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Characteristics of exhaust groove design for injection molds

Injection mold Characteristics of exhaust groove design for injection molds

Injection mold is a tool for producing plastic products; it is also a tool that gives plastic products complete structure and precise dimensions. Injection molding is a processing method used in the mass production of certain parts with complex shapes. Specifically, it refers to injecting heated and melted plastic into the mold cavity at high pressure from an injection molding machine. After cooling and solidifying, a molded product is obtained.

Mold composition

Although the structure of the mold may vary due to different types and properties of plastics, shapes and structures of plastic products, and types of injection machines, the basic structure is the same. The mold is mainly composed of pouring system, temperature control system, molded parts and structural parts. Among them, the pouring system and molded parts are the parts that are in direct contact with plastics and change with the plastics and products. They are the most complex and changing parts of the mold and require the highest processing smoothness and precision.

The injection mold consists of two parts: a movable mold and a fixed mold. The movable mold is installed on the movable template of the injection molding machine, and the fixed mold is installed on the fixed template of the injection molding machine. During injection molding, the movable mold and the fixed mold are closed to form the pouring system and cavity. When the mold is opened, the movable mold and the fixed mold are separated to facilitate removal of plastic products. In order to reduce the heavy workload of mold design and manufacturing, most injection molds use standard mold bases.https://doi.org/10.1016/j.addma.2021.102408

exhaust vent

It is a groove-shaped air outlet opened in the mold to discharge the original gas and the gas brought in by the melt. When the molten material is injected into the mold cavity, the air originally existing in the mold cavity and the gas brought in by the melt must be discharged out of the mold through the exhaust port at the end of the material flow. Otherwise, the product will have pores, poor connections, and The mold filling is not satisfactory, and the accumulated air may even burn the product due to high temperature caused by compression. Under normal circumstances, the exhaust hole can be located either at the end of the melt flow in the cavity or on the parting surface of the mold. The latter is to open a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on one side of the die.

1.The function of exhaust slot

During injection, a lot of molten material will not seep out of the vent hole, because the molten material will cool and solidify there and block the channel. The exhaust port must not be opened facing the operator to prevent the molten material from accidentally spraying out and injuring people. In addition, the matching gap between the ejector rod and the ejector hole, the matching gap between the ejector block and the stripping plate and the core can also be used to exhaust air.https://solidcomould.com/product/background-injection-mold/

The exhaust slot has two main functions: one is to remove the air in the mold cavity when the molten material is injected; the other is to remove various gases generated during the heating process of the material. The thinner the wall product is and the farther away it is from the gate, the more important it is to open the exhaust groove.

In addition, for small parts or precision parts, attention should also be paid to the opening of the exhaust groove, because in addition to avoiding surface burns and insufficient injection volume, it can also eliminate various defects of the product and reduce mold contamination. ​

So, how can the exhaust of the mold cavity be considered sufficient? Generally speaking, if the molten material is injected at the highest injection rate without leaving any scorch spots on the product, it can be considered that the exhaust in the mold cavity is sufficient.

2. Exhaust method

There are many ways to exhaust the mold cavity, but each method must ensure that: while the exhaust slot is being exhausted, its size should be designed to prevent materials from overflowing into the slot; secondly, it must also prevent clogging. Therefore, when measuring from the inner surface of the mold cavity to the outer edge of the mold cavity, the height of the exhaust groove part that is more than 6-12mm long should be increased by about 0.25-0.4mm. In addition, too many exhaust slots can be harmful. This is because if the clamping pressure acting on the part of the mold cavity parting surface without the vent groove is very large, it can easily cause cold flow or cracking of the mold cavity material, which is very dangerous. ​

In addition to exhausting the mold cavity on the parting surface, the purpose of exhausting can also be achieved by setting up an exhaust slot at the end of the material flow of the pouring system and leaving a gap around the ejector rod, because the exhaust slot If the selection of the depth, width and position of the opening is inappropriate, the flash and burrs produced will affect the beauty and accuracy of the product. Therefore, the size of the above gap is limited to prevent flash around the ejector rod.

Special attention should be paid here: when parts such as gears are vented, even the smallest flash may not be desired. It is best to vent this type of parts in the following ways:

(1) Completely remove the gas in the flow channel;(2) Use silicon carbide abrasive with a particle size of 200# to shot blast the mating surface of the parting surface. In addition, setting up an exhaust slot at the end of the material flow of the pouring system mainly refers to the exhaust slot at the end of the runner. Its width should be equal to the width of the runner, and the height varies depending on the material.

3. Design method

Based on many years of experience in injection mold design and product testing, this article briefly introduces several exhaust slot designs.For product molds with complex geometric shapes, it is best to determine the location of the exhaust groove after several mold trials. The biggest disadvantage of the overall structural form in the mold structure design is poor exhaust.

For the integral cavity core, there are several exhaust methods:(1) Use the slot or insert installation part of the cavity;

(2) Utilize the side insert seams;

(3) Partially made into a spiral shape;

(4) Install the slotted slat core in the longitudinal position and drill process holes;

(5) When exhaust is extremely difficult, use an inlay structure, etc.; if some molds have blind corners where it is difficult to open exhaust grooves, first the mold should be appropriately changed to inlay processing without affecting the appearance and accuracy of the product, so that , not only is it beneficial to process the exhaust groove, but sometimes it can also improve the original processing difficulty and facilitate maintenance.

4. Design of exhaust grooves during thermosetting plastic molding

Outgassing is more important for thermoset materials than for thermoplastic materials. First, the runner in front of the gate should be vented. The width of the exhaust groove should be equal to the width of the runner, and the height should be 0.12mm. The mold cavity should be vented all around, and the vent grooves should be 25mm apart, 6.5mm wide, and 0.075-0.16mm high, depending on the fluidity of the material. Softer materials should take lower values. The 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 the cylindrical surface of the ejector rod, and the direction of the grinding marks should be along the length of the ejector rod. Grinding should be carried out with finer-grained grinding wheels. The end face of the ejector rod should be ground with a 0.12mm chamfer, so that if any flash is formed, it will adhere to the workpiece.

5 Conclusion

Properly opening an exhaust groove can greatly reduce the injection pressure, injection time, holding time and mold clamping pressure, making plastic part molding from difficult to easy, thus improving production efficiency, reducing production costs, and reducing the energy consumption of the machine.

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