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The principle and composition of molds in injection molding

Time: 2021-05-17 Hits: 47


The principle and composition of molds in injection molding

 

Injection molds are parts that give plastic shapes and dimensions during molding. Although the structure of the mold may vary widely due to the variety and performance of plastics, the shape and structure of plastic products, and the type of injection machine, the basic structure is the same.

 

The injection mold is mainly composed of three parts: the gating system, the molded parts and the structural parts. Among them, the pouring system and molded parts are the parts that are in direct contact with the plastic and change with the plastic and the product. They are the most complex and the most variable parts in the plastic mold, requiring the highest processing finish and precision.

 

The gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main runner, cold material cavity, runner and gate, etc. Molded parts refer to the various parts that make up the shape of the product, including movable molds, fixed molds and cavities, cores, molding rods, and vents.


Industrial parts mould 2

 

Main sprue/runner

 

It is a passage in the mold that connects the nozzle of the injection machine to the runner or cavity. The top of the sprue is concave to connect with the nozzle. The diameter of the main runner inlet should be slightly larger than the nozzle diameter (0.8mm) to avoid overflow and prevent the two from being blocked due to inaccurate connection. The diameter of the inlet depends on the size of the product, generally 4-8mm. The diameter of the main runner should be expanded inward at an angle of 3° to 5° to facilitate the demolding of the runner.

 

Cold feed hole

 

It is a cavity set at the end of the main runner to trap the cold material generated between two injections at the end of the nozzle, so as to prevent the clogging of the runner or gate. Once the cold material is mixed into the cavity, internal stress is likely to occur in the manufactured product. The diameter of the cold material cavity is about 8-10mm, and the depth is 6mm. In order to facilitate demolding, the bottom is often borne by the demolding rod. The top of the stripping rod should be designed in a zigzag hook shape or set with a recessed groove, so that the sprue can be pulled out smoothly during demolding.

 

Branch runners

 

It is the channel connecting the main channel and each cavity in the multi-slot mold. In order to make the melt fill the cavities at the same speed, the arrangement of the runners on the mold should be symmetrical and equidistant. The shape and size of the cross-section of the runner have an impact on the flow of the plastic melt, the demoulding of the product and the difficulty of mold manufacturing.

 

If the flow of the same amount of material is used, the flow channel resistance with a circular cross-section is the smallest. However, because the specific surface of the cylindrical runner is small, it is unfavorable for the cooling of the runner redundant, and the runner must be opened on the two mold halves, which is laborious and easy to align.

 

Therefore, trapezoidal or semicircular cross-section runners are often used, and they are opened on half of the mold with a stripping rod. The runner surface must be polished to reduce flow resistance and provide a 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 cross-section width of the runners does not exceed 8m, the extra-large ones can reach 10-12m, and the extra-small ones are 2-3m. On the premise of meeting the needs, the cross-sectional area should be reduced as much as possible, so as to avoid adding debris in the runners and extending the cooling time.

 

Gate

 

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

 

The function of the gate is: A. Control the material flow speed: B. It can prevent backflow due to the early solidification of the melt in this part during injection: C. The passing melt is subjected to strong shear and the temperature is increased. , Thereby reducing the apparent viscosity to improve fluidity: D, to facilitate the separation of the product and the runner system. The design of the gate shape, size and position depends on the nature of the plastic, the size and structure of the product.

 

Generally, the cross-sectional shape of the gate is rectangular or circular, and the cross-sectional area should be small and the length should be short. This is not only based on the above effects, but also because it is easier for small gates to become larger, and it is difficult for large gates to shrink. The gate location should generally be selected where the product is thickest without affecting the appearance. The design of the gate size should take into account the properties of the plastic melt.

 

Cavity

 

It is the space where plastic products are formed in the mold. The components used to form the cavity are collectively referred to as molded parts. Each molded part often has a special name. The molded parts that make up the shape of the product are called concave molds (also called female molds), and the ones that make up the internal shape of the product (such as holes, grooves, etc.) are called cores or punches (also called male molds).

