Vacuum Holes: Diameter, Spacing and the Mark They Leave

Two curves, opposite directions

A vacuum hole has to do two incompatible things. It has to let air out fast enough that the sheet is pulled down before it cools, and it has to be small enough that the sheet does not sink into it and take a permanent dimple. Every hole in every mould is a compromise between those two requirements, and the compromise is decided by the hole diameter.

Time to full vacuum against hole diameter: 3.5 seconds at 0.5 mm, 1.8 at 1.0, 1.0 at 1.5, 0.7 at 2.0 and 0.5 at 3.0. The curve is steep at the small end because the conductance of a hole goes with the fourth power of the diameter in the molecular flow regime and with the square of the area in the viscous regime. Halving the diameter more than doubles the time.

Visible marks against the same diameters: none at 0.5 mm, 5 percent of parts at 1.0, 25 percent at 1.5, 60 percent at 2.0 and 95 percent at 3.0. A mark appears when the sheet can be drawn into the hole far enough to leave a dimple that survives cooling, and that depends on the sheet thickness, the temperature and the hole size together.

The window is 1.0 to 1.5 mm for most work

For a 1.5 to 2 mm sheet at normal forming temperature, 1.0 to 1.5 mm is where both curves are acceptable: full vacuum in one to two seconds, and marks on 5 to 25 percent of parts. That is the range most moulds are drilled to, and it is a compromise rather than an optimum.

Thin sheet moves the window down. A 0.5 mm sheet at 150 C will sink into a 1.5 mm hole on every part, so thin-gauge moulds are drilled at 0.5 to 0.8 mm and the draw-down time is bought back with more holes rather than bigger ones. Thick sheet moves the window up, because a 4 mm sheet does not sink into a 2 mm hole at all.

The practical rule is to size the hole against the sheet, not against the mould. A mould that will run 1 mm and 3 mm sheet on the same tool has to be drilled for the thin sheet, because the thin sheet is the one that marks.

Spacing changes the pattern, not just the count

Hole spacing controls how evenly the sheet is pulled down. Holes too far apart leave the sheet bridging between them, which shows as a shallow pillow between two dimples. Holes too close together make the marks into a visible pattern rather than isolated points, which is worse for appearance than fewer, better-placed holes.

The useful starting point is a grid at 25 to 40 mm for large flat areas, tightened to 15 to 20 mm at corners, at the bottom of deep draws and along any edge where the sheet has to wrap. Those are the places where the sheet arrives last and where it has the least time to be pulled in.

On a deep draw the holes have to be staged rather than simultaneous. A hole that is open from the start pulls the sheet against the mould before the plug has pushed it down, which thins the base and leaves the sidewall unformed. Sequencing the vacuum zones, or using a delay valve on the base holes, is what makes a deep part form evenly.

Table 1 — Vacuum hole diameter against the two things it controls

Making the mark disappear

The mark is a dimple where the sheet was drawn into the hole. It can be removed or hidden in five ways, and they are worth trying in this order because the first two cost nothing.

• Move the holes to a face that is not seen, or to a position that will be trimmed off.
• Reduce the diameter and add more holes to keep the draw-down time.
• Increase the sheet temperature slightly so the sheet is more extensible and recovers more after forming.
• Use a porous sintered insert instead of a drilled hole, which passes air over an area rather than a point.
• Add a slight counterbore on the back so the hole is short and the conductance is higher for the same diameter.

The counterbore is the least known and the most useful. A 0.8 mm hole that is 8 mm deep has poor conductance; the same 0.8 mm hole through a 1 mm wall with a 3 mm counterbore behind it flows almost as well as a 1.5 mm hole and marks like a 0.8 mm one. It is a machining decision, not a design compromise.

Holes block, and blocked holes look like a forming problem

Vacuum holes fill with material. A tiny amount of sheet is drawn into the hole on every cycle, and over a few thousand cycles the hole is partly closed by a skin of solidified polymer. The mould still pulls vacuum, the gauge still reads correct, and the parts start showing weak corners and poor detail.

The symptom is a progressive loss of definition that nobody notices until a part is compared with a first-off. The cure is a pin, a drill bit by hand, or a short blast of air from the back, done on a schedule rather than when a problem appears.

Table 2 — Forming faults that are actually hole geometry or hole maintenance

What to specify

Vacuum hole geometry belongs on the tool drawing, not in the toolmaker's judgement.

• Diameter, with the sheet thickness range it was chosen for.
• Pitch across flat areas, and the tighter pitch at corners and deep draws.
• Which zones are staged, and in what order they open.
• Whether holes are counterbored from the back, and to what diameter.
• Hole positions relative to trim lines, so that marks fall outside the finished part.
• The cleaning interval, in cycles, with the method.

The trim line item is the cheapest quality improvement available on a cosmetic part. Moving a row of holes 5 mm outside the trim costs nothing at the drilling stage and removes the mark from the part entirely.