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Melt Temperature And How It Is Achieved in Injection Molding

Many processors when asked will answer the question on what the melt temperature is by quoting the barrel temperature settings. However, there a few other processing parameters in the injection process that have a significant influence on the melt temperature; the temperature of the melt as it leaves the injection unit. 


 

How is the material heated in the process? 

The heat coming from the heater bands around the barrel supply some of the heat but heat transmission through the material is too slow to be sufficient for the process. A large part of the heat required for the melting process comes from the friction created by the screw rotation. Friction between the granules and also in the melt creates heat very much like rubbing your hands together in winter. The actual temperature of the melt is a function of the input of thermal energy and the amount of shear put into the material. It’s important to have the right balance. A too high melt temperature of level of shear may lead to an unacceptable level of material degradation and loss of properties.
 

The most important parameters are: 

• The temperature of the granulate as it enters the barrel 
• The temperature settings of the barrel heaters 
• The rotation speed of the screw 
• The level of back pressure applied 
• The injection speed 
• The geometry of the screw 


Certain
polymers, especially those in the engineering and specialty category need to be pre-dried at an elevated temperature. The melting and plasticizing behavior is influenced by the temperature of the granulate as less energy input is required when it is already at a certain temperature. 


Recommendations for the temperature setting of the barrel are usually provided by the material supplier, as is the recommended melt temperature.


 
The rotation of the screw and the applied back pressure provide the mechanical (frictional) energy input in the melting process. The rotation of the screw creates friction between the solid particles and also the melt, whereby the friction creates heat that increases the temperature and causes the transition from solids to melt.Too high friction caused by a high rotation speed and high back pressure causes locally a very high shear and subsequently material degradation. Possible defects caused by this include streaks, discoloration and loss of mechanical properties. 


 
For engineering thermoplastics a back pressure (hydraulic) of 5 – 10 bar (0,5 – 1 Mpa)is usually recommended, although this level may be adjusted as required by the process.

 

Screw circumferential speed recommended is around 0,25 m/s: the following formula will allow you to translate screw circumferential speed to machine rpm.: 


 

# rpm = (screw circumferential speed (m/s) x 60)/(screw diameter (m) x π) 


 

Screw circumferential Speed = 0,25 m/s


 

# rpm = 15/(screw diameter(m) x π) 


 

E.g.: this translates for a 32 mm screw to a screw rpm of about 150. 


 

As the melt is injected it is forced at considerable speed and pressure through the nozzle. Due to the shear forces caused by this there is an additional increase in (frictional) heat that in some cases may cause defects in the molded parts. 

 

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