How to Improve the Fluidity of Plastics
Fluidity is a key property of molten plastic that directly affects filling behavior, molding stability, surface quality, and process controllability. Poor fluidity often causes short shots, flow marks, weld lines, uneven filling, and high internal stress. Improving fluidity involves optimizing processing parameters, material selection, additives, mold design, and production environment. This article summarizes practical and effective methods to enhance plastic flow during injection molding.
Fundamental Principles
Fluidity refers to the ability of molten plastic to flow under heat and pressure. It is inversely related to melt viscosity: lower viscosity means better fluidity. The goal of improvement is to reduce viscosity, minimize flow resistance, stabilize melt delivery, and ensure complete and uniform cavity filling.
1. Optimize Injection Molding Process Parameters
Process adjustment is the most direct and cost‑free method to improve fluidity. Increasing barrel temperature, especially in the front and nozzle zones, helps fully melt the polymer, relax molecular chains, and reduce viscosity. However, excessive temperature must be avoided to prevent thermal degradation, discoloration, gas generation, or material decomposition.
Increasing injection speed promotes shear thinning, a phenomenon where melt viscosity decreases under high shear rate. This is especially effective for thin‑wall parts and long flow paths. Properly increasing injection pressure helps overcome flow resistance, but excessive pressure causes flash, high stress, and warpage. Multi‑stage injection profiles—slow at the start, fast in the middle, slow at the end—ensure stable filling without disturbing the flow front.
Raising mold temperature reduces the temperature difference between melt and mold, slows cooling, and extends the effective flow time. This is highly effective for materials with high viscosity or low flowability. Consistent mold temperature also improves surface quality and reduces residual stress.
2. Select Materials with Inherent Better Flowability
Melt index (MI) is the most direct indicator of flowability. For the same polymer type, grades with higher MI offer better flow. For example, high‑flow ABS, PP, PS, and PC are designed for thin‑wall, complex, or large parts. Reducing molecular weight also improves flow, although it may slightly reduce mechanical strength. For applications requiring both performance and flowability, specially modified high‑flow materials are available, including high‑flow flame‑retardant and reinforced grades.
3. Use Additives to Improve Flow
Lubricants and processing aids reduce internal friction between polymer chains and external friction at the metal interface. They effectively lower viscosity and improve flow without significantly changing base properties. Common additives include external lubricants, dispersants, and processing modifiers. Proper drying is also critical for hygroscopic materials such as nylon and polyester. Moisture causes degradation, inconsistent viscosity, and poor flow. Adequate drying not only improves fluidity but also eliminates silver streaks and bubbles.
4. Optimize Mold Design to Reduce Flow Resistance
Mold structure significantly influences flow resistance. Larger gates, fan gates, or side gates reduce pressure loss and allow smoother melt entry. Runners should be short, thick, and smooth with minimal bends and dead zones. Effective venting prevents trapped gas from blocking flow, which is a common cause of short shots and burn marks. For long or asymmetric parts, multiple gates or optimized gate locations shorten flow paths and reduce dependence on material flowability.
5. Stabilize Production Environment and Processing Conditions
Fluctuations in workshop temperature, uneven material feeding, moisture contamination, or inconsistent drying all affect melt viscosity and flow stability. Raw materials must be protected from moisture and cross‑contamination. Steady process settings avoid sudden changes in temperature, speed, or pressure. Long shutdowns require proper purging and warming to prevent cold slugs or degraded material from affecting the first shots.
Summary
Improving plastic flowability requires integrated optimization of process, material, additives, mold, and environment. Process adjustments provide immediate improvement. Material selection delivers fundamental enhancement. Additives fine‑tune flow without major reformulation. Mold design reduces resistance and improves filling stability. A stable production environment ensures consistent flow over time. Balancing these factors allows smooth filling, high productivity, low defect rates, and stable quality in injection molding.
