Key Notes for Injection Molding of Glass‑Filled PA66

Glass‑fiber‑reinforced PA66 is a widely‑used modified polymer in injection molding industry. Glass fiber filler greatly improves tensile strength, rigidity and heat‑distortion temperature of base material. Nevertheless, strong moisture absorption and deteriorated flowability will bring frequent defects such as glass‑fiber float, burning marks, dimensional fluctuation and mold abrasion. Raw‑material pretreatment, mold design, process parameters and equipment condition jointly determine finished‑product yield. Major notes for glass‑filled PA66 injection molding are summarized as below.

1. Raw‑Material Drying Control

PA66 absorbs moisture easily even after glass‑fiber modification. Excessive moisture inside pellets will generate silver streaks, bubbles and water marks on molded parts, and reduce impact strength significantly. Raw materials shall be sealed in storage and unpacked right before use.

Drying temperature shall be set at 80‑85℃ with drying duration of 4‑6 hours. Dried pellets shall be consumed within 2‑4 hours, otherwise moisture will be re‑absorbed from ambient air. Baking temperature higher than 90℃ is forbidden, for over‑heating will trigger material yellowing and degradation of glass‑fiber performance.

Re‑grind material adding ratio shall be controlled within 20%. Re‑grind pellets need drying together with virgin resin. Mechanical performance will keep declining after multiple recycling cycles, so excessive re‑grind is not allowed.

2. Injection Machine and Screw Configuration

Glass fiber will cause severe abrasion on screw and barrel. Bimetallic screw‑barrel assembly shall be selected for long‑term mass‑production. Ordinary nitrided screws will get scratched quickly and lead to unstable plasticization.

Screw length‑to‑diameter ratio shall be 20‑25:1 with compression ratio of 2.0‑2.5. Excessively high compression ratio will produce heavy shear heat and cause material decomposition and burning. Medium‑low screw rotating speed is recommended. High rotational speed will cut glass‑fiber length, weaken mechanical performance of finished parts and generate extra frictional heat.

Barrel temperature zoning reference: feeding zone 240‑250℃, transition zone 250‑260℃, nozzle 255‑265℃. Overall barrel temperature shall not exceed 270℃. Over‑high temperature will degrade PA66 matrix and make molded parts brittle and discolored.

3. Mold Design Essentials

Large‑size gates including fan gates, submarine gates and direct gates are preferred. Tiny pin gates will produce heavy shear and aggravate glass‑fiber float. Runner cross‑section shall be enlarged to decrease melt flow resistance and prevent glass‑fiber separation.

High‑hardness wear‑resistant mold steel such as quenched S136H or NAK80 shall be adopted. Ordinary P20 steel will be eroded rapidly by glass‑fiber flow and leave permanent scratch marks on cavity surface.

Mold temperature is critical. Recommended mold temperature ranges 80‑120℃. Low mold temperature will freeze melt rapidly. Glass fiber cannot be fully wrapped by resin matrix, resulting in severe glass‑fiber float and poor weld‑line strength. Higher mold temperature improves surface quality and mechanical property yet prolongs cooling cycle.

Sufficient vent slots shall be opened, vent depth 0.015‑0.025mm. Poor venting easily causes burn marks for glass‑filled PA66. Extra vents shall be arranged at weld‑line positions and melt terminals. Wall thickness shall keep uniform. Thickness of reinforcing ribs shall not exceed two‑thirds of main wall thickness to avoid sink marks. Demolding draft angle shall be enlarged to 1‑2° for shell‑type components. Large demolding resistance from glass‑filled material will scratch parts with insufficient draft angle.

4. Injection Process Parameter Tuning

Sufficient injection pressure is required for poor flowability of glass‑filled PA66. Injection pressure normally ranges 80‑120 bar according to part dimension. Medium injection speed is adopted. Too‑fast injection will induce jetting, glass‑fiber float and burning risk; too‑slow injection leads to premature melt cooling and short shots.

Holding‑pressure value shall reach 50%‑70% of injection pressure. Holding‑pressure duration shall match wall thickness to suppress sink marks and stabilize dimensional tolerance. Back‑pressure shall be kept at 0.8‑1.5 MPa. Low back‑pressure brings uneven mixing; excessive back‑pressure generates heavy shear heat and material degradation.

Adequate cooling time is necessary. Although glass‑filled PA66 crystallizes fast, thick‑wall parts require sufficient cooling to avoid post‑mold shrinkage and deformation after demolding.

5. Common Defect Control in Production

Glass‑fiber float is the most‑frequent defect for glass‑filled PA66. Improvement measures include proper raw‑material drying, higher mold temperature, reduced shear speed and enlarged gate‑runner size to decrease separation between glass fiber and resin matrix.

Mechanical strength drops heavily at weld‑line locations. Weld‑lines shall be kept away from force‑bearing zones for structural components. Higher mold temperature and better venting will relieve visible weld traces.

Dimension of finished parts changes with ambient humidity because of water absorption. Dimensional inspection shall be conducted under standardized environmental condition. During long‑time machine halt, melt inside barrel shall be purged. Glass‑filled PA66 shall not stay at high temperature inside barrel for long time, otherwise carbonization will cause black specks and burn marks after restarting production.

Conclusion

Main molding challenges for glass‑filled PA66 are moisture absorption, glass‑fiber float, mold abrasion and dimensional instability. Stable production can be achieved by complete raw‑material drying, wear‑resistant screw and mold steel, reasonable mold‑temperature setting and lowered melt shear. Water absorption of finished molded parts will modify dimension and mechanical performance. Such variation shall be taken into consideration in product design and dimensional acceptance specification.