Meaning
Precision sealing surfaces in an injection mould rely on tapered geometry to prevent the flow of molten plastic between moving components during the injection phase. This design feature is essential for maintaining the integrity of the tool and preventing the formation of flash on the finished part. A draft angle shut off involves two metal surfaces that come together at a slight angle, creating a tight seal that is easier to separate once the moulding cycle is complete.
It is commonly used in complex tools with slides, lifters or telescoping cores where a perpendicular shut off would cause excessive wear. The effectiveness of the seal depends on the precision of the machining and the thermal expansion of the steel.
Geometric Sealing
The taper allowed by the draft angle ensures that the surfaces only make contact at the final moment of the mould closing. This minimizes the friction and mechanical stress that would occur if the surfaces were perfectly vertical. When the mould is closed, the draft angle shut off creates a wedge like effect that provides a strong seal against the high pressure of the plastic melt.
This geometry is particularly useful for creating holes or windows in a part without the need for additional moving parts. The angle is typically between one and five degrees, depending on the material being moulded and the required life of the tool. If the angle is too small, the surfaces may gall or seize over time.
If it is too large, the seal may not be tight enough to prevent the polymer from leaking through.
Moulding Precision
Achieving a high quality finish on parts with complex internal features requires a very stable shut off mechanism. Any gap between the sealing surfaces will allow the polymer to enter, resulting in thin pieces of unwanted plastic known as flash. Draft angle shut off surfaces must be ground and lapped to a high degree of accuracy to ensure they fit together perfectly every time.
This precision is difficult to maintain in high volume production where the tool is subjected to constant heating and cooling. The choice of tool steel and its heat treatment are important factors in preserving the geometry of these surfaces. Regular inspection for signs of wear or deformation is necessary to prevent quality issues.
A well designed shut off will last for hundreds of thousands of cycles without needing repair.
Thermal Compensation
Operating an injection mould at high temperatures causes the steel components to expand, which can affect the fit of the sealing surfaces. Engineers must account for this expansion when designing the draft angle shut off to ensure it still functions correctly at the target melt temperature. If the fit is too tight when the tool is cold, it may become impossible to close the mould once it reaches its operating temperature.
Conversely, a fit that is too loose will result in flash during the initial stages of production. Some tools use cooling channels located near the shut off areas to maintain a more consistent temperature. This thermal management helps to extend the life of the tool and maintain the quality of the parts.
Proper lubrication of the sliding surfaces is also necessary to prevent premature wear.
