Gating system is one of five major systems in the design of plastic molds. Its design will affect product ranking, number of mold cavities, form of mold structure, choice of injection moulding machine size, level of injection molding costs, level of mold manufacturing costs, difficulty of mold manufacturing, pressure, post-processing methods and costs during manufacturing. Slight effects will cause product to appear burrs, shrinkage, deformation, air lines, pinching, trapped air, easy to crack, difficult to cut, difficult to repair, difficult to take, difficult to take, etc .; serious effects will make product unable to produce, unfilling injection, obvious and severe clamping, product burnt, damage to appearance, bending of mold core, dragging product to scrap, unable to perform post-processing, unable to package, unable to distinguish left and right, etc.

Gating system is composed of main runner, sub runner, cold material tank and water inlet (also called irrigation point, gate, etc.), as shown in Figure 3.1.

 

Gating system is channel for filling molten rubber into mold. In molten state, material compound is poured into mold through injection system under high pressure, high speed and high temperature state through injection nozzle of injection moulding machine. Quality of design quality of gating system directly affects molding quality and molding cycle of product. Although designer hopes that pressure of material flowing through gating system does not decrease, temperature does not decrease, and speed does not decrease, this is difficult to achieve. The only thing that can be done is to design gating system as perfect as possible, so that pressure loss is as small as possible and speed is as small as possible.
Consider following aspects when designing gating system:
(1) Determination of way of entering water.
(2) Determination of water entry position.
(3) Chain effect of water inlet mode on other mold structures.
(4) Difficulty and cost of mold manufacturing.
(5) Difficulty and cost of injection moulding.
(6) Characteristics of materials used.
(7) Places of deformation, pinch orientation and trapped air during production.
(8) Whether it is easy to handle, take, and package during injection production.
(9) Whether post-processing such as electroplating, painting and silk screen is required after injection moulding production.
(10) Product appearance requirements.
(11) Whether it is difficult to injection, injection pressure is high, and ejection is smooth.
(12) If this design is not possible, is there room for change?
(13) Whether processing after injection moulding, such as the water cut, water repair, trimming, etc. is convenient.
(14) Whether design is convenient for pressure compensation, flow, and cold storage.
(15) Whether design is convenient for venting, eliminating nicks, air marks, etc.
(16) Whether mold requires full-automatic or semi-automatic production during injection moulding production.

Design of gating system mainly includes three aspects: design of runner, way of water inlet and design of position of water inlet.

Design of runner will directly determine success or failure of gating system design. Design of flow direction of material also determines design of unit flow and flow rate of material. The larger runner, the larger flow rate and the faster injection speed. But at the same time, the more waste rubber is, if arc runner is used between runners, its pressure loss is much smaller than that of right-angle runner, but it is also much more difficult to process. Design of runner includes four aspects:
① Selection and design of cross section form of runner.
② Choice and design of main runner and sub runner.
③ Selection design of size of main runner and sub runner.
④ Design of plate on which runner is processed.

There are usually four forms of cross-section form of runner, see Table 3.1.

 

Figure shows runner with a circular cross section, which is the most widely used runner at present. Because circular cross section is actually like a pipe, its fluidity is very good, corner resistance is small, and pressure loss is small, which facilitates flow. This is main reason why it is widely used. It is also the only one of 4 runners forms that both plates need to be processed. When milling, a ball milling cutter is required for processing and processing volume is too large.

 

Figure shows B-shaped cross-section runner. U-shaped runner is directly processed on a plate. Whether it is processed in front mold or back mold, it can be determined according to mold structure. In short, it is easier to process than circular runners, and its sticking force is not large. A1 is best to be 10, minimum should not be less than 5, maximum should not be greater than 15, and processing should be performed with an inclined ball cutter.

 

Figure shows a runner with a trapezoidal cross section, and it is also one of commonly used runners. Trapezoidal runner is directly processed on a plate. Whether it is processed in front mold or back mold, it can be determined according to mold structure. It is better than circular runner and U-shaped runner, and its sticking force is not great. A2 is preferably 10, minimum is not less than 5, maximum is not greater than 15, R1 is preferably R1-R1.5. In order to facilitate processing, an inclined knife can be selected. Generally, trapezoidal cross-section runners are used for runners of orifice nozzle mold.

