The Calculation Formulas for Injection Molding 

Posted onMarch 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〒R2 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


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);


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


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)=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):  



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



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

d2::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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