Film Hinge Failure
Partial or complete breakage of the film hinge. The gate position, wall thickness and making the first movement while hot are decisive.
A film hinge is the thin plastic area connecting two sections in parts such as lids and boxes, and it is expected to open and close thousands of times. Breakage comes from the molecules in this area not being oriented correctly. The problem is usually not the material quality but how the hinge is filled.
How to Recognize It
- Partial separation, tearing or complete breakage of the hinge; at first use or after a few cycles.
- Stress whitening in the hinge area may accompany it.
- If the hinge does not fill at all (frozen material), the gate position is wrong.
Physical Causes
- A film hinge is a moving, permanent joint; its function relies on the elastic properties of the material. The highest load occurs with 180° bending + a high number of cycles.
- The gate position creates a melt flow stop or partial freezing at the hinge.
- There is a weld line in the hinge area — a weak point forms.
- Wrong wall thickness: if too thin it does not fill/breaks, if too thick excessive force is needed for movement.
- The hinge was flexed while cold (no molecular orientation was imparted).
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- If the hinge does not fill: increase the injection speed and the melt and mold wall temperatures.
- Overstretch the hinge IMMEDIATELY AFTER injection, while it is hot — this imparts molecular orientation and extends its life.
- For early breakage: increase the injection speed, and with semi-crystalline material LOWER the mold wall temperature (more elastic behavior).
- If there is stress whitening, work the hinge while it is hot.
Mold, Design and Material Solutions
- Move the gate AWAY from the hinge: the large-volume part of the piece should fill before the flow front reaches the hinge.
- The flow front should pass the hinge in PARALLEL — the best gate position can only be determined reliably by simulation.
- Remove the weld line from the hinge area.
- Adjust the hinge wall thickness (increase it if there is a filling problem, reduce it if movement is stiff); keep the maximum elongation of the material in mind.
- Consider an easy-flowing or higher-viscosity material (depending on the problem).
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Stress Whitening | Whitening can be seen in any stressed area. Film hinge failure occurs only in the hinge area and as a result of repeated bending. |
| Weld Line | If a weld line forms in the hinge area, breakage is almost certain; the two defects must be assessed together. |
Frequently asked questions
Why is molecular orientation important?
The strength of a film hinge comes from the molecules being oriented **perpendicular** to the hinge. This orientation only forms when the melt passes through the hinge quickly. Slow filling, or the hinge lying at the end of the flow, weakens the orientation.
Why is the first opening critical?
With materials such as PP, when the hinge is **fully opened and closed once** while the part is still hot, the molecular structure is permanently arranged and its life increases many times over. This step should be added to the production flow; doing it later on a cold part does not give the same effect.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice