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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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 Balance Layout for 6 cavity of mould

Picture below shown layout model for 6 cavity product, star layout used to connecting 3 cavity, then connected again with straight line. distance between one cavity to another is same

Top view


3D view


becouse flow at nearest sprue is faster than other, although the distance from the branch to gate is same, the flow of plastic will not good, nearest sprue will fill first and not same with another.

Improved runner layout
so we need another layout, below is improved runner layout for 6 product cavity, although is not the best, but it's more improved than first. but disadvantage of this runner layout is consume more material.

Top view


3D view

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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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Types of Runner depending temperature

there are various concept and type of runner, here we will give explain about runner type depending on the temperature control, we could divide in 3 type :
1. Standard runner system
2. Cold runner system
3. Hot runner system

standard runner system are directly machined into the mold plates (plates 2 or plate 3) but we can also make in special plate that called runner plate, this plate usually use in 3 plate mold type and attached in plate three(cavity plate)
at this picture below shown that runner directly machined in plate three


but picture below shown special plate called runner plate, those runner plate will attached in plate three, you can see from this plate that mold will have 4 cavity od product.


Cold runner system
this runner system have similar use with hot runner, but in different temperature, the temperature between 80-120 C, this Cold runner system used in reactive material such us rubber and thermosets type of plastic.
the difficulties in Cold runner system is
- high pressure consumption, so design more expensive
- because material is reactive, a little different temperature make different viscosities, this will make every cavity have different time to full fill.

Hot Runner System
this system should be viewed as extended nozzle in form of block, the hot runner system contains : sprue bush, runners, gate and extended nozzle, the advantage of hot runner is
- they can completely eliminate runner scrap, so there are no runners to sort from the parts,
- no runners to throw away or regrind and remix into the original material.
- no loss of melt and thus less energy and work input
- easier for fully automatic operation
- very good quality because melt can be transfered into the cavity at the optimum site

the disadvantages are :
- high cost
- thermal isolation from the hot runner manifold block is problematic

Hot runners are popular in high production parts, especially with a lot of cavities

there is 3 type of Hot Runner are :
1. the insulated hot runner,
Insulated runner molds have oversized passages formed in the mold plate
Heated type Hot runner
2. the internally heated hot-runner system,
3. the externally heated hot-runner system
internally heated and externally heated. The first is characterized by internally heated, with the heat being furnished by a probe and torpedo located in the passages. This system takes advantage of the insulating effect of the plastic melt to reduce heat transfer (loss) to the rest of the mold. The second consists of a cartridge-heated manifold with interior flow passages, look at picture below to learn more

this picture take from http://www.scudc.scu.edu

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Runner System and Construction

picture below show a simple runner system in mold construction, begin from nozzle until gate, nozzle for mold usually located in plate 1, plate 1 is upper plate,
when you design a runner system, basically like picture below, you must know location of product, is important when we decide where gate to be placed, here is the basic rule :
1. Look at drawing and 3D CAD data from the product, make sure you was choose where is cavity side and where is core side, give attention for tolerance of product
2. Do not locate gate in the precision tolerance, if is must to make a little grove in the product when you choose pin point gate type, Picture below use pin point gate type.
3. make sure the flow will steady for all side

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