How to Calculate Injection Molding Machine Tonnage

What size injection molding machine do I need?

Normally, most plastic injection engineers will rough calcualate the machine`s tonnage with the product`s surface multiply by a index number.


If you are using square inch as the surface, then you can make it 2.5 times (higher flow materials like PS, PE and PP) to 4 times (lower flow materials like PA, PC and PSU) as the index number.


While your are using square cm as the surface, then you should make it 0.48 times (higher flow materials like PS, PE and PP) to 0.72 times (lower flow materials like PA, PC and PSU) as the index number.


What is the formula to calculate tonnage?

Empirical Formula 1

Clamping force (T) = Clamping Force Constant Kp * Product Projected Area S (CM * CM) * Safety Factor(1+10%)


To properly calculate necessary clamp tonnage, you must first know the projected area of the molded part/s plus any runners.

Area can be calculated by multiplying length times width, A = L x W or A=πr2.


Kp experience value for square cm:

PS/PE/PP – 0.32;

ABS – 0.30~0.48;

PA – 0.64~0.72;

POM – 0.64~0.72;

Add glass fiber – 0.64~0.72;

Other engineering plastics – 0.64~0.8;

For example, a product has a projected area of 410 cm2 and material is PE, calculate clamping force.

Calculated by above formula: P = Kp * S = 0.32 * 410*1.1 = 141 (T), 150 ton injection molding machine should be selected.


Kp experience value for square Inch:


Most injection molders are make it 2.5 times(higher flow materials like PS, PE and PP) or 4 times (lower flow materials like PA, PC and PSU)

the surface square inches of the part and an additional 10% as a safety factor.


If you have a part that is 120 square inches, you would need a injection machine with 300 tons clamping tonnage for injection molding.


After added 10% safety factor, the required press size would be 330 tons of clamping force.Then you will need a 330 ton injection molding machine for your project.


Empirical Formula 2

Clamping force (T) = Material Molding Pressure * Product Projected Area S (CM * CM) * Safety Factor(1+10%)

350bar*S(cm^2)/1000 * (1+10%)

As above, 350*410/1000*1.1=160T, choose 160T injection molding machine.


Accurate Calculation Formula 3


Two important factors in calculating clamping force:

1. Projected area (S) is the largest area viewed along mold opening and closing.

2. Determination of cavity pressure (P)

Cavity pressure is affected by following factors

(1) Number and location of gates

(2) Gate size

(3) Wall thickness of product

(4) Viscosity characteristics of plastics used

(5) Injection speed


3.1 Grouping of thermoplastic flow characteristics

Group 1: GPPS HIPS TPS PE-LD PE-LLD PE-MD PE-HD PP-H PP-CO PP-EPDM

Group 2: PA6 PA66 PA11/12 PBT PETP

Group 3: CA CAB CAP CP EVA PEEL PUR/TPU PPVC

Group 4: ABS AAS/ASA SAN MBS PPS PPO-M BDS POM

Group 5: PMMA PC/ABS PC/PBT

Group 6: PC PES PSU PEI PEEK UPVC


3.2 Viscosity grade

Each of above groups of plastics has a viscosity (flow capacity) rating. Relative viscosity grades of each group of plastics are as follows:

Group multiplication constant (K)

Group 1 * 1.0

Group 2 * 1.3~1.35

Group 3 * 1.35~1.45

Group 4 * 1.45~1.55

Group 5 * 1.55~1.70

Group 6 * 1.70~1.90


3.3 Cavity pressure is determined by ratio of wall thickness, flow to wall thickness

Look up table P0*P=P0*K (multiplication constant)

clamping force


3.4 Determination of clamping force (F)

F=P*S= P0*K*S


For example, part: calculation of clamping force of polycarbonate (PC) lamp holder

As shown in figure is a round PC plastic lamp holder with outer diameter of 220mm, wall thickness range of 1.9-2.1mm, and pin-type center gate design. The longest process for parts is 200mm.

Place where flow resistance of melt is the largest occurs at position where wall thickness is thinnest (1.9 mm), so value of 1.9 mm should be used when calculating required injection pressure.


* Process / wall thickness ratio calculation

Process / wall thickness = longest melt process / thinnest part wall thickness = 200mm / 1.9mm = 105:1


* Application of cavity pressure/wall thickness curve

Relationship between cavity pressure and wall thickness, process/wall thickness ratio is provided. It can be seen from figure that cavity thickness of 1.9mm,

cavity pressure of flow/wall thickness ratio of 105:1 is 160Bar. Data is applied to the first group of plastics. For other groups of plastics,

we should multiply corresponding multiplication constant K.


* Determination of cavity pressure value of PC

Flow properties of PC belong to viscosity class of sixth group. Compared with the first group, viscosity of PC is 1.7-1.9 times of that,

different viscosities are reflected in cavity pressure, so cavity pressure of PC lamp holder should be 160 bar*K (the viscosity grade of PC). P = 160*1.9 bar = 304 bar for safety reasons, we take 1.9 times.


* Projection area value of PC lamp holder

S = π*lamp holder outer diameter 2 / 4 = 3.14* 22*22 / 4 (cm2) = 380cm2


* Clamping force of PC lamp holder

F=P*S=304bar*380cm2=304kg/cm2*380 cm2=115520Kg or 115.5Ton, so 120T can be used.

Soruce: https://guanxin-machinery.com/how-to-calculate-injection-molding-machine-tonnage/


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