Tuesday, 31 October 2017

How to Calculate the Residence Time in Plastics Injection Moulding [incl. online calculation tool]




The term residence time in injection moulding operations refers to the time that a plastic pellet takes from entering the injection moulding barrel until entering the injection mould. It relates to the amount of polymer material present in the cylinder of the injection unit, the shot weight and the total cycle time. Often, residence time is also referred to as Hold-Up Time (HUT).

Melting of plastics for processing is usually attained by bringing the plastics over a certain temperature, i.e., glass transition temperature for amorphous thermoplastic polymers and glass transition temperature as well as crystalline melting temperature for semi-crystalline thermoplastic polymers. For both types of thermoplastics longer than necessary heat exposure, especially in the presence of oxygen (air), may induce chemical degradation. Therefore, the residence time in injection moulding at polymer-sensitive melt temperatures needs to be optimal. In this context, residence time is especially important for polymers such as, for example, PVC, POM, ABS, PBT and PET.

Melt temperatures have to be chosen in a way that the material’s thermal stability during processing is ensured [1, 2]. Guidance about optimal residence time and residence time for different polymers is given by material manufacturers in processing and design guides.  In practice, tools for accurately calculating the melt residence time depending on the utilized machine and processing conditions are usually not available. This prevents processing engineers from making quick process assessments. For this reason, I have created a demo-sheet to calculate the residence time of your injection moulding operation. This can be used online or downloaded. The calculation is based on the formula below [3].

Formula for calculating the residence time in injection moulding

Here, number 8 represents the volume of the molten polymer in the barrel. This is the ratio between flight height and screw length, which for most injection moulding machines is approximately 8. Part A gives the number of shots in the barrel and Part B represents the cycle time to produce the part.

Finally, keeping the residence time at an optimum level will help you keeping materials’ degradation to a minimum and, consequently, the mechanical properties of your final moulded part to a maximum.


Successful moulding and thanks for reading!

Till next time!
Greetings, Herwig
New to my Find Out About Plastics Blog - check out my start here section



Literature
[1] http://www.solvay.com/en/binaries/Sulfones-Quick-Molding-Guide_EN-227546.pdf
[2] GE Plastics - Injection Moulding Guide
[3] Christoph Jaroschek - Spritzgießen für Praktiker
 


 


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