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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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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