Runner Design in Injection Mold,

in designing the runner, first-time we must consider items that affecting  runnersize, some of which
- Volume of part, the greater the volume takes a larger diameter runners anyway.
- Weight of part, the heavier parts, it takes a larger diameter runners.
- Plastic materials, plastic materials with low viscosity, aqueous requires a smallerdiameter runner.
- Wall thickness, the thicker the greater the diameter of the runner is needed.
- Length of flow path, the longer it takes the flow of large diameter runner.

picture above shown some factor that we must consider before choosing right diameter of runner, 

Cross Section Area

various cross section area use on the market, It depends on the heat loss, machining requirement, but the most popular cross section area are circular, parabolic and trapezoid section area, picture below shows three

Diameter

to determine the diameter, based on the type of material and wall thickness, the following chart can help, for similar materials PP, PA, POM, PC, PE can use the following graph, first select the appropriate thick wall thickness and look heavy parts, and then pull the line down, the approximate diameter required will be obtained

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runner components of injection mold

Runner components between the two mold plate and three plate mold because there are different functions of these components, but there are several components that can be used in general without looking at the type of mold construction.

1.Runner lock pin
The main function of the runner lock pin (RLP) is an pulling runner so
the gate can be disconnected/cut from the product, RLP fitted with a straight gate position, the head planted on the top plate, while in the body mounted on the cavity plate (3) up to the top
plate (1). RLP does not move during the injection process lasts, mold base plate
moves accordance with the mold opening sequence, the tip of the runner lock pin is the part that greatly affects the success or failure of the gate runner cutting process. Greater the undercut which is the stronger the ability of RLP to attract runners from the gate, the type of plastic material also influenced for selection of RLP.
2.RLP Lock
RLP lock have function to maintain RLP order not to loose from its position, there are various ways to lock them RLP
* closed with a locating ring or sprue bush or heat insulation board.
* using a plate
* use a screw plug
3. Bushing RLP
high cycle time mold or need durability are advised to use the bushing,the benefits include easy maintenance of RLP, more resistant to wear due to friction so it's can avoid direct contact between RLP and mold plate that can caused wears, this method will shorten maintenance time.
4. Ejector runner
Only used if the runners stuck on the stripper plate mold base or stuck on the cavity mold bases, the use of the runner ejector can help the runner release from the place that was not supposed to stick. Other components are collar to the head of RLP, and change flow components

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direct circular runner system and direct rectangle runner system in mold design

continuing previous post about sleeve ejector system in mold, now let's learn how to choose runner layout, and lay outing the cavity and core,

From picture above space that available for layouting cavity core is 40 mm X 40 mm, from those available space we must design effective and cheap runner cost, effective design for runner mean that total runner length designed as short as possible, and runner, product weight ratio is small, so it’s your product will contain only short runner, that can reduce cost of material. Here some layout that I tries to make from available space 40 mm X 40 mm.

Direct Circular Type
This type is easy to layout, because we only design for one runner, straight between center of sprue bush to gate position, then copy those runner base on amount cavity that can be layout, from space that available 40 mm x 40 mm, we can arrange the cavity until 16, so the angle between one cavity and other is 360 / 16 = 22.5, to arrange more detail copy and rotate your runner design with those angel (22.5) until you get 16 runner, each runner length is 28 mm, so total length of runner are 28 x 16 = 448 cm.
See picture below for detail construction and dimension of circular runner system for this mold



Top view


Direct rectangle type
Simple way to layout cavity core and runner is using direct rectangle, this type common use in imprecision part, 8 cavity will have length 28.6 mm from center sprue bush, and the 8 cavity others will have length 33.28 mm, see picture below for detail sketch of runner. From this condition total length of runner are (28.6 x 8) + (33.28 x 8) = 228.8+266.24=495.04 mm, this runner type have 47.04 longer than previous type.
The other’s disadvantage of this runner type is flow material will not same to all cavity, because distance between cavity and center of sprue bush for all cavity have 2 type, first type have 28.6 mm, second type have 33.28 mm in length, if the injection pressure too high or injection volume of material too much, it possibly flash at the first type (28.6) cavity. And if volume injection doesn’t enough shot mold possibly appear at second type cavity (33.28)



so, in this case, circular type of runner layout still the best to be implemented, how about the other runner layout type, such us H type, let's see on the next post

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Runner system, flowing part of plastic injection

After material injected by nozzle, runner systems will convey the molten material from the sprue to the gate. basic runner system consist of sprue, main runner, sub runner, cold slug, and gate. since the design of the runner system has large effect on molding quality, molding efficiency, cycle time, cooling time and other factor, it is important key when design of mold.

Runner System Construction

Sprue
sprue will contact first time with melt plastic, at the injection process, sprue always contact with nozzle from the injection machine, so consider when design the sprue consider the ball radius of nozzle, make sure it isn't make undercut on sprue.The dimensions of the sprue depend primarily on the dimensions of the molded part in particular the wall thickness, too little sprue will make sprue cold faster and make insufficient flow.





Main Runner
First time when design runner system, determine number of cavities, shape and gate types, then arrange the cavity layout,at the multi cavity of mold balancing of runner is important, balancing ensures virtually equal flow of plastic trough each gate.
To achieve balancing runner layout must be though,

Sub Runner
this part will continues flow from main runner until the gate, the dimension of sub runner and how much sub runner system will be, is very depending on how much cavity of the mold, and how the layout, what the layout use herring bone, H type, or star type. at the picture above use H type runner layout.

Cold Slug
when runner layout bent, clod slug may be required to continue plastic flow of better quality to the cavities.The cold slug well helps the flow of material through the runner system by stopping colder, higher viscosity material moving at the forefront of the molten mass entering into the cavity.The length of the well is usually equal to or greater about 1,5-2 time than the runner diameter.

Gate
Basic purpose of the gate is to limit the flow of the melted plastic, but it must allow that melted plastic flow and fill the cavity easily.
considering the fluidity of the material, gate should be made near the center of gravity of molding or the line to make flow uniform and melted material even, then make the gate thick but still considering melted plastic material, becouse thin part will cooled easily and faster.
the gate type is very various depend on part shape, mold layout, mold system, etc, here some explanation about design those gate type:
1. Direct gate
2. Side, edge gate
3. Tab gate
4. film gate
5. fan gate
6. Disc gate

Runner geometry
they are various geometry that we can use in runner, such us
1. full round
2. half round
3. trapezoid
use full round and trapezoid shape is recommended in various mold. Half round runners are not recommended because of their low volume to surface ratio
here some area calculation of cross section of runner (source : http://www.dsm.com)


Runner dimensions
The diameter of a runner depends on its length in addition to the part volume, part flow length, injection machine capacity, and gate size, Generally runner must never be smaller than the largest wall thickness of the product, it's usually make from 3 mm until 15 mm. picture below shown relation between runner diameter and maximum runner length (source : http://www.dsm.com)

The runner should be large enough to minimize pressure loss, yet small enough to maintain satisfactory cycle time, Large runners are not economical because of the amount of energy that goes into forming, and then regrinding the material that solidifies within them.

Runner layout
In general they are three type of runner layout
1. "H" bridge (branching) runner system
2. Standard (herringbone) runner system (conventional runner)
3. Radial (star) runner system
look at picture below shown standard runner and H bridge type runner.

(Source: http://www.hotmold.cn)
and below is star layout runner system (both bottom picture)
(source : http://www.dsm.com)
Balancing the runner system ensures that all mold cavities fill at the same rate and pressure, if the product have not similar product geometry, it is recommended to use mold analysis software like mold flow or C-Mold.

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