Match-Mould Forming: Two Halves for Parts That Need Both Faces

Two mould halves, one part

Match-mould forming, also called matched-die forming, presses a heated sheet between a male and a female tool that fit each other. The sheet is not pulled by vacuum or pushed by air; it is squeezed, and the gap between the two tools is the wall thickness.

That single fact explains everything the process can and cannot do. Because the wall is set by a mechanical gap rather than by how the material stretches, the thickness is nearly uniform regardless of the draw. Because both surfaces touch a tool, both surfaces can carry texture, detail or a moulded-in feature. And because the tools have to be pressed together, the machine needs a press rather than a vacuum pump.

The process sits between thermoforming and compression moulding, and it is chosen when a part needs two finished faces. A vacuum-formed enclosure has a good outside and a rough inside; a match-moulded one has two good faces and can be assembled without hiding the interior.

The wall barely moves with draw

The wall ratio, thinnest divided by thickest, is 0.92 at a 0.3 draw, 0.88 at 0.6, 0.84 at 0.9, 0.80 at 1.2 and 0.76 at 1.5. Vacuum forming over the same range falls from 0.62 to 0.31. At a deep draw the difference is a factor of two and a half.

The reason is that the material is not free to stretch. As soon as the sheet touches either tool it is constrained by friction, and the closing tools force the material to flow into the walls rather than thin across them. The wall thickness ends up close to the original sheet thickness minus a small amount of squeeze.

That is why a match-moulded part can be designed to the sheet thickness rather than to a thinned minimum. A 1.5 mm sheet gives a part that is 1.3 to 1.4 mm everywhere, where a vacuum-formed part of the same depth would have 0.5 mm at the base corners and 1.5 mm at the flange.

Table 1 — Wall ratio by draw, match mould against vacuum.

Clamp force is the whole machine

The clamp force is the forming pressure multiplied by the projected area. At a typical 600 kPa, a 0.1 square metre part needs 60 kN, a 0.25 square metre part 150 kN, a 0.5 square metre part 300 kN and a one square metre part 600 kN.

Six hundred kilonewtons is 60 tonnes. That is a hydraulic press, not a thermoformer, and it is the dominant cost in the decision. It also sets the tool requirement: the two halves have to resist that load without deflecting, or the wall will be thick in the middle of the part and thin at the edges.

The pressure itself is chosen to be just enough to form the detail and consolidate the wall, not as high as possible. Running at 1 MPa instead of 600 kPa gives a slightly sharper detail and doubles the machine requirement, which is rarely worth it. Most production runs sit between 400 and 800 kPa.

Two matched surfaces

A match-mould tool is two tools that have to fit each other to a wall thickness tolerance. That is a different manufacturing problem from a single-surface mould: the cavity and the core are machined separately and then spotted together, and the gap between them has to be uniform across the whole part.

Aluminium is the usual material for both halves, with a steel frame if the part is large. The female half carries the vacuum that holds the sheet while the tools close; the male half carries the detail. Both need temperature control, because a tool that runs cold on one side and hot on the other produces a part that warps as it leaves.

Tool life is shorter than for a vacuum tool because the edges take the closing load. A well-made aluminium match tool runs 50,000 to 100,000 parts with edge maintenance; a vacuum tool of the same size can run several times that, because nothing touches it except the sheet.

Table 2 — What changes when the tool becomes two halves.

Where match moulding pays

It pays where a part needs two finished faces and a consistent wall. Instrument and equipment housings that are assembled without a liner, automotive interior panels with a Class A face on both sides, and reusable trays that are stacked and seen from below are all typical.

It also pays where the material is expensive. Because the wall is uniform, a match-moulded part can be made from a thinner sheet than the vacuum equivalent, and on a high-temperature engineering polymer that saving can be 25 to 30 percent of the material cost. On a large part that can be the whole tool cost, amortised over the first year.

It does not pay for shallow parts, for parts where only one face is seen, or for volumes below a few thousand. A vacuum tool at a third of the cost will do the job on a shallow cosmetic panel, and the clamp force argument never arises.

What to specify

A match-moulded part is specified by the wall, the two surfaces and the material, in that order.

• Sheet thickness and the wall tolerance, which is set by the tool gap, not by the process.
• Both surface finishes, quoted to a standard, because both faces are visible.
• Draft on both halves, typically 2 to 3 degrees, and the split line position.
• Clamp force and press size, so the quote can be checked against the machine.
• Cycle time, which is longer than vacuum because the tools have to close and open.
• Trim allowance and the trim method, because a matched part usually needs edge trimming.

The split line is the item most often left to the toolmaker, and it is the one that shows. On a two-faced part, the split line runs around the whole perimeter, and where it sits decides whether it reads as a design feature or as a moulding defect.