Runner system is transition mechanism that guides molten plastic delivered by screw from gate to molding cavity. Appropriateness of runner system design determines functionality, processability, and economy of injection molded product. Runner system mainly includes: sprue, hot runner, cold runner, and gate. Injection molding is a technology that controls flow of molten plastic within a mold, and design of runner system directly affects dimensional accuracy, strength, molding cycle, and appearance of finished product.

Main function of runner system is to guide molten plastic injected by injection molding machine nozzle, allowing plastic to enter cavity evenly and quickly for molding. After being injected through nozzle, molten plastic enters runner system, passing through sprue, main runner, branch runners, and gate before entering mold cavity. Optimal gating system design must consider: gate location, injection molding technology (hot runner, cold runner, and process), gate and runner type, size, number, and relative dimensions of each part of gating system.

Main function of runner system is to guide molten plastic injected through injection molding machine nozzle, allowing plastic to enter mold cavity evenly and quickly for molding. After being injected through nozzle, molten plastic enters runner system, then passes through vertical runners, main runners, branch runners, and gates before entering mold cavity. As molten plastic flows through runner system, additional pressure is required to propel it forward, resulting in a pressure drop. Furthermore, as molten plastic flows through runner system, shear heat is generated due to friction, raising molten plastic temperature and facilitating its flow. Generally, larger runners can reduce pressure required to propel molten plastic, but they require a longer cooling time and generate more waste, increasing costs. Conversely, using a smaller runner system can improve shear strength, reduce viscosity, and increase material utilization efficiency, but it also increases injection pressure. Therefore, injection pressure limit of injection molding machine must be considered.
Therefore, finding a suitable runner system design is not easy. CAE mold flow analysis is a time-saving and labor-saving method. A well-designed runner system has following advantages:
1. Ensures that molten plastic can fill cavity, reducing product defects;
2. Optimal selection of number of cavities, producing high-quality plastic parts with maximum number of cavities;
3. Balanced filling of multi-cavity systems, reducing product differences between cavities;
4. Optimization of molding cycle time to ensure product quality and production efficiency;
5. Reduced waste;
When designing a runner system, appropriateness of number of cavities must first be considered. Number of cavities generally considers product's requirements, such as production time, part quality, part shape and size, and number of products required per batch. In addition, hardware limitations of injection molding machine itself must be considered, such as upper limit of injection pressure, plasticizing capacity, clamping tonnage, mold cost, requiring a comprehensive balance.
While a mold system with more cavities can increase output basket per cycle, runner system requires complex considerations and design, including filling uniformity, nature and state of molten plastic flowing into cavity. Any branch in runner may cause geometric asymmetry, thus affecting flow balance, cavity pressure, and melt temperature.
Design of vertical runner must not only facilitate convenient and reliable demolding of plastic part, but also ensure that vertical runner does not solidify earlier than other parts of plastic part during molding process, thus ensuring stable transmission of holding pressure. Recommended vertical runner design dimensions are shown in Figure 1.1. Root of vertical runner should be rounded (radius f2), as this rounded design facilitates plastic flow.

 

Figure 1.1 Recommended design dimensions for vertical runners
Runner systems exhibit different flow characteristics depending on their cross-section. Choice of runner cross-sectional shape is based on its flow efficiency and mold processing conditions. Cross-sectional area of runner generally requires minimal flow resistance and material temperature within channel, resulting in the slowest cooling rate. If runner is too large, it requires a longer cooling time, wasting more material and increasing material costs. If runner is too small, it easily leads to short shots, dents, poor product quality, and difficulty in stable control. If runner is too long, it easily causes excessive pressure drop (pressure loss), generating excessive waste, melt will over-cool before entering gate. Figure 1.2 shows recommended values for multi-cavity runner geometry design, and corresponding areas when using different runner cross-sections.

