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New processes overcome injection moulding challenges

Source:International Plastics News for Release Date:2021-09-01 3259
ChemicalPlastics & RubberOthersCompoundingRaw Materials & CompoundsMaterials Handling, Measuring & TestingMolds & ComponentsPlastics Machinery
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A coinjection process that ENGEL has developed in cooperation with Top Grade Molds enables high and completely encapsulated recycled material content in the production of pails

The use of recycled plastic materials is growing worldwide as demand from consumers and political pressure to protect the environment mount. Various countries already stipulate the minimum recycled content for certain products. In the Western US, for example, some market applications require 25% recycled content for pails. In addition, many brands do not want the dark coloured recycled core material to be visible at the injection point. As a result, several injection moulding techniques are gaining ground, breaking away from traditional processes, to meet the sustainability goals.

 

At the ENGEL live e-symposium held in June this year, several new technologies were introduced highlighting process improvement in support of cost-saving recycling and increased use of recycled materials. Strengthening the use of recycled materials is one of the essential prerequisites for establishing a circular economy for plastics, which ENGEL is intensively promoting together with its partner companies.

 

A coinjection process that ENGEL has developed in cooperation with Top Grade Molds enables high and completely encapsulated recycled material content in the production of pails, and is thus superior to conventional processes for sandwich injection moulding.  Another development is the two-stage process that makes it possible to process plastic waste as flakes in injection moulding directly after grinding, thus eliminating pelletising. 


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Very short cycle times, higher product quality

With a recycled material content of more than 30%, the 5-gallon pails produced during the ENGEL live e-symposium easily comply with the US specifications, which are very challenging as compared with other international standards.  ENGEL's development partner is Top Grade Molds from Mississauga, Canada, a mould manufacturer specialising in packaging applications. The pail mould for the world premiere is being provided by US pail manufacturer M&M Industries from Chattanooga, Tennessee. It is being deployed on an ENGEL duo speed injection moulding machine, which ENGEL developed specifically to meet the requirements of producers of pails and storage and transport containers. The large machine type combines productivity and efficiency with short cycle times. The coinjection process provides similar cycle times as producing pails in conventional single-component injection moulding. 

 

Coinjection makes it possible to concentrate the recycled material in the component's core and encapsulate it by a layer of virgin skin material. The virgin and recycled material are of the same type to ensure that the sandwich-moulded parts can also be recycled at the end of their useful life. HDPE is being used for the 5-gallon pails; the recycled material here originates from post-consumer collection.

 

The greatest challenge in sandwich injection moulding is to achieve high recycled content without compromising product quality or performance. One focus of the development was therefore on combining the two molten plastics in the pail mould without mixing them beforehand in the melt stream. As a result, the system switches between recycled and virgin material at the cavity injection point by way of a coinjection valve gate.

 

5-gallon pails are universally used in North America for a wide variety of end use markets. Since the core and skin material fractions are strictly isolated from each other in the production process, and the core is encapsulated by the skin, including the injection point, the new coinjection process also can be considered for the food industry.

 

The presentation of the coinjection process sees ENGEL further strengthen its expertise in the production of sandwich products with a core made of recycled material. While the ENGEL skinmelt process presented at the K show 2019 is primarily used for the production of technical parts, logistics boxes and pallets, the coinjection process developed with Top Grade Molds meets the requirements set by manufacturers of pails and other smaller packaging products and is also suitable for multi-cavity moulds. 

 

Efficient processing of plastic flakes 

Aside from the coinjection process, ENGEL also launched a two-stage process to a trade audience for the first time and demonstrated its great potential. The technology enables the processing of plastic waste as flakes in injection moulding directly after grinding. Since a complete process step, pelletising, is eliminated, the innovation significantly improves cost efficiency in plastics recycling. The technology also offers benefits in the production of very large and thick-walled components with high shot weights.

 

The key to shortening the recycling process is breaking down plasticising and injection into two independent process steps that are very well tuned with each other. In the first stage, the raw material, for example plastic flakes originating from post-consumer collection, is melted in a conventional plasticising screw. In the second stage of the process the melt is transferred to a second screw for injection into the cavity. The two-stage process makes it possible to integrate a melt filter and a degassing unit on the injection unit side of the injection moulding machine, so that products with a consistently high quality are obtained even from contaminated plastic flakes.

 

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ENGEL introduced a two-step proces that will shorten the recycling process. 

 

This innovation sees ENGEL make a further contribution to the establishment of a circular economy for plastics. Pelletising the recycled material is an energy-intensive process which typically also involves logistics overhead. If this step can be eliminated, the CO2 footprint is improved and recycling costs are also significantly reduced.

 

As an alternative to an injection screw, the plasticising screw used for creating the melt can be combined with a piston unit. This variant of the two-stage process is also very efficient for processing very large shot weights of up to 160 kg with a comparatively low injection pressure requirement. The two-stage process enables a more compact system design and a lower unit cost than is possible with a conventional single-stage plasticising and injection process.

 

ENGEL has developed a new piston design to eliminate the typical disadvantages of piston injection units when changing materials. The rheologically optimised piston tip supports uniform flushing around the piston, enabling fast material and colour changes. Typical applications are containers, pallets or even large fittings. Starting at a shot weight of 20 kg, processing efficiency increases significantly thanks to keeping the plasticising and injection processes separate.


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