injection molding calculation

8/10/2019 Injection Molding Calculation

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Introduction

When injection mold is fixed in a molding

machine, and molten polymer is injected into

the cavity from a nozzle, a high filling pressure

acts on the inside surface of the cavity.

Since the parting surfaces of the mold try to

expand outward due to this pressure, it is

necessary to clamp the mold so that it doesnot open instantaneously.

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The force of keeping the mold closed tightly iscalled required mold Clamping Force.

The unit for the force is ton (tonnage) or N(Newtons).

At the time designing a new mold, it is

necessary to obtain by theoretical calculationswhat is the optimum required mold clampingforce that the IM machine has to have for themold to be installed in it.

Example of required mold clamping force of112 tons was obtained by calculations, thenthe mold is installed in 80 tons capacity IM

machine, the molded part will be full of flash. 3


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Further, if the mold is installed in a molding

machine with a 350 tons capacity, even if the

molding operation is possible, the molding

operation becomes high in cost due to hourly

cost for 350 tons machine is higher than that

of a 80 tons machine.

Injection pressure (p) value in the range of 340

to 560 tons/inch2.

The value of pvaries depending on the type ofplastic, molded item wall thickness, cavity

surface temp., molding condition, etc.

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A projected area (A) of the cavity and the

runner with respect to the parting surface.Therefore, the value of Avaries depending on

the number of items molded and on the

placement of the runner.

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Clamping tonnage/force

Clamp tonnage or clamping force defines fromthe clamping mechanism which keep the moldclosed during injection.

Fig 1: Direction of clamping tonnage for clamping mechanism

Direction of clamping tonnage


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Clamp tonnage requirements are determined bythe melt flow index (MFI) or viscosity of thematerial being molded.

The melt flow index (MFI) is a measure of theease of flow of the melt of a thermoplasticpolymer.

It is defined as the mass of polymer, in grams,

flowing in 10 min through a capillary of a specificdiameter and length by a pressure applied viaprescribed alternative gravimetric weightsalternative prescribed temp.


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The method is described in the similar

standards ASTM D1238 and ISO1133.

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Melt Flow Index apparatus (MFI)


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Barrel

Insulation

Polymer Melt

Die

Thermometer inside

an oil filled well

Weight

Avg. weight of extruded

plastic in 10 mins.



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The bigger the value - the easier the flow.

Values can reflect relative chain lengths.

Low shear (not representative of injection moulding)

Spiral flow test better.

Used as a quality control for incoming material.

Specific test conditions must be used.


These flow index rating will have between 1 to 20tons/inch2.


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After the viscosity of the

intended plastic is

understood, projectedarea must be estimated,

against which the

injection pressure and

clamp tonnage will be

directed.

Visualizing the shadowof the part as if it was

viewed with a strong

light behind it does this.


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Determine required Clamp Force

=

Projected area of the part can be found based on the partgeometry.

Part thickness is important only if it is more than 1 inch(25.4 mm).

For every inch of thickness over 1 inch, the total clamp forcemust be increased by 10 %.

The injection pressure must vary with the materials flowability.

Typical value for injection pressure is between 2 to 8 tons perinch2(or 27.6 to 110.3 MPa)

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As rule of thumb, 4 or 5 tons/inch2(55 to 68.9

MPa) can be used for most materials.

Too low a clamping force can lead to flash or

non-filled parts.

Too high a clamping force can lead to mold

damage.

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The projected area is multiplied by a factor

(injection pressure) from 15 tons/inch2

(13.868.9 MPa) to determine the total

amount of clamp tonnage required. The exactnumber depends on material viscosity, with

the easy flowing materials needing only 13

tons/inch2(13.841.4 MPa). Stiffer materials needing between 2.55

tons/inch2(34.568.9 MPa).


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The following is an example of how to

determine proper clamp tonnage in U.S.

customary units.

Example 1:

The part to be molded is a square plague that is 6

x 6 inch and 0.060 inch thick. Determine the

clamp tonnage for both molding material which isNylon and Polycarbonate. Nylon factor: 2

tons/inch2& Polycarbonate factor: 5 tons/inch2.


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After, the total amount of tonnage is

determined, 10 % should be added as a safety

factor, and a machine that provides clamp

tonnage closest to that number should beused.


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For instance, in the polycarbonate case noted

earlier, there was a need for 180 tons clamp

tonnage.

Adding 10 % safety factor brings that number

to 198 tons. Ideally, a 200 ton machine would

be used or 225 or 250 tons could be used.


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The following is an example of how to

determine proper clamp tonnage in U.S.

customary units.

Example 1:

The part to be molded is a square plague that is 6

x 6 inch and 0.060 inch thick. Determine the

clamp tonnage for both molding material which isNylon and Polycarbonate. Nylon factor: 2

tons/inch2& Polycarbonate factor: 5 tons/inch2.


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Example 2:

Consider calculating the required mold clampingforce when four molded items are obtained using

Polybutylene terephthalate (PBT) plastic with 30% glass fiber added.

Let us assume that the assumptions for calculationare that the pressure inside the cavity is p = 2.13

tons/inch2

, the projected area of one cavity is A1 =2.37 inch2, and the projection area of the runneris A2 = 0.85 inch2.


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Thank You

&

Any Questions?


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Objectives

To determine specifically the clamp tonnage

required from the material being molded in

plastic injection moulding.


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Answer: The part to be molded is a square

plague that is 6 x 6 inch and 0.060 inch thick.

The projected area of that plaque would be

found by multiplying length times width, or 6

x 6 inch, which equals to 36 inch2.


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If the molding material is nylon, which is very easy

flowing, the lower range number of 2 tons/inch2

could be used; 36 inch2x 2 tons = 72 tons clamping

force is required to keep the mold closed againstnecessary injection pressures.

If the molding material is stiffer flowing resins, such

as polycarbonate, the projected area would need to

be multiplied times a factor of 5 instead of 2. Thatwould yield a total clamp tonnage requirement of 36

x 5, or 180 tons.

injection molding calculation

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