The Calculation Formulas for Injection Molding 

Posted on: March 23, 2022, By  GREFEE

Part 1

Clamping tonnage F (TON)    Formula: F=Am*Pv/1000

Clamping tonnage: TON      Am: Mold Cavity Projected area: CM2

Filling Pressure： KG/CM2

Generally, the plastic material filling pressure range is 150-350KG/CM2)

Good flowability takes bottom values, poor flowability takes top values.

Injection Pressure= filling pressure/0.4-0.6

For example: Mold Cavity Projected Area 270CM2, Filling pressure 220KG/CM2 

Clamping tonnage =270*220/1000=59.4TON

2. Injection Pressure   Pi（KG/CM2）Formula： Pi=P*A/Ao that is: Injection Pressure = Pump* active area of injection cylinder  + screw section area 

Pi: injection pressure  P: pump pressure  A: active area of injection cylinder 

A=π*D2/4       D: diameter  π: Pi  3.14159

Example 1: Known pump pressure, calculate the injection pressure? 

Pump Pressure = 75KG/CM2 active area of injection cylinder  =150CM2

Screw section area =15.9CM2（∮45mm）

^Formula: 2〒R^{2 that is:} 3.1415*（45mm÷2）²=1589.5mm²

Pi=75*150/15.9=707 KG/CM2 

Example 2: Known injection pressure, calculate the pump pressure? 

Required injection pressure = 900KG/CM2  

The effective area of injection tank =150CM2

Screw section area=15.9CM2（∮45）

Pump pressure P= Pi*Ao/A=900*15.9/150=95.4 KG/CM2 

3.Injection volume V（CM3）formula : V= π*(1/2Do)²*ST namely Injection volume = 3.1415*radius² * stroke

V： Injection volume （CM3）π：Pi 3.1415  Do： screw diameter CM 

ST: injection stroke CM 

Example: Screw diameter 42mm            Injection volume 165mm

V= π*（4.2÷2）²*16.5=228.6CM3

4. Injection weight  Vw(g) formula Vw=V*η*δ  that is: injection weight = injection volume*specific gravity* mechanical efficiency  

Vw： injection weight g      V: injection volume.   η: specific gravity   δ：mechanical efficiency 

Example: Injection Volume =228.6cm3   Mechanical Efficiency =0.85  Specific Gravity =0.92

Injection weight Vw=228.6*0.85*0.92=178.7G 

5. Injection speed S（CM/SEC）formula: S=Q/A that is: injection speed= pump output ÷ active area of injection cylinder 

S: injection speed CM/SEC

A: Active area of injection cylinder CM2

Q: Pump output amount CC/REV formula: Q=Qr*RPM/60  (per min/L) that is: Pump output amount = pump output per revolution * motor revolution(s) per minute (RPM)

Qr: single revolution pump output amount (per revolution/CC)

RPM: revolution(s) per minute 

Example: motor rotating speed 1000 RPM

Single revolution pump output amount 85 CC/RPM

Active area of injection cylinder 140 CM2

S=85*1000/60/140=10.1 CM/SEC 

6. Injection rate Sv(G/SEC) formula: Sv=S*Ao that is: injection rate= injection speed*screw section area 

Sv: injection rate G/SEC   S: injection speed CM/SEC   Ao: screw section area 

Example: injection speed=10CM/SEC    screw diameter ∮42 

Area =3.14159*4.2*4.2/4=13.85CM2

Sv=13.85*10=138.5G/SEC 

Part 2

1. Theoretical volume:(π/4=0.785)

(1) screw diameter²*0.785* injector stroke= theoretical volume(cm³);

(2)Theoretical volume/0.785/screw diameter=injection stroke (cm).

2. injection weight

Theoretical volume*plastic percent*injection constant（0.95）ideal=injection weight（gr);

3. Injection pressure 

Injection cylinder area²/screw area²*system maximum pressure(140kg/cm²)²=injection pressure(kg/cm²);

Injection cylinder diameter²/screw diameter²*system maximum pressure(140kg/cm²)=injection pressure(kg/cm²);

Maximum material pipe combination injection pressure*actual used pressure(kg/cm²)/maximum system pressure (140kg/cm²)=injection pressure(kg/cm²)

4. Injection rate 

(1)screw area(cm²)* injection speed(cm/sec)=injection rate (cm³/sec)

(2)screw diameter (cm²)*0.785* injection speed (cm/sec)=injection speed(cm³/sec).

