Molding 101
Polymers
Polymer Viscosity
Properties of Plastics
Injection Molding
Screw and Check Ring
Commonly Used Calculations
Injection Molding

a. What is Injection Molding?

 Injection Molding is the technique of injecting molten plastic into a cold mold and forming a part. A schematic of the main elements of a molding machine are shown below. It consists of a hopper that holds the raw plastic pellets. The hopper feeds the barrel with the plastic. The plastic is melted in the barrel and with the help of the screw (piston) is injected into a mold. The mold being colder than the plastic rapidly cools and solidifies the plastic. The mold is then opened and the part is ejected out of the mold.

 b. What is the importance of melt viscosity in Injection Molding?

 During Injection Molding, the material is subjected to large amount of shear forces during the cavity filling stage. The shear rate is proportional to the injection speed. If the shear rates are in the non-Newtonian region of the curve, then small variations in the shear rate will cause a large shift in the viscosity. This will make the mold filling inconsistent resulting in shot to shot inconsistency. It is therefore important to find the Newtonian region of the curve and set the injection speed (therefore shear rate) in this region. The viscosity curve can be generated at the molding machine for any given mold. The term ‘In-Mold Rheology’ is another term used for developing the viscosity curve. The effect of shear rate on viscosity is far greater than that of temperature. Therefore as long as the actual melt temperature is in the within the recommended range you will get a similar profile during the development.

 

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