Dec 02,2025
Mold exhaust system design: how the invisible“Breathing system” affects filling and defects
Mold exhaust system design: how the invisible“Breathing system” affects filling and defects
Die exhaust, a detail often considered at the end of a design, is the“Silent level” that determines filling success or failure. The exhaust efficiency must be as high as the melt speed. A neglected exhaust problem, can make the most sophisticated mold and the best process to fall short.
Typical symptoms and causes of dysventilation
Diesel Effect: trapped air is compressed so rapidly that it heats up and reaches the plastic's burning point, causing local carbonation and blackening. This is the most visible sign of poor exhaust.
Insufficient filling: the gas at the end of the cavity can not be discharged, forming an“Air cushion” to prevent the melt completely filled.
Shrink mark: the gas hinders the effective transmission of holding pressure, resulting in insufficient feeding.
Low Weld Line Strength: trapped gas at the confluence of the melt, resulting in poor fusion, forming a fragile zone.
Surface flow marks: the gas interferes with the smooth flow of the melt front.
Key elements of exhaust system design
The depth of the exhaust slotーー the golden rule
The depth design is based on the viscosity (flowability) of the plastic melt. The basic principle is to allow gas to pass through but prevent the melt from escaping.
General Purpose Plastics (ABS, PP, PS) : venting slot depth is recommended as 0.02-0.03 mm.
High viscosity engineering plastics (PC, PMMA) : can be properly deepened to 0.03-0.05 mm.
Low viscosity plastics (PA, POM) : need to be more shallow, 0.01-0.02 mm, and even use semi-exhaust steel.
Width and layout of exhaust slot
The width should be adequate, usually 5-8 mm, and evenly distributed along the perimeter of the cavity.
Key layout area: the opposite side of the gate, the last filling of the cavity, the melt confluence, the gap between the insert and the thimble, and the inside of the slider core-pulling mechanism.
Back-end drainage of vents
Vents must be connected to the outside of the die through drains that can be deepened to 0.5-1.0 mm to allow gas to be quickly removed or escaped.
Advanced Exhaust Technology and materials
Porous Steel: a sintered material filled with pores that allow gas to pass through but keep out the melt. Especially suitable for deep ribs, thin walls, etc. can not open the traditional exhaust slot“Trapped gas dead”.
Exhaust inserts: make the key exhaust points as independent inserts for easy replacement and maintenance after damage. The insert itself can be either exhaust steel or plain steel with a precision exhaust slot.
Vacuum exhaust system:
Principle: before injection, the vacuum pump will be forced out of the cavity of the air, the formation of negative pressure.
Value: the complete elimination of any defects due to the presence of air significantly reduces injection pressure, improves fiber orientation, and reduces internal stress. Is the optical parts, medical equipment and other high-demand mold the ultimate solution.
Conclusion:
A good mold designer looks ahead to the exhaust system as a functional unit as important as runner and cooling. By following the principle of scientific depth, carrying out a comprehensive layout, and introducing advanced technologies such as exhaust steel and vacuum exhaust when necessary, it is possible to ensure that the mold“Breathes” smoothly in the era of high-speed injection molding, and to improve the quality of the mold, clear the last hurdle for the stable production of high-quality products.
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