Perimeter, not area
The flange scales with the perimeter of the parting line, and the tooling face scales with area, and those two things pull apart fast as a shape gets long and thin. A 10 foot by 3 foot part has a parting line 26 feet long. At a three inch flange that is 6.5 square feet, plus a quarter of a foot for the four corners, against a tooling face of perhaps 48 square feet — so about an eighth of the tool before any split is counted. Squash the same area into a long strip 20 feet by 18 inches and the perimeter goes to 43 feet and the flange to 10.8 square feet on the same face area. Nothing about the part changed except its shape.
That is the case for measuring rather than estimating. A flange is invisible on a drawing and obvious on an invoice.
What a split costs
Splitting a mould does not add one flange, it adds two, because each half needs a face to bolt against. A three foot split line at a three inch flange is another 1.5 square feet of tooling laminate on top of the 6.6 for the outer flange, and it brings its own bolts, its own keys and its own leak path. It is often the right decision — a part with a return or an undercut simply cannot come out of a one piece tool — but it is a decision with a number attached, and the number is bigger than the length of the split line suggests.
The bolt count on a split run counts both ends, because a run that stops has to be clamped at the ends. The outer flange is a closed loop, so it does not get the extra one.
Parting board waste is high and worth entering
The temporary flange the first half gets built against is a strip cut to follow the parting line, and that strip comes out of a rectangular sheet. On a straight sided part the waste can be modest. On anything curved it is severe, because the offcut inside a curved strip is a shape nobody wants. The default 35 percent on the form is a placeholder to replace with what your own nesting achieves, and the difference between 20 and 50 percent is often a whole sheet.
The thing this cannot count
Whether the flange is stiff enough. A mould flange has one job beyond carrying the bolts, which is not to flex between them, because a flange that opens between bolts leaves a witness line down the middle of every part that comes out. The fix is stiffness — a wider flange, a thicker tooling laminate, a stiffener bonded along the back — rather than more torque. How much is needed depends on the size of the mould, the laminate, and how roughly it gets handled, and no amount of arithmetic on a perimeter answers it.
Likewise the flange width itself. Three inches is on the form because a number had to be, not because it is right. It has to carry a bolt with washers, leave room for a hand and a spanner, and have enough material outside the bolt line not to crack. Whoever has built moulds for that kind of part has the number.
Questions people ask
How much extra tooling does a mould flange add?
On the default shape — a 120 by 36 inch parting line with a 3 inch flange, four outside corners and one 36 inch split line — the flange comes to 8.25 square feet against a 48 square foot tooling face, so 14.7 percent of the tool. On a long narrow shape with the same face area it goes considerably higher, because flange scales with perimeter and the face scales with area.
Why does a split line add two flanges?
Because both halves need a face to bolt against, and they sit either side of the same line. A 36 inch split at a 3 inch flange adds 1.5 square feet of tooling laminate, not 0.75, and it brings its own bolts and alignment keys with it. Splitting a mould is often unavoidable on a part with a return or an undercut, but it is worth costing rather than assuming.
How far apart should the bolts on a mould flange be?
That is your call and it depends on how stiff you built the flange, which is why it is a field with your own number in it. The thing the spacing has to achieve is that the two halves do not open between bolts, because a flange that flexes leaves a line down every part. Closer bolts and a stiffer flange are the two levers, and more torque on widely spaced bolts is not one of them.
How many sheets of parting board will I need?
For the 8.25 square feet of flange in the default case, 35 percent cutting waste puts 11.1 square feet on the saw, and one 4 by 8 sheet covers it with room to spare. The waste figure is the one to think about rather than the area — a flange is a long thin strip cut from a rectangle, and on a curved parting line the offcuts are unusable. Nesting the strips is what brings the number down.
Should I use the footprint as the tooling face area?
Only if the plug really is flat. The area that gets laminated is the developed surface, and a deep or curved shape can be a great deal more than its plan view — a hull, a fairing or anything with a return can run to twice its footprint. The honest way to get it is a tape run over the surface. The footprint option on the form is there for genuinely flat tooling and for a first rough pass.
Does this include the tooling gelcoat and the laminate schedule?
It gives you the area and stops there. What goes onto that area — how many plies, which fabric, what surface coat — is a decision about the tool and how many parts it has to make, and it is not something this page has any view on. Take the area to the resin quantity calculator with whatever schedule you were given and it will turn it into cloth and resin.