A crack in HMI touch glass is an open front face, not a cosmetic flaw. Test the whole face: 9 tap points, 3 drags and a 5-minute watch for phantom input. A clean panel can finish the wait in restricted service; any dead or phantom touch takes it out of the control path the same day.
A crack across the glass of an HMI touch panel is an open front face, not a scratch. The lens is part of the seal that keeps washdown spray, oil mist and dust off the display and touch layers, and a through-crack is a gap in it. The decision is testable at the panel in about 15 minutes, most of that the 5-minute watch: tap 9 points across the face, drag 3 lines across it, and count what fails. A panel that passes can stay in restricted service until its replacement lands. Any dead point, broken drag or phantom touch takes it out of the control path the same day, because a panel that presses its own buttons can write start, jog or setpoint values to the PLC.
What each kind of damage points to
| What you see | What it points to | What to do |
|---|---|---|
| Hairline crack, glass solid, display clean | The front face is open along the crack, and the crack will grow | Order a replacement and run the touch test below |
| Loose shards or sharp edges | Cut hazard at operator reach | Barrier the station and handle with cut gloves; test with the shards contained |
| Dark lines or blotches spreading from the crack | The crack has reached the display layer | Replace now; run the test anyway, touch often outlives the image |
| Moisture droplets or oil visible behind the lens | Liquid is inside; corrosion and erratic behavior follow | Out of service now, replacement ordered |
| Touches nobody makes | Phantom input; the panel can write to the PLC | Out of the control path before any test |
A crack is one branch of the wider repair-or-replace call. The case where touch dies while the display behind it still works is worked through in Broken HMI touch screen with a working LCD: repair or replace.
Put the machine in a safe state first
The test works by pressing the screen, and pressing a live screen can change a value. Stop the machine or put it in its stopped mode, tell the operator what you are doing, and open a blank screen, one with no buttons and no numeric inputs. If the project has no blank screen, use a text-only screen and never press a real control. One panel never reaches the test: one that shows touches nobody makes. Remove it from the control path and run the machine from hardwired pushbuttons or a spare until the replacement is in.
The touch test that decides it
- Clean and dry the face. A film of coolant or cleaner fakes both failure modes: it bridges between touches, and it can register input with no finger. If a protector film is already on the face, take it off for the test. Wipe with a lint-free cloth and dry fully.
- Open the blank screen from the safe-state step.
- Tap 9 points: the four corners, the four edge midpoints, the center. Mark any point that fails to register and where it sits against the crack.
- Drag 3 lines: one left to right across the full face, one top to bottom, and one slow drag along the whole crack. A drag that breaks or jumps marks a dead strip.
- Watch 5 minutes with hands off. Any movement, value change or touch indicator is phantom input and ends the test.
A tap that lands wrong or a drag that breaks has one other common cause: calibration. Recalibrate with a stylus and retest before you blame the crack, the way HMI touch button only responds to half: recalibrate with a stylus works through it. Phantom input on a panel with no crack at all points elsewhere, usually heat, and that triage is its own job: HMI ghost touch at high temperature: check before replacing.
The only pass is a clean sweep: all 9 points register, all 3 drags run unbroken, and the 5-minute watch records nothing.
Example: diagonal crack on a washdown-area panel
Inputs, and where each comes from:
- Panel: an HMI on a washdown filling line, crack running from the lower left corner to the middle of the right edge. A typical case for this failure; the measures below are the test steps defined above.
- Tap result: 9 points tested, 7 register, 2 dead, both beside the crack ends. (measured at step 3)
- Drag result: the crack-line drag breaks twice; the two full-face drags run clean. (measured at step 4)
- Ghost watch: 5 minutes untouched, 0 inputs. (measured at step 5)
Rule in words: the panel keeps its place in the control path only when dead points = 0 and phantom inputs = 0, with a broken drag counting as dead.
Arithmetic:
- Dead points: 9 tested - 7 registered = 2 dead. Allowed 0. 2 > 0: fail.
- Drags: 1 of 3 broken. Allowed 0: fail.
- Phantom inputs: 0 in 5 minutes. Allowed 0. 0 = 0: pass.
- Any single failure decides, and this panel has two.
Decision: viewing only. Operators work the machine from hardwired pushbuttons or a spare panel, film goes over the crack as shard control, spray is directed away from the face, and the replacement is ordered the same day. Had the tap come back 9 of 9, the drags clean and the watch quiet, the same panel could have run in restricted service for the wait: pressing allowed away from the crack, film on, spray off, order still placed. The threshold is the same in both directions: 0 dead and 0 phantom keeps it in service, any positive count moves operators off it.
Why tape, epoxy and a bare glass swap fail
- Clear film or tape: it holds shards in place, and that is its whole job. The face stays open along the crack, so film goes on after the test as shard control, never as a seal.
- Glue or epoxy in the crack: adhesive wicks along the crack into the touch layer and can turn a small dead zone into a dead half of the screen. It adds no barrier either.
- A bare glass swap: the front assembly is lens, touch sensor, bonding or gasket and bezel built as one matched part. Most makers treat it that way, so ask the support line whether a glass-only repair exists for the model, with the model and serial from the rear label at hand. The common answer is no, and a front you reseal yourself has no published rating behind it.
Replace the panel and verify it before release
- Back up first. Find the project file and note the software version that saved it. If no file exists and the cracked panel still powers up, read the communication settings from its setup screens and photograph them, and check whether the programming software can upload the project. Label and photograph every rear connection: power, communication cable, ground.
- Record the PLC-side settings the panel talks to: node or station address, baud rate, and IP address where the link is Ethernet.
- Order the same model where one exists; the project loads with no layout changes. A different model changes screen size and possibly resolution, so retarget the project and check every screen for objects that fall off-screen or overlap.
- Fit the new panel with the gasket that ships with it and never reuse a compressed or torn one. Tighten the mounting hardware evenly in the pattern the installation manual gives.
- Verify before release: run the whole touch test again on the new panel, confirm live PLC data with no communication fault, then, machine in a safe state, exercise each screen's controls one at a time and confirm the expected bit or register changes. Step through every screen once for on-screen objects and alarm banners.
Recommendation: test before you decide and replace either way. A cracked panel that passes the full touch test can stay in restricted service until its replacement lands, with film over the crack and spray kept off the face. A dead point, a broken drag or a single phantom touch moves operators to hardwired pushbuttons at once, and moisture behind the lens takes the panel off the wall immediately. Back up the project and photograph the wiring before the old panel comes down, because the replacement arrives blank.
Questions
Is the fault in the touch layer or the display layer behind the crack?
Put a test pattern or an even-colored screen on the panel and look before you touch. Dark bands, blotches or lines spreading from the crack are display damage, and the panel replaces regardless of how touch tests. A clean image with taps that fail at certain points is touch-layer damage, and the nine-point tap test is what decides whether it can stay in service.
How long can a panel that passed the test stay in service?
For the wait, and there is no figure that stretches it. A passing test says the touch layer worked when you tested; it says nothing about how fast the crack grows through the next hot-to-cold cycles or washdown shifts. Retest at the start of each shift the panel is used, and pull it the moment a point goes dead or a phantom touch appears.
The replacement is a different model. What has to change?
Screen size and possibly resolution change, so the project rarely loads unchanged. Retarget the project in the programming software, then open every screen and look for objects that fall off the edge or overlap. Confirm the communication settings against what you photographed on the old panel, and with the machine in a safe state, press each control once and watch the expected bit or register change in the PLC.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.