Injection Molding Considerations for PSU Resin

PSU (Polysulfone) is a high-temperature resistant, high-rigidity, high-viscosity amorphous engineering plastic. Its injection molding difficulty is higher than that of ordinary ABS/PP. The core is to grasp the five key points: drying, temperature, mold, process, and post-processing. Below is a detailed explanation of the precautions from the whole process of raw material pretreatment, injection molding process, mold design, defect solution, and post-processing, balancing mass production stability and quality control.

I. Raw Material Pretreatment: Absolute Drying to Eliminate Bubbles and Silver Streaks

PSU is extremely hygroscopic (equilibrium water absorption 0.25%, saturation up to 1.2%), and insufficient drying is the primary cause of processing failure, which must be strictly implemented:

Drying requirements: A hot air circulation dehumidifying dryer must be used; ordinary hopper drying is strictly prohibited. The temperature is 120–140℃, the time is 4–6 hours, the material layer thickness is ≤50 mm to avoid local overheating and degradation. The moisture content after drying is controlled at ≤0.02% (detected by a moisture meter; below 0.03% is prone to silver streaks, bubbles, and delamination).

Storage and secondary drying: Seal and prevent moisture after opening; if exposed to air for more than 2 hours, it must be dried again. If the machine is shut down for more than 1 hour, the remaining material in the barrel must be emptied. Before starting the machine again, the new material should be supplemented with drying for 1–2 hours to prevent moisture absorption and re-moisture.

Taboos: Do not mix with moisture-absorbing materials; the drying air must be dehumidified (dew point ≤-30℃) to avoid secondary moisture absorption caused by water-carrying hot air.

II. Injection Molding Parameters: Precise Temperature Control, Low Shear, and Slow Mold Filling

PSU has high melt viscosity and good thermal stability, but it is prone to oxidative degradation and shear sensitivity at high temperatures. The core of the parameters is high temperature, low pressure, slow injection, segmented temperature control, reasonable holding pressure, and sufficient cooling:

Barrel temperature: Set accurately in segments, gradually increasing from the hopper to the nozzle to avoid local overheating; rear section 280–300℃, middle section 310–320℃, front section 320–335℃, nozzle 325–340℃. For glass fiber reinforced PSU (GF30), take the upper limit of 330–340℃; for pure PSU, take the lower limit of 310–325℃. It is strictly prohibited to exceed 350℃; exceeding will cause yellowing, brittleness, and toxic gas decomposition.

Nozzle and mold: The nozzle adopts an open large diameter (≥4 mm); small diameter and self-locking type are prohibited (easy to block and high shear). The mold temperature must be high, 80–120℃ (controlled by a mold temperature machine; cold water direct cooling is strictly prohibited). For thin-walled parts, 100–120℃; for thick-walled parts, 80–100℃. Insufficient mold temperature will lead to high internal stress, cracking, decreased transparency, and obvious weld lines.

III. Mold Design: Adapt to High Viscosity, Prevent Internal Stress, and Facilitate Demolding

The mold is the key to stable PSU molding, focusing on solving the problems of difficult mold filling, high internal stress, and easy cracking during demolding:

Runner and gate: The runner adopts a circular/trapezoidal large cross-section, shortening the runner length to reduce resistance. The gate selects side gate, fan gate, and large-diameter point gate; small pin-point gate is prohibited (high shear and easy to block). Gate size: width ≥1.5 times the product wall thickness, thickness ≥0.8 times the wall thickness, avoiding shear overheating and gate cracking caused by too small gate.

Venting: PSU has slow mold filling and easy air entrainment. Venting grooves must be opened at the end of the cavity, weld line, and bottom of the deep cavity. The depth of the venting groove is 0.02–0.04 mm, width 5–10 mm, ensuring smooth air discharge to prevent bubbles, burning, and missing glue.

Demolding and ejection: PSU has high rigidity and sensitivity to internal stress, so the demolding slope should be large: 1.5–3° for the cavity, 2–4° for the core; take the upper limit for deep cavity/ribs. Ejection adopts a combination of ejector pins + top plate + ejector sleeves; the number of ejector pins is large and evenly distributed to avoid excessive local ejection force. The end face of the ejector pin is polished, and an ejector pin guide sleeve is added to prevent white top and cracking. The surface of the mold cavity/core is mirror-polished (Ra≤0.2μm) to reduce demolding resistance and avoid scratches.

Cooling system: The mold cooling water channels are evenly arranged, close to the cavity, ensuring stable mold temperature of 80–120℃, avoiding internal stress and deformation caused by local temperature difference; rapid cooling is prohibited, and the water channel diameter is ≥8 mm to ensure smooth circulation.

IV. Common Defects and Solutions: Targeted Adjustment to Avoid Repeated Mold Trials

Silver streaks/bubbles: The core is insufficiently dried raw materials (re-dry to moisture content ≤0.02%), too high material temperature (reduce by 5–10℃), poor venting (deepen venting groove), and too fast injection speed (reduce speed).

Cracking/brittleness: High internal stress (increase mold temperature, reduce injection pressure, extend holding pressure, annealing treatment), too small gate (enlarge gate), unbalanced ejection (adjust ejector pins), and raw material degradation (control temperature, reduce shear).

Obvious weld lines: Increase mold temperature and material temperature, slow down injection speed, add venting, and optimize gate position to reduce welding.

Deformation and warpage: Uneven cooling (optimize water channels), too low mold temperature (increase mold temperature), improper holding pressure (adjust holding pressure/time), and uneven product wall thickness (optimize product structure).

Yellowing/burning: Material temperature exceeds 350℃, too long residence time (shorten cycle, empty remaining material), excessive screw shear (reduce speed, reduce back pressure), and too high nozzle temperature.

V. Post-Processing and Storage: Eliminate Internal Stress and Ensure Long-Term Stability

PSU has high internal stress after injection molding, which is easy to crack during use (especially when exposed to solvents/high temperatures), so post-processing is necessary:

Annealing treatment: Immediately put the product into an oven at 140–160℃ after ejection, keep warm for 2–4 hours, and cool to room temperature with the furnace. It eliminates internal stress, improves crack resistance and solvent resistance; annealing is necessary for precision parts/force-bearing parts.

Storage: Seal and package after annealing, avoid moisture absorption, exposure to sunlight, and contact with organic solvents (alcohol, acetone, gasoline, etc. will cause stress cracking).

VI. Safety and Equipment Maintenance

The processing temperature is high; prevent scalding from the barrel/nozzle, and ensure good ventilation. PSU decomposes at high temperature to produce a small amount of toxic gas, so the workshop exhaust must be turned on.

When shutting down: First empty the PSU in the barrel, clean the barrel with PE/PP, prevent residual PSU from degrading and carbonizing at high temperature, and blocking the nozzle when starting the machine next time.