 

Figure 1.2 Recommendations for runner dimensions
Figure 1.3 shows common runner cross-sections including: circular runners, trapezoidal runners, modified trapezoidal runners (a combination of circular and trapezoidal), semi-circular runners, and rectangular runners. In selecting and using runners, three types are generally recommended: circular runners, trapezoidal runners, and modified trapezoidal runners. Considering maximum volume-to-surface area ratio, circular runners are optimal cross-sectional design, offering higher pressure conductivity, lower flow resistance and heat loss. However, using circular runners requires machining on both sides of mold plate, increasing mold processing costs and difficulty. It's also crucial to ensure semicircles on both sides are aligned during mold clamping. Using runners with other cross-sectional shapes, such as semicircular, rectangular, trapezoidal, and modified trapezoidal runners, avoids need for double-sided machining. Trapezoidal and modified trapezoidal runners are commonly used in three-plate mold runner systems because of their excellent heat transfer efficiency. With careful design, trapezoidal runners can also achieve satisfactory melt delivery efficiency. Semicircular runners are generally not recommended, although they offer better demolding performance than circular runners. Fully circular runners offer the best efficiency in reducing heat and pressure loss.

 

Figure 1.3 Runner Cross-Sectional Dimensions and Shape
Furthermore, hot runner systems offer advantages such as producing plastic parts with uniform density, eliminating need for runners, preventing flash and gate waste, making them an ideal injection molding method. In a hot runner system, plastic that hasn't entered mold cavity remains molten within runner until next part is filled, thus preventing gate waste.
For runners with different cross-sectional shapes, flow resistance can be evaluated using hydraulic diameter (p), as shown in Figure 1.4. A larger hydraulic diameter results in lower flow resistance. Hydraulic diameter is defined based on runner cross-sectional area (a) and perimeter (P).

 

Figure 1.4 Equivalent hydraulic diameter of differential runner cross-section
Diameter and length of runner system affect melt flow. A runner system with higher flow resistance will generate a greater pressure drop during filling process. To reduce flow resistance, runner diameter can be increased, but this requires a longer cooling time, increasing production cycle and consuming more plastic, thus raising production costs. In runner system design, pipe diameter can be initially planned using empirical formulas, followed by fine-tuning using mold flow analysis software to achieve an optimized melt transfer system. A preliminary empirical formula is as follows:

 

Where D is runner diameter, W is product weight, and L is runner length.
Many design data are available for reference and will not be discussed here. This course focuses on using CAE tools to assist in confirming optimized design.

Gate is channel through which molten plastic enters mold cavity via runner. It is channel with the smallest cross-sectional area and shortest flow length in runner system. Design of gate has a significant impact on mold design, quality of plastic parts, and production rate. Different gate designs affect flow properties of melt, including changes in shear rate, shear stress, local pressure loss, frictional temperature rise, and plastic viscosity. These changes, in turn, affect molecular orientation, crystallinity, fiber orientation, residual stress, and appearance of molten polymer. Gate design encompasses type, size, location, number of gates, and is influenced by factors such as product design, mold design, part specifications, plastic type, mold type, and economics. Therefore, designing a suitable gating system is a significant challenge.
Generally, unless melt flow length ratio exceeds limit of plastic, necessitating use of a multi-gate system to reduce flow length ratio, a single-gate system should be used whenever possible. This is because a single-gate system reduces weld line problems and ensures more uniform distribution of material, temperature, pressure, resulting in better molecular orientation. Furthermore, a single-gate system reduces runner waste, thus lowering production costs.
Determining gate location requires considering: ideally, injecting material into a thicker section of product to avoid dents; whether gate location is on a critical surface; material flow path (flow length ratio); equal length and uniformity of filling path; volume of material to be filled (filling time/maximum shear rate); location of weld lines; molecular orientation effect caused by flow path; ease of post-processing (gate cutting); and for multi-cavity molds, ensuring simultaneous injection from all gates and simultaneous filling of all cavities.
Figure 1.5 illustrates weld line formation, a common cosmetic defect. This area typically exhibits weaker structural strength and is formed by convergence of two molten plastic flow waves, creating surface flow marks, also known as weld lines or weld lines.

