Because cross-sectional area of gate is very small, it causes injection pressure loss. To avoid excessive pressure loss, gate should be designed in a thicker part of wall for better pressure holding. Other design considerations include:
1. Flow direction: Avoid molecular orientation of molten plastic, which can cause severe warping;
2. Weld lines: Try to make flow distance of molten plastic within cavity equal to reduce gas encapsulation or weld lines;
3. Filling: Appropriate placement can increase turbulence to reduce chance of jetting and flow marks.

Selection of number of gates mainly depends on flow length ratio of plastic and pressure holding effect. As shown in Figure 3.30(a), a single gate needs to handle a maximum transfer distance of 250mm with a wall thickness of 1mm, so flow length ratio is 250. In Figure 3.30(b), case is increased to two gates, which shortens flow length ratio to 180, but product has risk of weld lines. Flow length ratio is crucial and must be designed in conjunction with flow properties of plastic to determine appropriate number of gates (see Table 3.1). CAE analysis should be used to check flow balance, pressure loss, filling end, and weld line location under different gate numbers.

 

Figure 3.30 Flow lengths with different numbers of gates: (a) single gate and (b) two gates

Gate location determines pressure holding level of different parts of product. As shown in Figure 3.31, areas near gate have better pressure holding and less shrinkage, and vice versa.

 

Figure 3.31 Shrinkage difference due to uneven shrinkage
Gate location is basically determined by product specifications. Avoid placing it on product's exterior surface, and also avoid weld lines where molten plastic meets appear on exterior. As shown in example in Figure 3.32, three gates are set. CAE analysis predicts three potential weld line locations, as indicated in figure. If these locations are important exterior surfaces, gate location must be adjusted, thus moving weld line location.

 

Figure 3.32 Weld line prediction from mold flow analysis
Furthermore, gate location should also avoid weld lines appearing at structural weaknesses. As shown in Figure 3.33, changing gate location can move weld line from short side to a corner.

 

Figure 3.33 Weld angles at different gate positions: (a) Gate located on short side; (b) Gate located on long side.
For large parts, multiple gates are necessary. As shown in Figure 3.34, a two-cavity mold design, original design (Figure 3.34(a)) has two gates per cavity. This results in a weld line appearing in the center of product. Because flow path of molten adhesive is already long when it meets, temperature is lower, leading to a more noticeable weld line. Modifying design to a three-gate system (Figure 3.34(b)) reveals a new weld line location. Since this is a high-temperature weld, weld line is less noticeable. Therefore, CAE analysis can be used to adjust appropriate gate location to achieve a better molten adhesive bonding quality.

 

Figure 3.34 Using mold flow analysis to inspect weld line quality: (a) Two gates per product and (b) Three gates per product
Appearance quality is the first priority in gate location design, followed by smoothness of flow. As shown in Figure 3.35, there are five gate points: A, B, C, D, and E. Near Gate A, there is a thin tab design. It can be observed that if gate A is used for gate entry, flow competition effect causes stagnation at tab area. This is because when molten plastic encounters a branch, it flows more easily along thicker wall, resulting in selective flow. If gate D is used for gate entry, this problem is avoided, achieving complete filling. This is same principle as placement of reinforcing ribs mentioned in Figure 3.15. Therefore, using CAE to assist in checking appropriateness of gate location, through simple gate location change simulation analysis, can quickly provide advantages and disadvantages of various preliminary design schemes and a basis for selection.

 

Figure 3.35 Mold Flow Analysis Prediction of Filling at Different Gate Locations
Another consideration for gate location is that when molten plastic passes through a narrow gate, elastic effect becomes significant, and molten plastic will exhibit a momentary swaying flow behavior. If mold cavity does not restrict flow of molten plastic in flow direction, a jetting effect will occur, necessitating a change in gate location or gate type. For example, an overlapping gate can be used instead of a side gate, as shown in Figure 3.36.

 

Figure 3.36 Gate Design to Avoid Jetting
When using fiber-reinforced plastics, gate location is crucial because molten plastic flow pattern influences fiber distribution and orientation, as shown in Figure 3.37, thus affecting structural strength of product. The higher degree of fiber alignment in a particular direction, the stronger strength along that direction, but the weaker strength perpendicular to that direction. If subjected to external forces perpendicular to this direction, cracking along fiber direction is more likely to occur. Generally, a more uniform fiber distribution and orientation results in products with more consistent strength and quality.

 

Figure 3.37 Fiber orientation at different gate locations

There are many types of gates. Appropriate gate type is usually determined based on product appearance requirements, gate removal methods, product geometry, mold manufacturing, and plastic properties. As shown in Figure 3.38, various gate types, including direct or pin-type horn gates, fan gates, overlapping gates, submarine gates, and pin-point gates, are listed. Appropriate timing for each gate type is briefly described in Table 3.3.

 

Figure 3.38 General gate types
Table 3.3 Common Gate Types

Besides location and form, size of gate is also crucial in gate design, primarily depending on filling and holding time, shear rate plastic can withstand, and gate removal method, as shown in Figure 3.39.

 

Figure 3.39 Effect of gate size on plastic part size
Gate shear rate is estimated as  for circular gates and  for rectangular gates. Where Q is flow rate, r is gate radius, w is gate width, and t is gate thickness. See Table 3.4 for allowable molding shear stress and shear rate of plastics.
Table 3.4 Maximum Shear Stress and Shear Rate

 

Injection system, as shown in Figure 3.40, includes vertical runners, runners, and gates. Main task of runner is to uniformly deliver molten plastic from nozzle to product cavity. Quality of runner design directly affects filling path, cycle time, and consequently, product quality.

 

Figure 3.40 Melt delivery system
Runner design considers four main aspects:
1. Runner shape: For example, circular, parabolic, or trapezoidal cross-sections determine pressure loss of molten plastic before entering product, mold processing costs, and demolding considerations.
2. Runner dimensions: Primarily determines molten plastic flow rate, also affects pressure loss and demolding mechanisms.
3. Number of cavities: Attention must be paid to geometric balance of multi-cavity systems, volume that can be handled per mold run, production capacity, and cost.
4. Overall runner configuration: Determines flow balance, demolding design, and manufacturing costs. Common runner designs include sequential, H-shaped symmetrical, circular symmetrical, and X-shaped symmetrical, as shown in Figure 3.41.

 

Figure 3.41 Typical flow channel configuration

Common cross-sectional forms for runner shape and dimensions are circular, trapezoidal, and parabolic, as shown in Figure 3.42. From a fluid dynamics perspective, a circular shape is most recommended because it offers the best flow efficiency and lower pressure loss. However, disadvantage is that it requires mold processing on both sides, resulting in higher costs. Trapezoidal cross-sections are relatively easy to machine and are therefore common. Parabolic cross-sections offer better compatibility with parting lines, but their machining cost is higher than trapezoidal cross-sections.

 

Figure 3.42 Cross-sectional view of a typical flow channel type
Due to different viscosity characteristics of various materials, runner is responsible for smoothly transporting molten plastic from nozzle to gate. Plastics with good flowability allow for narrower flow channels, so runner size must also take into account material's flow characteristics. Table 3.5 lists recommended runner dimensions for common materials.
Table 3.5 Recommended Runner Dimensions

 

Figure 3.43 shows flow imbalance phenomenon in a flow channel system with symmetrical geometry
If using a hot runner system, common runner diameter is 8-20mm, depending on manufacturer's design.