ideal process parameters in injection moulding

IDEAL PROCESS PARAMETERS

In injection Moulding

BY DEVANG NAIK

• Injection Molding: -

It is a cyclic process, where a heat soften material injected in to close mold, which gives the desired shape.

•Plastics :- It is an organic substance

which can be converted in to any shape by application of heat and pressure.

• Terms used in injection molding machine.

• Injection Pressure.

• It is a maximum pressure excreting on plastic melt at screw tip.– measuring unit kg/cm2 or bar.

• Speed:- Rate of displacement of screw

per second. Measuring unit gm/sec, cc/sec, mm/sec.

• Back Pressure :- It is a pressure /force which

force the screw backward direction even as it is conveying polymer forward.

This back pressure controlled by throttling the oil behind injection cylinder.

It is not more than 14 bar.

• Screw:- It is a mechanical device

(helical flighted shaft) which convey the heat soften material.

• Nozzle:- It is a tube like mechanical

device which provide mechanical & thermal connection from not cylinder (barrel) to much colder mold.

Types of nozzle:- 1. open channel. 2. Check valve type. 3. Shut off valve nozzle.

• Size of orifice with respect to sprue bush

• M/C• Up to 180T 4 mm 3 mm• 350 T 5 mm 4 mm• 650T 6 mm 5 mm• 850T 10mm 8 mm

• How to start machine ? Check points…Sprue bush dia.Nozzle orifice.Cavity pressure.Cooling line looping.Water pressure & Water temp. Delta

T : 2 to 5 degreeTemp. profileScrew surface speed (SSS) .Speed profile.Hold on position.Melt temp.

• Process Setting. Mould MachineMold temp. 1. Injection

pressure Cavity pressure. 2. System

PressureSprue bush dia 3. Barrel temp. System Flow 4. Hot runner Air pin/Ejector pinWater coolingGate mechanismRaw material

Materials Drying• Hygroscopic Non Hygroscopic• Polymers with high Very little or no affinity• Affinity for moisture. For moisture.• Nylon, ABS Polystyrene.• Polycarbonate Polyethylene.• Polyester PVC,PP.• Polyurethane Acetal.

L/D and Compression

ratio.

Non return valve

• 1 Set up m/c to run standard parts.• 2 Turn of pack and hold time pressure.• 3 Weigh the parts and record weight.

Mold Venting

• Why Vent?• Evacuation of latent air that is in the closed mold.• Allow the evacuation of gases produced by low molecular weight polymers and additives.

Problem associated with poor venting.

• Burn marks.• Poor mold filling.• Weak wield lines.• Internal bubble and non-fill

area.• Sink mark.• Longer cycle time.• Stress cracking.

Temperature• What is important barrel temp or melt temp?• Optimum MELT TEMP is the key to successful

molding.• Factors affecting melt temp.• Barrel temp setting.• Screw speed.• Screw back pressure.• Residence time.

Key point• In order to reproduce the same process on multiple machines .•Melt temperature is one variable that must be controlled and duplicated.

Flow Rate

• Plastic materials viscosity decreases as shesr rate increases.

• WHY IS THIS IMPORTANT?• Screw speed----Lower viscosity at higher

RPM.• Inj speed---Flow rate easier with higher inj

speed.• Flow Rate=Time in sec,measured from start

of injection to transfer to pack/hold.

How does velocity profiling help?

• Allows the mold to be filled as fast as possible.

• Reduces burning, splay and other aesthetic issues.

• Helps with weld line.• Surface finish.• Best speed for each area of the segment

can be selected.

Pressure• What is pressure?• Pressure is Resistance to FLOW.• Injection pressyre-----1st Stage pressure.• Packing pressure-------2nd Stage

pressure.• Back pressure.• Cavity pressure--------Most Important

Cavity pressure

• Injection pressure must always be set per requirement of cavity pressure. This can be done by gradually increasing the set inj. Prs. On monitor and observe the increment in pr.gauge.The set point after which the gauge pr. Does not change is your require injection pressure.

Hydraulic Pr.Vs Plastic Pr.

• Hydraulic pressure: Measure of how much force a machine can generate against the ram.

• Plastic (Melt) pressure: Pressure generated in the nozzle of a molding machine usually derived from the intensification ratio of the machine.

• Cavity Pressure: Actual pressure in the cavity MOLD).

Time

• Injection• Pack and Hold-----Gate freeze study.• Cooling.• Mold open.• Mold close.• Cooling Time= 150 * Thickest wall of the

part.• = 150*.100=15sec.

Screw surface speed

• Screw surface speed=0.052 * Screw Dia*Screw RPM.For HDPE and PP 750.SSS is low than definable require higher barrel temp.

Melt Temperature

• There are few processing parameters in the injection process that have significant influence on the melt temp., the temp of the melt as it leaves the injection unit.

• The temp setting of barrel heaters.• RPM of screw.• Back pressure applied.• Residence time.• Cycle time.

EDUCATIONIs the key to

successful implementation

of injection molding.

Thank you.