 

Figure 1.5 Causes of weld lines
Improving weld lines requires reviewing gating method and number of gates to achieve flow balance—meaning each gate injects a relatively uniform flow area. Goal is to minimize number of weld lines possible. To improve weld line quality, modify number and location of gates to avoid excessively long flow paths or overcooling of plastic through a single gate. While multi-point gateing may produce more weld lines, the shorter flow paths, higher material temperatures, and higher effective pressures usually make weld lines less noticeable. Modify runner and gate dimensions to improve flow characteristics of plastic and utilize viscous heating to raise plastic temperature, thus improving weld strength.
The best solution for weld lines is, of course, to completely avoid their formation. However, in most cases, weld lines are unavoidable (e.g., around multi-point gates, holes, etc.). Therefore, consider following:
1. Aesthetic Considerations: Move weld lines to areas with less important aesthetic characteristics, such as non-visible surfaces, internal surfaces, hidden surfaces, or textured areas.
2. End-Use Performance Considerations: Move weld lines to non-load-bearing surfaces, thicker walls, or structural reinforcements to avoid stress concentration and damage, and to compensate for potential weld strength reduction. This consideration is particularly important for weld line formation in plastic parts with assembly holes. Figure 1.6 shows differences in weld lines formed during molding of a plastic part with a circular hole due to variations in number and location of gates. In terms of product strength, hot weld lines are fewer and stronger, making it a superior design. Since higher temperatures increase activity and diffusion between molecular chains, resulting in stronger weld lines, welding should ideally occur in high-temperature regions. Avoiding cold welds is the first priority, followed by hot welds.

 

Figure 1.6 Differences in weld lines near holes caused by different gate locations
Furthermore, design of gate location and size should avoid jetting. This can be achieved by increasing gate size to reduce plastic flow rate at gate, or by improving gate location and type (e.g., using overlapping gates). Figure 7.7 shows that to reduce jetting and flow marks, gate should ideally be perpendicular to runner. Allowing molten plastic to impact mold wall instead of directly entering cavity can improve jetting. Gate size must provide adequate filling pressure and speed for molten plastic. Larger parts are typically designed with multiple gates; excessively long gate spacing should be avoided to prevent excessive pressure loss. Gate length should be minimized. Gate location should allow molten plastic to flow from thicker walls to thinner sections, be located away from areas prone to impact and stress, and allow gas within cavity to escape directly to vent, preventing encapsulation.

 

Figure 1.7 Gating design to avoid jetting
Compared to part and runner, gate cross-section is typically small, about 1/2 to 2/3 of part's wall thickness. Therefore, gate can be easily removed from part, avoiding gate marks. Figure 1.8 illustrates that gate size affects rate of plastic solidification, thus impacting effectiveness of holding pressure stage. Placing gate in a thinner area can lead to surface depressions or even internal vacuum bubbles in thicker areas. A larger gate cross-section results in a smaller pressure drop, which has a better impact on appearance, stress concentration, and dimensional accuracy at gate. However, an excessively large gate increases difficulty of gate removal and cooling time, affecting production efficiency.

 

Figure 1.8 Proper gate location
Type of gate is closely related to mold design and runner configuration. Selection of gate type should consider product geometry and size, production rate, and cost. Different gate types have corresponding application ranges; for example, fan-shaped gates and thin-film gates for flat geometries; disc-shaped gates and annular gates for cylindrical geometries. Furthermore, two-plate and three-plate mold designs also limit choice of gate type. Therefore, gate design requires comprehensive evaluation and consideration to determine optimal gate system for different products.
Gate types are usually distinguished by method of gate removal, can be divided into two categories: manually trimmed gates and automatically trimmed gates.

