In the material selection list for plastic molds, 718 mold steel has always been a "low-key powerhouse." It doesn't boast corrosion resistance like S136, nor does it carry the hot-work banner like H13. But in large appliance housings, automotive interior parts, and daily-use packaging—high-volume plastic mold applications—718 shows up with remarkable frequency. The problems come along too: many treat 718 as a "pre-hardened steel, just rough and finish machine it," only to find it won't polish bright, won't texture evenly, or shows inconsistent hardness mid-machining. Today, let's talk about the details in 718 machining that are easy to overlook.
1. Understand 718's "Temperament" First
718 is a pre-hardened plastic mold steel, typically supplied at HRC 28-34. This hardness range is clever: it allows direct milling, drilling, and tapping, while being hard enough to support medium-volume injection production—no heat treatment needed, shortening mold lead time.
But "pre-hardened" does not guarantee perfectly uniform hardness. For poorly-manufactured thick plates, core-surface hardness deviation of several HRC points may occur. If your cavity is deep with large machining allowances, measure hardness at different locations after roughing, and confirm the material's specific condition with your supplier to avoid sudden tool wear during finishing.
2. Polishing: Can 718 Achieve a Mirror Finish?
This is one of the most frequently asked questions. The answer: Yes, but with conditions. 718's cleanliness and microstructure uniformity directly determine polishing results. Premium 718, vacuum-degassed with low inclusion content, can achieve a high surface finish suitable for most appliance housings and automotive interior parts. But if you're aiming for optical-grade mirror surfaces, 718 is not the best choice—S136 or NAK80 would be more appropriate.
Another key to polishing is avoiding "over-burning." Although 718 is pre-hardened, prolonged high-speed polishing generates heat, softening the surface and creating soft spots—resulting in an "orange peel" effect that worsens with more polishing. Use gradual polishing steps with adequate cooling; don't rush it.
| Comparison Items | 718 Mold Steel | P20 Mold Steel |
| Hardness (Delivery) | 28-34 HRC, pre-hardened | 28-32 HRC, pre-hardened |
| Impact Toughness | Good | Very good |
| Wear Resistance | Fair | Good |
| Size Stability | Stable, batch-sensitive | Good |
| Cost | Medium to high | Medium |
| Weldability | Fair | Fair |
| Main Use | Large plastic molds, automotive interior parts | General injection molds, textured parts |
As shown above, 718 mold steel offers a good balance of hardness, toughness, and polishability for large and high-volume plastic molds, but it demands attention to batch consistency and welding control. P20 mold steel, on the other hand, is more forgiving in polishing and texturing, making it a solid all-round choice for general injection molds. Neither is universally "better"—the right choice depends on your specific plastic mold application, surface requirements, and production volume.
3. Acid Etching (Texturing): A Hidden Test for Pre-Hardened Steel
718 generally performs stably in texturing, but there's a trap: different batches of 718 can show variations in etch depth and uniformity. This is because texturing results are influenced by hardness, microstructure uniformity, and inclusion content. If the mold cavity has been welded before texturing, the welded zone will almost certainly texture differently from the base material, leaving visible "patch marks."
So, when using 718 for plastic molds requiring texturing: first, complete all welding and repairs before texturing; second, confirm hardness and batch consistency with your texturing vendor; third, don't blindly chase "deeper is better"—excessively deep etching on 718 tends to expose internal inhomogeneities.
4. Common Misconception: 718 Is Not a "Universal Pre-Hardened Steel"
The most dangerous misconception: treating 718 as a "no heat treatment, weld anything, modify anything" universal material. In reality, 718 has mediocre weldability; welded zones tend to form hard, brittle martensite, causing problems in subsequent machining and polishing. If welding is necessary, preheat and use matching filler metal, followed by local tempering.
Also, 718 is not suitable for transparent-part mold cavities, nor for long-term service with highly corrosive plastics like PVC. Material selection has no absolute good or bad—only fit.
In one sentence: machining 718 mold steel is about "respecting its pre-hardened state"—don't treat it like soft steel, and don't expect it to do everything. Control polishing, texturing, and welding, and it will deliver stable returns in your plastic molds.
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FAQ
Q1: What is the delivered hardness of 718 mold steel?
A: Typically HRC 28-34, depending on plate thickness and the supplier's melting process. Core hardness in thick plates may be slightly lower than the surface.
Q2: Can 718 mold steel be polished to a mirror finish?
A: Premium 718 can achieve a high surface finish suitable for appliance and automotive interior appearance parts, but consistently reaching optical-grade mirror finish is difficult. For optical parts, choose S136 or NAK80.
Q3: What should I do if 718 mold steel textures unevenly?
A: First check for welded areas—welded zones will always texture differently from the base material. Then confirm batch hardness and microstructure uniformity. Complete all welding and repairs before texturing.
Q4: Is 718 mold steel suitable for PVC plastic molds?
A: Not really. PVC releases corrosive gases during processing, and 718's corrosion resistance is insufficient for long-term exposure. Choose corrosion-resistant mold steel like S136 instead.