718H Steel for Injection Molds

What 718H is

718H is a pre-hardened plastic mould steel supplied at roughly 33–36 HRC, corresponding to DIN/EN 1.2738. It is the same broad family as P20 but with a modified analysis and a cleaner melt practice, which buys a few points of hardness and a noticeably better polish.

It is the standard choice for a large cosmetic cavity that has to hold a high-gloss surface through a long campaign. Because it arrives pre-hardened in large plates, the tool can be machined, polished and put into production without a hardening cycle that would distort a big block. That combination - hardness, polish and plate size - is what 718H is bought for.

It is still a low-alloy steel, not a stainless grade. It does not resist PVC or aggressive flame retardants, and it is not the right answer for a glass-filled production tool. Those limits are shared with P20 and should be read before the block is ordered.

Hardness, and what it does not buy you

The extra hardness over P20 is worth real money on a cosmetic tool. It resists light wear, holds a polish longer and is less prone to indentation at the parting line. On a long run of unfilled resin, that translates into fewer re-polish stops and a more stable surface.

It does not buy abrasion resistance against glass fibre. At 33–36 HRC, 718H will still wash out at a gate carrying a filled melt, and a hardened grade at 48–52 HRC remains the correct answer. The gap between 30 and 34 HRC is meaningful for polish and light wear; it is not meaningful against glass.

Hardness also does not buy corrosion resistance. The alloy has no significant chromium content in the stainless sense, so a wet tool room or a corrosive resin will attack it. Treating 718H as a stainless grade because it is harder than P20 is a common and expensive mistake.

Corrosion and polishability

Corrosion behaviour is close to P20: low resistance, no protection against PVC or aggressive flame retardants, and a real risk of rust during storage. The difference is that a 718H cavity is usually a high-value cosmetic surface, so the cost of a corrosion pit is higher. A rust bloom on a polished 718H face means a re-polish, and a pit means welding and re-polishing.

Polishability is where 718H earns its place. The cleaner steel takes a finer, more uniform finish than a standard P20 block, and it holds that finish through more shots. For an appliance face, a business-machine cover or an automotive interior part with a high-gloss requirement, that is the difference between a tool that needs re-polishing mid-campaign and one that does not.

The polish still has a ceiling. 718H is a cosmetic steel, not an optical one. A clear lens or a mirror surface needs a stainless grade with a finer inclusion structure, and no amount of polishing time will turn 718H into that. Specify the surface standard by its intended function rather than by the polish step.

The practical routine is the same as for any pre-hardened tool: dry the cavity after every run, protect it in storage, keep the cooling water treated, and avoid leaving the mould closed and wet. A 718H tool that is looked after will hold its polish; one that is stored wet will not.

By production volume

Below roughly 50,000 shots, 718H is often more than the programme needs and P20 is the cheaper route. Between 50,000 and 500,000 shots, 718H is the natural choice for a cosmetic unfilled part: enough hardness to hold the polish, and a plate size that suits a large cavity.

Between 500,000 and a million shots, 718H is still viable for an unfilled resin with good maintenance, but the re-polish interval becomes part of the planning. Above that, a hardened or stainless grade usually wins on total cost, especially where the surface is critical.

The volume figure has to be read with the resin. A million-shot ABS campaign is a 718H job. A 200,000-shot glass-filled campaign is not, because the gate will wear out long before the cavity does.

By resin

718H is at its best in unfilled cosmetic resins: ABS, acrylic, polystyrene, polypropylene and polyethylene. These carry no abrasive filler and release nothing corrosive at normal temperature, so the tool's life is governed by polish retention rather than by wear.

Unfilled PC is a qualified match. The steel can run a PC mould at 80–110 °C for a long campaign, and the polish suits a clear or high-gloss part, but a true optical requirement still points to a stainless grade. For a pigmented PC housing, 718H is a sensible choice.

Glass-filled resins such as PA+GF are a mismatch. The gate and vent land will wear, and a replaceable insert is the minimum mitigation. PVC and aggressive flame-retardant grades are also a mismatch, because corrosion arrives first.

Heat treatment, welding and repair

718H is supplied pre-hardened, so the normal tool skips hardening entirely. That is the point of the grade. If a local area needs to be harder, the usual route is a separate insert in a through-hardening steel rather than a re-hardening cycle on the whole block.

Welding follows the same rules as P20 but is less forgiving on a polished face because the surrounding finish is finer. Preheat, match the filler, control the heat input and cool slowly. The heat-affected zone will polish to a slightly different lustre, and on a high-gloss cavity that difference is visible. Plan the repair away from the visible face where possible.

Gate repair is the most common weld on a 718H tool. Where the damage is repetitive, a replaceable gate insert is the better design: it turns a cavity repair into a small block change and protects the polish on the rest of the tool.

Nitriding is a reasonable upgrade where light abrasion is expected. It hardens the surface without changing the bulk, and it is compatible with the pre-hardened condition. As with any pre-hardened steel, the cavity should be dimensionally settled before the treatment, because correcting a nitrided surface is harder.

Surface treatment and coatings

Nitriding is the first option for a 718H cavity that needs more wear resistance at the gate or the vent land. It keeps the bulk toughness and adds a hard case, and it is a routine treatment for a tool that will see a long run.

Chrome plating improves release and corrosion resistance, and is useful on a cavity that is stored between campaigns or runs a mildly aggressive resin. It adds lead time and can chip at a sharp edge, so the geometry should be reviewed first.

PVD coatings are better suited to a hardened substrate. On a 34 HRC block the coating can fail by substrate deformation, so nitriding is usually the more reliable first step for this grade.

Treatments are an extension, not a substitution. A coating will not turn 718H into a glass-filled production steel, and it will not turn it into a stainless grade.

Cost per shot vs cost of steel

718H costs more than P20 per block, and the difference is small relative to the machining and polishing that a large cosmetic cavity already carries. The extra cost buys a longer polish life and fewer re-polish stops, which is usually worth more than the block price difference on a high-gloss part.

Against a hardened grade, 718H avoids the heat-treatment step and the distortion risk on a large plate. That can shorten the tool build and remove a schedule dependency, which often matters more than the per-shot cost on a medium-volume programme.

The calculation flips when the resin is abrasive or corrosive. In a glass-filled or PVC job, the cheaper block is not cheaper: the maintenance and downtime erase the saving. Read the resin first, then the volume, then the block price.

Common selection mistakes

• Treating 718H as stainless. It is not. A PVC or flame-retardant resin will corrode it, and a wet tool room will rust it.
• Using it for a glass-filled production tool. The gate will wear, and 34 HRC is not enough to resist it. A hardened grade or a replaceable insert is required.
• Expecting optical polish. 718H is a high-gloss cosmetic steel, not an optical one. A clear lens needs a cleaner stainless grade.
• Welding a visible face without planning the finish. The heat-affected zone polishes differently, and the repair shows on a high-gloss surface.

What to send us

Send the 3D part, the resin grade and filler content, the annual volume and campaign length, the surface standard, and the cavity count. If the part has a visible face, say which face and what gloss or texture it carries, because that is what decides whether 718H is enough or whether a stainless grade is required.