The Principle And Composition Of Molds In Injection Molding

Update:27-05-2021
Summary:

  An injection mold is a part that has a plastic shape […]

  An injection mold is a part that has a plastic shape and size during the molding process. Although the structure of the mold may vary greatly depending on the type and performance of the plastic, the shape and structure of the plastic product, and the type of injection molding machine, the basic structure is the same.

 

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

 

  The gate 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.

 

  Main gate/runner

 

  It is a channel in the mold that connects the nozzle of the injection machine to the runner or cavity. The top of the gate is concave to connect with the nozzle. The diameter of the sprue inlet should be slightly larger than the nozzle diameter (0.8 mm) to avoid overflow and prevent blocking of both due to incorrect connection. The diameter of the inlet depends on the size of the product, usually 4-8mm. The diameter of the main runner should expand 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 capture the cold material generated between two injections at the end of the nozzle to prevent the runner or gate from being blocked. Once the cold material is mixed into the cavity, internal stress is likely to occur in the finished 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 usually supported by a demolding rod. The top of the stripping rod should be designed as a zigzag hook shape or provided with a groove, so that the gate can be pulled out smoothly during the demolding process.

 

  Branch runner

 

  It is the channel connecting the main channel and each cavity in the multi-slot mold. In order to make the melt fill the cavity 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 will affect the flow of the plastic melt, the difficulty of product demolding and mold manufacturing.

 

  If the same amount of material flow is used, the flow path with a circular cross section has the least resistance. However, due to the small specific surface area of ​​the cylindrical runner, it is not conducive to redundant cooling of the runner, and the runner must be opened on the two mold halves, which is laborious and easy to align.

 

  Therefore, trapezoidal or semicircular cross-sectional runners are often used, which are opened on half of the mold with a stripping rod. The surface of the runner 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-sectional width of the runner does not exceed 8m, the cross-sectional width of the super-large runner can reach 10-12m, and the cross-sectional width of the ultra-small runner is 2-3m. On the premise of meeting the demand, the cross-sectional area should be reduced as much as possible to avoid adding debris in the flow channel and prolong 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 channel (or branch channel), but it is usually reduced. Therefore, 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 flow rate of the material: B. It can prevent the backflow caused by the premature solidification of the melt in this part during the injection process: C. The melt passing through is subjected to strong shear and the temperature increases . , Thereby reducing the apparent viscosity to improve fluidity: D, to promote the separation of the product and the runner system. The design of the shape, size and position of the gate 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 small gates are easier to grow larger, while large gates are difficult to achieve. shrink. Generally, the gate location should be selected where the product is thickest and does not affect the appearance. The design of the gate size should take into account the characteristics of the plastic melt.

 

  Number of cavities

 

  This is the space where the plastic product is formed in the mold. The components used to form the cavity are collectively referred to as molded parts. Each molded part usually has a special name. The molded parts that make up the shape of the product are called die (also called die), and the molded parts that make up the inner 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 characteristics 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 ventilation hole, and the demolding method according to the determined structure. Finally, according to the size of the control product, determine the design of each part and the combination of each part. When the plastic melt enters the cavity, high pressure is generated, so the molded parts should be selected reasonably and the strength and rigidity should be checked.

 

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

 

  exhaust vent

 

  It is a trough-shaped air outlet opening in the mold, used to discharge the original gas and the gas introduced by the molten material. When the melt is injected into the cavity, the air initially stored in the cavity and the gas introduced by the melt must be discharged from the mold through the exhaust port at the end of the material flow, otherwise the product will have holes, poor welding, and damage to the mold. The filling is unsatisfactory, and even the accumulated air will burn the product due to the high temperature generated by the compression.

 

  Under normal circumstances, the vent can be located 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. 2 mm, the width of one side of the cavity is 1.5-6 mm. During the injection process, there will not be a large amount of molten material in the vent, because the molten material will cool and solidify at this location and block the channel. The open position of the exhaust port should not face the operator to prevent accidental spraying of molten material and injury.

 

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

 

  Structure

 

  It refers to the various parts that constitute the mold structure, including: various parts for guiding, demolding, core pulling and separation. For example, front and rear splint, front and rear gusset template, bearing plate, bearing column, guide column, stripping template, demolding rod and return rod.

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