The Ultimate Guide to Core Insert Hardness Selection for Injection Molds

In injection mold manufacturing, core insert hardness is not just a material parameter—it determines mold lifespan, part surface quality, dimensional stability, wear resistance, maintenance frequency, and total production cost. Many mold makers choose hardness only by habit, which leads to problems like rapid wear, chipping, cracking, rust, flashing, or short service life.

In this professional guide, we explain the complete, practical principles for selecting core insert hardness, based on plastic material, production volume, appearance requirements, mold structure, and component functions. This is the standard followed by high-quality mold factories worldwide.

1. Core Hardness Must Be Matched to Plastic Material

The plastic resin you use is the first and most important factor for setting hardness. Different plastics create different levels of abrasion and corrosion.

1.1 Standard General-Purpose Plastics

• ABS, PP, PE, PS, PMMA (non-abrasive)

• Low wear, no sharp particles

• Recommended hardness: HRC 28–38

• Typical steels: P20, 718H, NAK80

1.2 Glass-Filled & Highly Abrasive Plastics

• PA+GF, PBT+GF, PET+GF, LCP, PP+GF

• Glass fiber acts like sandpaper → extremely abrasive

• Minimum hardness: HRC 48–54

• Must use heat-treated steel: H13, SKD61, 2344

• Low hardness will cause grooves, size deviation, and short mold life

1.3 Corrosive & Acidic Plastics

• PVC, POM, flame-retardant materials, transparent engineering plastics

• Release corrosive gas during molding

• Prioritize anti-corrosion stainless steel

• Recommended hardness: HRC 48–52

• Typical steels: S136, 2316

2. Adjust Hardness Based on Expected Production Volume

Mold life directly depends on total shot count. Higher volume requires higher wear resistance.

2.1 Low Volume (Prototype / Small Batch)

• Under 100,000 shots

• Hardness: HRC 28–32

• Advantage: easy machining, low cost, short delivery time

2.2 Medium Volume

• 100,000 – 500,000 shots

• Hardness: HRC 32–42

• Balanced performance: wear resistance + toughness

2.3 High Volume / Long Life Mold

• Over 500,000 shots

• Hardness: HRC 48–52

• Full quenching + tempering required

• Stable size, low maintenance, long service life

3. Surface Appearance & Precision Requirements

Surface finish and precision are deeply affected by core insert hardness.

3.1 Matte / Etching / Texture Surface

• Needs uniform grain and good adhesion

• Best hardness: HRC 30–38

• If too hard → texture is uneven; if too soft → texture wears off quickly

3.2 High-Gloss / Mirror / Transparent Parts

• Requires polishing to #1, #2, or mirror finish

• Hardness: HRC 48–52

• Stable steel structure, no pitting, no deformation during polishing

• Must use stainless steel or high-quality mold steel

3.3 High-Precision Components

• Tight tolerances, complex dimensions

• Uniform hardness is more important than high hardness

• Hardness tolerance: ≤ ±2 HRC

• Avoid internal stress causing deformation after heat treatment

4. Balance Hardness and Toughness (Critical for Complex Structures)

Higher hardness = higher brittleness

This is the most common mistake in mold design.

When to Use Lower Hardness (Better Toughness)

• Deep cores, narrow pins, thin steel areas

• Sharp corners, undercuts, complex inserts

• High ejection force areas

• Hardness: HRC 35–42 to avoid chipping or breaking during production

When to Use Higher Hardness

• Large flat surfaces

• Simple structure, no thin steel

• High-wear areas (gates, runner areas)

5. Hardness Differences Between Cavity, Core, and Moving Components

Different mold parts serve different functions → different hardness standards.

5.1 Cavity Side (A Plate – Appearance Surface)

• Directly forms product exterior

• Needs good surface retention

• Hardness: HRC 32–52 (slightly higher than core side)

5.2 Core Side (B Plate – Structure Side)

• Withstands clamping force, ejection, and plastic flow impact

• Hardness: HRC 30–48

• Balances toughness and wear resistance

5.3 Sliders, Lifters, Inserts, Pins

• Reciprocating motion, high friction

• Hardness: HRC 35–45

• Not too hard (easy to break) or too soft (quick wear)

6. Common Mistakes to Avoid

1. Believing “harder is better”

   Too hard causes chipping, cracking, difficult machining, and higher cost.

2. Using low hardness for glass-filled materials

   Mold wears out within months.

3. Inconsistent hardness in the same mold

   Causes size mismatch and short lifespan.

4. Skipping proper heat treatment

   Leads to deformation, rust, and unstable hardness.

5. Using non-corrosion-resistant steel for corrosive plastics

   Mold rusts and damages part surface.

7. Summary of Professional Selection Principles

1. Start with plastic type → set minimum hardness.

2. Match production volume → decide wear resistance level.

3. Follow appearance & precision → choose polishing or etching suitability.

4. Balance hardness & toughness → avoid breakage in complex structures.

5. Set different hardness for cavity, core, and moving parts.

6. Always use proper heat treatment to ensure stability.