This article mainly addresses following four questions.
Six common injection methods and their advantages and disadvantages?
Basic considerations for choosing an injection method?
Impact of injection methods on product structure and quality?
How to determine injection point?
Basic Concepts
Runner: Also known as main sprue or injection runner, it is part where injection molding machine nozzle contacts main runner bushing of mold. This is the first part through which molten plastic flows after entering mold.
Gate sprue: Refers to part from runner to mold cavity (product).

 

1-Main flow channel; 2-Diversion channel; 3-Cold material well; 4-Inlet; 5-Product
Common Gate Types and Characteristics
Large Gate (Side Gate)
Generally, side gates are located on parting surface of mold, allowing material to enter from side of product. This gate method is widely used.
Advantages:
Simple structure: Mold processing and manufacturing are easy, and cost is relatively low. Side gates only need to be opened on the side of mold, without complex mechanisms, making processing and maintenance convenient.
High adaptability: Suitable for various plastic materials and products of different shapes and sizes.
High molding efficiency: Gate size is large, flow resistance is low, and it can quickly fill cavity, suitable for mass production.
Disadvantages:
Noticeable gate marks: Due to large gate size, after product is demolded, a noticeable mark will be left at gate, which may affect product's appearance. For products with high appearance requirements, further processing may be required.
Prone to weld lines: When molten plastic enters mold cavity through a side gate, it encounters cores, inserts, or other structures during its flow, causing melt front to separate and easily form weld lines at point of convergence, reducing product strength.

 

Narrow gate injection (point gate, point injection)
This special type of injection has a very small gate size. Its advantage is that after removing gate, trace left on product is inconspicuous, and sprue can be automatically broken off upon mold opening. However, injection pressure loss is relatively large.
Advantages:
Good appearance quality: Extremely small point gate size results in inconspicuous gate marks after demolding, minimizing impact on appearance, making it ideal for products with high appearance requirements.
Automated production is possible: Narrow gate molds generally use a three-plate mold structure, allowing gate to automatically detach during mold opening, facilitating automated production and improving production efficiency.
Increased melt flow rate: Point gates increase flow rate of molten plastic, enabling rapid filling, helping to improve surface quality and molding effect of product, especially suitable for thin-walled products.
Disadvantages:
Complex mold structure: Three-plate molds are more complex than two-plate molds, increasing design and manufacturing difficulty and cost.
High pressure loss: Due to small gate size, pressure loss when raw material passes through point gate is high, requiring higher injection pressure and placing higher demands on performance of injection molding machine. This may also lead to higher internal stress in product.
Prone to stringing: During injection molding, plastic melt at point gate may string, affecting product quality. Proper control of injection parameters is necessary.

 

Fan-shaped gate:
Gradually thinning and widening along feeding direction, with the thinnest point at product junction, it can quickly fill product with relatively low pressure loss. However, it leaves a larger gate mark, requiring manual gate removal.
Advantages:
Uniform melt distribution: A fan-shaped gate ensures uniform distribution of plastic melt, reducing defects such as warpage and shrinkage marks. It is suitable for large-area, thin-walled flat products.
Reduced shear rate: Fan-shaped gate results in a relatively low shear rate of melt flow, improving consistency of plastic molecular orientation, thereby improving dimensional stability and mechanical properties of product.
Disadvantages:
Difficult gate removal: Gate width is relatively large, making removal difficult and leaving a wide mark on product surface, increasing post-processing costs and time.
Complex mold processing: Irregular shape of fan-shaped gate makes mold processing difficult, requiring high-end processing equipment and processes, thus increasing mold costs.

