Heating of plastic pellets in plastic injection molding-A small discussion

Shear Heating vs. Conductive Heating in Polymer Extrusion

Think barrel heat is the primary force in polymer extrusion? Think again. While many assume the barrel’s heat melts most of the polymer, that’s far from reality. Why? Because polymers are terrible heat conductors.

The heat from the barrel takes ages to penetrate thick polymer layers, especially in larger screws with deeper channels. If you’re relying on barrel heat alone, you’ll be waiting… and waiting… with only the outer layer melting. That’s where shear heating takes over.

So, What Is Shear Heating?

It’s the hidden hero in polymer extrusion. Shear heating happens when the screw rotates inside a barrel filled with viscous polymer, generating heat through friction and mechanical energy. This heat builds rapidly, especially with larger screws, making shear heating the real powerhouse behind melting.

But here’s the twist…
The bigger the screw, the more shear heating dominates.
In small screws with shallow channels, barrel heat has a fighting chance—it transfers more effectively because the polymer layer is thinner. However, in high-output applications, conductive heating quickly becomes impractical. It’s like trying to cook a thick steak with a weak flame: the outer surface cooks, but the inside stays raw.

Screw Design: The Game-Changer

Here’s where it gets interesting.
The right screw design can mean the difference between smooth, high-speed production or frustrating inconsistency. For large screws, viscous dissipation—mechanical energy from screw rotation—provides nearly all the melting power. Meanwhile, small screws can make better use of barrel heat and consume less energy per pound of output.

But don’t be fooled.
Relying too much on barrel heat can backfire. Inconsistent melting, poor mixing, and reduced melt quality are just a few of the risks. Different parts of the polymer melt at different rates, leading to uneven performance and potential defects.

Key takeaway?

• Large screws: Prioritize shear heating for high-output efficiency.
• Small screws: Use barrel heat wisely, but adjust temperatures carefully to maintain stability.

Fountain Flow: The Secret to High-Quality Injection Molding

Let’s shift gears to injection molding. Ever heard of fountain flow? It’s the key to understanding how high-quality parts are made. When molten plastic enters the mold, it forms a thin, solid skin layer upon contact with the cold steel. But that’s just the beginning…

Here’s where it gets fascinating.
The plastic at the front keeps flowing inward, forming successive layers. This inside-out process, resembling a water fountain, gives fountain flow its name. But it also creates challenges.

During the fill phase, the outer layer thickens as the melt progresses, reducing the flow channel size. Pressure builds, and in smaller channels, it can hit the machine’s limit. This is known as pressure-limited molding, which can slow the process and affect part quality.

And then comes the pack-and-hold phase…

Avoiding Common Defects

Here’s where things can go wrong—fast. Without proper control, common defects can ruin your parts:

• Splay: Those annoying shiny streaks? They’re caused by trapped volatiles, like moisture or air, that disrupt the surface texture.
• Bubbles: Inadequate venting traps air in the part, preventing full packing. Proper venting solves this.
• Voids: Insufficient packing pressure leaves hollow spaces, especially in thick sections of materials like polycarbonate.

But it doesn’t stop there.
Molecular orientation can throw another curveball. As molten plastic flows, molecules align in the flow direction and freeze in that state. In thick parts, this creates a surface layer with aligned molecules and a core with random orientation. Fiber-filled materials, like glass-reinforced plastics, behave similarly—fibers align on the surface, impacting the part’s strength.

The Takeaway

Understanding and controlling fountain flow is a game-changer. It can help you avoid defects like voids, splay, and bubbles, while improving part stability and overall quality.

Want flawless results? Focus on fine-tuning parameters like pressure, temperature, and venting, and you’ll be on the path to process perfection.

How band heaters function in plastic injection molding?

• Power Supply to Band Heater
  ⬇
• Electrical Energy Conversion to Heat
  ⬇
• Heat Transfer to Barrel Surface
  ⬇
• Barrel Heats the Plastic Inside
  ⬇
• Plastic Melts to the Required Consistency for Injection
  ⬇
• Plastic Ready for Injection into the Mold

How thermal pins work?

• Heat Absorption from Mold Surface
  ⬇
• Working Fluid Vaporizes Inside Thermal Pin
  ⬇
• Heat Travels as Vapor to Cooler End of Thermal Pin
  ⬇
• Condensation Occurs at Cooler End
  ⬇
• Heat Dissipated Away from the Mold (or Transferred Back for Heating)
  ⬇
• Working Fluid Returns to Original State and Repeats Cycle

Heating with Screw

• Plastic Pellets Enter the Feed Zone
  ⬇
  Pellets are fed into the barrel through the hopper.
• Screw Rotation Generates Friction and Shear Heat
  ⬇
  As the screw rotates, it pushes the plastic forward while applying mechanical energy, generating friction and shear heat.
• Barrel Heaters Provide Additional Heat
  ⬇
  External heaters on the barrel supply consistent heat to assist the melting process.
• Polymer Melts Gradually
  ⬇
  The combination of shear heat and barrel heat softens and melts the plastic progressively as it moves along the screw.
• Homogeneous Melt is Ready for Injection
  ⬇
  By the end of the screw, the polymer is a uniform molten state and ready for injection into the mold.