided to prevent waste in processing and cost, and only upgraded for high-gloss, high-transparency MDPE products or ultra-large-scale production.

2. Graded Steel Selection Scheme by Production Volume & Product Appearance

Small-batch Trial / Prototype Mold (Volume < 50,000 shots, simple structure, low appearance requirements)

Such molds focus on rapid prototyping and low cost. Low-carbon mold steel with easy machining and no complex heat treatment is preferred. The cavity and core can use 45# carbon steel, normalized or quenched and tempered to 28–32 HRC, ensuring no obvious wear in short-term production. The mold base adopts standard S50C, and ejector pins, guide pins and guide bushings use SUJ2 bearing steel. No special steel is required for venting grooves or cooling channels. The overall processing cycle is short and cost is extremely low, suitable for small-batch trials of general MDPE structural and non-appearance parts, avoiding waste from pre-hardened or mirror steel.

Medium-batch Production Mold (50,000–500,000 shots, standard appearance products, no high-gloss requirements)

This is the most common mass production scenario for MDPE parts, balancing wear resistance, polishability and cost performance. P20 (718S) pre-hardened plastic mold steel is the first choice for cavity and core, with pre-hardened hardness of 30–35 HRC. It can be directly polished to Ra 0.8–1.6 μm after cutting without subsequent heat treatment, suitable for standard MDPE appearance parts without pits or orange peel defects. If the product has slight undercuts, thin-wall structures or requires extended service life, 718H pre-hardened steel with hardness 33–38 HRC can be used for better wear resistance and polishability. The mold base remains S50C, and slides and inserts use P20 or NAK80. Cooling channels use ordinary brass or stainless steel tubes, providing high stability for mainstream MDPE products such as home appliance parts, packaging containers and daily plastic products.

Large-batch Production Mold (Volume > 500,000 shots, high-appearance / high-gloss MDPE products)

For long-term mass production and high-surface-quality MDPE products (e.g., high-gloss housings, thin-wall containers, weld-line-free precision parts), improved mirror polishability and long-term wear resistance are required. NAK80 mirror mold steel is preferred for cavity and core, with pre-hardened hardness 37–43 HRC, polishable to mirror finish Ra 0.2–0.4 μm without porosity or impurities, suitable for high-gloss MDPE molding with no heat treatment and small machining deformation. For higher wear resistance and dimensional stability, S136 stainless steel (annealed then quenched to 48–52 HRC) can be selected, which offers excellent polishability and slight rust protection for humid production environments. The mold base uses hardened S50C, slides and inserts use VIKING wear-resistant steel, ejector pins use SKD61 hot-work die steel, and cooling channels use stainless steel tubes to ensure stable production with no obvious wear and consistent dimensional accuracy over one million shots.

MDPE Molds with Special Structures (Complex cavities, multi-slides, deep-cavity products)

Deep-cavity, multi-undercut and multi-slide MDPE molds require balanced toughness and anti-deformation performance to avoid cavity deformation or slide jamming during long-term production. The main cavity uses 718H or NAK80, while slides, moving components and inserts use SKD61 hot-work die steel quenched and tempered to 45–48 HRC, with balanced toughness and wear resistance and excellent thermal fatigue resistance for repeated opening and closing. Sprue bushings and runner bushings use SKD61 or H13 to prevent runner wear from long-term melt scouring, ensuring smooth MDPE flow without stagnation or burning.

3. Matching Steel Selection Details for Various Mold Components

Mold base: S50C carbon structural steel; tempered treatment available for mass production molds to ensure rigidity and dimensional stability.

Cavity / Core: 45# steel for small batches; P20 / 718H for medium batches; NAK80 / S136 for large batches or high-gloss applications.

Slides / Inserts / Ejector pins: P20 / 718H for general use; SKD61 for stressed areas; VIKING for high-wear scenarios.

Guide pins / Guide bushings / Locating pins: SUJ2 bearing steel, quenched to 58–62 HRC to ensure guiding precision.

Cooling channels: Brass tubes for standard molds; 304 stainless steel tubes for long-life molds to resist rust and clogging.

Sprue bushings / Runners: SKD61 or H13, resistant to melt erosion and thermal deformation.

4. Core Principles for MDPE Mold Steel Selection

Priority on economy: MDPE is non-corrosive and low-abrasive; blind selection of high-alloy corrosion-resistant steel or high-speed steel is strictly prohibited. Graded selection by production volume meets requirements sufficiently.

Adaptability to machinability: Pre-hardened steels are preferred to avoid quenching deformation and shorten mold manufacturing cycle.

Appearance matching: P20 / 718H for general appearance; NAK80 / S136 for mirror finish; excessive polishing is unnecessary.

Mass production stability: Emphasize hardness and wear resistance for large-batch production; focus on processing cost and efficiency for small and medium batches.