 

Submarine Gate (Hidden Gate)
Runner is designed on parting surface, and gate is located on the side of product or on internal ribs, in a concealed location that does not affect appearance. It is at a certain angle to runner (generally not exceeding 45°), submerged below (or above) parting surface, and enters cavity at an angle. It automatically detaches from product during demolding.
Advantages:
Minimal impact on appearance: Gate is hidden inside or on edge, leaving no obvious marks on product surface.
Automatic gate cutting: Solidified sprue is automatically cut off when product is ejected, enabling automated production and improving production efficiency.
Wide adaptability: Suitable for various product structures. For products with high appearance requirements and where point gates cannot be used, submarine gates are a better choice. Disadvantages:
Limited gate location: Gate location usually needs to be at a specific position on the edge or inside product, which limits product structure design. Not all products can easily accommodate a submarine gate.
Prone to cavitation: Because gate location is relatively concealed, air may not be able to escape smoothly during filling process, easily forming cavitation near gate, affecting product quality. A well-designed venting system is required.

 

Loading Ejector Pin Injection
This is also a type of submarine injection. When product's appearance does not allow for injection marks or there are no internal ribs for injection, injection can be moved to an ejector pin.
Advantages:
Avoids appearance defects: Gate is located at internal ejector pin position, suitable for products with extremely high appearance requirements.
Facilitates venting: Combination of gate and ejector pin position facilitates gas venting, reducing defects such as air bubbles and cavitation inside product.
Reduces weld lines: Molten plastic enters cavity directly from ejector pin position, allowing for more uniform filling and reducing weld lines.
Disadvantages:
Complex ejector pin structure: Requires a specially designed ejector pin structure to achieve injection function.
High requirements for injection molding process: When using ejector pin injection, flow state and pressure control of molten plastic are critical, requiring high precision in injection molding process parameters.

 

Loading Horn Injection
An evolution of submarine injection, used when product's appearance has specific requirements. This type of gate is unsuitable for tough plastics (such as PA) or brittle plastics (such as PS). The former is difficult to cut, the latter is prone to breakage and clogging.
Advantages:
Good appearance quality: Horn gates are similar to submarine gates, with inconspicuous gate marks, suitable for products with high appearance requirements.
Automatic demolding: Solidified material from gate can automatically detach from product, enabling automated production and improving efficiency.
Low pressure loss: Compared to point gates, horn gates are relatively larger, resulting in less pressure loss. Lower injection pressures can be used, reducing internal stress in product.
Disadvantages:
High mold processing difficulty: Complex shape of horn gates requires high precision mold processing, necessitating specialized processing techniques and equipment.
Limited gate location selection: Choice of gate location needs to consider product structure and mold demolding method, which is relatively limiting. Not all products are suitable for horn gates.

 

