Gating system of an injection mold refers to channel through which molten plastic flows from injection molding machine nozzle to cavity entrance. Gating systems are generally divided into two types: conventional gating systems and hot runner systems. Conventional gating systems typically consist of several parts, including main runner, branch runners, gate, and cold slug well (cavity). Gating system of a two-plate mold is shown in Figure 5-1.

 

Figure 5-1 Gating system for a two-plate mold
Gating system design is a crucial part of mold design. Gating system design includes selecting main runner, determining cross-sectional shape and dimensions of branch runners, selecting gate location, determining gate type and cross-sectional dimensions, and designing cold slug well. Before designing, a comprehensive analysis should be conducted on plastic part material, dimensions, geometry, potential defects, appearance requirements, production volume, and whether fully automated injection molding production is used.
During injection molding, molten plastic enters mold cavity through gating system. Type of gating system chosen largely determines mold structure; for example, two-plate and three-plate molds correspond to different gating systems. Design of gating system has a significant impact on quality and production efficiency of injection mold. If runner cross-section is too large, more plastic is consumed and cooling time is long; if runner cross-section is too small, excessive pressure drop will occur, resulting in insufficient pressure during cavity filling. Furthermore, a small runner cross-section can cause polymer degradation due to excessive viscous heat. Therefore, it is crucial to rationally select shape and size of runner cross-section and length of runner. For multi-cavity injection molding, arrangement of main runner and branch runners, as well as dimensions of each cross-section and gate size, must be rationally arranged to ensure that each cavity is filled with same pressure at the same instant. For single-cavity molds with multiple gates, different gate sizes and positions, as well as different runner systems, will change weld line position, thus affecting mechanical properties of plastic part. Regardless of whether it's a single-gate single-cavity, single-gate multi-cavity, multi-gate multi-cavity, or multi-gate single-cavity layout, a correct and reasonable gating system design is crucial.

1. When considering cavity layout, following points should be noted: Use a balanced layout whenever possible to ensure molten plastic fills each cavity evenly. Consider number of cavities and ensure cavity layout is as symmetrical as possible to mold centerline. Cavity layout and gate placement should ensure even stress distribution during injection molding. Cavities should be arranged as compactly as possible. Choose the shortest flow path to shorten filling time and reduce mold dimensions.
2. Minimize heat and pressure losses: Select an appropriate runner cross-section. Determine reasonable runner dimensions. Within a certain range, using a larger runner system can help reduce flow resistance. However, when pressure drop in runner system is small, smaller dimensions should be prioritized. This reduces material usage in runner system and shortens cooling time. Minimize bends in runner system; surface roughness Ra should be between 1.6 and 0.8 μm.
3. Gating system should be able to collect cooler material to prevent it from entering cavity and affecting quality of molded part.
4. Gating system should smoothly guide molten plastic to fill all corners of cavity, allowing for smooth escape of gases.
5. Prevent defects in product, avoiding incomplete filling, shrinkage marks, flash, undesirable weld line positions, residual stress, warpage, uneven shrinkage, etc.
6. Gate should be designed to achieve the best possible product appearance quality, avoiding defects such as burn marks, serpentine patterns, and shrinkage cavities on product's surface.
7. Gate should be located in a concealed position, easily removable, and leave no obvious marks on product. Ensure gate location does not affect appearance or interfere with surrounding parts, and avoid direct impact on core inserts or small inserts to prevent bending or breakage.
8. Consider whether injection molding can be fully automated.
9. Consider subsequent processes, such as processing, assembly, and management requirements. For example, screen printing, electroplating, and spraying may require multiple parts to be connected through runners.

Theoretical basis of gating system design is polymer rheology. Gating system optimization involves using mold flow analysis software to comprehensively analyze flow of plastic.
A cavity with more than one gate is called a multi-gate system. An ideal gate design allows for rapid and uniform plastic flow with appropriate gate solidification time. Generally, the fewer gates, the better, as long as cavity can be filled on schedule. Each gate should cover the largest plastic part area within flow ratio achievable by plastic flow force. In some large products, multiple gates are the best option. Number of gates is generally determined by flow path and product volume. For any given plastic part, there may be countless possible gating system designs. Goal of optimization is to ensure that melt in cavity reaches filling boundary simultaneously, within the allowable injection pressure and with the fewest possible number of gates. Design of gate location and number needs to consider time required for melt to flow to end of cavity. Analyzing this process helps find optimal flow state to achieve melt flow balance, resulting in a high-quality plastic part. Gate location is determined under flow balance conditions, and flow pattern during filling must be considered when selecting gate location. To ensure product quality, number of gates should be minimized, thereby reducing number of weld lines. Location of weld lines must be controlled within area that least affects quality of plastic part.
Gating system has a significant impact on appearance, precision, production efficiency, and raw material consumption of plastic part. Therefore, it is essential to apply mold flow analysis technology to analyze gates and runners. Many software programs with good performance are now available. However, this technology can only simulate melt flow for pre-designed gates and runners. Based on the simulation results, gate and runner can be improved, thus avoiding need for modifications after problems are discovered during trial molding following mold manufacturing. However, a good basic design for gate and runner is essential to reduce workload of simulation. Therefore, injection mold designers still need to master basic principles of gate and runner design, initial simulation schemes themselves need to be of a high standard; otherwise, it's just choosing the best among the worst and failing to fully utilize mold flow analysis software.
Quality of injection molded products largely depends on mold design. Location and number of gates are important mold structural parameters. An unreasonable gate location often causes uneven melt filling, leading to over-holding pressure, high shear stress, and severe warpage. Number of gates has a significant impact on injection pressure and weld lines. More gates result in a shorter melt flow path in cavity and lower injection pressure, but may increase number of weld lines. Conversely, fewer gates may reduce number of weld lines, but due to longer flow path, higher injection pressure is required, resulting in higher residual stress in part and potentially causing warpage. Therefore, optimizing number and location of gates is of great significance.

