When designing an injection molding system (gating system), the first thing to do is to determine gate position. Its rationality plays a decisive role in molding quality of product and smoothness of injection process. Selection of gate position should follow following principles:
1. Runner design: When constructing gating system, runner should reduce bends as much as possible to ensure smoothness of fluid flow. At the same time, runner surface roughness should be controlled between Ra1.6 - Ra0.8μm to reduce flow resistance and heat generated by melt friction.
2. Mold layout adaptation: Design of gating system should fully consider whether mold is a one-cavity or multi-cavity structure. System planning is carried out according to cavity layout, strive to maintain symmetry with center line of mold to achieve balanced distribution and flow of melt.
3. Product area considerations: For plastic products with a large projection area, when designing pouring system, it is necessary to avoid opening a gate on one side of mold to prevent uneven force during injection molding process, causing deformation or defects of product.
4. Convenience of gate removal: During design process, it is necessary to focus on the ease of gate removal, and when correcting gate, it is necessary to ensure that there is no trace left on the surface of plastic product to ensure appearance quality and integrity of product.
5. Multi-piece mold matching: In design of a mold with multiple pieces, it is strictly forbidden to place plastic products with significant size differences in same mold to avoid uneven product size accuracy and appearance quality due to uneven injection pressure, cooling rate and other factors.
6. Main runner protection: When designing main runner, it is necessary to avoid molten plastic from directly impacting small diameter core and insert. Through reasonable main runner structure design and size optimization, impact stress can be reduced to prevent bending or breaking of core and insert.
7. Process optimization: Under premise of meeting requirements of plastic molding process and exhaust conditions, the shortest process path should be selected to shorten filling time, improve production efficiency, reduce heat loss and pressure drop of melt during flow process.
8. Filling stability: Pouring system should have good guiding performance, be able to smoothly fill molten plastic into various parts of cavity, effectively suppress occurrence of plastic eddy currents and turbulence during filling process, ensure that gas in cavity can be smoothly discharged out of mold, and avoid defects such as pores and looseness in product.
9. Production cycle control: In large-scale production of plastic products, under premise of ensuring product quality, by optimizing pouring system design, reasonably selecting mold materials and cooling methods, etc., cooling time and molding cycle can be shortened, production efficiency can be improved, and production costs can be reduced.
10. Main channel accuracy guarantee: Since main channel part is prone to shrinkage during injection molding process, if plastic product has high dimensional accuracy requirements at this part, it is necessary to reserve appropriate processing allowances or correction allowances when designing main channel for subsequent precision processing and size adjustment.
11. Gate position optimization: Gate position should be set accurately to ensure that when plastic flows into cavity, it can directly face spacious and thick-walled part of cavity, thereby improving flow state of melt, reducing flow resistance, promoting smooth and uniform flow of plastic.
12. Weld mark control: By rationally planning gate position and number, optimizing melt flow path, and adjusting injection process parameters, try to avoid weld marks on product. If it cannot be completely eliminated, weld marks should be generated in unimportant parts of product to reduce impact on mechanical properties and appearance quality of product.

Figure (I) shows form and design parameters of vertical main channel:
d——Main channel small end diameter: That is, diameter of main channel at contact point with injection machine nozzle, which is calculated as injection machine nozzle aperture plus (0.5 - 1) mm.
L——Main channel length: Determined during design based on specific structure of mold.
A——Taper of the main channel: A is usually selected in range of 2° - 4°. For plastics with higher viscosity, A can be 3° - 6°. However, due to limitation of standard taper reamer, standard values should be selected as much as possible, or standard gate sleeves should be selected. (Specific size can be designed according to gate sleeve part size formulated by company, combined with performance of plastic and size of plastic part, and appropriate specifications can be selected. Note: Minimum size of front end of gate d≥3.5mm). Generally speaking, size D is about 10-20% larger than D1.

 

During mold design process, due to constraints of product characteristics and mold's own structure, or limitations of gating system and number of cavities, main channel often deviates from mold center, sometimes deviation distance is large. This causes mold to face many problems during use stage.
First, in ejection process, since ejection position is not in the center of mold, push plate and ejector fixing plate are prone to ejection deviation, which in turn causes ejector breakage, product deformation or damage and other problems.
Secondly, if main channel is not centered, size of single-sided seam will be too large, resulting in overflow. Although three-plate mold structure can solve above problems to a certain extent, this structure will significantly increase mold cost.
In view of this, in above situation, use of inclined main channel design can effectively avoid or improve these shortcomings.

 

Inclined main channel design parameters
As shown in Figure (2), relevant design parameters of inclined main channel are shown. Among them, value of inclination angle α is closely related to performance of plastic material.
For plastic materials such as PE (polyethylene), PP (polypropylene), PA (polyamide), and POM (polyoxymethylene), maximum inclination angle α can reach 30°;
For plastic materials such as HIPS (high-impact polystyrene), ABS (acrylonitrile-butadiene-styrene copolymer), and PC (polycarbonate), maximum inclination angle α is 20°.
It should be noted that SAN (acrylonitrile-styrene copolymer) and PMMA (polymethyl methacrylate) materials are not suitable for inclined main channel design.
In addition, other design parameters of inclined main channel are consistent with design requirements of vertical main channel.

Do you often encounter problem of how to choose gate position in your work, and don’t know where to set gate?