Basics of Mold Processing

1. Molding Method – Selection can be made from two fundamental types of material.

A) Hot-work tool steel, which can withstand relatively high temperatures encountered in die casting, forging, and extrusion.

B) Cold-work tool steel, which is used for blanking and shearing, cold forming, cold extrusion, cold forging, and powder compacting.

Plastic Materials – Some plastic materials generate corrosive by-products, such as PVC. Corrosion can also be caused by factors such as prolonged residence of plastic material in the injection machine barrel, condensation due to downtime, corrosive gases, acids, cooling/heating cycles, water, or storage conditions. In such cases, the use of stainless steel is recommended.

Mold Size – Large molds often utilize pre-hardened steel. Through-hardened steel is frequently used for small molds.

The summary of Mold Service Life 

Molds intended for long-term use (>1,000,000 cycles) should be made of high-hardness steel with a hardness of 48–65 HRC.

Molds for medium-to-long-term use (100,000 to 1,000,000 cycles) should use pre-hardened steel with a hardness of 30–45 HRC.

Molds for short-term use (<100,000 cycles) may be made from mild steel with a hardness of 160–250 HB.

Surface Roughness – Many plastic part mold manufacturers prioritize excellent surface finish. For instance, when sulfur is added to improve machinability, surface quality is compromised. High-sulfur steel also becomes more brittle. These sulfide inclusions act as hard particles; during subsequent polishing of the mold cavity, they can be pulled out or leave microscopic pits, leading to surface defects such as "pinholes" or "orange peel," which prevent achieving a high-gloss mirror finish. Therefore, for mold designers and purchasers, material selection should not focus solely on "ease of machining." Priority must be given to the final mold's surface quality requirements and overall service life, thereby avoiding the use of high-sulfur steel, which would be penny-wise and pound-foolish.

2. What is the Primary Factor Affecting the Machinability of Materials?

The chemical composition of the steel is crucial. Generally, the higher the alloy content of the steel, the more difficult it is to machine. As the carbon content increases, machinability decreases.

The structure of the steel is also very important for machinability. Different structures include forged, cast, extruded, rolled, and machined. Forgings and castings often have surfaces that are very difficult to machine.

Hardness is a major factor influencing machinability. The general rule is that the harder the steel, the more difficult it is to machine. High-Speed Steel (HSS) can be used to machine materials with a hardness up to 330-400 HB; HSS with Titanium Nitride (TiN) coating can handle materials up to 45 HRC; for materials with a hardness of 65-70 HRC, it is necessary to use cemented carbide, ceramic, cermet, or Cubic Boron Nitride (CBN) tools.

Non-metallic inclusions generally have a detrimental effect on tool life. For example, Al2O3 (alumina), which is a pure ceramic, is highly abrasive.

Finally, residual stress can cause machinability issues. A stress-relief operation is often recommended after rough machining.

3. Impact of Steel on Molds and Future Trends

The performance, lifespan, and cost of molds are directly influenced by steel selection. Key factors include:

Chemical Composition and Structure: Carbon and alloy content determine hardness, toughness, and wear resistance. High-alloy steels offer wear resistance but are difficult to machine, requiring a balance between performance and cost. Surface defects from forging or casting increase machining challenges.

Hardness and Machinability: High hardness improves wear resistance but raises machining costs. Tool materials (e.g., HSS, carbide, CBN) must be selected based on mold lifespan (e.g., >1 million cycles require HRC 48-65 steel).

Surface Quality Requirements: High-polish demands (e.g., mirror finish) necessitate low-sulfur, clean steels to avoid defects caused by sulfide inclusions.

Adaptability to Special Environments: Corrosive plastics (e.g., PVC) require stainless steel; large molds often use pre-hardened steel to minimize heat treatment distortion.

4 . Mold Machining Future Trends

High-Performance Material Development: New mold steels (e.g., powder metallurgy steels) combining high hardness, toughness, and machinability.

Advanced Surface Treatments: Coatings (e.g., TiN, DLC) to enhance wear and corrosion resistance, reducing reliance on extreme base material properties.

Digital and Intelligent Material Selection: Big data and AI simulations to precisely match steel properties with mold operating conditions, optimizing cost and lifespan.

Sustainable Manufacturing: Promotion of repairable mold steels and low-carbon alloys to reduce resource consumption.