A vacuum leak reduces the pressure differential available to move the sheet and slows air evacuation from the tool. The part may show soft detail, incomplete corners, inconsistent draw, or a gradual loss of performance during a production run.
Leaks can occur in hoses, fittings, valve seats, receiver connections, mold seals, pressure-box gaskets, baseboards, and unused ports. A system may still reach a strong final vacuum while responding too slowly for the forming cycle, so a static gauge reading alone is not enough.
Isolate the leak
The vacuum curve should be observed from valve opening through the end of forming. A slow pressure drop, poor hold, or rapid recovery toward atmospheric pressure indicates leakage or excessive system volume. The machine circuit should then be separated into sections so the pump, receiver, distribution line, and tool can be tested independently.
Damaged flexible lines and worn seals should be replaced rather than compensated for with longer heating or more pump run time. After repair, the same test should be repeated with the mold installed and sealed. A leak-free machine circuit can still lose performance through the tool face or a distorted clamping interface.