Main runner is channel connecting injection molding machine nozzle to sub-runner inlet, and it's the first place molten plastic passes through when entering mold cavity. Its size is closely related to plastic flow rate and filling time. If it's too large, too much cold material is recycled, cooling time is increased, air is trapped, easily causing bubbles and a loose structure, it's prone to eddies and insufficient cooling; if size is too small, heat loss increases, fluidity decreases, injection pressure increases, and molding becomes difficult. Generally, main runner is formed inside sprue bushing of mold. Its shape and size are determined by sprue bushing, which is a standard part, usually made of high-quality steel and heat-treated. Selection must be compatible with injection molding machine used. There are various types of sprue bushings, which can be selected according to different mold structures. They are generally fixed to mold plate to prevent sprue bushing from rotating or being carried out during production.

1. Main Runner Cone Angle: To facilitate removal of solidified material from main runner, it should be conical. Cone angle of inner hole of sprue bushing should be 2-4°. An excessively large cone angle reduces pressure, generates turbulence, easily introduces air, creating porosity, wastes raw materials, prolongs cooling time, affects injection molding cycle, and increases manufacturing cost of plastic part. An excessively small cone angle increases flow rate, making injection difficult.
2. Main Runner Diameter: Size of main runner inlet diameter affects flow rate of plastic melt and mold filling time. If inlet diameter is too small, heat loss and pressure loss increase, fluidity decreases, and molding becomes difficult. If inlet diameter is too large, runner volume increases, plastic consumption increases, leading to prolonged cooling and solidification time, decreased production efficiency. Furthermore, if main runner is too large, it can easily cause turbulence and eddies in flow of plastic melt, resulting in air bubbles inside product. Therefore, main runner inlet diameter must be designed appropriately. Typically, inlet diameter of main runner is 3-8mm. If melt flow is good and product is small, diameter can be designed to be smaller; conversely, it should be designed to be larger.
3. Surface Roughness of Main Runner: Surface roughness Ra of inner wall of tapered orifice of main runner is 0.4-0.63pm. Edge of large end of tapered orifice should have a rounded transition with r=1-3mm to reduce flow resistance when melt changes direction.
4. Spherical Radius of Main Runner: Part of main runner inlet that contacts injection molding machine nozzle head is generally made into a concave spherical surface to match spherical radius of injection molding machine nozzle head. Injection molding machines from Japan, South Korea, mainland my country, Taiwan, and North America all have spherical nozzle heads, therefore their sprue bushing inlet ends are also correspondingly concave spherical. In various injection molding machines commonly used in Europe, nozzle head is flat, and corresponding sprue bushing head is also flat. Many French companies use tapered nozzle heads for their injection molding machines, and correspondingly, inlet of mold's sprue bushing is also tapered.
For spherical injection molding machine nozzle heads, different countries and companies have different spherical radii. Since spherical radius SR of injection molding machine nozzle head is fixed, to ensure molten plastic completely enters main runner from nozzle without overflowing, concave spherical surface of sprue bushing end face should have good contact with spherical surface of injection molding machine nozzle. Generally, concave spherical radius of sprue bushing is SR2 = SR1 + (1~2) mm, concave spherical depth is L1 = 3~5 mm, and small end diameter d1 of sprue bushing conical hole should be larger than inner diameter d of nozzle, generally d1 = d + (0.5~1) mm, see Figure 5-5.

 

Figure 5-5 Relationship between spherical sprue bushing and injection molding machine nozzle
5. Length of Main Runner: While ensuring product molding, length of main runner should be as short as possible to reduce pressure loss, heat loss, and waste. If main runner is too long, temperature of molten plastic will drop, affecting mold filling. Length L of main runner should ideally not exceed 60 mm.
6. Main Runner Grading: Grading should be avoided as much as possible for main runner. If a joint is necessary due to mold structure, a method as shown in Figure 5-6 should be used, i.e., designing a small sprue bushing. This small sprue bushing does not extend into mold core. Smaller diameter D of conical hole inside mold core should be larger than larger diameter d of conical hole inside sprue bushing, generally D = d + (0.4~0.5) mm, to prevent difficulty in removing solidified material from main runner due to misalignment of two conical holes.

 