 

When designing molded parts, the overall structure of the cavity must first be determined according to the properties of the plastic, the geometry of the product, the dimensional tolerances and the requirements for use. The second is to select the parting surface, the position of the gate and the vent hole and the demoulding method according to the determined structure. Finally, according to the size of the control product, the design of each part and the combination of each part are determined. The plastic melt has a high pressure when it enters the cavity, so the molded parts should be selected reasonably and checked for strength and rigidity.

 

In order to ensure the smooth and beautiful surface of plastic products and easy demoulding, the roughness of the surface in contact with plastic should be Ra>0.32um, and it should be corrosion-resistant. Formed parts are generally heat treated to increase the hardness, and are made of corrosion-resistant steel.

 

exhaust vent

 

It is a trough-shaped air outlet opened in the mold to discharge the original gas and the gas brought in by the molten material. When the melt is injected into the cavity, the air originally stored in the 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 welding, Dissatisfaction with the filling of the mold, and even the accumulated air, which burns the product due to the high temperature generated by compression.

 

Under normal circumstances, the vent 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 a shallow groove with a depth of 0.03-0. 2mm and a width of 1.5-6mm on one side of the cavity. During injection, there will not be a lot of molten material in the vent hole, because the molten material will cool and solidify at the place and block the channel. The opening position of the exhaust port should not be facing the operator to prevent accidental spraying of molten material and hurting people.

 

In addition, the matching gap between the ejector rod and the ejector hole, the matching gap between the ejector block and the stripper plate and the core can also be used for exhaust.

 

Structural parts

 

It refers to the various parts that constitute the mold structure, including: various parts for guiding, demolding, core pulling and parting. Such as front and rear splints, front and rear buckle templates, bearing plates, bearing columns, guide columns, stripping templates, demolding rods and return rods.

                  

Heating or cooling device

 

This is a device for solidifying and shaping the melt in the mold. For thermoplastics, it is generally the channel for the cooling medium in the male and female molds, and the cooling medium is circulated to achieve the purpose of cooling. The cooling medium introduced varies with the type of plastic and the structure of the product, including cold water, hot water, hot oil and steam. The key is high-efficiency and uniform cooling. Uneven cooling will directly affect the quality and size of the product. According to the thermal properties of the melt (including crystallization), the shape of the product and the mold structure, the arrangement of the cooling channels and the selection of the cooling medium should be considered.

 

Introduction and composition of injection mold

 

Plastic products are usually produced in batches or in large quantities. Therefore, molds must be used with high efficiency and high quality, with little or no processing after molding. Therefore, mold design must consider:

 

1. Determine the parting surface and gate location according to the use performance and molding performance of the plastic part.

 

2. Consider the manufacturability of the mold manufacturing project, determine the design plan according to the equipment status and technical strength, to ensure that the mold is easy to process from the whole to the parts, and it is easy to ensure the dimensional accuracy.

 

3. Consider injection productivity, increase the number of injections per unit time, and shorten the molding cycle.

 

4. The size and structure of holes, pillars, convexities and concaves with precision requirements are displayed in the mold, that is, the plastic parts are not processed or processed less after being formed.

 

5. The mold structure strives to be simple and applicable, stable and reliable, with a short cycle and low cost, which is convenient for assembly, maintenance and replacement of wearing parts.

 

6. Selection and processing of mold materials.

 

7. Standardized production of molds: try to use standard mold bases, commonly used ejector pins, guide parts, sprue sleeves, positioning rings and other standard parts.

 

Basic composition of injection mold

                                             

1. Pouring system: The channel through which the molten material flows from the nozzle of the injection machine into the cavity, including the main flow channel, the runner channel, the gate, the cold slug well, the draw rod, etc.

 

2. Molded parts: parts of molded plastic parts, such as cores, cavities and other auxiliary parts.

 

3. Temperature control system: used to adjust the temperature of the mold.

 

4. Plastic parts ejection system: including side parting mechanism, secondary ejection mechanism, first reset mechanism and fixed distance parting mechanism.

 

5. Installation part: the part where the mold body is reliably installed on the injection machine.

 

6. Connection system: the connection system that combines the various structural parts into a whole.

 

7. Guiding system: to ensure the accuracy of the movement of each structural member, such as guide posts, guide chutes, etc.