Circular H type Runner Layout in Mold Design

It’s called H type because the runner shape from the top similar with “H”, it’ use in both circular layout and rectangle layout system, in circular type the runner doesn’t similar with H but it more similar with fork, circular type use in various type of product, like gear, cylinder shape, etc.
Total length of H circular type in each part of runner
8.801 x 16 = 140.816
7.491 x 8 = 119.856
18.83 x 8 = 150.64
Total length of runner in H circular type runner system
140.816 + 119.856 +150.64 = 410.912
Let’s compare with previous type runner system, direct runner system both circular and rectangle type,
Direct circular type = 448 mm
Direct rectangle type = 495.04 mm
H circular type = 410.912 mm
So the conclusion is from three example runner systems for these part and layout type, H circular type has the minimum length of runner,


Advantage
1.Flow length in the runner more stabile, cause we can add runner slug before branched the runner, see picture above
2.When designing the runner, runner volume is made small as possible, this type is smallest than two type runner before.
3.All the cavities fill at the same time possibility is high, balanced runner have an equal flow length and runner size form sprue to all cavities also same, so that each cavity can fill in nearly condition.

Read More

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

Read More

Fill time runner balance on 6 cavity of mold

runner balance is important when we works with more than 1 cavity in mold design, unbalancing runner can cause various like unfill mold (short shot), weld line, air traps, burning and others.

from previous post that talk about design of runner balance in 6 cavity, now let's prove it by simulation of filling time.

Unbalance Flow Definition
Unbalanced flow is plastic completely filling some flow paths in the mold before other flow paths have filled. we can see from result of simulation, if color is same in cavity (for example at the end and gate) that mean it have good flow.

1. Unbalance Runner System


from picture above we can consider that flow of plastic in various cavity doesn't fill in the same time, it can cause flashing, short shots, high cycle time, density differences throughout the part, warpage, air traps and extra weld lines.
flashing will occur in cavity that fill earlier, becouse cavity that fill earlier will continue receive plastic flow until other cavity filled (it depend on injection pressure and cycle time set up).
Short shots happened at cavity number that fill more slowly then others cavity then the pressure and cycle time doesn't enough to fill the last cavity.

2. Balance Runner of 6 cavity
although this runner layout is not the best for 6 cavity layout, it better in fill time than runner layout before. look at the picture below


by changing the layout we can get runner that more balance with previous, from cavity number 1 until 6 the fill time is almost same.

Read More

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

Read More

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.

Read More