9.Four basic knowledge points of injection mold

Injection molding technology is widely used. I hope this article can give some inspiration to friends who want to understand this knowledge.——-Vito.

The injection mold is composed of a moving mold and a fixed mold. The moving mold is installed on the moving template of the injection molding machine, and the fixed mold is installed on the fixed template of the injection molding machine. In injection molding, the moving mold and the fixed mold are closed to form the pouring system and the mold cavity, and the moving mold and the fixed mold are separated to remove the plastic products.

Although the structure of the mold may vary due to the variety and performance of plastic, the shape and structure of plastic products and the type of injection machine, the basic structure is the same. The mold is mainly composed of pouring system, temperature control system, forming parts and structural parts. The casting system and molding parts are directly in contact with the plastic, and change with the plastic and products. They are the most complex and the most varied parts in the plastic mold, and require the highest machining finish and precision.

The gating system refers to the part of the flow channel before the plastic enters the cavity from the nozzle, including the main flow channel, cold material hole, shunt channel and gate, etc. Forming parts refers to various parts that constitute the shape of the product, including moving die, fixed die and cavity, core, forming rod and exhaust port.

1. Pouring system

The gating system, also known as the runner system, is a set of feed channels that lead the plastic melt from the nozzle of the injection machine to the mold cavity. It is usually composed of a main channel, a shunt, a gate and a cold material hole. It is directly related to the molding quality and production efficiency of plastic products.

1. The mainstream way

It is a section of the mold connecting the injection nozzle to the shunt or cavity. The top of the main channel is concave to connect with the nozzle. The diameter of the inlet of the main channel should be slightly larger than the diameter of the nozzle (O.8mm) to avoid overflow, and to prevent the blockage of the two due to inaccurate connection. The diameter of the import depends on the size of the product, generally 4-8mm. The diameter of the main passage should be expanded inward by an Angle of 3° to 5° to facilitate the demoulding of the flow passage tautologies.

2. Cold material

It is a hole at the end of the main channel to capture the cold material produced between two injections at the nozzle end, so as to prevent the blockage of the shunt or gate. If the cold material once mixed with the cavity, the product is easy to produce internal stress. The diameter of the cold material hole is about 8 l0mm, and the depth is 6mm.

In order to facilitate the demoulding, the bottom is usually borne by the demoulding rod. The top of the demoulding rod should be designed into a zigzagging hook shape or set a trench, so that the main channel can be pulled out smoothly when demoulding.

3. The shunt

It is the channel connecting the main channel and each cavity in the multi-groove mold. In order to make the melt fill the cavity at the same speed, the arrangement of the shunt on the mold should be symmetrical and equidistant distribution.

The shape and size of the shunt section affect the flow of plastic melt, the demoulding of products and the difficulty of mold manufacturing. If the flow of the same amount of material, the flow passage with circular section has the least resistance.

However, because the specific surface of the cylindrical flow channel is small, it is not good for the cooling of the taunt, and this kind of flow channel must be set up on the two halves of the die, which is laborious and easy to align. Therefore, it is often used to trapezoidal or semicircular section of the shunt, and opened in a half of the mold with a stripping rod.

The runner surface must be polished to reduce flow resistance and provide faster filling speed. The size of the runner depends on the type of plastic, the size and thickness of the product. For most thermoplastics, the width of the shunt section is not more than 8m, the extra large can reach 10-12m, the extra small 2-3m. Under the premise of meeting the needs, the cross-sectional area should be reduced as far as possible, so as not to increase the diversion and prolong the cooling time.

4. The gate

It is connected to the main channel (or shunt) and the cavity of the channel. The cross-sectional area of the channel can be equal to that of the main channel (or shunt), but it is usually reduced. So it has the smallest cross-sectional area in the whole runner system. The shape and size of the gate have great influence on the quality of the product.