 

Figure shows a semicircular cross-section runner, which is rarely used in main runner. But it is often used in final sub runner of small products. For small products, flow rate and pressure compensation of semi-circular runner are sufficient. There is no need to do circular runner, because processing of semi-circular runner is only half of volume of circular runner. It is also directly processed on a plate. Whether it is processed in front mold or back mold, it can be determined according to mold structure. A spherical milling cutter is used during processing.
Runner section specifications are φ2.0mm, φ2.5mm, φ3.0mm, φ3.5mm, φ4.0mm, φ4.5mm, φ5.0mm, φ6.0mm, φ8.0mm.
Commonly used specifications of main runner are φ4mm ～ φ6mm, minimum should not be less than φ4mm, and maximum need not be greater than φ8.0mm. Sub runner: size of first sub runner is φ4mm ～ φ6mm; size of second sub runner is φ3mm ～ φ5mm; size of third sub runner is φ2.5mm ～ φ4mm; size of fourth sub runner is φ2mm ～ φ3.5mm .

Design of sub runner has many methods and forms. In the case of multiple cavities or multiple products sharing a set of molds, each product is required to be injection uniformly, and injection paths are approximately equal. Otherwise, it will appear that one has been injection-molded and the other has not been injection-molded to half. When this half is also injection-molded, original injection-molded workpiece will be burr. Form and analysis of path of main runner and sub runner are shown in Table 3.2 and Figure 33 to Figure 38.

 

Analysis: This runner is ranked in a non-uniform layout, that is, injection pressure, injection flow, and injection temperature of 1st and 2nd, 3rd and 4th, and 5th and 6th positions are different. If 1st injection molding is completed and 2nd is definitely out of sync, it will either be advanced or backward, which will cause differences in products produced. For products with higher requirements, this design is not allowed, but this design has a compact structure, takes up less space in mold, and has a lower mold cost. Moreover, it has only two-stage runner, which reduces pressure loss and high speed during injection. Therefore, for products with relatively low requirements, this runner design method can be adopted.

 

Analysis: This is almost same design as in Figure 33, the only difference is that products are layout vertically. Compared with Fig. 33, mold is more square and runner is shorter. Water inlet is on short side of product, that is, A side or B side. Mold in Figure 33 is relatively slender, and runner is long. Inlet is on long side of product, that is, C side or D side. Design of these two runners is almost equivalent. Choice can be based on comprehensive consideration of impact of product or other parts of mold.

 

Analysis: This runner is layout in a uniform manner, that is, injection pressure, injection flow, and injection temperature of position 1, 2, 3, 4, 5, 6, 7, and 8 positions are same. If it is full, it will be full together. In this way, there can be no difference in injection molded products, which can meet requirements of higher products. However, design of this runner has three-stage runners, loss of injection pressure and injection temperature will be greater, waste will be more.

 

Analysis: This is almost same design as in Figure 35, the only difference is that products are layout vertically. Compared with Fig. 35, mold in Fig. 36 is more square and runner is shorter. Water inlet is on long side of product, that is, C side or D side. While Figure 3 shows that mold is relatively slender and runner is longer. Water inlet is on short side of product, that is, A side or B side. Design of these two molds is almost equivalent, and selection can be determined based on the comprehensive consideration of product or other parts of mold. In general, runner design of Figure 35 and Figure 36 is more widely used in actual mold design. Because uniform injection pressure is the biggest advantage of this structure, and large loss of injection pressure is its disadvantage, improvement method is usually to change right-angle runner to arc runner. Take design of Figure 3 as an example for comparison, see Figure 37 and Figure 38.

 

Analysis: Because first and second manifolds are connected by arcs. It can be known from fluid mechanics that its pressure loss is much smaller than vertical bend angle. During injection, rubber material can enter second runner smoothly, and resistance is very small, which is very advantageous for injection molding. Disadvantage is that machining of runner needs to be processed by a CNC milling machine, and cost is high. When using a curved runner, it is best to use a vertical runner on last runner, see Figure A in Figure 37. Because runner from gate sleeve to A is connected smoothly with an arc. There is no cold material location. If even the last stage is connected with an arc (see B in Figure 37), cold material will be directly injected into product, resulting in an increase in defective rate of product. Therefore, it is necessary to use a vertical runner on the last stage runner and design a cold storage level (see D position in Figure 37), so that cold material is hidden at D position, and hot rubber material flows into mold. Length E of cold storage material is preferably E = (1.5-2) * d. For example, when d = 5mm, E = （1.5-2） * 5mm = 7.5-10mm, it is same in other runner structure design,
E = (1.5-2) * d (see position D in Figure 38).

 

Analysis: Sub runner at all levels intersect at 90 degrees. It can be known from fluid mechanics that his pressure loss is quite large, which is very disadvantageous for injection molding.
From Table 3.2 and figures, it can be seen that layout is exactly same, but it has a completely different runner design, of course, it also brings completely different results. Following examples illustrate comparison of effects of various runner design.

