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〒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
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
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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