It is a groove-shaped air outlet opened in mold to discharge original gas and gas brought in by melt. When melt is injected into cavity, air originally stored in cavity and gas brought in by melt must be discharged out of mold through exhaust port at the end of material flow, otherwise product will have pores, poor connection, underfilled mold, and even accumulated air will burn product due to high temperature due to compression.

 

Under normal circumstances, vent hole can be set either at the end of flow of melt in cavity, or at parting surface of mold. The latter is a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on one side of die.
During injection, there will not be a lot of molten material oozing out of vent hole, because molten material will cool and solidify there, channel will be blocked. Opening position of exhaust port should not face operator to prevent accidental ejection of molten material and injury. In addition, matching gap between ejector rod and ejector hole, matching gap between ejector block and stripper plate and core can also be used to exhaust air.

There are two main functions of exhaust groove: one is to remove air in mold cavity when molten material is injected; the other is to remove various gases generated during heating process of material. The thinner product is, the farther it is from gate, opening of exhaust groove is particularly important.
In addition, for small parts or precision parts, attention should also be paid to opening of exhaust grooves, because it can not only avoid surface burns and insufficient injection volume of products, but also eliminate various defects of products and reduce mold pollution.
So, how is exhaust of mold cavity considered sufficient? Generally speaking, if melt is injected at the highest injection rate without leaving a focal spot on product, venting of mold cavity can be considered sufficient.

There are many ways to exhaust mold cavity, but each method must ensure that: when exhaust groove is exhausted, its size design should prevent material from overflowing into groove; secondly, it should prevent clogging. Therefore, measured from inner surface of cavity to outer edge of cavity, height of vent groove with a length of 6-12mm or more should be increased by about 0.25-0.4mm.
Also, having too many vent slots is detrimental. Because, if clamping pressure acting on unvented part of cavity parting surface is very large, it is easy to cause cold flow or cracking of cavity material, which is very dangerous.
In addition to venting mold cavity on parting surface, venting can also be achieved by setting a venting groove at the end of material flow of gating system and leaving a gap around ejector rod, because venting groove can achieve purpose of venting. If selection of depth, width and position of opening is not appropriate, resulting flash burrs will affect beauty and accuracy of product. Therefore, size of above-mentioned gap is limited to prevent flash around ejector rod.
Special attention should be paid here: When exhausting parts such as gears, even the smallest flash may not be desirable. It is best to exhaust such parts in following ways:
Completely remove gas in flow channel;
Mating surface of parting surface is shot peened with silicon carbide abrasive with a particle size of 200#;
Venting groove is set at the end of material flow of gating system, mainly referring to venting groove at the end of runner, its width should be equal to width of runner, and height varies depending on material.

Based on years of experience in injection mold design and product testing, this article briefly introduces design of several exhaust grooves.
For product molds with complex geometric shapes, it is best to determine opening of exhaust groove after several mold trials. The biggest disadvantage of the overall structure in mold structure design is poor exhaust.
For integral cavity core, there are following exhaust methods:
Utilize slot or insert mounting portion of cavity
Utilize side insert seams
Partially made into a spiral shape
Fitting slotted slat cores in longitudinal position, opening process holes
When exhaust is extremely difficult, mosaic structure is used, etc.
If dead corners of some molds are not easy to open exhaust grooves, first of all, without affecting appearance and accuracy of product, mold should be appropriately changed to inlaid processing, which is not only conducive to processing exhaust groove, but also can sometimes improve original processing difficulty and facilitate maintenance.

 

Venting of thermosets is more important than thermoplastics. First, runners in front of gate should be vented. Width of exhaust slot should be equal to width of runner, and height should be 0.12mm. All sides of cavity should be exhausted, and each exhaust slot should be separated by 25mm, with a width of 6.5mm and a height of 0.075-0.16mm, depending on fluidity of material.
Softer materials should take lower values. Ejector rod should be enlarged as much as possible, and in most cases, 3-4 planes with a height of 0.05mm should be ground on cylindrical surface of ejector rod, and direction of wear scar should be along length of ejector rod. Grinding should be carried out with a finer-grained grinding wheel. End face of ejector rod should be ground with a chamfer of 0.12mm, so that if a flash is formed, it will adhere to workpiece.

Properly opening exhaust groove can greatly reduce injection pressure, injection time, pressure holding time and clamping pressure, so that molding of plastic parts is changed from difficulty to easy, thereby improving production efficiency, reducing production costs and reducing machine energy consumption.