Gate is the most critical part of gating system, its type and size are directly related to quality of product and cost of mold. There are many gate structures of injection molds, different types of gates have slightly different sizes, characteristics and application conditions.
Shape, size and feeding position of gate have a great influence on molding quality of plastic parts. Gate design is unreasonable, so correct gate design is an important part of improving quality of plastic parts.
Gate design is related to factors such as plastic properties, plastic part shape, section size, mold structure and injection process parameters. General requirement is to make molten material enter and fill cavity at a faster speed, and at the same time, it can be cooled and closed in time after filling, so gate section should be small and length should be short, which can increase material flow, quickly cool and seal, facilitate separation of plastic part and gate condensate, without leaving obvious gate marks, ensuring appearance quality of plastic part.

When wall thickness of plastic parts differs greatly, under premise of avoiding spraying, in order to reduce flow resistance and ensure that plastic pressure is effectively transmitted to thick-walled parts of plastic parts to reduce shrinkage, gate should be opened at the thickest part of plastic part, which is also conducive to filling. If there are reinforcing ribs on plastic parts, reinforcing ribs can be used as flow channels to improve flow conditions.
As shown in figure below, select (a) gate location, plastic part will have dents due to severe shrinkage. In Figure (b), gate is selected at thick wall of plastic part, which can overcome dent defect. In Figure (c), selection of direct gate greatly improves filling conditions and improves quality of plastic parts, but it will leave a large trace of de-gate.

 

Location of gate should also be conducive to exhausting. Usually, location of gate should be far away from exhaust part. Otherwise, plastic melt entering cavity will close exhaust system prematurely, so that gas in cavity cannot be discharged smoothly, which will affect molding quality of plastic part. Location of gate as shown in left view of following figure. When filling mold, melt will quickly close exhaust gap at parting surface of mold, so that gas in cavity cannot be discharged, which will cause a large filling resistance and easily form air bubbles, glue burning and other undesirable problems. If position shown on the right in figure below is used instead, such defects can be overcome and exhaust efficiency can be improved.

 

 

Determination of gate position not only determines flow direction and flow balance of polymer, but also plays an important role in shape and pressure-holding effect of melt flow front.

Setting of gate position also considers deformation and shrinkage of plastic part. For crystalline plastics, shrinkage in plastic flow direction and shrinkage perpendicular to flow direction are very different, degree of deformation is also different when different forms of gates are used. Of course, number of gates should also be considered, but sometimes setting of gate position is more important than setting of number of gates.

 

Location of gate affects orientation of polymer molecules and shrinkage of part. Image above shows that if a product is long and thin, and absolute straightness is required, then a gate needs to be placed at the end of product. Same is true for guarantee of roundness of round products, and gate needs to be set at the center of circle, not side.

Gate position will always leave gate traces, so gate position should be opened as far as possible in parts that do not affect appearance of plastic part, such as edge, bottom and inner side of plastic part, especially for plastic parts with high appearance quality requirements.

 

When determining gate position of plastic parts, maximum flow distance ratio allowed by plastic, referred to as flow ratio, should also be considered to ensure that melt can fill cavity. Maximum flow distance ratio refers to ratio of maximum length of melt flowing in cavity to thickness of runner. Allowable value of flow ratio varies with melt properties, temperature, injection pressure, etc. If calculated flow ratio is greater than allowable value, it is necessary to increase thickness of product or change gate position, or use a multi-gate method to reduce flow ratio.

(1) Gate should be set at thick wall of molded product, so that plastic flows from thick wall to thin wall to reduce loss of pressure. Plastic melt should be injected quickly into cavity with the shortest path, minimum heat and pressure loss, and injection molding is completed.
(2) Gate should be set in the place where product can be most easily removed, and try not to affect appearance (such as flow marks may occur).
(3) Position of gate should be such that when plastic flows into cavity, it can flow evenly along parallel direction of cavity, and is conducive to exhaust of cavity.
(4) Gate position should be set to avoid welding marks and flow marks from important parts of product and reduce strength of product.
(5) When a mold has multiple cavities, size and position of gate should be opened according to distance between gate and main channel, size of product.
(6) Setting of gate should prevent plastic from directly impacting weak core, insert, row, etc., to prevent deformation of product.
(7) Setting of gate should take into account different shrinkage of product in horizontal and vertical directions. Temperature and pressure of melt entering cavity from each gate should be same to ensure same shrinkage rate of products in each cavity.
(8) Turning point of runner should be transitioned by an arc, and connection with gate should be processed into an inclined plane to facilitate flow of melt.