---
title: "What is 718h material in plastic injection molding?"
description: "A purchasing director evaluates high-cost 718h mold steel for a plastic housing project. The analysis clarifies 718h as a pre-hardened steel, details its longevity benefits for high-volume runs, and provides criteria to justify the investment based on cycle life and part quality."
url: "https://www.jaterson.com/qa/718h-material-plastic-injection-molding.html"
language: "en"
type: "Q&A"
category: "Injection Molding Q&A"
datePublished: "2026-10-18"
dateModified: "2026-10-18"
brand: "OK TOOL"
manufacturer: "OK TOOL"
answerCount: 9
---

# What is 718h material in plastic injection molding?

## Question

 I'm the purchasing director for a mid-sized electronics manufacturer, and we're sourcing a critical plastic housing for a new product line. The annual volume is projected at 300,000 units, and we need the injection mold to last for at least 500,000 cycles without significant wear or maintenance downtime. I've received several quotes, and the variance in mold cost is staggering—one supplier specifies their mold will be made from "718h" steel, which is 40% more expensive than the P20 steel others are proposing. My engineering team is skeptical of the added cost, but I've been burned before by a mold that degraded quickly, causing flash and dimensional issues that delayed production and increased scrap rates. I need to make a data-driven decision. Is 718h just a marketing term for a premium steel, or does it offer tangible longevity and part-quality benefits that justify the investment for a high-volume run like ours? What specific performance metrics should I request from the supplier to validate their claims, and are there any hidden costs or trade-offs I should be aware of when opting for a higher-grade mold material? 

## Answers
                            
### Answer 1 — Best Answer

Your concern is common and critical. The term "718h" refers not to a plastic resin but to a specific grade of pre-hardened mold steel, a common point of confusion. It is a nickel-chromium-molybdenum alloy known for excellent polishability, good hardness in the pre-hardened state (typically around 33-37 HRC), and better corrosion resistance than standard P20. For a 500,000-cycle project, the material choice is not a marketing gimmick but a fundamental engineering decision that directly impacts tool life, part consistency, and total cost of ownership.

The core issue you've faced—premature mold wear leading to flash and dimensional drift—is often a result of selecting a mold steel with insufficient hardness or wear resistance for the required cycle count. P20 is a good, economical steel for lower-volume runs or prototypes, but its lower inherent hardness means it will wear faster under abrasive conditions or high clamp pressures. 718h offers superior resistance to abrasion and polishing wear, which maintains the critical sealing surfaces of the mold cavity longer. This directly translates to a longer period of producing parts within specification, reducing the frequency of mold maintenance and the risk of unscheduled downtime.

To make a data-driven decision, you must request concrete metrics from your supplier. First, ask for a material certification for the 718h block, confirming its hardness. Second, inquire about the expected polishability (often cited as a # mirror finish capability) and how it correlates to the surface finish required on your housing. A key metric is the supplier's projected cycle life to first major maintenance for your specific plastic material—abrasive filled resins like glass-filled nylon will wear a mold faster than plain ABS. Finally, request their warranty or guarantee terms regarding core and cavity wear over a specified cycle count.

The justification for the higher upfront cost lies in the cost per part over the mold's life. Calculate the potential cost of production stops, manual de-flashing, part scrap, and mold refurbishment with a less durable steel. For 300,000 annual units, even a few hours of unplanned downtime or a 1% scrap rate adds significant cost. The trade-off with 718h is primarily the initial investment. There are no major hidden processing costs, as it is machinable in its pre-hardened state. However, ensure your supplier has experience machining it to avoid issues with internal stresses or achieving fine details. For your volume and quality requirements, 718h is a prudent, justifiable choice that prioritizes production stability over short-term savings.

To prevent future issues, establish clear acceptance criteria for the mold. This should include a first-article inspection report with critical dimension checks and a documented mold sampling process using production-grade material. Implement a preventive maintenance schedule with the supplier from the start, based on cycle counts rather than visible defects. This proactive approach, enabled by a more robust steel like 718h, turns your mold from a cost center into a reliable asset.

**status:** accepted
**Author:** Sophia Wang
**Date:** 2026-10-18

### Answer 2

The thermal properties of the mold steel directly influence the injection molding process window. 718h has a consistent microstructure that provides good thermal conductivity and stability. This means heat is dissipated from the plastic melt more uniformly compared to some lower-grade steels.

For your housing, this translates to more predictable and consistent cooling times, which is a primary driver of cycle time and part dimensional stability. A mold that cools unevenly can cause warpage or sink marks. The higher hardness of 718h also ensures that the mold's internal cooling channels maintain their shape and surface finish over hundreds of thousands of cycles, preventing a gradual loss of cooling efficiency that can creep into production and cause quality drift.