The most crucial step in optimizing injection mold gating system is determining location and number of gates. Based on Moldflow technology, determining location and number of gates involves a series of steps, including selecting optimal gate location, developing a gate design scheme, flow simulation analysis, warpage analysis, comparing comprehensive simulation results.
1. Determining Gate Location. Using Moldflow optimal gate location analysis module, a reasonable gate location area is simulated, and this is used as optimal gate location for comprehensive analysis of research object.
2. Developing an Optimal Gate Design Scheme. Based on usage environment and quality requirements of plastic part, and combined with practical experience, 3-5 gate design schemes are developed. Then, through comprehensive analysis, design scheme with the best simulation results and the most representative characteristics is selected.
3. Flow Simulation and Warpage Analysis. Flow simulation utilizes Moldflow to simulate and analyze flow of melt within mold. By analyzing filling time, injection pressure, temperature, and clamping force, optimal design is determined to avoid problems such as cavitation and weld lines. Furthermore, impact of warpage on plastic part must be analyzed to ensure part quality.
4. A comprehensive comparison of various design schemes is conducted to determine the best design.

After determining location and number of gates, dimensions of main runner, branch runners, and point gates are set based on shear rate of melt at different locations. Flow balance in gating system is achieved to ultimately achieve good melt filling. Based on specific structure of plastic part, branch runners are designed with a balanced approach whenever possible.
Gate design is a crucial factor in determining product quality, including design of number, location, type, and size of gates. Gate location, in particular, determines balance of flow and has a significant impact on product quality. With advancements in Moldflow technology, mold design has made significant progress based on accumulation of empirical data.

Review of gating system solidification refers to review of sample after trial molding, not review before mold design. Sample review generally includes four parts: part size review, appearance review, gating system solidification review, and mold operation stability review. Gating system design is also one of main factors directly affecting injection molding cycle. Purpose of gating system solidification review is to identify and summarize mold injection problems by observing and analyzing gating system solidification. After the first trial molding, sample will more or less have some problems that need improvement. Besides finding cause in sample itself, many problems are related to gating system. Issues such as venting, cold slug, mold filling, and injection cycle can all be solved through gating system review.
Common problems with mold gating system include: runner shape and size being too small, insufficient gating system machining roughness, main runner sticking to mold, cold slug well being too small, runner venting not being opened or being too small, poor ejection of gating system, air entrapment, etc. Occurrence of sample appearance defects and other problems is related to gating system. Reviewing gating system sprue reveals the overall mold operation, which is crucial in injection molding practice. Simultaneously, sample reviews require continuous experience accumulation and meticulous data recording.

During injection molding, injection molding machine fills mold with molten plastic through sprue bushing to form product. After product cools, sprue sprue, commonly known as sprue waste, is generated within gating system. This waste material, after being crushed, can be added to raw materials for next batch of products. In principle, sprue from product from which product originated should be mixed into raw materials to continue making that product, or some colorant can be added to create other products with darker colors. Adding too much sprue reduces costs but also degrades product performance; therefore, amount of sprue added in production generally does not exceed 25%, and sprue is not allowed for high-requirement or transparent products. Customers now specify proportion of sprue in finished product to control quality.

 

Figure 5-2 Material identification of runner of two-plate mold

 

Figure 5-3 Material markings for three-plate mold flow channel
Sprue sprue is produced along with product; modern factories use ERP information management systems to manage and encode sprue sprue. Coding of sprue material can be automatically generated by system-defined rules based on coding of rubber compound and color powder. Therefore, to facilitate the classification, sorting, and recycling of sprue material, mold design needs to mark sprue material for easy recycling. Material marking for a two-plate mold runner is shown in Figure 5-2, and material marking for a three-plate mold runner is shown in Figure 5-3.

To easily identify which cavity finished product came from after injection molding, cavities need to be numbered. Additionally, some molds have cavity numbers marked on ejector pins, which are difficult to identify once mold is disassembled. Therefore, cavity markings need to be placed next to mold core for sample review and mold maintenance.
Arrangement of cavity numbers is shown in Figure 5-4. Rules are as follows: Based on the entire mold set; Based on moving mold, from left to right and from top to bottom.

 

Figure 5-4 Cavity Numbering Rules