Injection Mold Design Tutorial, Technology and Engineering

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.

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

continuing from previous post about sleeve ejector system, below the explanation of cavity and core constructions

Cavity Construction
Basically cavity consist of : cavity main insert, and cavity insert, for this product, we use cavity main insert shape like pin, and to release air trap, we put cavity insert pin (pink color) with gas vent land. Gate located at cavity main insert that one position with parting line

Gas vent construction
At this mold type contractions gas vent very important, to prevent product burning and unfill (shot mold), see at those picture between gate position and top product distance is to high, base on my experience more than 10 mm plastic material can not fill until top of part (shot mold), although it depend on thickness of product, this product have thickness only 0.5 mm in each side, they are various ways to make gas vent for pin construction, picture below shown the other one.
At the top of product we cut to make gas vent land, give distance about 5-10 micro mm, see picture below, P1 as pin length have maximum length 19.992 mm (orange color) mm and P2 hole at the grey pin have length 20.000 mm (grey color), so distance between other is only 8 micro mm, gas can exit with this distance but plastic material can’t fill at this distance, almost plastic material in injection process, can fill if 20 mm space available, it cause plastic product defect that called flash.



























other picture for gas vent land and groove construction



At second picture, gas vent groove release at cavity main insert pin, gas vent groove depth is about 0.1 – 0.5 mm. when designing gas vent the important point is make sure that gas can release to out of mold.

Cavity insert

The main part of cavity insert is only cavity pin, because this product can be formed only by pin at the cavity side (see picture below), cavity pin located at center of product (grey color), product color is yellow, don’t forget to make some taper at this pin.