Plastic injection molding is cornerstone of modern manufacturing, and molds are core factor determining product quality, efficiency, and cost. This article uses clear logic, illustrated language, and engineering examples to systematically analyze three key technology modules—gating (injection) system, cooling system, and mold/product dimensional design principles—to help you quickly grasp underlying logic.

 

Gating is the only channel for molten plastic to enter mold cavity. Its type, location, and size directly affect uniformity of filling, internal stress distribution, appearance quality, and difficulty of subsequent mold repair. Mainstream gating systems can be divided into four categories according to structure and function:
1. Edge Gate
a. Structural characteristics: Located on parting surface, connecting runner to edge of product; cross-section is mostly rectangular.
b. Applicable Scenarios: Highly versatile, suitable for medium to large-sized products such as flat panels and boxes; facilitates automated shearing and post-processing.
c. Points to Note: May leave noticeable marks at injection point; thick-walled parts are prone to jetting marks or weld lines.
2. Pin-point Gate
a. Structural Characteristics: Typically 0.8–1.5 mm in diameter, located on the top surface of product or in a concealed position, achieving automatic breakage through a submerged or needle valve structure.
b. Applicable Scenarios: Products with high appearance requirements (such as mobile phone cases, appliance panels); commonly used in multi-cavity molds, facilitating balanced injection.
c. Advantages: Minimal residue, automatic demolding, beneficial for pressure holding; can eliminate cold sprue when used with a hot runner.
3. Submarine Gate
a. Structural Characteristics: Cuts obliquely into side or bottom of product, automatically breaking off by shearing force during mold opening; gate is concealed inside product or on a non-visible surface.
b. Applicable Scenarios: Products requiring strict surface finish (e.g., transparent lens holders, medical casings); suitable for high-flow materials (e.g., PP, PS).
c. Design Considerations: Recommended tilt angle is 15°–30°, length controlled within 1–3 mm to avoid stringing or residue.
4. Direct/Spur Gate
a. Structural Characteristics: Melt flows directly into cavity center via main runner, without branch runners, resulting in a large cross-sectional area.
b. Applicable Scenarios: Large, thick-walled, single-cavity products (e.g., car bumpers, barrel containers); emphasizes filling rate and shrinkage compensation.
c. Risk Warnings: Difficult to remove, significant residue; prone to localized overheating and shrinkage marks, requiring adequate holding pressure and cooling.

Cooling accounts for 60%–70% of injection molding cycle, directly affecting production cycle time, dimensional stability, and surface gloss. A cooling system is not "the more the better," but rather a precisely designed system based on thermal balance and flow symmetry.
Mainstream Cooling Methods Classification

Mold dimensions ≠ drawing dimensions. Actual molding is affected by multiple factors including shrinkage, deformation, wear, and process fluctuations. Therefore, a "process-oriented" approach must be used to define critical dimension chain.
1. Shrinkage rate is not a constant, but a variable.
a. Different materials have significantly different shrinkage rates: PP (1.0–2.0%), ABS (0.4–0.7%), PC (0.5–0.7%), POM (2.0–2.8%);
b. For same material, shrinkage is anisotropic due to differences in filling direction (flow vs. vertical), wall thickness, and holding pressure parameters;
c. Practical suggestions: Before the first trial molding, use median value from material supplier's TDS data for initial design; reserve a 0.02–0.05 mm allowance for critical dimensions, and revise using data from three trial moldings.
2. Unified Datum: Mold Datum = Measurement Datum = Assembly Datum
a. All dimensions of cavities, cores, sliders, and ejectors must be modeled using mold center or main locating pin as a unified datum;
b. Avoid "local closed-loop dimensional chains," for example: height of a snap-fit is formed by insert A + slider B → easily leads to cumulative errors due to assembly mistakes;
c. Golden Rule: Individual part dimensions take precedence over assembly dimensions; mating dimensions (e.g., H7/g6) must be completed on mold, not relying on machining tolerances.
3. Wear Compensation: Providing "Growth Space" for Future Lifespan
High-frequency contact areas (such as parting surfaces, ejector pin end faces, and slider ramps) require pre-added wear allowance based on the expected lifespan:
a. General molds (500,000 cycles): Leave a 0.01–0.02 mm grinding allowance on parting surface;
b. High-gloss molds (200,000 cycles): Leave a 0.005 mm allowance on polished surface to avoid distortion from repeated polishing;
Clearances between moving parts (such as guide pillars/sleeves) are designed for H7/f7 fit, but actual machining allows for a running-in space based on H7/e7.
4. Process Window Mapping to Dimensional Tolerances
a. If customer requires ±0.05 mm, mold manufacturing tolerance is recommended to be ≤±0.015 mm, ensuring this dimension is within injection molding process stability window (e.g., holding pressure time ±0.5s does not cause out-of-tolerance).
b. For dimensions sensitive to out-of-tolerance (e.g., gear tooth pitch, lens focal length ring), adjustable mechanisms (e.g., inlaid gear rings, fine-tuning shims) must be installed on mold.

 

Gate determines "how to enter," cooling determines "how to stabilize," and dimensional design determines "how to be precise." These three are not isolated modules, but a dynamically coupled system: point gates need to be matched with high-response cooling; thin-walled high-speed molding forces conformal water channels to work in tandem with low-shrinkage materials; precision gear molds further require gate position to avoid tooth area and cooling to uniformly cover the entire gear train.
True masters do not dwell on "which gate to choose," but consider "how to achieve optimal balance between melt, heat, and dimensions within 0.1 seconds." In next issue, we will dissect a real-world DFM case study of an automotive door handle mold, showing you the entire decision-making process from 2D drawings to a 99.2% mass production yield.