Tooling for injection moulding: cavity, steel and cost

What a mould quote is made of

A mould quote is not one number. It covers the tool design, the steel, the standard components, the cavity and core machining, the runner system, any side actions, the surface finish, the trial shots and the documentation. Two quotes for the same part can differ because one includes a hot runner and the other does not, or because one assumes a single cavity and the other assumes eight.

The first step in comparing quotes is to normalise the scope. Ask for the cavity count, the steel grade, the runner type, the ejection method, the expected cycle, the tool life, the spare parts and the trial allowance. A lower price with a shorter tool life or no spare core is not a saving if the programme runs for five years.

Cavity count and the volume equation

Cavity count is usually chosen from the annual volume and the press size. A single-cavity tool suits prototypes, low volume and parts with a long cycle or a difficult fill. A two or four cavity tool is common for medium volume. Eight, sixteen or thirty-two cavities are used where the part is small, the volume is high and the press can carry the tool.

More cavities do not automatically lower the piece price. The tool cost rises, the press has to be larger and the runner has to balance. A cavitation study should compare the tool cost, cycle, scrap and press rate for two or three options, then choose the lowest cost per good part.

Cavity balance matters as much as cavity count. If one cavity fills late, it produces a short shot or a weak part while the others are complete. A balanced runner or a hot runner with individual tip control costs more, but it protects the yield. For a tight-tolerance part, a family tool with cavities that can be adjusted separately is often worth the extra cost.

Steel grade follows the resin and the volume

The mould steel has to resist wear, corrosion and heat. P20-type pre-hardened steel is common for low to medium volume and unfilled resins. H13-type hardened steel suits high volume and abrasive fillers. A stainless grade such as S136 suits corrosive resins, including some flame-retardant grades and PVC.

Abrasive fillers change the calculation. Glass fibre and mineral fillers wear the gate, the runner and the cavity surface. A soft steel may last for a short programme, but it will lose its edge and its dimensions. For an abrasive resin, specify a harder steel, a replaceable gate insert or a surface treatment, and include the maintenance interval in the tool cost.

Surface finish is part of the steel choice. A polished face needs a steel that can take a polish without pitting. A textured face needs a steel that can be etched consistently. If the part has a visible face and a sealing face, the two may need different finishes and different maintenance.

Hot runner or cold runner

A cold runner leaves a sprue and runner that are ejected with the part and either reground or scrapped. It is simpler and cheaper to build, but it wastes material, lengthens the cycle and leaves a gate mark. For a small part at high volume, the runner can be a significant part of the material bill.

A hot runner keeps the melt hot in the runner and injects directly into the cavity. It removes the runner scrap and can shorten the cycle, but the tool costs more, the control system is more complex and the colour change is slower. A hot runner also needs a careful thermal layout, because a cold slug or a degraded pocket can produce defects.

The choice depends on the resin, the part size, the volume and the colour strategy. A hot runner is often justified for a technical part with a small shot weight and a high volume. A cold runner is often the better choice for a large part, a low volume or a programme that changes colour often.

Side actions, lifters and undercuts

Every feature that cannot be ejected in the line of draw needs a side action, a lifter or a split. A side action adds a slide, a wear plate, a hydraulic or mechanical drive and a maintenance point. It also adds tool height and can limit the number of cavities that fit in the press.

Avoid an undercut if a simpler geometry will do the job. A snap fit can often be redesigned as a pass-through feature. If the undercut is essential, keep it small, give it draft and make the slide accessible, because a buried side action is expensive to service.

Lifters are an alternative for an internal undercut. They move with the ejector and release the feature as the part leaves the core. They are cheaper than a hydraulic slide but they need space and they can mark the part. The tool designer should show the lifter path and the clearance in the design review.

An illustrative cost split

The chart below is an illustrative split for a mid-size technical part in a four-cavity tool. It is not a quote and it will not match every supplier, but it shows where the money goes. Design and DFM, steel and standard parts, cavity machining, the runner system, side actions and trials all carry a share.

Ask for the split in the quote. A supplier who can show the cost build is easier to compare and easier to challenge. If the split is not available, ask for the assumptions behind the price: cavity count, steel grade, runner type, tool life and trial quantity.

Lead time, trials and the cost of being late

A mould programme usually runs through design, steel order, machining, fitting, trial and correction. The design and the steel order are often the longest fixed items, and a late design change can restart part of the sequence. The trial is not the end of the programme; it is the point where the real part is measured and the tool is corrected.

Allow for at least one correction cycle after the first trial. A tool that is delivered on time but needs two extra weeks of correction is later than a tool that is quoted with a realistic trial allowance. Ask for the trial plan, the sample quantity and the measurement report that comes with the samples.

The cost of a late tool is not just the delay. It can include expedited freight, a line stop, a temporary tool or a bridge supply. Those costs belong in the business case for the tool, even if they do not appear on the purchase order.

Compare tooling on total cost

A total cost model adds the tool price, the piece cost over the programme volume, maintenance, scrap and the cost of trial and correction, then divides by good parts produced. Compare that number, not the tool price alone.

Ask for the expected tool life in shots, the spare parts list, the maintenance interval and the ownership of the tool. If the tool is not yours, the exit cost matters. If it is yours, the maintenance record matters. A tool with a known history is worth more than an unknown tool with a lower price.

Finally, check the documentation. A tool that comes with a steel certificate, a dimensional report, a trial record and a maintenance manual is easier to transfer, insure and maintain. That documentation is part of the asset, not an optional extra.