Tuesday 22 December 2020

Rule of Thumb for Polymer Processing: The Importance of Melt Uniformity in Injection Moulding


In this rule of thumb, we discuss the importance of melt uniformity in injection moulding.

In a previous rule of thumb, we showed that there is a 20% variation in viscosity due to the intrinsic polymer structure (here thefull post). Additionally, non-molten plastic pellets and non-uniformity of melt can cause several more problems in the final part.

Here are some of the problems related to improperly molten resin pellets:

1. Decrease in part performance and increase in part failures

2. Increase of part warpage issues

3. Shot-by-shot filling not uniform, combined with short shots

4. Decrease in weld-line strength

5. Black specs caused by material degradation (dead spots)

20% of part failure can be related to processing. However, the biggest causes for part failure are poor specification and materialmisselection [1].

How to improve melt uniformity?

There are several ways to improve the melt uniformity in injection moulding: changing the injection moulding screw, the use of in-situ monitoring devices such as ultrasonic-based systems, and the processing conditions itself (temperatures, pressures).

General purpose is no purpose and why screw design matters

Let us focus on the change of injection moulding screw. Most moulding machines have a three section screw consisting out of solid conveying, compression, and metering zone. It is commonly referred to as General Purpose (GP) screw. And there it starts the challenge. Often people say that the general purpose screw is a no-purpose screw. Following the pellet melting model of a GP screw, it can happen that not all pellets are melted up. This can cause solid-bed break ups, which in turn decrease the overall mixing quality. Furthermore, material degradation and black specs can occur.

However, there are alternatives and all the major injection moulding machine manufacturers put a lot of focus and resources into developing improved screws. A well-known example is the barrier-screw, which allows the formation of a more homogeneous melt. Also, there is no need of having mixing elements anymore. Barrier screws are double flighted screws and they allow the pinpointing of the location where melting is completed. The aim is to separate the melt from the solid polymer by using a smaller diameter on one of the screw flights. Allover, plasticizing is easier; however it is not suitable for all polymers.

In conclusion, using special screws in injection moulding operations can lead to a reduction of melt temperatures, reduction of backpressure, removal of coloring problems due to non-melted pellets, reduction of screw cleaning operations and reduction of cycle times. Due to the aforementioned advantages, specialized screws pay off already within several months.

Thanks and #findoutaboutplastics,

Greetings and happy Christmas holidays,


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[1] https://www.findoutaboutplastics.com/2020/11/plastic-part-failure-part-1-reasons.html

[2] Saechtling-Kunststoff-Taschenbuch, Volume 30, Hanser, 2007

[3] https://www.ptonline.com/articles/improve-quality-productivity-with-advanced-screw-design

[4] https://www.ptonline.com/articles/the-melting-precision-of-barrier-screws

[5] https://www.findoutaboutplastics.com/2016/06/polymer-injection-moulding.html


  1. great knowledge - thanks for sharing and Merry Christmas!

    1. @Plastics Expert Witness Many thanks and wish you happy Christmas holidays!