In injection molding, melt shear is a core physical process that determines fluidity and uniformity of plastic melt. Injection speed and back pressure are the two key parameters for controlling melt shear. By altering intensity and distribution of melt shear, they directly affect product's appearance, mechanical properties, and production stability, making them key technical skills that process engineers must master.

Injection speed is flow rate of melt as it enters mold cavity under screw's thrust. It has the most direct and significant impact on melt shear. Core logic can be summarized as "speed determines shear intensity, which in turn affects melt state."
- Positive Correlation with Shear Intensity: Faster injection speed increases melt's flow velocity at barrel nozzle, mold gate, and cavity slits. This intensifies relative motion with runner walls, increases shear rate, and significantly enhances shear intensity.
- Advantages: Moderately high shear can reduce melt viscosity (especially for shear-sensitive materials such as PP and PE), improve fluidity, ensure complete mold filling for thin-walled and complex-structured products, reducing defects such as underfill and cold spots.
- Risks: Excessive shear intensity (e.g., excessive injection speed) can lead to two problems: First, excessive shear heat generation can cause material degradation (e.g., black spots in ABS at high temperatures, fiber breakage in PA66 reinforced parts), impacting mechanical properties of product; second, unstable melt flow can create "jet streams," leading to uneven mixing of melt within mold cavity, bubbles, and delamination, ultimately causing silver streaks on product surface or internal stress concentrations.
- Process Adaptation Principles: Injection speed must be matched to product structure and material properties. Thin-walled products require higher speeds to ensure mold filling, while thick-walled products require lower speeds to avoid excessive shear. Materials insensitive to shear (e.g., PC) can be injected at a higher speed, while heat-sensitive materials (e.g., PVC) require stricter speed control to prevent overheating.

 

Injection speed linearity test

Back pressure is reverse pressure exerted on melt by barrel tip during screw retraction phase during plasticization stage. Its effect on melt shear is more of a "fine-tuning" process, primarily by increasing internal friction within melt, thereby improving shear uniformity.

 

Back Pressure Generation Principle

 

- Optimizing Shear Uniformity: When back pressure increases, screw's retraction resistance increases, which intensifies extrusion of melt within barrel. This increases friction between melt's molecular chains and between melt and barrel wall, resulting in more complete shear.
- Advantages: Sufficient shear promotes more complete melting of plastic pellets (reducing unmelted pellets) and more uniform mixing of masterbatch, additives, and base resin, preventing color spots and streaks in finished product. Furthermore, moderate back pressure can expel gases from melt (such as moisture in raw materials and volatiles produced during plasticization), reducing bubble defects in finished product.
- Risks: Excessively high back pressure can lead to two problems: First, accumulation of shear heat, similar to high injection speeds, can cause material degradation. Second, it reduces plasticizing efficiency (slower screw retraction speed), prolonging molding cycle and reducing production efficiency. It can also cause melt to reside excessively in barrel, further exacerbating thermal degradation.
- Process Adaptation Principle: Back pressure setting should be based on raw material conditions. For highly hygroscopic materials (such as PA and PC) or products requiring high color mixing requirements, back pressure can be appropriately increased. For low-viscosity, easily degradable materials, back pressure should be kept at a low level to avoid excessive shear.
Shear rate simulation analysis results

 

Injection speed and back pressure do not act independently. Their effects on melt shear exhibit a synergistic effect, requiring coordinated adjustment to achieve process balance.
- Positive synergy scenario: When using a high injection speed (requiring strong shear to ensure mold filling), back pressure can be appropriately reduced to avoid cumulative shear heating that can lead to material degradation. Conversely, if product requires high uniformity (such as transparent parts), increasing back pressure is necessary to enhance shear mixing. In this case, injection speed should be reduced to prevent the total shear strength from exceeding specified value.
- Reverse imbalance risk: If high injection speeds and high back pressures are used simultaneously, both melt shear strength and heat generation will exceed specified value, which can easily lead to material degradation and increased stress within part. If low injection speeds and low back pressures are used together, uneven melt plasticization and insufficient mold filling can occur, resulting in defects such as material shortages and color spots.
Balancing speed and back pressure with "shear control" as core
Controlling melt shear with injection speed and back pressure is essentially a process of "balancing strength and uniformity." Process personnel need to break away from "single parameter adjustment" mindset and combine product quality requirements (appearance, strength, precision) with material properties. By adjusting the two in a coordinated manner, they can control melt shear within "moderate range" - ensuring complete melt filling and uniform mixing, while avoiding quality risks caused by excessive shear, ultimately achieving dual guarantees of product quality and production stability.