Sub runner is material flow channel between main runner and gate. It is transition section where plastic melt flows from main runner into cavity and is responsible for smoothly changing flow direction of melt. In multi-cavity mold, it plays role of distributing melt to each cavity. Generally, no sub runners are set in single cavity molds

1) With the shortest distance and the smallest heat pressure loss, melt can be quickly and smoothly input into cavity.
2) Melt must be able to feed into cavity from each gate at the same time under same temperature and pressure conditions.
3) Although larger cross section of runner is conducive to filling mold and ensuring sufficient holding pressure, from perspective of saving plastic materials, cross-sectional area should be as small as possible, and larger cross-sectional area will increase cooling time.
4) In order to save materials and facilitate cooling, surface area to volume ratio of runner should be as small as possible.

Shape and size of runners mainly depend on size of product, mold structure, and type of plastic processed. Generally speaking, with increase of product size and wall thickness, since resistance of melt flowing in large cross-section runner is smaller than that in small runner, large cross-section runner can promote filling process of mold more. If runner is long, process is long and viscosity of plastic should be smaller. Cross-sectional shape of runner is shown in Figure 1, and Table 1 shows recommended diameters of commonly used plastic runners.

 

 

There are two types of shunt layouts, balanced and unbalanced. Balanced layout requires that length, shape, and cross-sectional dimensions of runners from main runner to each cavity must be correspondingly equal to achieve thermal balance and plastic flow balance of each cavity, as shown in Figure 2. Using non-balanced layout, plastic enters each cavity first and later, and time for each cavity to fill is different. Products produced by molding of each cavity are quite different, but cavity layout can be more compact and mold plate size can be reduced, total length of runner is shortened. When unbalanced arrangement is adopted as shown in Figure 3, in order to achieve purpose of filling each cavity at the same time, gate can be designed in different sizes.

 

 

Surface roughness value of sub runner should not be too low to prevent cold material from being brought into cavity. Generally, Ra value is required to be 1.6 μm.

Sub runner and gate are usually connected by inclined planes and arcs, which is conducive to flow and filling of plastic melt, reduces flow resistance.

Main runner refers to a section of material flow channel from contact point of injection machine nozzle with mold to sub runner, which is responsible for introducing plastic melt into mold from nozzle. In order to facilitate pulling out of aggregate from sprue, sprue is usually designed in a conical shape (as shown in Figure 1), with a cone angle α=2°～4°, and 6°～10° for plastics with poor melt fluidity, surface roughness of inner wall is generally Ra 0.8 μm. Usually diameter of inlet end of main runner is 4-8 mm. If melt fluidity is good and product is small, smaller value can be used, and larger value can be used on the contrary. Table 1 lists recommended values of cross-sectional diameter of main runner.

 

 
Figure 1 Shape and size of main runner

 

Note: D1 is diameter of inlet end; D2 is diameter of outlet end.

Due to repeated contact between sprue and plastic melt, repeated collisions between inlet and nozzle, sprue is often designed as a detachable sprue bushing, which is made of better steel and heat treated. Generally, it is made of T8 and T10, heat treatment hardness is 52～56 HRC. Bushing mouth should be made into a spherical concave, spherical radius should be 1~2 mm larger than spherical radius of nozzle head, recessed depth should be 3~5 mm, and inlet diameter should be 0.5~1 mm larger than nozzle aperture. Bushing mouth should have a taper, generally taper is 4°～7°, and outlet should also be rounded to make material flow smoothly. Fit between main runner bushing and mold plate can be H7/k6. Small mold can design sprue bushing and positioning ring into a whole. Structure of main runner bushing and positioning ring is shown in Figure 2.

 

1. Positioning ring 2. Main runner bushing 3. Fixed mold seat plate 4. Fixed mold plate
Figure 2 Main runner bushing and positioning ring