Air is the only thing between the sheet and the tool
Vacuum forming works because the air under the sheet is removed, letting atmospheric pressure push the sheet onto the tool. If the air cannot leave fast enough, the sheet arrives before the air does, and the result is a bubble of trapped air holding the sheet off the surface.
That trapped air shows up as three defects: read-through (the shape of the air pocket visible on the outside), incomplete draw (a rounded, undefined corner), and poor surface (the sheet never touches the polished tool, so it keeps its own texture).
The sizing rule: the vents must move the volume of air under the sheet in less time than the sheet takes to descend. In practice that means vents every 25–50 mm on flat areas, larger and more frequent at corners and in deep draws, and a channel system behind the vents that can carry the flow away.
Hole size: the compromise
Vent holes are a trade-off between air flow and witness marks. A bigger hole evacuates faster but leaves a more visible mark on the part.
| Hole diameter | Air flow | Witness mark | Użycie |
|---|---|---|---|
| 0.3 – 0.5 mm | Niski | Invisible | Cosmetic parts, thin sheet, textured surfaces |
| 0.5 – 1.0 mm | Średni | Faint, visible under gloss | General purpose |
| 1.0 – 1.5 mm | Wysoki | Visible dimple | Thick sheet, non-cosmetic parts |
| 1.5 – 2.5 mm | Very high | Clear mark, may need post-finishing | Very thick sheet, structural parts |
| Slots (0.5 × 5 mm) | Wysoki | Line mark | Where a channel can be routed behind |
Table 1 — Vent hole sizing. The move from 0.5 to 1.0 mm roughly quadruples the flow area but makes the mark visible on a glossy part.
The rule of thumb is that the hole should be no larger than 60% of the sheet thickness for cosmetic parts, and can be up to the sheet thickness for non-cosmetic work. On 1 mm sheet that means 0.6 mm vents for visible surfaces.
Spacing: how close is close enough
Air under the sheet has to travel horizontally to reach a vent. The further it has to travel, the longer it takes, and the more likely it is to be trapped by the descending sheet.
| Area | Typical spacing | Reason |
|---|---|---|
| Large flat areas | 25 – 50 mm | Air travels along the surface to the nearest vent |
| Shallow curves | 20 – 40 mm | Slightly longer path as the sheet conforms |
| Deep draws (bottom) | 10 – 25 mm | Air is compressed as the sheet descends |
| Inside corners | 5 – 15 mm, clustered | Air is trapped in the corner geometry |
| Around ribs and bosses | 10 – 20 mm | Each feature traps its own air pocket |
| Along seal edges | Continuous channel | Prevents a ring of trapped air |
Table 2 — Vent spacing by feature. The clustering at corners is the single most important rule: a corner without a vent will not form properly no matter how many vents the flat area has.
A useful sanity check is to count vents on the drawing before the tool is cut. A 600 × 400 mm flat panel at 40 mm spacing needs about 150 vents. If the design has 20, it will not work — and the tool will be re-drilled by hand at commissioning, badly.
Placement: the geometry that traps air
Air does not distribute evenly under a descending sheet. It is pushed towards the last points to be formed, which are the deepest and the most enclosed. Those are where the vents must be concentrated.
| Location | Why air traps there | Vent strategy |
|---|---|---|
| Inside corners | Three surfaces converge; air has nowhere to go | Cluster of 3–5 small vents |
| Bottom of deep draws | Sheet arrives last; air is compressed | Dense array, 10–15 mm spacing |
| Around ribs | Sheet bridges across the rib, trapping air on both sides | Vent at the base of each rib, both sides |
| Text and logos | Fine detail traps a thin air layer | Vents inside the letters, not around them |
| Seal surfaces | A ring of air cannot escape past the descending sheet | Continuous groove with drilled outlets |
| Under plug-assist areas | Plug pushes sheet down, trapping air ahead of it | Vents concentrated at the plug contact zone |
Table 3 — Where air actually traps. The text case is counterintuitive: vents around the letters do not help, because the air is trapped inside the letter shapes.
The text and logo case catches out a lot of first tools. A raised logo on the tool creates a small enclosed volume that the sheet bridges over. Vents drilled around the logo do nothing for that volume. The vents have to be inside the logo's own outline, which means holes small enough not to be visible through the letter.
Behind the vents: the channel system
A vent hole is only as good as what is behind it. If the hole opens into solid aluminium, it vents the volume of the hole itself and nothing more.
| Metoda | How it works | Koszt | Najlepsze dla |
|---|---|---|---|
| Through-holes to atmosphere | Drilled right through the tool and backing plate | Lowest | Simple tools, flat backing |
| Channel plate | Machined grooves on the back face, covered by a plate | Niski | Most aluminium tools |
| Vacuum box | Sealed chamber behind the tool, evacuated as a whole | Średni | Large tools, many vents |
| Vented tool material | Porous aluminium or composite; air passes through the bulk | Wysoki | Cosmetic parts, no witness marks |
| Laminated screen | Sintered metal sheet between tool and backing | Średni | Curved surfaces |
Table 4 — Vent backing methods. The channel plate is the standard: cheap, effective, and easy to clean when it clogs.
