Hey there! As a supplier of injection molding screws, I've seen firsthand the unique challenges that come with using screws for high - viscosity plastics in injection molding. In this blog, I'm gonna break down these challenges and share some insights on how we can tackle them.
Understanding High - Viscosity Plastics
First off, let's talk about high - viscosity plastics. These plastics have a thick, sticky consistency, kind of like honey compared to water. They don't flow easily, which is a big deal in injection molding. In injection molding, we rely on the plastic to flow smoothly through the screw and into the mold cavity to take the shape we want. High - viscosity plastics like polycarbonate, polyphenylene sulfide (PPS), and some grades of polyethylene terephthalate (PET) are known for their resistance to flow.
Challenges in Plasticizing
One of the major challenges is plasticizing these high - viscosity plastics. The screw in an injection molding machine is responsible for melting and homogenizing the plastic. With high - viscosity plastics, it's much harder to get them to reach a uniform molten state.
Heat Transfer Issues
High - viscosity plastics are poor conductors of heat. When the screw rotates, it tries to transfer heat from the barrel to the plastic. But because these plastics don't let heat pass through easily, it takes longer to melt them. This can lead to uneven melting, where some parts of the plastic are fully molten while others are still in a semi - solid state. As a result, the quality of the final product can be compromised, with issues like streaks or inconsistent mechanical properties.
Shear Heating
Another aspect is shear heating. As the screw rotates, it applies shear stress to the plastic. In high - viscosity plastics, this shear stress can generate a lot of heat. While some heat is good for melting the plastic, too much can cause thermal degradation. Thermal degradation means the plastic breaks down chemically, which can change its properties and make it unsuitable for the intended application.
Challenges in Feeding
Feeding high - viscosity plastics into the screw is also a headache. These plastics tend to clump together, and they don't flow freely from the hopper into the feed section of the screw.
Bridging
Bridging is a common problem. The plastic forms a bridge or arch in the hopper, preventing it from dropping down into the screw. This can lead to inconsistent feeding, which in turn affects the quality and quantity of the plastic being processed. If the feeding is not consistent, the injection molding process becomes unstable, and the parts produced may have variations in size and weight.
Friction in the Feed Section
The high - viscosity plastics create more friction in the feed section of the screw. This increased friction can make it harder for the screw to pull the plastic into the barrel. As a result, the screw may need to work harder, which can lead to increased wear and tear on the screw and the drive system.
Challenges in Metering
Metering is the process of accurately measuring the amount of plastic to be injected into the mold. With high - viscosity plastics, metering becomes more difficult.
Backflow
Backflow can occur when the high - viscosity plastic tries to flow back along the screw flights during the injection phase. This backflow can cause inaccurate metering, as the amount of plastic actually injected into the mold may be less than what was intended. It can also lead to issues like short shots, where the mold is not completely filled.
Pressure Buildup
High - viscosity plastics require higher pressures to be pushed through the screw and into the mold. This increased pressure can put a lot of stress on the screw and the entire injection molding machine. If the machine is not designed to handle these high pressures, it can lead to mechanical failures, such as screw breakage or barrel leakage.
Solutions and Our Products
So, what can we do to overcome these challenges? Well, at our company, we offer a range of specialized screws designed to handle high - viscosity plastics.
Bimetallic Screws
Our Bimetallic Screw for Injection Molding Machine is a great option. These screws have a hard, wear - resistant outer layer and a tough inner core. The hard outer layer can withstand the high shear forces and abrasion caused by high - viscosity plastics. It also helps with heat transfer, as it can conduct heat more efficiently to the plastic, reducing the melting time and improving the uniformity of the molten plastic.
Coated Screws
We also have HVOF Coating Screw and Pvd Coating Screw. These coatings provide a smooth surface, which reduces friction between the screw and the plastic. This helps with feeding and also reduces the risk of shear heating and thermal degradation. The coatings are also very hard and wear - resistant, which extends the lifespan of the screw.
Conclusion
Using a screw for high - viscosity plastics in injection molding is full of challenges, from heat transfer and feeding issues to metering problems. But with the right screws and solutions, these challenges can be overcome. At our company, we're committed to providing high - quality injection molding screws that can handle the toughest plastics.
If you're facing problems with high - viscosity plastics in your injection molding process, don't hesitate to reach out. We can help you find the perfect screw for your needs and ensure that your production runs smoothly. Let's work together to achieve the best results in injection molding!
References
- Throne, J. L. (1996). Plastics Process Engineering. Hanser Publishers.
- Osswald, T. A., & Turng, L. - S. (2007). Injection Molding Handbook. Hanser Gardner Publications.