The purpose of the gate is:

Control material flow velocity

In the injection can be stored in this part of the melt early coagulation to prevent backflow

The passing melt is subjected to strong shear and the temperature is increased, so as to reduce the apparent viscosity and improve the fluidity

Easy to separate the product from the runner system. The design of gate shape, size and location depends on the nature of the plastic, the size and structure of the product

The general gate section shape is rectangular or circular, the cross-sectional area should be small and the length should be short, not only based on the above effect, but also because it is easier for a small gate to become large, while it is difficult for a large gate to shrink. The location of the gate should be selected as the thickest product in process and does not affect the appearance of the place. The size of the gate should take into account the nature of the plastic melt. The cavity is the space in the mold where plastic products are formed.

The components used to form a cavity are collectively called molded parts. Each forming part has a special name. Forming parts of the shape of the product called concave die (also known as the negative die), forming the internal shape of the product (such as holes, slots, etc.) called the core or punch (also known as the positive die). The design of molding parts should first be based on the performance of plastic, product geometry, dimensional tolerance and use requirements to determine the overall structure of the cavity.

Secondly, the position of parting surface, gate and vent and demoulding mode are selected according to the determined structure. Finally, according to the size of the control product, the design of each part and determine the combination between the parts.

Plastic melt into the cavity has a high pressure, so molding parts should be reasonable material selection and strength and stiffness check. In order to ensure the smooth and beautiful surface of plastic products and easy demoulding, where the surface contact with plastic, its roughness Ra>0.32um, and corrosion resistance. Molded parts are generally heat-treated to improve their hardness and made of corrosion-resistant steel.

Two, temperature control system

In order to meet the mold temperature requirements of injection process, a temperature regulating system is needed to adjust the mold temperature. For injection moulds for thermoplastics, a cooling system is designed to cool the moulds.

The common way to cool the mold is to open a cooling water channel in the mold and use the circulating cooling water to take away the heat of the mold. In addition to hot water or steam in the cooling water channel, electric heating elements can also be installed in and around the mold.

Three, molding parts

The forming part is composed of a core and a concave die. The core forms the inner surface of the product, and the concave die forms the outer surface shape of the product. After closing the mold, the core and cavity constitute the cavity of the mold. According to the process and manufacturing requirements, sometimes the core and die are composed of several pieces, sometimes they are made as a whole, and inserts are used only in the parts that are easy to damage and difficult to process.

Exhaust port It is a groove shaped air outlet opened in the mold to discharge the original and molten gas. When the molten material is injected into the cavity, the air originally stored in the cavity and the gas brought in by the melt must be discharged to the mold through the exhaust port at the end of the material flow. Otherwise, the product will have pores, poor connection, dissatisfaction with the mold filling, and even the accumulated air will be burned due to high temperature caused by compression.

In general, the vent can be located at the end of the molten flow in the mold cavity or on the parting surface of the 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 the die. During injection, there will not be much melt leakage from the vent, because the melt will cool and solidify there and block the channel. The opening position of the exhaust port should not be opposite to the operator, in order to prevent melting accidental ejection injury. In addition, it can also use the clearance between the ejector rod and the ejector hole, and the clearance between the ejector block and the template and the core to exhaust gas.

Four, structural parts

It refers to the various parts of the mold structure, including: guide, demoulding, core pulling and parting of various parts. Such as front and rear splints, front and rear buckle template, pressure plate, pressure column, guide column, strip template, strip rod and return rod, etc.

1. Guide component

In order to ensure that the moving die and the fixed die can be accurately aligned when the die is closed, a guide part must be set in the die. In injection mold, four sets of guide posts and guide sleeves are usually used to form the guide, and sometimes it is necessary to set the inner and outer cone surfaces on the moving mold and the fixed mold respectively to assist positioning.

2. Launch institutions

In the process of mold opening, a pushing mechanism is required to push out or pull out the plastic products and the coagulant in the flow channel. The fixed plate and push plate are introduced to hold the push rod. In the push rod is generally fixed with a reset rod, the reset rod in moving, fixed die closing to reset the push plate.

3. Side core pulling mechanism

Some plastic products with side concave or side hole, before being pushed out must be first side parting, the lateral core can be pulled out smoothly, at this time, the need to set up a side core pulling mechanism in the mold.

4. Standard formwork

In order to reduce the heavy mold design and manufacturing workload, injection mold mostly adopts the standard mold frame.





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