Injection Moulding The
Injection Moulding The bioplastics handbook for injection molders T his article is an excerpt of “The bioplastics handbook for injection molders”, created and published by Green Dot Bioplastics (Emporia, Kansas, USA), edited by Michael Thielen. Download the full handbook via the link given at the end [1]. Processing bioplastics Bioplastics can be processed with the same equipment and, in many cases, similar cycle times as traditional plastics. As with switching from one traditional plastic to another traditional plastic (e.g., polyethylene to a polystyrene), though, it’s important to understand the processing parameters of the new material. And if you plan to use the same mold, it’s important to choose a bioplastic with similar processing characteristics to the material you’re currently using. Because of the custom nature of many materials, materialspecific processing considerations vary formulation by formulation. Still, there are category-specific processing considerations injection molders would find helpful to know before using a bioplastic. In this guide, Green Dot Bioplastics offers general considerations for these two overarching bioplastic categories: Biocomposites (plastics made with a matrix resin — either petroleum-based or renewable and a reinforcement of natural fibers or fillers) and biodegradables (plastics which are metabolized into organic bio-mass after use). See info-box for details about Green Dot Bioplastics’ materials. Injection molding with biocomposites Biocomposites can replace as much as 65 % (in cases even up to 70 %) of petroleum-based content with renewable materials such as wood-, flax-, hemp-, bamboo- etc. fibers or starch — a compelling bio story in a time when plastics bans and restrictions are heightening. Most biocomposites do not deviate significantly from traditional plastics and can be processed on the same equipment without major modifications to the injection molding process. As long as injection molders are aware of the small ways in which biocomposites differ from traditional plastics —and the small ways that difference affects the injection molding process— processing problems are easy to avoid. And as with any material change, it’s paramount to follow the processing characteristics laid out by the material manufacturer. Lower temperatures, slower injection speeds Biocomposites are filled with organic fillers and fibers, which are especially sensitive to high temperatures and shear buildup. If the material gets above about 205°C (400°F), there’s potential for degradation of the cellulose and burn streaks in the finished part. Changes in the process (e.g., lower temperatures, slower injection speeds) and tooling (e.g., opening a cooling line, enlarging gates) may be needed to alleviate shear stress and prevent the material from overheating. Although injection speeds might need to be lower for biocomposites, cycle times should remain within an acceptable range because they are being processed at lower temperatures meaning less cooling time is required. And the organic filler adds a degree of dimensional stability to the material, allowing it to be removed from the mold at higher temperatures than conventional plastics and further reducing cycle times. Follow drying recommendations The organic fillers (e.g., natural fibers, wood, starch, etc.) in a biocomposite readily absorb moisture from the environment and must be dried in a desiccant dryer prior to processing. Processing biocomposites at moisture levels above about 0.5% can result in a host of processing problems such as drooling from the nozzle or runner and the formation of voids within the part. Account for differing shrinkage characteristics The higher the ratio of organic fiber and fillers in a biocomposite, the lower the shrink rate. Injection molders who currently make a part out of a traditional plastic (e.g., polyethylene) and want to switch to a biocomposite (e.g., a wood fiber polyethylene composite) need to be conscious of the differing shrinkage characteristics between the two materials. A higher shrink rate isn’t necessarily good, and a lower shrink rate isn’t necessarily bad. But molds are designed with draft angles to accommodate the shrinkage of one plastic material — and if the same mold shall be used for the new material, a material with a similar shrink rate needs to be chosen. Injection molding with biodegradables Experience shows that the vast majority of processing problems with biodegradable plastics stem back to moisture and temperature. Processing biodegradable plastics at above the recommended temperature will cause the material to degrade. And if excess moisture is present, the resulting parts will be brittle, weak and have a reduced shelf life. If all processing recommendations of the raw material suppliers (e.g. Green Dot Bioplastics) are followed, though, the biodegradable plastic will remain strong and functional until subjected to the specific conditions in which it is designed to biodegrade. Most often, this is in industrial composting facilities or, in special cases, backyard composting settings — not while a product is on a store shelf or in use. 34 bioplastics MAGAZINE [03/19] Vol. 14
Injection Moulding Drying time and moisture levels It’s essential to follow the drying instructions specified by the material manufacturer to ensure the material is at or below the recommended moisture level prior to processing. Moisture levels above 0.1% will not only cause problems during the injection molding process (e.g., foaming, drooling, voids) but also product performance problems. The presence of excess moisture can cause the plastic to hydrolyze during the product lifecycle, resulting in a loss of molecular weight and mechanical performance. This can cause parts to be brittle and weak and, ultimately, fail. Lower processing temperatures Many biodegradable plastics tend to be more shear-sensitive and have lower melting points than most biocomposites and traditional plastics. Because of this, most biodegradable materials have to be dried and processed at much lower temperatures, often well below 175°C (350°F). Processing biodegradable plastics above the manufacturer recommended temperature can result in material degradation. Slower injection speeds and larger gates / runners may be required to further reduce the temperature and shear stress depending on the mold and material you are using. Meet injection molders who have made bioplastics work In a separate publication by Green Dot Bioplastics, four injection molders provide valuable and promising insights on working with bioplastics. Although each had technical issues at the initial stages, they were eventually able to run the materials successfully in their respective facilities [2]. A research-driven guidebook to processing bioplastics A special guide published by IfBB – Institute for Bioplastics and Biocomposites (Hanover University of Applied Sciences and Arts) and supported by FNR (specialist agency renewable resources) to the processing of bioplastics gives a straightforward explanation of processing considerations for a variety of materials and processing procedures based on years of research [3]. The texts were contributed by IAP – Fraunhofer Institute for Applied Polymer Research, IfBB, SKZ, SLK, Chair of Lightweight Structures and Polymer Technology, Technical University Chemnitz. [1] The bioplastics handbook for injection molders (complete pdf) The Bioplastics handbook for injection molders tinyurl.com/bioplastics-handbook [2] N.N.: Injection molders who have made bioplastics work; tinyurl.com/injection-moulders [3] Offers, J.; Lack, N. (Edts): Processing of Bioplastics – a guideline – tinyurl.com/bioplasticsprocessingguideline Green Dot Bioplastics materials at a glance Each of Green Dot’s Terratek® material lines don’t describe a discrete material, but rather a class of materials. • Terratek BD, is a class of materials that are both bio-based and biodegradable. Within the Terratek BD category, there are near-infinite possibilities to customize a formulation by alloying different polymers and additives to meet customer-specific performance requirements. These grades are more rigid and similar to traditional polyolefin resins. • Terratek Flex is a class of biodegradable elastomers that is rubber-like and soft to the touch, Terratek BD and Terratek Flex, are certified compostable according to ASTM D6400 and EN 13432, meaning they will disintegrate within 12 weeks and biodegrade within 180 days in an industrial composting facility. • Terratek WC is a class of wood-plastic composites. WC contains up to 60% wood particles • Terratek SC is a class of starchplastic composites with starch-toplastic ratios ranging from 30 to 65%. Terratek WC and SC replace a significant portion of traditional plastic with renewable, reclaimed organic fillers. The organic material adds dimensional stability, stiffness and a natural appearance to plastic products without sacrificing performance or processability. The value of Green Dot Bioplastics lies in the material science expertise which gives the supplier the flexibility to customize their material lines to match the unique requirements of each client. Green Dot Bioplastics offers custom formulations for each category, alloying different polymers, additives and fillers to achieve a broad range of performance and processing parameters. www.greendotbioplastics.com bioplastics MAGAZINE [03/19] Vol. 14 35