Channel plates need enough cross-section. A 3 mm deep × 6 mm wide groove running 200 mm to the outlet has a flow resistance that becomes significant when many vents feed into it. The general rule is that the total channel cross-section should be at least 20 times the total vent hole area.
Venting by material and thickness
| Material / thickness | Vent size | Spacing | Uwagi |
|---|---|---|---|
| PS, 0.5 – 1 mm | 0.3 – 0.5 mm | 30 – 50 mm | Easy material; forgiving |
| PVC, 0.5 – 2 mm | 0.5 – 0.8 mm | 25 – 40 mm | Witness marks show; keep small |
| ABS, 1 – 3 mm | 0.6 – 1.0 mm | 25 – 40 mm | Standard |
| PETG, 0.5 – 2 mm | 0.4 – 0.7 mm | 25 – 40 mm | Sticky; vents can plug with polymer |
| PC, 2 – 6 mm | 1.0 – 1.5 mm | 20 – 30 mm | Stiff sheet; air traps easily |
| PP, 1 – 4 mm | 0.8 – 1.2 mm | 20 – 35 mm | Sags; needs fast evacuation |
| HDPE, 2 – 8 mm | 1.0 – 2.0 mm | 20 – 30 mm | Thick, stiff; large vents needed |
| Textured surfaces | 0.3 – 0.5 mm | 15 – 25 mm | Texture hides marks, so small holes are possible |
Table 5 — Venting by material. Thick, stiff sheet needs larger vents because the sheet descends faster and compresses the air harder.
PETG deserves its own note because it is sticky. Vents that work perfectly on PS will slowly plug with PETG as polymer is drawn into the holes. The fix is slightly smaller vents with a larger channel behind them, and a periodic cleaning routine.
Reading the defect
| Wada | What it looks like | Vent diagnosis |
|---|---|---|
| Read-through | Faint circles or blobs on the outer surface | Air pockets at those exact positions — add vents there |
| Incomplete corner | Rounded instead of sharp | No vent at the corner; cluster needed |
| Matt patch on a glossy tool | Local loss of gloss | Sheet did not contact the tool — air held it off |
| Dimples at vent positions | Small depressions | Vents too large, or vacuum applied too long |
| Part sticks to tool | Hard release, distortion on demould | Vents blocked, or vacuum held after cooling |
| Uneven draw on one side | One side forms, the other does not | Channel blocked on that side |
Table 6 — Diagnosing venting problems from the part. The read-through pattern is a map of where the air was trapped, which makes it the most useful diagnostic of all.
Read-through is diagnostic because it is a negative image of the air pockets. If the part shows two faint circles 60 mm apart, the air was trapped in two places 60 mm apart, and vents are needed there. It is the fastest way to fix a new tool without guessing.
How big should vacuum forming vent holes be?
0.3–0.5 mm for cosmetic parts on thin sheet, 0.5–1.0 mm for general purpose, and 1.0–2.0 mm for thick or non-cosmetic work. The upper limit is set by the visible witness mark, the lower limit by how fast the air has to move.
How far apart should vents be?
25–50 mm on flat areas, 10–25 mm at the bottom of deep draws, and clustered at 5–15 mm in inside corners. Corners without vents will not form regardless of how many vents the flat areas have.
What happens if a tool is undervented?
Air is trapped between the sheet and the tool. You get read-through (the air pocket shape visible on the surface), rounded corners, incomplete draw, and matt patches where the sheet never contacted the polished tool. Underventing is the most common first-tool failure.
What happens if a tool is overvented?
Visible dimples at every vent position, and a surface that looks peppered on a glossy part. It also weakens the tool surface and makes cleaning harder. Overventing is less common than underventing because it is visible immediately.
Can I drill extra vents after the tool is made?
Yes, and it is often necessary. Drilling from the back into the cavity with a small bit, then chasing the hole through the channel plate, is standard commissioning work. Do it on a drill press with a depth stop, not freehand — a through-hole in the wrong place leaves a mark on every part.
Why do my vents keep clogging?
Either the polymer is being drawn into them (vents too large, or vacuum applied while the sheet is too soft), or the channel behind them is blocked with debris. PETG and other sticky materials plug vents more readily. Clean the channels periodically and consider smaller vents with a larger plenum behind.
What is a vented tool material?
A porous aluminium or composite that lets air pass through the bulk of the material instead of through drilled holes. It eliminates witness marks entirely, which matters on high-gloss cosmetic parts. It costs significantly more and is more fragile than solid aluminium.
Does venting matter for pressure forming?
More, not less. Pressure forming pushes the sheet down 4–7 times harder, so the air has to escape in a fraction of the time. Vents that are adequate for vacuum forming will cause read-through under pressure. Pressure tools need more vents and bigger channels.
Getting read-through or rounded corners?
Send us the part drawing, the material and gauge, and photos of the defect. We will tell you where the air is trapping and what the vent layout should be — or quote a tool with venting designed in.
PL 
EN 
ES 
FR 
DE 
PT 
AR 
RU 
JA 
KO 
IT 
NL 
TR 
ID 