Gating System Design Principles:
① For molds with multiple cavities, a geometrically balanced layout, or a runner layout, must be used to ensure simultaneous filling of all cavities, as shown in Figure 1-1. For special cases requiring an unbalanced layout or multiple-point water entry, sprue width or runner size should be adjusted based on mold flow analysis results to ensure simultaneous filling of all cavities. (See Figures 1-2 and 1-3.)

 

Figure 1-1: Runner balance in a multi-cavity mold.

 

Figure 1-2: Balance number of runner in a single-cavity, multi-point water-entry mold.

 

Figure 1-3 Unbalanced layout
② Runners should be as short and straight as possible to minimize heat and pressure loss, and corners must have rounded corners to ensure smooth flow. However, for transparent parts such as PC and PMMA, an "S"-shaped runner must be used to prevent cold material from directly entering cavity and causing flow marks, as shown in Figure 1-4.

 

Figure 1-4 "S" shaped runner for transparent products
③ Generally, 320# or higher sandpaper should be used on the runner surface to minimize gloss. However, 600# sandpaper should be used on mirror-faced parts to minimize gloss.
④ All runners must have cold wells at their ends to prevent cold slugs from entering mold cavity and affecting appearance of part. Note that entry point of small sprue should not be directly opposite main runner, as shown in Figure 1-5.

 

Reasonable runner form

 

Unreasonable runner configuration
Figure 1-5: A cold well must be designed at the end of runner.
⑤ Gating system should correctly guide molten material to fill every corner of mold cavity, allowing for smooth discharge of trapped air.
⑥ Gating system should prevent defects such as underfill, shrinkage, deformation, flashing, and dimensional deviation, as shown in Figure 1-6.
Original sprue position

 

Improved sprue position

Figure 1-6 Comparison of filling results before and after adjustment of nozzle position and size
⑦ Sprue should be easily removable (automatically removable sprues should be used if possible), ensuring that removal does not affect part's appearance.
⑧ Runner must be removable using a robot to improve production efficiency.
⑨ Runner must be designed in a round or U-shaped form for easy processing.
⑩ Runner weight should be considered during design. It should not exceed quotation and ratio of sprue weight to the total weight should be controlled below 25%. In special cases, it should not exceed 50%. If runner weight exceeds quotation, written information should be submitted to contact design model person in charge. 
(11) For a mold with multiple water points in one cavity, number of sprues should be designed according to ratio of flow distance L to glue position thickness T (flow length ratio), mold flow analysis results should be referred to to prevent position and size of weld mark from affecting strength and appearance of product; flow length ratios of commonly used plastics are shown in table below.

(12) For a mold with multiple cavities and different parts, size of sprue and runner should be determined according to mold flow analysis results to ensure that each cavity can be filled at the same time; as shown in Figure 1-7.
Good sprue design

 

Figure 1-7: Flow balance in a single mold with multiple cavities and different parts.
 (13) For a mold with multiple cavities and different parts, a sprue insert should be made to switch to a certain cavity to meet production needs when necessary, as shown in Figure 1- 8(a). At the same time, in order to ensure filling balance by adjusting runner size during forming process, a runner regulating valve is made on the side of product that is easy to fill, as shown in Figure 1-8(b):

(14) Layout of a mold with multiple cavities should be symmetrical, compact and prevent eccentric loading to reduce mold frame specifications, reduce nozzle weight and molding machine tonnage, as shown in Figure 1-9:
Layout shown in Figure a is not optimal, resulting in flash and poor dimensional accuracy when injecting glue from different parts. However, sometimes, due to limited sprue location, special cases may occur. As shown in figure below, option c is more reasonable than option b in terms of sprue weight. In such cases, it is necessary to comprehensively consider various factors and evaluate pros and cons of each option to find the most optimal layout.

 

Figure 1-9: Choosing a reasonable layout for a multi-cavity mold
☆ When mold needs to be eccentric due to structural reasons, eccentric direction is preferably top and bottom sides, and eccentricity S is less than 10% L (L is length of mold). As shown in Figure 1-10: When mold is eccentric, ejector hole must follow eccentricity and more than two ejector holes should be used as much as possible to ensure ejection balance;

 

Figure 1-10 Eccentric Mold Design