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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shringkage of mold calculation, mold calculator

as the request, lot of people need engineering and calculation of injection mold and mold design, so i try to make little software, the name is mold calculator, it's freeware so download it and you can publish it, now it still under development, in this beta version this software only provided various shrinkage formula.

what is shrinkage ?
Shrinkage is inherent in the injection molding process. Shrinkage occurs because the density of polymer varies from the processing temperature to the ambient temperature
There are linear and volumetric shrinkage. Both methods are difficult to measure accurately.
see more information about shrinkage and what the main thing that affecting shrinkage in http://plastics-engineering.blogspot.com/search/label/shrinkage

Relation Between Injection parameter setting and shrinkage
picture below shown relation between shrinkage and injection machine parameter.


Mold Calculator software beta 1
calculating shrinkage is basic when designing mold, so
i try to make little software, more than 4 year never learn or make some program again, then now i still learn .net frame work, as the practice i try make this software, becouse I'm not professional software designer please don't hesitate to contact me, give some advice both in programming and mold calculation.

this software is still under development, on this version you can calculate, shrinkage value, dimension after and before shrinkage, and exact shrinkage scale to input in 3D modeling software.

to download those mold calculator, i have 2 version, In Indonesian language version and English language version, download at

English version

Indonesian version

you also need download .net framework 3.5 from Microsoft

Basic Formula of Shrinkage

D' = D x(1+shr)
with
D' is dimension after shrinkage
D is dimension before shrinkage
shr is shrinkage (in persen %)

so to calculate shrinkage we can use formula

shr = (D'/D)-1

from this basic formula we can develop by substitute with other formula

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3D and Drawing ejector simple construction in mold with video

simple and cost effectively ejector system i try to design, it using very minimum part, using this ejector system just have 3 ejector pin, by understanding minimum part to make ejector system, we can develops more complex ejector system. see picture below



this ejector system is same with previous post, i just completely the picture with isometric picture, drawing sample and 3D movie, so i hope we could more understand in ejector system. below is drafting picture of those 3D picture



little different from previous post, this ejector system use long ejector rod, ejector rods type is very depend the type of machine, stack mold, basic injection mold have different ejector rod type, although the basic system is same.



video above shown simple 3D model of ejector system, at those video is not include bolts that connected ejector plate and ejector baking plate, also bolts that connected bottom plate to core plate.
i hope by using those video is more easy to learn mold design.

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Ejector Component List for ejector pin type

Each type of mold have their own ejector system characteristics, such as : one step ejector, two step ejector, stripper ejector system, pin and blade ejector system, gas ejector, and vacum ejector. from various ejector system that available all of that type have main part or main component. picture below shown common and main component for ejector system. this image size about 700X500 px, click the picture to enlarge.



picture above shown side view of ejector system, the target is to eject the product (black) from core insert after mold open. when the ejector system work and push the product to outer parting line the side view picture is like below



Ejector working system
with above system, how could the ejector push the product? below i will try to explain step by step
1. after cavity of mold injected with plastic material, and cooled moldbase will open, and product have to eject from the core insert.see the open condition of molds.
2. after mold base in open condition like picture below

rod from injection machine will push the ejector rod,because ejector rod connected to ejector plate, ejector plate will move forward.
3. ejector guide pin will guide those moving, to prevent scratch and guide the moving more easily use ejector guide bush, ejector pin and ejector guide bush will have little friction.
4. ejector pin clamped between two ejector plate, because that ejector pin will also move forward push the product.
5. after product take it out from cavity, mold base will moving close again.
6. and because at the return pin have spring, spring and closing mold will guide ejector back to the position (1)

Ejector Component List
this list is standard minimum to make ejector system
1. Ejector plate, place for ejector and ejector guide bush, shoulder bolts and return pin.
2. Ejector Backing Plate, place for clamping the ejector and push back the ejector.
3. return pin, push the ejector plate and ejector backing plate to first position.
4. spring in return pin, to make sure and holds the ejector plate so it's always in back position, when in assembling process, it can lift up the ejector pin, and ejector plate so the position always in back, give the little preloads to ejector plate to make sure the ejector plate position.
5. shoulder bolts, first time lock in assembling molds
6. ejector pin, eject and push the product from core.
7. ejector guide pin, guide the moving of ejector plate.
8. ejector guide pin bush, to make sliding and prevent large contact when moving.
9. support pillar, to prevent large deflection of core plate when injecting the plastic material process held.
10. ejector rod, contact with rod in injection machine, this also passing force from injection machine to ejector plate.
11. ejector bolts, to tie up between ejector plate and ejector backing plate.

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