Selection Criteria for Glue Gate Method
Appearance Requirements
Products with High Appearance Requirements: For products with extremely high appearance quality requirements, such as mobile phone casings and high-end cosmetic packaging, where obvious gate marks are unacceptable, a narrow gate is ideal choice. Ejector pin gates or horn gates are also good options.
Products with Moderate Appearance Requirements: For products with less stringent appearance requirements, such as ordinary plastic storage boxes and industrial parts, a large gate is more suitable.
Although side gates leave more noticeable marks, mold structure is simpler, cost is lower, and molding efficiency is higher.
Product Structure and Dimensions
Thin-Walled Products: For thin-walled products such as mobile phone frames and ultra-thin plastic panels, rapid filling is required to avoid uneven cooling and deformation.
Narrow gate gates can increase melt flow rate, suitable for these products;
Ejector pin gates also facilitate uniform melt filling of thin-walled cavities, reducing risk of deformation.
Large Flat Products: For large-area, thin-walled flat products, a fan-shaped gate is a better choice to ensure uniform melt distribution and prevent warping. It allows melt to enter cavity evenly along its width.
Complex Structural Products: When a product has many cores, inserts, or a complex internal structure, impact of gate method on melt flow must be considered.
Submarine gates allow for injection into product from within or at edges where it is not easily noticeable. This does not affect molding of complex structures and automatically cuts off gate, making it suitable for these products.
Product Strength and Performance:
Products Requiring High Strength: For products subjected to significant external forces, gate methods that result in obvious weld lines that could affect strength should be avoided.
Weld lines can be reduced by optimizing gate location and method, such as using multiple gates to allow melt to fill simultaneously, choosing a fan-shaped gate to improve melt flow uniformity.
Preventing product deformation: For products with high dimensional stability requirements and prone to deformation, impact of gate method on internal stress distribution must be considered.
Improperly designed large gates (side gates) may cause product deformation due to uneven internal stress.
Hot runner systems combined with point gates can better control temperature and pressure, reduce internal stress, and prevent product deformation.
Production efficiency and cost:
Mass production: For mass production, impact of gate method on production efficiency must be considered.
Large gates (side gates) have large gate sizes and high molding efficiency; while small gates, although requiring more complex molds, can be automated and are suitable for large-scale production.
Small Batch Production: For small batch production, to reduce costs, priority should be given to injection methods with simple mold structures and low manufacturing costs, such as large gate injection (side gate).
Material Characteristics
High Viscosity Materials: For high-viscosity plastic materials, such as some engineering plastics, higher injection pressure is required for them to flow.
Injection methods with larger gate sizes and lower pressure loss should be selected, such as large gate injection (side gate) or fan-shaped injection, to ensure smooth melt filling of cavity.
Heat-Sensitive Materials: Heat-sensitive materials are prone to decomposition or discoloration at high temperatures. During injection, residence time of melt in runner should be minimized.
Hot runner systems combined with point gate injection allow for precise temperature control, suitable for molding these materials.
Impact of Injection Method and Location on Product Quality
Melt Flow Balance: Proper injection location ensures uniform melt flow and synchronous filling of cavity.
Improper location will lead to flow imbalance, with higher pressure at the first filling area and insufficient material or air trapping at later filling areas.
For flat products, uneven filling and inconsistent thickness on both sides can occur if glue injection is off-center.
Weld Line Impact: Weld lines form when melt encounters obstacles such as mold core. Injection location determines location and extent of these weld lines.
Weld lines at critical stress points reduce product strength, while weld lines in appearance areas affect aesthetics.
For products with holes or protrusions, injection point should minimize negative impact of weld lines.
Internal Stress Distribution: Injection location affects melt flow, leading to uneven internal stress distribution within product.
Improper injection can cause excessive localized internal stress, making product prone to warping, deformation, or even cracking during demolding or use.
For thin-walled products, large differences in melt flow rate due to injection can cause warping due to uneven internal stress.
Appearance Quality: Injection location affects gate mark, impacting product's appearance.
For products with high appearance requirements, injection point should be in an inconspicuous location.
For example, for mobile phone back covers, injection point is often placed at the edge or inside, without affecting front appearance.
Methods for Determining Glue Injection Location
Product Structure Analysis
Shape and Size: For small, regular products, injection can be done centrally or symmetrically; for complex, large products, injection location should be determined according to shape. For example, for long, strip-shaped products, multiple injection points can be set at both ends or in the middle to ensure rapid and uniform filling.
Wall Thickness Distribution: Thick-walled areas cool slowly and are prone to shrinkage marks. Injection should allow melt to fill thin-walled areas first, then thick-walled areas, utilizing thick walls to compensate for shrinkage. For products with reinforcing ribs, avoid injecting directly at reinforcing ribs; inject from sides or in a parallel direction.
Product Usage Requirements
Appearance Requirements: For products with high appearance requirements, injection should be selected in areas that will not affect appearance, such as edges, bottoms, or interior. For transparent products, avoid injection that affects transparency. For optical lenses, ensure there are no internal defects.
Strength Requirements: For products subjected to high stress, prevent weld lines at critical stress points.
Simulation Analysis Software
Simulated Melt Flow: Simulate melt flow. Analyze flow front, pressure, temperature, and weld lines from results, adjust injection location accordingly to predict and resolve quality issues.
Optimize Glue Gap Design: Evaluate quality indicators such as warpage, weld lines, and shrinkage marks, and select optimal solution.
Refer to Case Studies
Industry Experience: Learn from glue gap design experience of similar or identical structural products.
Case Analysis: Analyze production quality problem cases to understand the defects caused by improper glue gap design and their solutions. Learn to avoid making mistakes, draw inspiration from successful cases, and optimize glue gap design.