What are common problems with sight glasses?

What Are the Most Common Problems with Mold Flow Indicators?

You’re pulling your hair out over a mysterious quality issue. The parts have sink marks, but only in one corner. You check the cooling manifold, and every flow indicator shows the process is getting plenty of coolant.

This is a classic trap. The most common problems with flow indicators are not that they fail completely, but that they lie. Debris, improper installation, or material degradation can cause them to give false readings, leading you to chase problems that don't exist while ignoring the real issue.

This reminds me of a time I visited a medical device molder who was getting a high scrap rate on a complex part. The mold was brand new and beautifully made. The process was textbook perfect. And all sixteen of their brand new, ball-type flow indicators showed a healthy flow of 3 gallons per minute to each circuit. Yet, one side of the part was consistently showing signs of overheating. We spent half a day checking everything. Finally, I asked the operator to shut off the water supply. To our shock, half the balls in the indicators stayed right where they were. A fine layer of mineral scale from their hard water supply had deposited itself on the inside of the tubes, creating a tiny ledge that the balls were resting on. The indicators were lying. We installed good filters before the indicators, and the problem was solved for good.

What are common problems with sight glasses?

Your flow indicator is showing perfect flow, but the mold surface is still hot to the touch in that area. You trust your instruments, but the steel is telling you a different story.

This frustrating conflict is usually caused by an internal jam. A tiny piece of rust or scale can wedge the indicator's moving parts, or installing it in the wrong orientation can render it useless.

The number one reason for a false reading is debris jamming the internal mechanism. This is why the filter must always come before the indicator. A close second is incorrect installation—many visual indicators are designed for vertical (upward) flow only and will not read correctly if installed horizontally.

Let's break down how this happens. Most flow indicators used in molding rely on a simple visual cue: a spinning impeller, a floating ball in a tapered tube, or a moving piston. Each is vulnerable to contamination. For a spinner, a single piece of grit lodged in a blade can stop it from turning, making you think you have zero flow when you actually have good flow. More dangerously, with a ball-in-tube indicator, scale can build up on the inside walls or a particle can get wedged under the ball. This props the ball up, indicating high flow even when the channel is almost completely blocked. Installation orientation is also critical. A ball or piston indicator often relies on gravity to return the moving part to its zero position. If a unit designed for vertical installation is placed on its side, the ball will just rest on the side of the tube, giving no meaningful reading at all.

Common Causes of False Readings

What are common problems with sight glasses?

You walk past a mold and notice a foggy haze on the inside of a flow indicator, or worse, a tiny but persistent drip from its base. It's easy to ignore these "minor" issues.

These are not minor issues; they are warnings of imminent failure. A hazy or clouded lens means the plastic is under chemical attack, and a leak indicates a seal failure or a cracked body, often from over-tightening.

A cloudy indicator is an unreadable indicator, making it useless for process control. Leaks from cracked plastic bodies not only create a slip hazard but can also spray coolant onto your mold or electronics, leading to much bigger problems than a simple drip.

The body of most affordable flow indicators is made from plastic, like polycarbonate or polysulfone, to keep them transparent. While tough, these plastics are not invincible. The biggest enemy is chemical incompatibility. Certain rust inhibitors, glycol mixtures, or water treatment chemicals can attack the plastic, causing it to become brittle and develop tiny surface cracks, a phenomenon called "crazing." This is what makes the lens look foggy. Over time, these micro-cracks can grow and lead to a catastrophic failure under pressure. The other common killer is mechanical stress. Most indicators use NPT (tapered pipe) threads. The instinct when there is a small leak is to grab a wrench and give it another turn. This outward wedging force is immense and very easily cracks the plastic housing. The leak gets worse, and the indicator is now destroyed. It's always better to disassemble, clean the threads, apply a proper liquid sealant, and re-tighten gently.

Diagnosing Physical Indicator Failures

Conclusion

Don't let your flow indicators lie to you. Prevent false readings, leaks, and clouding by placing them after filters, choosing compatible materials, and avoiding aggressive over-tightening during installation.