5. Injection speed 

(1) injection rate(cm³/sec)/screw area(cm²)=injection speed(cm/sec); 

Pump volume per revolution (cc/rev)*motor rotating speed(rev/sec)/60(second)/injection area(cm²)=injection speed(cm/sec).(motor rotating speedRPM:60HZ——1150,50HZ—–958)

6. Injection cylinder area 

(1)injection pressure(kg/cm²)/maximum system pressure(140kg/cm²)*material pipe area(cm²)=injection cylinder area(cm²);

Single cylinder—(injection cylinder diameter²-displacement ram diameter²)*0.785=injection cylinder area(cm²);

Twin cylinder—(injection cylinder diameter²-displacement ram diameter²)*0.785*2=injection cylinder area(cm²).

7. hydraulic motor single revolution volume

Injection cylinder area(cm²)*injection speed(cm/sec)*60secs/motor rotating speed=single revolution pump volume(cc/sec).

(motor rotating speedRPM: 60HZ——1150,50HZ—–958)

8. Screw rotating speed & hydraulic motor single revolution volume 

(1)single revolution pump volume(cc/rec)*motor rotating speed(RPM)/single revolution hydraulic motor volume =screw rotating speed;

(2)pump volume per revolution(cc/rec)*motor rotating speed(RPM)/screw rotating speed=hydraulic motor single revolution volume. 

Overall injection pressure 

(1)maximum system pressure(kg/cm²)*injection cylinder area(cm²)=overall injection pressure(kg);

 (2)injection pressure(kg/cm²)*screw area(cm²)=overall injection pressure(kg).

10.Ounce & related units conversion 

(1)1ounce(oz)=28.375g (gr);

(2)1lb(ib)=16oz(oz);

(3)1kg(kg)=2.2lb(ib); that is：1kg=1.1lb；

(4)1lb(ib)=454g(gr)=0.454kg(kg).

11. Die-closing force 

(1) crank type 

Mold closing cylinder area (cm²)*maximum system pressure(140kg/cm²)/1000*crank magnification(20-50)=mold-closing force(Ton)

（2）Single-cylinder direct-pressure type 

Mold closing cylinder area(cm²)*maximum system pressure(140kg/cm²)/1000=die- closing force(Ton)

12. Pillar diameter & mold-closing tonnage formula

Pillar diameter²(cm²)*0.785*Young’s modulus (E)(scm4about1000kg/cm²)*4=approximate die- flossing force value(Ton).

13. End products arrangement projected area & die-closing tonnage conversion formula 

End products arrangement projected area(inch²)*standard thickness(1.5mm)/average end product thickness(mm)*constant of used material/PS raw material constant(1)=die-closing force(Ton);

（1）The end product arrangement projected area is centered by injection gate, long side as radius to calculate diameter out. 

Arranged diameter²(inch²)*0.785=end product arrangement projected area (inch²).

（2）Use constant of raw material and calculates with approximate experience point, flowability better than PS is classified as “below 1” flowability worse than PS is classified as”over 1” .

ABS  1.05; AS  1.2;  PMMA  1.3;  PC  1.6;  PBT  0.9;  

PP   0.7;  PE  0.7-0.8; plastic steel 0.8;  NILON  0.7-0.9¨¨¨etc.

Different raw materials are classified as different grades, should know more about the properties of the materials. 

The height of the cup is calculated by 30% of the projected area. 

14. Demolding tonnage 

Demolding cylinder area(cm²)*maximum system pressure(140kg/cm²)/1000=demolding force(Ton)

15. Electric unit 

1 h.p(HP)=0,754 kw(KW)

1kw(KW)=1.326h.p(HP)=1000w(W);

1kw(KW)=1km measurement unit (1KW/Hr).

16. Pump size and horsepower relationship 

P=maximum operating pressure (eg:125kg/cm².140kg/cm²);

Q=oil pump output amount per minute(L/min);

Q=oil pump output per revolution (cc/rec)*motor rotating speed (RPM)/1000=oil pump output per minute(L/min);

Applicable horsepower (HP):  

P*Q/540=HP;

P*Q/612=KW.

Matching with the motor can reach the largest pressure no need speed declination. 

HP*450/Q=LP;

KW*612/Q=LP.

17. Calculation of electricity usage 

(Motor capacity+electrothermal capacity+dryer capacity)*electric constant (约40%)= power consumption per hour (kilowatt-hour,KW/Hr).

