Rule of Thumb for Plastics Injection Moulding: Usage of Regrind

 

In this rule of thumb post, we discuss the motivation for using regrind, what to be aware of when using regrind and which levels of regrind can be applied.

Motivation to use regrind

Regrind is used to mix it with virgin resin or completely use it for new parts to decrease the thermoplastic resin costs. One common source for obtaining regrind are sprues from moulded parts. They will be collected after an injection moulding project is set properly and parts are of good quality. However, also rejected parts can be transformed to regrind.

Thermal history

By adding regrind, thermal history is important. High melt temperatures in combination with too long residence times in the plasticizing unit can lead to thermal degradation of the thermoplastic. Adding the first time i.g. 20 % of regrind to the virgin material is fine. However, when using the sprues or parts of this 20%/80% compound and adding new regrind, decrease of mechanical properties due to thermal degradation may occur. Therefore, contacting your resin supplier is useful and check how often a chosen thermoplastic can be moulded without losing its mechanical properties by more than 10%.

Ratio of regrind

The amount of regrind added to virgin resin is between 20-25%. This ratio is valid for most plastics. In moulding operations, some parts allow using even 100% of regrind and other parts allow only for 100% virgin materials.

Thanks and #findoutaboutplastics

Greetings

Herwig

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Check out my other rule of thumb posts: 

Rule of Thumb for Plastics Injection Moulding: 20% Material Viscosity Variation over Time

Rule of Thumb for Plastic Part Design: Increase HDT & Modulus With Fillers

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Rule of Thumb for Injection Moulding - " What happens in the cavity..."

The 5P’s – Rule of Thumb for Plastic Design and Processing Operations

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Literature:

[1] https://knowledge.ulprospector.com/8055/pe-regrind-resin-qa/#:~:text=Generally%2C%20the%20molding%20community%20targets,100%20percent%20regrind%20during%20production.