Stripper Ejector Construction (step by step mold design) part 8 : add return pin

In ejector system, return pin is important, i was post about ejector system, include the function of each part, see it at this link, previous post is adding ejector pin and ejector guide bush in the mold, now next step is adding return pin, when we use mold standard or mold base blank, return pin always include in those mold base blank, if you have the catalog, return pin symbol is always RP, with dimension from X direction and Y direction, see picture below, yellow pin is return pin.



picture below is standard mold base, i get from Lun Kee mold base standard we can show dimension of mold base standard 150 mm x 150 mm with flange 200 mm, return pin diameter 12 mm, with the position 57 in Y direction from 0,0. and 24 in X direction, see the green line at picture below



add the return pin between ejector plate, and ejector stripper plate, the height of return pin same with surface of core plate. make hole in Ej. plate and core plate. like picture below



Spring for Return Pin
this spring will help move back ejector plate and ejector stripper plate after ejecting process, by calculate load of ejector system we can choose the best spring for return pin, too hard spring will difficult to assembly, but too weak spring will give some problem in ejecting process, this spring also help to prevent ejector stuck after ejecting process. see the construction like picture below
some mold designer prefer make hole of spring until half of core plate. which one do you use it depend on load calculation of spring.



Next step are add ejector rod and support pillar
To prevent large deflection in center of cavity plate, we must add support pillar as much as we can, arrange it so the deflection of cavity mold when plastic injecting is minimum, deflection can make gap between cavity plate and core plate, so flashing maybe appear, maximum allowable deflection depend on material, but generally it about 20-30 micron.
In this case I put 4 support pillar symmetrically

To move ejector system, we must add ejector rods, in little mold generally use only 1 pcs of ejector rods until 80 ton still save use only 1 pcs. but if your construction more than 100 ton or mold base size more than 400 mm x 400 mm use more than one pcs of ejector rods.
picture below shown 1 pcs construction of ejector rods.

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Stripper Ejector Construction (step by step mold design) part 7

Ejector Guide Pin and Bush
in this part, we will design ejector guide pin, EGP bush, bolts for stripper plate and bottom plate. first learn ejector system and part at link below

3D ejector drawing

hope you can understand ejector system and ejector guide pin (EGP) function, Ejector guide pin located between ejector plate and ejector stripper plate, to fit the ejector pin, we can add bolt at the bottom of EGP, and clamp it in bottom plate of mold



Bolt for spacer block
To make position of spacer block fit between backing support plate or core plate and bottom plate, add bolts from bottom plate to spacer block, see picture below for more detail. In each spacer block, add 2 pcs of bolts, the other function is when disassembling process spacer block and bottom plate became one part, it make dissembling easier




Ejector Puller Bolts
to push the stripper plate when ejecting proses begin, we need connector from ejector plate to stripper plate at core,in this case, i use ejector puller bolts, you also can use other standard part,just make sure that you can connect ejector plate to stripper plate at core side. see picture below for detail construction.

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Stripper Ejector Construction (step by step mold design) part 6

now, after put sprue bush and locating ring at top plate, let's design core side, like cavity block, core block also need bolts to attach the core block to core plate, see picture below


at those picture i use M10 bolts type, this bolts use to attach the core block to mold base plate, don't forget to give chamfer of each side of core bolts, this chamfer very useful in assembly mold processing, this chamfer will become guide of core block when assembling to mold base plate.
for safety reason, you must also consider chamfer for other side to prevent scratch accident of operator when doing assembly process.



at last make packet at mold core plate, at those pocket give some Radius for machining process, this radius is must equal or more bigger than milling cutter radius that you want use in machining process, commonly more deep your pocket more large the radius of milling cutter.

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Stripper Ejector Construction (step by step mold design) part 5

next step let's put sprue bush at top plate until directly to product, beside sprue bush i put locating ring, this two part important become guide between mold and injection machine, sprue bush will guide molded plastic to runner, because this product use direct gate, runner also direct from runner to product. see picture below for detail



Locating Ring Type
i use 120 mm diameter of locating ring, with height 15 mm, from that height, 5 mm i put on moldbase, and 10 mm become guide when mold base attach to injection mold, 3D picture below will give briefly image of locating ring.



Sprue Bush

What is Gate ?
The entrance through which molten plastic flows into the cavity of the mold from the runner. This is where the flow or melting of molten plastic is controlled by control type of gate or gate dimension. It is positioned at the thickest area of a part.
other function of gate is the gate makes it easier for the runner, which will be unnecessary after molding, to be separated from the part.is recommended to consider gating options early in part design.

Direct Gate
direct gate or sprue gate we can define as connecting directly end sprue to the molding and used as a gate,
this type of gate is commonly used for single cavity molds, where sprue feeds the material to cavity molds directly rapidly with minimum pressure drop.
see more explanation of direct gate at link below
http://mould-technology.blogspot.com/2008/02/direct-gate-or-sprue-gate.html

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Stripper Ejector Construction (step by step mold design) part 4

on the step below we design bolts position to hold the cavity insert to top plate of mold base, i use M10, or you can use other bolts base on your strength calculation and available market, there are various bolts type available on the market, i prefer use socket head screw in mold design, see link below for reference about this type of bolts
http://www.engineersedge.com/iso_socket_head_screw.htm

Picture below shown Socket Cap Head Screw.























picture below shown cavity main insert and bolts position, i attach the bolts in 4 side symmetrically,



for easy machining process, don't forget this little tips but very important when designing mold, at corner side of pocket make sure you make Radius or half circle, so pocket can be process use in MC easily. i use half circle type in each corner.



Chamfer in Insert
to prevent accident when assembly process, safety reason and easy assembly of mold we need to make little chamfer in each edge of mold, with chamfer it also became guidance when assembly the cavity insert to mold base. see picture below for detail picture, i use C5 in symmetric chamfer and 3-10 in asymmetric chamfer.

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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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