These gates require secondary processing by operators to remove them, usually because gate is too large and needs to be manually removed after demolding. It is important to note that for plastics highly sensitive to shear stress, or when product requires fiber orientation, automated gate removal should be avoided; manual removal is preferable.
Manually removed gates include following forms: direct gate, side gate, overlapping gate, fan gate, film gate, disc gate, ring gate, and radial gate.
To summarize design key points from a results-oriented perspective, following should be checked:
Stress: Area near gate is where stress concentration is most severe. Avoid designing it near points subject to external forces or prone to damage.
Pressure: To avoid excessive pressure loss, gate should be designed in a thicker area for better pressure retention.
Flow Direction: Avoid molecular orientation of molten plastic, which can cause severe warping.
Weld Lines: Try to ensure equal flow distance of molten plastic within cavity to reduce gas encapsulation or formation of weld lines.
Filling: Appropriate placement can increase turbulence, reducing chance of jetting and flow marks.
Gate types can generally be divided into following:

A direct gate, also known as a spruegate, as shown in Figure 1.9, is typically used in two-plate mold designs with one cavity per mold. This type of gate design easily leaves gate marks on product's geometric surface after manual removal. Therefore, gate location should be designed on surfaces that do not require specific product appearance. Furthermore, direct gate design allows molten plastic to fill cavity with minimal pressure drop, increasing efficiency of pressure transmission during filling and holding.
Advantages: Simple construction, low pressure loss, good filling performance, accurate dimensions, and high quality;
Disadvantages: Post-processing of gate will leave marks, affecting product appearance; only one molded part can be formed at a time;

 

Figure 1.9 Direct gate
Design recommendations for direct gate are shown in Figure 1.10. Size of direct gate is related to injection molding machine nozzle and product's geometric wall thickness. Diameter of gate's interface with machine nozzle should be at least 1 mm larger than injection molding machine nozzle diameter, while diameter of interface with finished product is recommended to be twice product's geometric wall thickness (at least greater than 1.5 mm). Because gate size is larger than product's geometric wall thickness, shrinkage at direct gate will increase residual stress at gate. Recommended draft angle for sprue is between 1° and 2.4°. A draft angle that is too small may prevent runner from disengaging from runner bushing during product ejection; while a draft angle that is too large will waste excessive plastic and increase cooling time.

 

Figure 1.10 Direct Gating Design Recommendations
a = Draft Angle (1°~4°), S = Wall Thickness, d = Gate Diameter, D>1.5mm+S; d>0.5mm+Nozzle Diameter

Edge gate is a basic type of gate, as shown in Figure 1.11(a). Also known as a standard gate, this type of gate typically has a rectangular cross-section, with its edge extending to the side, top, or bottom of plastic part. Gate overlap is often located at parting line of mold. Edge gates are frequently used in multi-cavity mold designs. Direction and position of runner connection to cavity facilitate automated post-processing steps such as assembly, finishing, and inspection. Recommended thickness for a typical edge gate is two to ten times thickness of plastic part. Gate length should generally not be too long, otherwise it will cause a significant pressure drop.
Advantages: Easy separation of gate from molded part; prevents plastic backflow; gate generates shear heat, further increasing melt temperature and promoting filling; gate size is easy to process.
Disadvantages: High pressure loss; poor flowability; prone to flow marks on flat or large-area molded parts.

An overlap gate (or lapped gate) is a variation of edge gate. As shown in Figure 1.11(b), gate overlaps with sidewall or surface of plastic part. Overlap gates can prevent jetting, reducing surface defects in defective products. However, gate's placement, orientation, product's geometry still need to be designed and considered. Advantages and disadvantages of this gate type are similar to those of edge gates, but overlap gates are not easily removed in a single operation. Therefore, it is recommended to place gate in a location where outer surface is not critical, or in a location where gate removal is not required.
Recommended dimensions for overlapping gates are similar to those for edge gates. Gate thickness should be 60-75% of part thickness, or 0.4-6.4 mm. Width should be two to ten times part thickness, or 1.6-12.7 mm. Gate face length should not be too long, ideally exceeding 1.0 mm, with an optimal recommended value of 0.5 mm.

