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.
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.

 

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 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 be undesirable. Gear parts are best exhausted in following ways:
(1) Completely remove gas in runner;
(2) Mating surface of parting surface is shot peened with silicon carbide abrasive with a particle size of 200.
In addition, opening of exhaust groove at the end of material flow of gating system mainly refers to exhaust groove at the end of runner, its width should be equal to width of runner, and the height varies depending on material.

For product molds with complex geometries, 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:
(1) Use groove or insert installation part of cavity;
(2) Utilize insert seam on the side;
(3) Partially made into a spiral shape;
(4) Install slotted slat core and open process hole in longitudinal position.
When it is extremely difficult to exhaust, a mosaic structure is used. 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 insert processing, which is not only conducive to processing exhaust groove, but also sometimes improves original processing difficulty and facilitates 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, making molding of plastic parts easier than difficult, thereby improving production efficiency, reducing production costs and reducing machine energy consumption.
In fact, not all must be exhausted through exhaust slot. There are several other ways of exhausting:

For molds for forming large and medium-sized plastic parts, there is a large amount of gas that needs to be removed. Usually, an exhaust groove should be opened, and exhaust groove is usually opened on the side of die on parting surface. Position of exhaust groove is preferably at the end of melt flow, and size of exhaust groove is based on principle that gas can be discharged smoothly without overflowing. Width of exhaust groove is generally about 3-5mm, depth is less than 0.05mm, and length is generally 0.7-1.0mm.

For small molds, parting surface gap can be used to exhaust, but parting surface must be at the end of melt flow.

For combined die or cavity, combined gap can be used to exhaust.

Use clearance between push rod and mold plate or core to exhaust, or intentionally add clearance between push rod and mold plate.

Powder unsintered alloy is a material sintered with spherical particle alloy, which has poor strength but loose texture and allows gas to pass through. Placing a piece of such alloy in the part to be vented can meet requirements of venting, but diameter of vent hole at the bottom should not be too large to prevent cavity pressure from squeezing and deforming it.

On the outside of plastic melt junction, a cavity is arranged to allow gas to be discharged into it, and a good exhaust effect can also be obtained.

In the part where gas is closed, set exhaust rod. This method has a good exhaust effect, but it will leave traces of rod on plastic part, so exhaust rod should be set in hidden place of plastic part.