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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How to Calculate Clamping force

as we know clamping unit purpose is to keep mold closed against force when part injected with plastic material, because of that clamping force at least more than injection force, we can applied this clamp pressure both in injection machine with hydraulic or with mechanical toggle.

Injection molding machines are usually characterized by the tonnage of the clamping force that they provide. The clamping force, indicates the amount of force that the clamping unit can apply to the mold to keep it securely closed during the injection of the molten plastic, picture below shown injection machine from Nissei with type NEX80 (80 ton)



formula
the clamping force is proportional to the projected area of the moulding and runner, and must be opposed by the clamping force. although a proportion of the pressure produced by the injection cylinder is transmitted to the cavity, various losses occurring in the heating cylinder, nozzle and gate we can consider that force acts on the mold to make it open can be calculated from the following formula:

Force (lb.) = Pressure (lb/in2) x Projected Area (in2)

what is projected area ? projected area is area that one line with mold base parting line, for complete calculation, calculate the runner projected area and part projected area is more precision. see picture below to get the imagine og projected area


Clamping force calculator
below are clamping force calculator using java script, the script i get from http://www.custompartnet.com, with little modification so it's more suitable with blogger post page.

Clamping Force

Number of cavities: Part projected area (in²): Runner projected area (in²): Shot projected area (in²): Cavity pressure (psi): Separating force (tons): Factor of safety: Clamping force (tons):

this picture below also have good visualization to explain projected area, clamping force and injection pressure


if you prefer see original calculator go to this webpage Clamping force calculator
but why i set safety factor 0.8, the result will more lower than without safety factor, in clamping force those formula is before correction for pressure drop and other, so the injection pressure always more lower than those manual calculation.

or we can use graph or table to define clamping pressure, like this one from niir.org

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