 

Figure 1.11 (a) Side gate; (b) Overlapping gate

Fan gate is similar to side gate, but its width and thickness vary with flow direction. This gate type offers a large filling area, providing a uniform and stable filling flow front cross-section for large, flat parts with uniform cross-sections. Application of a fan gate stabilizes and diffuses flow front, ensuring uniform flow direction and reducing residual stress in product geometry, thus preventing cooling warping and dimensional inaccuracies. However, like side gate, this gate is difficult to remove. Recommended maximum thickness for a fan-shaped gate is within 75% of wall thickness, and recommended width is at least 25% of length to side edge of cavity. For narrow front-strip products, width can be equal to side edge of product, as shown in Figure 7.12(a).
Advantages: Uniform filling prevents deformation of molded part, resulting in a molded part with a good appearance and virtually no defects;
Disadvantages: Difficult post-processing, difficult gate removal, large gate residue;

A film gate, also known as a flash gate, functions similarly to a solid gate, both being designed for large-area, uniformly shaped acrylic plates, as shown in Figure 1.12(b). Film gate overlaps to geometrically straight edge of product, and gate width can span the entire edge of cavity or part of cavity. Through film gate, molten plastic flows evenly and uniformly, thus minimizing product warpage. Compared to fan-shaped gates, although film-type gates require less space and are smaller in size, their gate design is extremely sensitive to factors such as gate thickness, runner diameter, and filling rate. A gate thickness of 50% to 70% of geometric thickness is recommended, and gate length should be short.
Advantages: More uniform flow, fewer weld lines, prevention of warpage in molded parts;
Disadvantages: Difficult gate removal, affects product appearance;

 

Figure 1.12 (a) Fan-shaped gate (b) Membrane gate

Disk gates (Diskgate/Diaphragm gate), as shown in Figure 1.13(a), are commonly used for cylindrical bodies with openings on both sides or circular, coaxial geometric products with openings on inner side. Disc gate design is used in three-plate molds. Molten plastic flows through coaxial vertical runner and fills cavity via gate, resulting in uniform plastic filling of product. This increases predictability of shrinkage, reduces warpage and axial displacement. However, removing a disc gate is more difficult and time-consuming. Generally, gate thickness is between 0.25-1.27 mm.
Ring gate has similar applications and purposes to disc gate, as shown in Figure 1.13(b). It is also used for coaxial plastic parts with cylindrical or circular geometry. Ring gates are used in two-plate mold cold runner designs. Molten plastic first flows around mold core and then flows downwards along cylindrical tube wall, allowing for uniform plastic filling and avoiding warpage, deformation, and weld lines. However, due to diversity of geometric shapes and properties of plastics, it can easily increase uncertainty and unevenness of filling. Therefore, before using this type of gate, it is advisable to use analysis software for evaluation and testing. Recommended thickness range for annular gates is typically 0.25-1.6 mm.
Spokegates, also known as four-point gates or crossgates, as shown in Figure 1.13(c), are generally used in multi-cavity three-plate molds or hot runner systems. Spokegates are similar to disc and annular gates and are mainly used for tubular plastic parts. Spokegates offer advantages such as easy gating and plastic saving. However, they are more prone to weld lines and poorer geometric roundness in product. Gate geometry is often a uniform rectangular or tapered circular gate, with a recommended thickness of 0.8-4.8 mm and a width of 1.6-6.4 mm.

 

Figure 1.13 (a) Disc gate: (b) Ring gate (c) Radial gate

Automatic removal gates, in coordination with mold movements, remove gate and runner from product geometry during demolding and ejection, avoiding need for manual secondary gate removal. This design reduces gate size and product residue, while maintaining consistent cycle times for accurate production rate estimation. Automatic removal gates include following types: pin gates, submarine gates, hot runner gates, valve gates, and internal gates.

