How To Calculate Injection Moulding Plasticising Rates : Invotec Solutions

Everyday information the injection moulder requires from Invotec Solutions.

The below article can be used for machine setters and machine operators of injection moulding machines.

Discover our full range of plasticising components or for the injection moulder.

View our other articles form our setter and operators guide for injection moulders. Taken from our moulders catalogue (Toolbox Edition)

EXAMPLE: POLYCARBONATE

(Injection rates using two different polymers i.e PC and PP on the same moulding machine)

Calculate injection rate values given the following data

Screw diameter = 35mm
Material being processed = Polycarbonate (PC)
Maximum available injection velocity = 220mm/sec
Maximum screw stroke = 240mm
Maximum shot volume = 231cm3
Injection speed selected = 80mm/sec
Final screw stop position (including decompression distance) = 147mm
Changeover position = 10mm
Actual screw stroke used: Final screw stop position – changeover position / 147 – 10 = 137mm

Conversion into actual shot volume used = 131.8cm3

Maximum available shot weight in PC: Maximum shot volume available x Melt Density of PC

Therefore: Maximum available shot volume x (Room Density of PC x 0.85)
= 231 x (1.21 x 0.85)
= 231 x 1.029
= 237.70g of PC

Actual shot weight used for PC = Actual screw stroke volume x Melt Density of PC = 131.8 x 1.029 = 135.62 g

Predicted injection time = 137/80 = 1.56 seconds

Injection rate in PC in cm3/sec
= Actual shot volume used / injection time
= 131.8cm3 / 1.56 = 84.49cm3/sec

Actual injection rate in grammes/sec
= Actual shot weight used / injection time
= 135.62g/1.56 = 86.94g/sec

EXAMPLE POLYPROPYLENE

(Injection rates using two different polymers i.e PC and PP on the same moulding machine)

Calculate injection rate values given the following data

Screw diameter = 35mm
Material being processed = Polypropylene (PP)
Maximum available injection velocity = 220mm/sec
Maximum screw stroke = 240mm
Maximum shot volume = 231cm3
Injection speed selected = 80mm/sec
Final screw stop position (including decompression distance) = 147mm
Changeover position = 10mm
Actual screw stroke used = Final screw stop position – changeover position = 147 – 10 = 137mm

Conversion into actual shot volume used = 131.8cm3

Maximum available shot weight in PP = Maximum shot volume available x Melt Density of PP

Therefore: Maximum available shot volume x (Room Density of PP x 0.85)
= 231 x (0.90 x 0.85)
= 231 x 0.765
= 176.72g of PP

Actual shot weight used for PP = Actual screw stroke volume x Melt Density of PP = 131.8 x 0.765 = 100.83g

Predicted injection time = 137/80 = 1.56 seconds

Injection rate in PP in cm3/sec
= Actual shot volume used / injection time
= 131.8cm3 / 1.56 = 84.49cm3/sec

Actual injection rate in grammes/sec
= Actual shot weight used / injection time
= 100.83g/1.56 = 64.63g/sec

EXAMPLE POLYACETAL (POM) which is a high density material

Calculate injection rate values given the following data

Screw diameter = 40mm
Material being processed = Polyacetal (POM)
Maximum available injection velocity = 180mm/sec
Maximum screw stroke = 260mm
Maximum shot volume = 326.77cm3
Injection speed selected = 50mm/sec
Final screw stop position (including decompression distance) = 106mm
Changeover position = 10mm
Actual screw stroke used = Final screw stop position – changeover position = 106 – 10 = 96mm

Conversion into actual shot volume used = 120.65cm3

Maximum available shot weight in POM = Maximum shot volume available x Melt Density of POM

Therefore: Maximum available shot volume x (Room Density of POM x 0.85)
= 326.77 x (1.40 x 0.85)
= 236.77 x 1.19
= 388.86g of POM

Actual shot weight used for POM = Actual screw stroke volume x Melt Density of POM = 120.65 x 1.19 = 143.57g

Predicted injection time = 96/50 = 1.92 seconds

Injection rate in POM in cm3/sec
= Actual shot volume used / injection time
= 120.65 cm3 /1.92 = 62.84cm3/sec

Actual injection rate in grammes/sec
= Actual shot weight used / injection time
= 143.57g/1.92 = 74.78g/sec

How To Calculate Injection Moulding Plasticising Rates