A: What is the injection ability of the injection molding machine 

Injection ability =injection pressure（kg/cm²）×injection volume（cm³）/1000

B: What is the injection horsepower of the injection molding machine 

Injection hP PW（KW）=injection pressure（kg/cm²）× injection rate（cm³/sec）×9.8×100%

C: What is the injection rate of the injection molding machine 

Injection rate V（cc/sec）=p/4×d²×g

d^2：：material pipe diameter      g： material intensity

Ｄ： what is the injection thrust of the injection molding machine?

glue injection thrust=（kgf）=p/4（D₁²－D₂²）×P×2

D₁：Injection cylinder bore diameter    D₂：piston rod outlet diameter   P： system pressure 

E: what is the injection pressure of the injection molding machine?

Injection pressure P（kg/cm²）=[p/4×（D₁²－D₂²）×P×2]／（p/4×d²）

What is the plasticizing capacity of the injection molding machine?

Plasticizing capacity W（g/sec）=2.5×(d/2.54)²×(h/2.54)×N×S×1000/3600/2

h=tooth depth at the front of the screw （cm）  S=raw material intensity 

G: what is system pressure? What is the difference between system pressure and injection pressure?

System pressure （kg/cm²）= maximum operating pressure set in the hydraulic circuit

H: injection speed?

H: speed= stroke/time 

Part 4

1． Clamping force  F(TON) formula： F=Am*Pv/1000 namely： clamping tonnage= cavity projected area*filling pressure÷1000

F： Clamping tonnage TON     Am： Cavity projected area CM2   

Pv： Filling pressure KG/CM2 

(Filling pressure of ordinary plastic material is 150-350kg/CM2)

(Good flowability takes the bottom value, poor flowability takes high value) 

Filling pressure/0.4-0.6=injection pressure 

Example: cavity projected area 270CM2, filling pressure 220KG/CM2

Clamping tonnage = 270*220/1000=59.4TON

2. Injection pressure  Pi（KG/CM2）formula： Pi=P*A/Ao that is: injection pressure=pump pressure *active area of injection cylinder÷screw section area

Pi: injection pressure    P: pump pressure    A: active area of injection cylinder  Ao: screw section area

A=π*D2/4       D: diameter  π: pi 3.14159

Exapmle1: known pump pressure, how much is the injection pressure? 

Pump pressure =75 KG/CM2  active area of injection cylinder =150CM2 

Screw section area=15.9CM2（∮45) 

Pi=75*150/15.9=707 KG/CM2 

Example 2: known injection area, how much is the pump pressure?

Required injection pressure = 900KG/CM2   active area of injection cylinder = 150CM2

Screw section area=15.9CM2（∮45）

Pump pressure P= Pi*Ao/A=900*15.9/150=95.4 KG/CM2 

3． injection volume V（CM3）formula： V= π*Do2/4*ST that is： injection volume=screw section area*injection stroke

V： injection volume CM3  π：pi    Do： screw diameter CM 

ST： injection stroke CM e.g： screw diameter  42mm     injection stroke165mm 

V= π*4.2*4.2/4*16.5=228.6CM3

4． injection weight  G formula： Vw=V*η*δ that is： injection weight=injection volume*specific gravity*mechanical efficiency 

Vw： injection weight  G    V： injection volume    η：specific gravity   δ：mechanical efficiency 

Example: injection volume= 228.6 CM3     mechanical efficiency=0.85       specific gravity=0.92

Injection weight Vw=228.6*0.85*0.92=178.7G 

5． Injection speed  S（CM/SEC）formula： S=Q/A, that is： injection speed=pump output amount÷active area of injection cylinder 

S： Injection speed CM/SEC        Qr： pump output amount（per revolution/CC）CC/REV

A： Active area of injection cylinder  CM2   Q=Qr*RPM/60 (per min/L)

Q： Pump output amount   RPM： revolution per minute 

Example: motor rotating speed 1000RPM       pump output amount 85 CC/REV

Active area of injection cylinder140CM2

S=85*1000/60/140=10.1 CM/SEC

6． injection rate  Sv（G/SEC）formula： Sv=S*Ao, that is： injection rate=injection speed*screw section area

Sv： injection rate G/SEC  S： injection speed CM/SEC  Ao： screw section area

Example： injection speed=10CM/SEC  screw diameter∮42

Screw section area=3.14159*4.2*4.2/4=13.85CM2 

7. Calculation of circle 

Area of a circle S=πR²(R-radius of circle) 

Circumference of a circle=2πR(R-radius of circle) or πH(H- diameter of circle)

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