Pin gates automatically separate runner and gate from product during demolding or ejection, as shown in Figure 1.14. They are typically used in multi-cavity three-plate mold designs, where runner system is located between upper fixed plate and cavity on one side of runner platen, while cavity is located between main male and female platens. Demolding involves two stages: demolding along main parting line and demolding along secondary parting line, automatically ejecting product geometry and runner waste. Diameter of pin gate needs to be small enough to avoid damaging product geometry during demolding and removal of runner system. Due to small gate size, a larger runner pressure drop occurs, and additional shear heat may re-plasticize melt. A typical pin gate diameter is approximately 45-50% of geometric wall thickness.
Advantages: Molded part and runner/gate system can automatically separate during mold demolding or ejection without manual intervention; multiple gates can be designed, allowing for selection of optimal injection locations.
Disadvantages: Higher pressure drop when plastic enters mold cavity; faster gate freezing, reducing effectiveness of holding pressure on molded part; larger amount of runner waste generated; higher mold cost.

 

Figure 1.14 Needle-point gate

A submarine gate is also called a tunnel gate or chisel gate. Figure 1.15(a) is a schematic diagram of a Submarine/Tunnel/Chiselgate. This type of gate is often used in two-plate cold runner mold design systems with small cavities. Runner is located at parting line, a special feature is inclined conical gate between end of runner and cavity, which allows molten plastic to fill cavity. After molten plastic has cooled, ejection system automatically separates gate from plastic part. To achieve automated gate separation, gate size should not be too large to prevent damage to gate surface of plastic part during gate separation; however, a gate that is too small should also be avoided, as this can lead to increased pressure drop, excessive gate shear stress, and premature gate cooling. Recommended diameter for a typical submarine gate is 40-70% of wall thickness, but gate size is still closely related to plastic and application. Plastic material should avoid excessive shearing to prevent breakage during runner ejection; common materials include PS, PA, POM, and ABS. Several submarine gates can be designed inside a double-opening cylinder, replacing disc gate. This provides automatic gate removal, avoiding manual secondary gate removal. However, it can affect roundness of molded part and create weld lines.
Another option is knock-outpin submarine gate, as shown in Figure 1.15(b). This gate is used in conjunction with ejector pin for injection, and is particularly useful when design changes are needed to prevent gate marks from appearing on product's surface.
Advantages: Automatic gate-to-part separation; minimal gate residue; gate position can be freely set to outside or inside of molded part;
Disadvantages: High pressure loss; suitable for simple molded parts.

 

Figure 1.15 (a) Submersible gate (b) Knock-out pin submersible gate

A typical cashew gate has a diameter of 0.25-1.5 mm, as shown in Figure 1.16. Cashew gates are difficult to manufacture, tapering from coarse to fine, with end connecting to injection gate on the bottom of cavity. Horn gate is a variant of tunnel gate. It allows for gate entry in areas inaccessible by standard tunnel gates and features automatic gate removal. Its main limitation is that curved design can cause significant deformation of material near gate during ejection.
Advantages: Easy post-processing, can be automated;
Disadvantages: High pressure loss, complex mold structure, high cost;

 

Figure 1.16 Horn-shaped gate

A hot runner valve gate uses a movable, concealed pin within hot nozzle to control opening and closing of gate. As shown in Figure 1.17. Valve opens during filling and holding, allowing plastic to fill mold cavity, and closes before solidification, thus reducing steps of melting molding runner and cutting gate. Application of valve gates allows for more diverse manufacturing processes. However, due to design and implementation difficulties of valve gates, as well as higher mold development costs, valve gates are typically used for larger products with high-quality geometry. Even with larger gates, valve gates do not leave gate marks on plastic part. Because holding pressure cycle is controlled by needle valve, using a valve gate can achieve a better holding pressure cycle and more stable part quality. Size of needle valve gate is closely related to application of needle valve, plastic material, product geometry, and number of gates.
Advantages: Material savings, shorter molding cycle;
Disadvantages: Leaves marks on product surface;

 

Figure 1.17 Hot runner valve gate