How the two processes differ

Molded grating is made by placing chopped-strand mat in a mould, pouring in resin and curing the whole panel in one shot. Pultruded grating is made by pulling continuous glass roving through a heated die, curing a straight profile and then joining the profiles into a grid with cross bars.

One is cast, the other is pulled. A molded panel has its grid, joints and surface formed as a single piece with nothing assembled. A pultruded bearing bar is a continuous profile, and the cross bars are fixed with pins, clips, adhesive or bolts. That single difference drives every performance gap that follows.

Both processes are mature and both have a place. What actually decides the choice is where the load travels, which face sees the chemical attack and how far the span has to reach. For a point-by-point view, see the molded versus pultruded comparison

Fibre direction sets the load path

Glass fibre carries tensile load; the resin bonds the fibres and transfers shear between them. Fibres aligned with the load give the highest tensile strength and modulus. Fibres spread at random give similar properties in every direction, but at a much lower absolute level.

In pultruded grating the roving runs continuously along the bearing bar, so strength and stiffness along that bar are far higher than across it. Walkway panels are normally laid with the bearing bars across the supports, so the load travels down the short span, which is exactly the path pultruded grating handles best.

In molded grating the fibre is random in the plane, so the two directions behave similarly and there is no strong axis. That suits platforms with two-way load, many openings, irregular shapes and a lot of site cutting.

Two-way does not mean twice as strong. Molded grating usually carries 25 to 35 percent glass by weight, while pultruded grating can exceed 60 percent; at the same section, pultruded tensile modulus is often an order of magnitude higher. Published data governs the real numbers, so do not infer capacity from the process name alone.

Reading load and deflection

Choosing grating is not a question of how many kilograms it holds. Span, load type, allowable deflection and support conditions all have to be settled. Manufacturer load tables usually give uniform, concentrated and line loads separately, each with a stated deflection limit.

Deflection often governs before strength does. A common walkway limit is span divided by 160, or one quarter of an inch, whichever is smaller. That limit comes from how the panel feels underfoot and how it drains, not from material failure.

Pultruded grating has deep bearing bars and continuous fibre, so it meets deflection limits more easily on long spans. Molded grating works as a grid, which suits short spans, close supports and irregular platforms. When comparing two load tables, first confirm that span, support width and deflection limit use the same basis.

For uniform, concentrated and line load data at a given span, see the FRP grating load and deflection data

Panel sizes and cutting

Molded grating commonly comes in 1.22 by 3.66 metre panels, with metric and larger sizes also available. Pultruded bearing bars can run well over ten metres, with widths built up in modules of roughly 300, 500 or 1000 millimetres. Long walkways favour pultruded to reduce joints; small platforms favour molded for easier layout.

Both types can be cut on site, at different costs. Molded grating can be cut in either direction, but the grid opens at the cut and needs an edge trim or banding. Pultruded grating tolerates cuts along the bearing bar; cutting across a bar severs the continuous fibre and must be avoided or reinforced.

Opening position matters as well. A hole in the middle of a bearing bar is better than one at its edge, and the hole should keep a generous distance from the support. Several openings close together push the load onto a few bars, and that is where local failure starts.

Panel size, bar spacing, edge trim and opening limits are set out in the FRP grating specification guide

Resin systems and corrosion

Either process can use orthophthalic, isophthalic, vinyl ester or phenolic resin. Molded grating carries more resin, so in aggressive service the resin itself is the main barrier. Pultruded grating carries more glass, so protection depends more on the surface veil and the integrity of the resin layer.

Vinyl ester usually outperforms polyester against alkali, chlorine and some solvents, but it costs more and demands different processing. A resin cannot be chosen from the words acid and alkali alone; the specific medium, concentration and temperature have to be matched.

Cut and drilled faces are the weak points. After site machining, the cut fibres should be sealed with the specified edge resin. This step is often dropped on chemical and coastal projects, and it is usually where staining, blistering and exposed fibre appear first.

Temperature changes resin behaviour too. Polyester resins typically have a heat deflection temperature between 60 and 100 degrees Celsius, and sustained operation above that band reduces load capacity. For hot duty, confirm the resin grade and the derating factor with the manufacturer instead of applying ambient data.

Surface and slip resistance

Molded grating forms a concave top surface as it cures, and grit can be added in the mould. Pultruded bearing bars usually have a grooved or gritted top, with anti-slip plate as another option. Each surface suits a different duty.

A concave surface relies on its profile keying into the sole and is adequate dry or lightly wet. Grit relies on particles bonded into the resin, which is more reliable with oil, slurry or a slope, but uncomfortable underfoot or when work is done seated. Slip ratings must come from test reports on the actual product, not from the surface name.

When molded wins, when pultruded wins

Molded grating suits aggressive corrosion, one-piece protected platforms, trench covers, flooring and equipment enclosures. It also suits irregular shapes that need heavy site cutting, and short spans with close supports.

Pultruded grating suits long walkways, heavily loaded platforms with tight deflection limits, installations where straightness and appearance matter, and structures where the load clearly runs one way along defined bearing bars.

Mixed layouts are common: pultruded for the main walkway to reach the span, molded for the corners and irregular areas. The two must share height, bar spacing and support detail, and the joint needs a transition support so it does not become a step or a water trap.

Where a project demands both high load and harsh chemistry, do not assume pultruded is automatically better. It has more glass but a thinner resin layer; under continuous strong acid or alkali spray, the thicker resin of molded grating can last longer. On that kind of duty, soak coupons of both before deciding.

What to put in the inquiry

State the process, resin type, grating depth, bearing bar height and spacing, panel size, colour, surface type, edge treatment and support span. An inquiry that says only FRP grating leaves the supplier quoting a standard product that is often not the one the job needs.

If the drawings are not fixed yet, list the span, the support spacing and the chemical medium first, then ask the supplier to work back to depth and process. Choosing a panel first and changing the structure later usually means rework on deflection and supports.

• Fix span and deflection limit before choosing process or depth.
• Write the chemical medium, concentration and temperature into the specification.
• State site cutting, openings and edge sealing requirements up front.
• List fasteners, banding and transition supports as separate items.