In-Mold, Hand Lay-Up or Compression Moulding: Choosing a Helmet Shell Process

Helmet buyers usually negotiate price before they understand which shell process the quote is built on. That is the wrong order. In-mold, hand lay-up, compression moulding and resin transfer moulding produce shells with different wall thickness control, void content, tooling cost and cycle time, and those differences decide what the helmet can pass in the lab and what it costs at volume.

This guide compares the four routes from a buyer's perspective: what each one is good at, where it breaks down, and which questions separate a real process capability from a sales claim.

Start With the Shell Route, Not the Price

A helmet is a shell, an EPS liner, a retention system and comfort padding, assembled and tested as one unit. The shell's job is to spread a point load across the EPS and to hold the liner in shape after the impact. How the shell is made decides how evenly it does that job from part to part.

When two suppliers quote the same helmet format at very different prices, the gap is usually in the shell route, not in labour rates alone.

The Four Shell Routes at a Glance

The table is a starting point, not a verdict. A low-volume program can still justify a matched die if the buyer intends to scale, and a high-volume program can stay on hand lay-up if the geometry changes every season.

Where In-Mold Fits, and Where It Does Not

In-mold means a pre-formed thermoplastic shell, usually polycarbonate or ABS, is placed in the EPS mould so the expanding bead bonds to it in one cycle. The result is a light, thin shell with excellent graphics capability and no separate bonding step. It remains the default for most mass-market bicycle helmets.

Its limits are structural. The shell is a thermoplastic skin, typically thinner than a millimetre, so it contributes little bending stiffness of its own.

If a supplier offers "in-mold basalt", ask what the shell material actually is. Basalt and carbon laminates need cure temperatures and pressures that a thermoplastic in-mold cycle does not provide, so most composite helmets are moulded separately and then bonded to the EPS liner with structural adhesive.

Hand Lay-Up: Flexibility Paid For in Labour

Hand lay-up places dry fabric or prepreg into an open mould, wets it out or debulks it, and cures under vacuum bag pressure. Tooling is cheap and fast to change, which makes it the natural route for prototypes, small series, extra-large sizes and low-volume custom work.

The cost shows up in labour and variance. Resin content depends on the operator, thickness drifts across the part, and void content is higher than in a matched-die process. Those are not cosmetic issues, because they change how the shell behaves in a drop test.

Small-series custom work, including an Adjustable Basalt Fiber Bike Helmet variant with a dial fit system and integrated lighting, often starts on this route before a buyer commits to production tooling.

Buyers should require a written laminate schedule, meaning fabric weight, fibre orientation and ply count per zone, plus per-part thickness records. Without them, "hand lay-up" describes a technique rather than a specification.

Compression Moulding: The Volume Route for Basalt and Carbon

Compression moulding uses matched metal dies, heat and pressure to consolidate prepreg or a preform. The result is a high fibre volume fraction, low void content, repeatable wall thickness and a clean moulded surface on both faces. Cycle times are short compared with open-mould work, and inserts, threaded bosses and split lines can be moulded in.

The barrier is tooling. Matched dies cost more and take longer to build than an open mould, so the route only pays back once annual volume is predictable. A Custom Carbon Fiber Motorcycle Helmet program that expects to run the same shell for several seasons is a typical candidate.

This is also where composite bicycle programs land. Once a buyer moves past a thermoplastic skin to a woven shell, whether a Carbon Fiber Bicycle Helmet or a basalt equivalent, the choice narrows to hand lay-up or compression moulding.

Compression moulding gives the buyer something valuable in certification work: repeatability. When a lab measures a shell at four points around the crown, the numbers from cavity one and cavity four should be close.

Resin Transfer and Infusion: The Middle Ground

Resin transfer moulding and vacuum infusion use a closed mould and a dry fabric preform, with resin drawn or injected in. Both faces come out smoother than hand lay-up, styrene exposure drops, and fibre volume fraction is easier to control. Tooling sits between the two extremes.

The trade-off is cycle time and process discipline. Resin flow, preform compaction and injection pressure all have to be controlled, and poorly degassed resin shows up as porosity in exactly the zones that carry impact load.

How the Route Changes Impact Test Results

Bicycle helmets are tested to EN 1078 in Europe and to CPSC 16 CFR 1203 in the United States. Motorcycle helmets face ECE 22.06 in Europe, which added rotational impact testing, and FMVSS 218 in the United States. All of these rely on guided drops onto flat and shaped anvils, plus retention, roll-off and field-of-vision checks.

In a drop test, the shell spreads the striker load into the EPS. A shell that is too flexible indents locally and crushes the liner in a small area. A shell that is too stiff transmits load faster and can push the liner past its useful crush distance, which is why shell stiffness and liner density have to be developed together.

Void content and fibre volume fraction decide how consistently a shell does that spreading job. Ask for fibre volume fraction by burn-off, cure degree by DSC, and a thickness map showing where the supplier measures.

Tooling, MOQ and the Real Cost Curve

MOQ in composites is rarely a warehouse decision. It is the volume at which the tool, the material minimum order and the learning curve stop dominating the unit price.

Finishing and Assembly Consequences

Composite shells need surface preparation before paint: sanding, primer, and often a gel coat or in-mould coating to close pinholes. Thermoplastic in-mold shells take decoration directly. The difference matters to a buyer planning a multi-colour graphic program across several markets.

Shell-to-EPS bonding is the other hidden variable. Adhesive choice, surface cleanliness and bond-line coverage determine whether the shell stays attached after impact. Ask how the bond line is inspected and what happens to a shell that fails the check.

Supplier Questions Worth Asking Early

• Which shell route is this quote based on, and is it the route you will actually run?
• Can you show a laminate schedule and a thickness map for the shell?
• How do you control resin content and void content, and what do you record per batch?
• Which cavities and which sizes were tested, and can I see reports for each?
• What tooling do you own, and would my tool be exclusive to this program?
• How do you handle a design change after the die is cut?
• What is your rework and scrap rate on this route?

A Practical Decision Rule

Choose in-mold when the shell is cosmetic, thin and thermoplastic, and volume is high. Choose hand lay-up when geometry changes, volume is low, or the program is still proving itself. Choose compression moulding when the shell must repeat, the volume is committed, and the tool can be amortised over more than one season.

Then test the choice, not the brochure. One shell pulled from a production cavity and dropped on a calibrated rig tells a buyer more than any process description.