Table 3.3 Examples and analysis of runner design

 

Figure 39: This runner is designed to feeding directly. There is no cold material position, and cold material will be directly injected into mold.
Figure 310: Both designs of runner are designed to hide cold material. During injection, cold material is hidden at position of cold material, so that hot rubber material is injected into mold, which guarantees quality of product. Although there are more design wastes, it is still an excellent runner design solution. After all, quality of product is first.

 

Figure 311: Design of this runner is to directly feeding, there is no cold material position, cold material will be directly injected into mold, usually not used, but because of its simple processing, low pressure loss, and fast injection, it can also be used for less demanding products.
Figure 312: Design of this runner is also to directly feeding. There is no designed cold material level. Cold material will be directly injected into mold. Usually it is not used. Moreover, this runner is difficult to process, cost is high, and practical use is not significant. .
Figure 313: Design of this runner is a right-angled runner with a cold storage level. It is simple to process and fast, it guarantees quality of product. Therefore, it is the most commonly used runner design on this mold.

 

Regardless of Figure 314 or Figure 31, design of runner is non-uniform. Although there are cold storage position, it is not convenient for injection molding production. There are many other runner design solutions, but they cannot be balanced and uniform. They are not listed here, so in actual design, try not to use number of mold cavities of this solution, and try not to use this runner design solution. If it is unavoidable, Figure 314 and Figure 315 runner design can be used, of which Figure 314 is more concise.

 

Figure 316: Mold structure is the most compact and mold size is the smallest, but its runners are non-uniformly arranged, feeding and pressure is not uniform, there is a cold material position, and it can be used for products with lower requirements.
Figure 317: Runner is designed to be uniformly arranged. Injection and pressure in each cavity are same. There is also a cold material position. It is often used for relatively easy injection molding products. Runner processing is very simple.

 

Figure 318: Runner is designed to be uniformly layout. Injection and pressure in each cavity are same. Cold material poition is designed. It is often used for difficult injection molding products and demanding products, but processing of runner is difficult. Only CNC milling machines can be used for processing, and processing cost is much higher.
Regardless of figure 319 or 320, design of runner is non-uniformly ranked. Although there is a cold material position, it is not good for injection molding production. Therefore, in actual design, try not to use number of cavities of this solution. When it is unavoidable, runner design solutions of Figure 319 and Figure 320 can be used. Among them, Figure 319 is more concise and can be used for products with relatively low requirements. However, for products with relatively high requirements, it is best to use design solution of Figure 320 runner. Although it is non-uniformly layout, compared with Figure 319, it has been greatly improved, and quality of production is also much stabilized during injection molding.

 

 

Figure 321: Mold structure is the most compact, mold size is the smallest, injection molding machine used for injection production is the smallest, and cost is the lowest, but his runner design is non-uniform, injection is very unbalanced, and pressure of each inlet is It is also not balanced. Difficulty of adjusting machine during injection molding is quite large. Although it is designed with a cold storage position, it is only used for products with lower requirements.
Figure 322: It is also a non-uniform layout, non-uniform runner design. Compared with Figure 322, it is a bit lighter, but its mold structure is not compact, so its application is not many and its value is not high.
Figure 323: Runners are designed to be uniformly arranged. Injection pressure, injection temperature and injection flow rate of each cavity of mold are same, and sub runners are also designed at each stage. Quality of product is guaranteed, but pressure loss is a little bit large. It is often used for products that are easier to injection. It is one of the most commonly used runner design solutions for products that are relatively easy to injection. What’s more, processing of runner is very simple.
Figure 324: Runner is designed to be uniformly layout. Injection pressure, injection temperature and injection flow rate of each cavity in mold are same. It is also designed with a cold material tank, which guarantees quality of product and loss of injection pressure is very small. It is one of the most commonly used runner design for products that are more difficult to injection, but it is more difficult to process runner. It needs to be processed by CNC milling machine.

 

 

Figure 325: This is a runner design according to general thinking. It can completely inject product. Product has no surface texture or air texture, its strength and toughness are full. However, this is a button type product, which needs to be post-processed after injection molding, such as painting, silk printing, electroplating, etc. At this time, product is a button, and elasticity of key bar is very high. It can be imagined that keys will bounce constantly, even be deformed, and cannot be fixed at all, let alone positioning, which brings great inconvenience to painting and silk printing. 325 has no layout and cannot be electroplated. Therefore, if product needs to be post-processed after injection molding, auxiliary runner design is generally required.
Figure 326: On the basis of Figure 325, a square ring-shaped auxiliary runner (also called a continuous frame, a reinforced frame) has been designed. Ring-shaped auxiliary runner is connected with product and sub runner as a whole. For production injection, do not cut product, But flow it into a whole mold, which makes it very convenient for painted, silk printed or electroplated. It can also protect the product from deformation and breakage. In Figure 326, there are also 4 packaging positioning columns, as shown in Figure 329. This is designed for injection molding to facilitate stacking packaging. Main purpose is to be convenient, fast and not hurt the product when injection packaging.
Figure 328: This is based on design of Figure 326. Auxiliary runner is designed with plating hooks. Its main function is to make it easy to hang when plating. If product does not require post-plating treatment, it is not necessary to design this auxiliary runner.

