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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Mold Cavity Core Construction for Sleeve Ejector System

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.

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Sleeve ejector system

This system use when part have tolerance or good dimension so, so we can’t put ejector in those face, or ejector prohibited in that face other reason is part have hole and face that can touch with ejector is very small and surrounding the hole. Look at picture below

Red face indicated that ejector is prohibited, for that reason the only way to eject those part is using sleeve ejector that will located at green face (look at picture)

Basic construction of sleeve ejector system is like picture below

 

How Sleeve Ejector Works

Basic mechanism of sleeve ejector is how to make insert pin doesn’t move when sleeve ejector move to eject the product.

Picture above shown step by step when sleeve ejector moving to eject part, the main part on those picture are product (yellow) sleeve ejector (green), core insert pin (white at center of sleeve ejector) mold base core (light blue at side sleeve core), cavity insert (dark blue), cavity pin insert (pink at center of dark blue), and cavity mold base (light blue surrounding cavity insert pin.

A.     After plastic material injected at cavity insert then cooling water/liquid will flow to transfer heat from mold to liquid. At this position mold base still close, just seconds before ejecting process begin.

B.     Ejecting process begin, first mold will open, core insert include cavity insert will move to backward, because shrinkage of material always move to inner side, product will stick in core insert pin.

C.     Sleeve core move forward when ejecting process begin, sleeve core will eject the product, and the product will leave core insert pin, because core insert pin doesn’t move forward.

After sleeve core move same as ejector stroke, product will drop, and.. Gotcha.. you get the product.

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Stripper Ejector Construction (step by step mold design) part 3

after make insert like in posting number 2, let's make main construction of ejector stripper plate, simple ejector stripper plate consist of three main part
1. ejector plate
2. bolts for clamp
3. ejector pin

those plate main function is to push product when ejecting, isn't need much part of plate surface that contact with bowl surface, only 0,8 mm product will able to eject from core insert side.


clearance and dimension tolerance
below is picture and some explanation where and when we set the clearance between insert part.


1. between insert core and core block, fit tolerance about 0.010 mm with height about 15mm from parting line, and after that make clearance about 0,5 until 2 mm.
2. between stripper and core block, give it free tolerance about 0.5 mm each side
3, between stripper plate and insert core, give it fit tolerance about 0.005 mm until 0.010 mm, it to prevent plastic product flash.
4. between ejector pin bolts and stripper plate, give it free tolerance from 0.5 mm until 1 mm.
5. ejector core pin and core block, give it fit tolerance about 0.010 in top side with height 10 mm, after that give free clearance about 0.5 mm in each side.

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Step by Step Mold Design (part 2), Insert Design

continuing with the previous post, now let's make some insert in core side, this insert very simple and easy in machining process. from the core block side we divide at the circle line of side of bowl, why ?, because we will eject the product using stripper plate, stripper plate construction will eject the product by touching the thin face in all around of bowl.

insert function
there are two function in of insert in this mold construction
1. to provide re change part, if the bowl shape doesn't good, we don't need to change all core side, but only the insert part. so this system is more cheaper when the part need good shape and good tolerance.
2. provide gas vent, and stripper construction, because the product need gas vent in bottom the side, and gas vent in stripper is not good, gas vent only can make with insert in core.

below is construction of insert part


Gas Vent Dimension and construction
gas vent can divide into 2 part, the accurate tolerance and not accurate tolerance part, the dimension of accurate tolerance is depending from plastic material that injected in molding, but generally is about 0,005 mm until 0,020 mm


Collar
at the end of insert part, there was a collar to prevent part move in Z axis, the dimension of collar is about diameter + 2mm and height is 4 mm

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step by step mold design of ejector plate type

PRODUCT
by sample complate design of bowl i'll try to explain easily how to design a mold, start with simple product a bowl like picture below


by looking the bowl, we can consider that bowl only need core cavity side without undercut release.

PARTING LINE
choosing the right parting line is important, parting line will separate core side and cavity side, becouse we only need core and cavity, the parting line is only one, between cavity and core, to make it, first,make the block then from subtract the product and split it by parting line that you define before


some CAD software also have instant cavity and core making feature, split and sweep feature so make the mold is easy by using those feature, after split it into cavity and core we have cavity side and core side of the mold.

Cavity Side

and below picture is core side


now we have cavity and core side, it still rough, without bolt, without gate, ejector and other part, the next step is make some insert part.

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