The thinner product, the farther away from gate, opening of exhaust groove becomes particularly important. In addition, for small parts or precision parts, opening of venting groove should also be paid attention to, because it can not only avoid surface burns and insufficient injection volume, but also eliminate various defects of product and reduce mold pollution.
Function of exhaust groove mainly has two points: 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 can exhaust of cavity be considered sufficient? Generally speaking, if melt is injected at the highest injection rate, but no focal spots are left on product, it can be considered that exhaust in cavity is sufficient.

 

There are many ways to exhaust mold cavity, but each method must be guaranteed: size of exhaust slot should be designed to prevent material from overflowing into slot while exhaust slot is exhausted; secondly, it should prevent clogging. Therefore, when measuring from inner surface of cavity to outer edge of cavity, height of venting groove above 6-12mm should be about 0.25-0.4mm.
In addition, too many exhaust grooves are harmful. Because if clamping pressure acting on parting surface of mold cavity without opening vent groove is very large, it is easy to cause cold flow or cracking of mold cavity material, which is very dangerous.
In addition to venting mold cavity on parting surface, it is also possible to achieve purpose of venting by setting venting grooves at the end of material flow of gating system and leaving gaps around ejector rods, because depth, width and position of venting groove are selected, if not appropriate, flash burrs produced will affect appearance and accuracy of product. Therefore, size of above gap is limited to prevent flash around ejector rod.
Special attention should be paid here: when a part such as a gear is exhausted, even the smallest flash may be undesirable. It is best to use following method to exhaust gear part:
(1) Completely remove gas in the flow channel;
(2) Shot peening mating surface of parting surface 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 height varies depending on material.

For product molds with complex geometric shapes, it is best to determine opening of vent slot after several mold trials. The biggest disadvantage of overall structure in mold structure design is poor exhaust.
For integral cavity core, there are following exhaust methods:
(1) Use groove or insert installation position of cavity;
(2) Use side insert seams;
(3) Partially made into a spiral shape;
(4) Install slotted slat core and open craft hole in longitudinal position.
When exhaust is extremely difficult, mosaic structure is adopted. If dead corners of some molds are not easy to open exhaust groove, first of all, without affecting appearance and accuracy of product, mold should be appropriately changed to inlay processing. This will not only facilitate processing of exhaust groove, but sometimes can also improve original processing difficulty and facilitate maintenance.

Venting of thermoset materials is more important than that of thermoplastic materials.
First of all, runners in front of gate should be exhausted. Width of exhaust slot should be equal to width of runner, and height should be 0.12mm. All sides of mold cavity should be exhausted. Exhaust slots should be 25mm apart, 6.5mm wide, and 0.075-0.16mm high, 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 flat surfaces with a height of 0.05mm should be ground on cylindrical surface of ejector rod, direction of wear marks should be along length of ejector rod. Grinding should be carried out with a fine-grained grinding wheel. End face of ejector rod should be ground with a chamfer of 0.12mm, so that if there is a flash formed, it will adhere to workpiece.

Properly opening exhaust groove can greatly reduce injection pressure, injection time, holding time and clamping pressure, so that molding of plastic parts is changed from difficult to easy, thereby improving production efficiency, reducing production costs, and reducing machine energy consumption.
In fact, it is not necessary to exhaust air through exhaust groove. There are several other ways to exhaust:

For molds for forming large and medium-sized plastic parts, a large amount of gas needs to be removed, and an exhaust groove should usually be opened. Exhaust groove is usually opened on the side of concave mold on parting surface. Location of exhaust slot is better at the end of melt flow, size of exhaust slot is based on the principle that gas can be smoothly discharged without overflow. 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 for exhaust, but parting surface must be located at the end of melt flow.

For combined dies or cavities, split gaps can be used to vent.

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

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

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

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