 

Figure 330, Figure 331:
After disassembling and assembling a part product, it is a toy car. When packing, it is a whole board and a whole board stacked. Big and small parts of this toy car are connected by auxiliary runners, which can be conveniently sold in the entire board in box.

 

In Figure 332, if auxiliary runner is not designed to strengthen product, it will be deformed due to shrinkage after injection molding. When it is assembled, either side A is high or side B is low, and it is naturally unusable. If auxiliary runner is designed, side A and side B are connected. After injection, do not need to cut off, wait for a while after cooling, and then cut off, so that deformation of product is eliminated.

 

In Figure 333 and Figure 334, if auxiliary runner is not designed, product may stick to front mold at the moment when front and rear mold plates are opened, so mold cannot be produced. Auxiliary runner is designed, and product is pulled to back mold through dummy nozzle. This is role of this auxiliary runner. Of course, when mold is ejected, it can also be used as a ejector pin, which can serve two purposes.

Both sides of circular runner are processed, so T-shaped flow runner is selected to explain selection design of runner channel. Choice of which plate runner is processed on is based on principle that it does not affect material flow, is easy to take out when mold is opened, and does not hinder movement of other moving parts on the mold.

1. Processing of runner on the orifice nozzle (ie, three-plate mold), as shown in Figure 3.35.

 

Runner drawn by solid line in Figure 3.35 is processed on the front mold plate, and runner drawn by dotted line in Figure 3.35 is processed on nozzle plate.
If runner is processed on nozzle plate, when Ⅰ-Ⅰ is parting, point A is disconnected, and when Ⅱ-Ⅱ is parting, points C and B are also disconnected, which is normal. During Ⅲ-Ⅲ is parting, front and rear templates are separated, and product remains in rear mold, which is normal, and mold is completely opened. However, runner cannot be removed because runner is always buried in nozzle plate, and there is no power or mechanism to pull runner away from nozzle plate. Therefore, if runner is processed on nozzle plate, runner cannot be removed. Mould design failed.
If runner is processed on the front mold plate, when Ⅰ-Ⅰ is parting, point A is disconnected, and when Ⅱ-Ⅱ is parting, points C and B are also disconnected, which is normal. During Ⅲ-Ⅲ parting, front and rear templates are separated, product remains in rear mold, which is normal, and mold is completely opened. Runner can be removed, because runner is buried in the front template before mold is opened. During Ⅰ-Ⅰ parting, pulling needle not only breaks point A, but also pulls runner away from front mold plate and attaches it to nozzle plate. During Ⅱ-Ⅱ parting, nozzle plate scrapes off points C and B. At this time, runner has completely loosened and can be removed. Mold design is successful.
Therefore, in small nozzle mold, runner processing should be on the bottom surface of front mold plate, not on nozzle plate.

2. Processing of runner of multi-hole push plate mold of large nozzle, as shown in Figure 3.36. Runner processing drawn by solid line in Figure 3.36 is located on the front die insert plate, and runner processing drawn by dotted line in Figure 3.36 is on push plate.

 

If runner is processed on push plate, when Ⅰ-Ⅰis parting, that is, front mold plate and push plate are parting, product remains on rear mold insert, normal, when Ⅱ-Ⅱis parting, launch product, pull off pull pin, take out product, it is normal, if it can't take out runner, it is abnormal. Because runner is always buried on push plate, there is no power to pull it out during parting. Therefore, if runner is processed on push plate, runner cannot be taken off, and mould design fails.
If runner is processed on front template insert, when Ⅰ-Ⅰ parting is performed, that is, front template and push plate are parted, product remains on the rear die insert, and nozzle runner is also pulled away by pulling needle. Front template insert is attached to push plate, which is normal; when Ⅱ-Ⅱ is parting, push out product, pull needle, take out product, at the same time, runner automatically falls down, normal. Mould design was successful.
Therefore, in multi-hole push plate mold of large nozzle, runner processing should be on front template insert and not on push plate.