**status:** suggested
**Author:** Rachel Huang
**Date:** 2026-10-18

### Answer 3

From a tooling perspective, selecting 718h involves specific design and machining considerations. Its pre-hardened state allows for machining of the cavity and core inserts without subsequent heat treatment, avoiding the risk of distortion. However, this requires precise machining strategies.

The expected mold life for 500k cycles is achievable, but it depends heavily on part geometry. Sharp corners and thin ribs will experience higher stress concentrations. With 718h, you have a material that can better withstand this fatigue. A key recommendation is to review the mold design with the supplier to ensure adequate draft angles and appropriate steel thickness around high-wear areas, leveraging 718h's properties to their fullest.

**status:** suggested
**Author:** Eric Zhao
**Date:** 2026-10-18

### Answer 4

Machining 718h requires specific parameters to achieve the required tolerances and surface finish for a high-gloss housing. While it is machinable, tool wear is a factor to monitor. A skilled shop will use coated carbide tools and optimized feed rates to produce a cavity surface that requires minimal manual polishing.

The achievable surface finish from the CNC process directly impacts the final part appearance and the effort needed for mold texturing. Request data on the typical surface roughness (Ra) they can achieve after milling and prior to any polishing. This is a tangible indicator of their capability to handle the material and deliver a mold that meets your aesthetic requirements consistently.

**status:** suggested
**Author:** Amy Li
**Date:** 2026-10-18

### Answer 5

The choice of mold steel ultimately affects the functional performance of the assembled product. For an electronics housing, consistent wall thickness and critical fit dimensions like snap-fit locations or screw boss diameters are paramount.

A wear-resistant steel like 718h maintains these cavity dimensions far longer than softer steels. This reduces tolerance stack-up issues in assembly over the production life of the mold.

When validating the mold, perform a capability study (Cpk) on critical dimensions not just on the first samples, but again after a simulated high-cycle run. This will prove the steel's ability to maintain functional integrity.

**status:** suggested
**Author:** Emily Chen
**Date:** 2026-10-18

### Answer 6

On the production floor, the durability of 718h translates directly to operational efficiency. A stable mold minimizes the need for process adjustments to compensate for wear, allowing for a more automated and consistent production line. You can expect a more reliable cycle time, as there is less likelihood of issues like sticking parts or flash that require operator intervention or machine stoppages.

This consistency is crucial for integrating the molding cell into a larger automated assembly process. The higher initial cost is offset by reduced labor for monitoring and troubleshooting, leading to a lower cost per part in a high-volume setting.

**status:** suggested
**Author:** Jason Zhou
**Date:** 2026-10-18

### Answer 7

The decision between 718h, P20, or a stainless steel like S136 is a classic cost-performance trade-off. 718h sits in a sweet spot for high-volume production of engineering plastics. It offers better corrosion resistance than P20 for materials that release mild acids during molding, but it is not as corrosion-resistant as a full stainless steel.

For your application, unless you are using PVC or other highly corrosive resins, 718h provides the optimal balance. The cost premium over P20 buys you extended tool life and part quality consistency, while avoiding the significantly higher cost and potentially more challenging machining of a stainless grade that you may not fully utilize.

**status:** suggested
**Author:** Daniel Yang
**Date:** 2026-10-18

### Answer 8

Consistent part dimensions from shot to shot are critical for automated assembly. Mold wear causes dimensional creep, which manifests as variation in features like pin diameters or slot widths.

This variation accumulates in the assembly fixture, causing misalignment or failed snaps. A robust steel like 718h dramatically slows this wear-induced variation.

When approving the mold, request a dimensional analysis report from a series of consecutive shots (e.g., 50 parts) to establish a baseline variation. This data, more than a single sample, demonstrates the mold's initial stability and predicts its performance in volume assembly.

**status:** suggested
**Author:** Olivia Chen
**Date:** 2026-10-18

### Answer 9

Implementing a quality plan for a 718h mold involves defining specific wear-related checkpoints. During initial sampling, establish tight control limits for critical part dimensions. In production, the In-Process Quality Control (IPQC) should include periodic checks for early signs of wear, such as a slight increase in part weight (indicating flash beginning) or a shift in a non-critical but easily measured dimension.

This proactive monitoring allows for scheduled maintenance before defects affect assembly. The key is to use the mold's known durability to move from reactive defect sorting to predictive maintenance based on SPC trends.

**status:** suggested
**Author:** David Zhang
**Date:** 2026-10-18

## Related Resources

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