Figure 5-6 Small gate sleeve and main channel

1. High-Efficiency Sprue Bushing Design
Main runner is formed within sprue bushing. In today's advocacy of green manufacturing, measures to save raw materials are widely valued; therefore, innovative sprue bushing design is crucial. MISUMI of Japan has designed a high-efficiency sprue bushing. This bushing adds a recessed groove at the opening to store molten plastic, based on a standard sprue bushing. Sprue bushing inlet is no longer determined by injection molding machine, but by size and specifications of plastic product being molded. Therefore, inlet of sprue bushing can be designed to the smallest possible size to achieve cost savings.
2. Effects of High-Efficiency Sprue Bushings
This high-efficiency sprue bushing has significant effects on reducing costs, increasing productivity, reducing molding material (resin tank), and shortening molding cycle. Specific effects are as follows:
① Reduced plastic material in sprue and runner sections. By reducing diameter of sprue bushing and designing runner diameter to minimum size required for part molding, amount of plastic material used in a single molding cycle can be reduced. Practice shows that up to 70% of runner system consumption can be reduced.
② Shortened molding cycle. Due to reduced diameter of sprue bushing and runner, cooling efficiency is improved, thus shortening molding cycle. Cooling time is significantly reduced.
③ Reduced stringing. Reduced sprue bushing inlet diameter reduces cooling and solidification time, thereby reducing stringing.
④ Molten plastic in melt tank can act as a gasket to isolate nozzle, inhibiting plastic leakage.
⑤ Center of molten plastic remains in a semi-molten state, preventing poor demolding of main runner.
⑥ Molding can be performed without removing plastic from melt tank during molding cycle.
3. Comparison of High-Efficiency Sprue Bushings and Ordinary Sprue Bushings
See Table 5-1 for a comparison of high-efficiency and ordinary sprue bushings.
Table 5-1 Comparison of High-Efficiency Sprue Bushings and Ordinary Sprue Bushings

 

4. Precautions for Using High-Efficiency Sprue Bushings
① High-efficiency sprue bushings are only suitable for injection molding machines with a nozzle tip diameter of approximately 3mm.
② Optimal melt tank depth F varies depending on plastic used and molding conditions, as shown in Figure 5-8.

 

Figure 5-8 Selection of melt bath depth for different plastics
③ Due to use of high-efficiency sprue bushings, sprue bushing inlet diameter and runner can be set to be finer, but it is essential to ensure that sprue bushing inlet diameter and runner diameter are sufficient for stable molding.
④ Due to reduced inlet and runner diameters of sprue bushing, flow path for molten plastic becomes narrower. Therefore, molding conditions previously set for traditional sprue bushings are no longer suitable for high-efficiency sprue bushings. Injection pressure and molding temperature must be adjusted accordingly based on type of plastic used.
⑤ Before molding begins, allow injection molding machine nozzle to contact sprue bushing. Preheating for approximately 1 minute is sufficient. If molding is performed without preheating, plastic accumulated in molten pool will cool and solidify prematurely, making it difficult for main runner to exit sprue bushing.
⑥ If nozzle tip temperature is low, plastic at nozzle orifice and in molten pool may solidify before filling mold cavity.
⑦ Do not allow nozzle to retract during molding. Otherwise, injection molding machine nozzle will detach from sprue bushing contact area, causing plastic in molten pool to solidify, preventing normal injection molding.
⑧ If nozzle remains in contact with sprue bushing without normal injection molding, heat from nozzle will cause plastic in melt bath to carbonize and decompose. In this case, plastic in melt bath needs to be removed before molding.
⑨ When molding stops or ends, or when plastic raw material is changed, plastic in melt bath needs to be removed promptly.
⑩ It is easy to remove before plastic is completely solidified. After plastic solidifies, it should be reheated before removal. When removing solidified plastic with tools, be careful not to damage sprue bushing and avoid being burned by molten plastic.
5. Key Parameter Design of High-Efficiency Sprue Bushings
Key parameter of a high-efficiency sprue bushing is depth of melt bath. Quality of molding ultimately depends on depth of melt bath. If melt bath depth is too shallow, front end of runner will not disconnect within melt bath and will be pulled out from nozzle front, resulting in stringing, as shown in Figure 5-9. When melt channel depth F is too deep, plastic in melt channel and solidified material in main runner solidify together, resulting in poor demolding of solidified material in main runner. This material sticks to sprue bushing, affecting continuation of injection molding, as shown in Figure 5-10. Therefore, a shallower melt channel is recommended.

 

Melt channel depth data shown in Figure 5-8 is from experimental data and is used as a reference when selecting melt channel depth in mold design. Different manufacturers and batches of plastic raw materials may have performance differences, and different injection molding machines also have different performance characteristics. Therefore, selection of melt channel depth should be based on actual situation of factory, continuously accumulating experience and forming big data to guide production.

Because main runner is repeatedly rubbed by high-temperature molten plastic and hardened plastic, injection mold must be equipped with a main runner bushing that is easy to disassemble. If nozzle contact part of sprue bushing is not precisely machined to match shape of injection molding machine nozzle tip, plastic raw material will leak out during injection. Furthermore, if sprue bushing does not reach sufficient hardness, nozzle contact part will wear down during repeated contact during injection. Therefore, sprue bushing must be made of high-quality steel and processed and heat-treated separately. When main runner passes through several mold plates, without a sprue bushing, overflow due to gaps between mold plates will prevent solidified sprue from being removed. Sprue bushing is often pushed out by back pressure of molten material in cavity or runner. Therefore, it should be reliably connected to mold panel and act as a stopper.
To facilitate smooth demolding of cooled and solidified runner material from sprue bushing, a tapered hole is provided in sprue bushing. If taper is too large, runner diameter will increase, and cooling time will be correspondingly prolonged. Therefore, appropriate taper must be selected according to type of plastic material and expected molding conditions. In addition, surface of tapered hole should have good surface roughness to avoid undercutting, which would make it difficult for solidified runner material to be removed. Currently, sprue bushings can be selected according to standards and do not need to be custom-made.