In aluminum profile extrusion production, die lines are one of the most common surface defects. They appear as longitudinal concave-convex streaks along the extrusion direction on the profile surface. Minor die lines impair surface texture, while severe ones cause problems such as color difference and incomplete coverage in subsequent surface treatments like anodizing and powder coating, directly reducing product yield and delivery grade. As a typical quality pain point in the extrusion process, die lines arise from multiple dimensions including die condition, process parameters and billet quality, and require systematic root-cause analysis and targeted control solutions.
Core Causes of Die Lines
Die lines are essentially the result of interaction between the die bearing surface and metal flow as the alloy exits the die orifice. All extruded profiles have die lines to varying degrees, and they are classified as defects when streak depth exceeds quality standards. The main triggers fall into three categories:
1. Die-Related Factors
The die is the core source of die lines, and the condition of the bearing surface directly determines profile surface quality.
Insufficient bearing surface finish If the bearing surface fails to meet mirror polishing requirements, with machining defects such as wire-cut marks, tool marks and pits, these flaws will be directly replicated onto the profile surface during metal flow, forming continuous longitudinal streaks. The standard requires bearing surface roughness within Ra 0.4 μm; die lines become significantly noticeable when roughness exceeds Ra 0.8 μm.
Insufficient bearing hardness and wear Inadequate die nitriding results in surface hardness below HV900, leading to rapid bearing wear during production and the formation of furrow-like wear marks. After long-term operation, peeling of the nitrided layer and base material damage will also cause a sudden deepening of die lines.
Unreasonable bearing design Excessively long bearing length increases metal friction stroke, intensifying aluminum pickup and surface scratching. Large step gaps between bearing sections without arc transition cause abrupt metal flow, forming local raised or grooved die lines.
Aluminum pickup and foreign matter on the bearing When extrusion temperature is too high, aluminum alloy particles adhere to the bearing surface and form buildups. These periodically detach and re-adhere during production, leaving discontinuous or continuous scratches on the profile. Foreign matter such as graphite and metal debris remaining in the die cavity can also directly scrape longitudinal grooves.
2. Extrusion Process Factors
Mismatched process parameters amplify the impact of die defects, inducing or worsening die lines.
Excessively high extrusion temperature When billet and die temperatures exceed specifications, aluminum alloy viscosity decreases and plasticity increases, making metal particles more likely to adhere to the bearing surface and form pickup-type die lines. Meanwhile, high temperature accelerates die surface oxidation and annealing, reducing hardness and wear resistance.
Excessively fast extrusion speed Too high an exit speed increases shear stress of metal flow and raises friction in the bearing adhesion zone, which not only worsens aluminum pickup but also causes die micro-vibration, forming wavy die lines. Sudden speed changes can also create instant impressions and stop marks.
Uneven die temperature Insufficient die preheating and large temperature differences across zones result in uneven metal flow and overloaded local bearing areas, leading to inconsistent wear and aluminum pickup, and forming longitudinal streaks of varying severity.
3. Billet and Equipment Factors
Billet quality defects Billet composition segregation, high impurity content and insufficient homogenization lead to uneven plastic flow. Hard particles will scratch grooves when passing through the bearing. Oxide scales and oil stains on the billet surface, if not cleaned before extrusion, will be carried into the die cavity and scratch the profile surface.
Equipment accuracy issues Extruder spindle vibration, unstable puller operation and insufficient die support pad stiffness can cause periodic vibration of the die or profile, forming chatter marks and wavy die lines on the surface.
Targeted Solutions for Die Lines
To address the above causes, coordinated governance across die control, process optimization and production management is required to achieve effective control of die lines.
1. Full-Process Die Quality Control
Improve bearing machining and polishing precision Strictly control the bearing machining workflow. Perform step-by-step polishing after wire cutting to ensure surface roughness Ra ≤ 0.4 μm, with no sharp edges or steps. Adjust the bearing exit angle within -1° to -3° to reduce adhesion zone height and frictional resistance.
Standardize nitriding treatment Adopt a high-quality gas nitriding process to ensure bearing surface hardness of HV900–1200 and uniform nitrided layer depth. Re-inspect die hardness after a set number of production shifts. Re-nitride and re-polish dies when the nitrided layer peels off; damaged dies are prohibited from production.
Optimize bearing design Design bearing length reasonably according to profile wall thickness and alloy characteristics, avoiding excessively long or short dimensions. Apply smooth arc transitions between bearings of different thicknesses to reduce abrupt metal flow. Balance flow velocity via feeder hole design for complex sections to reduce local bearing load.
Establish a die maintenance mechanism Keep usage records for each die, and conduct regular off-machine polishing and maintenance based on extrusion tonnage. Promptly remove dies for cleaning when aluminum pickup occurs during production; forced operation is not allowed. After repair, verify bearing finish and dimensions before returning to production.
2. Extrusion Process Parameter Optimization
Precisely control extrusion temperature Set optimal billet and die temperatures according to alloy grade. For 6063 alloy, billet temperature is generally controlled at 460–500℃ and die temperature at 420–450℃. Keep temperature fluctuation within ±10℃ to avoid over-temperature production.
Match reasonable extrusion speed Follow the principle of "low temperature with low speed, high temperature with high speed". Appropriately reduce exit speed for profiles with high surface quality requirements. Maintain steady extrusion speed, avoid frequent start-stops and sudden speed changes, and minimize stop marks and instant impressions.
Preheat dies fully and uniformly Preheat dies thoroughly for 2–4 hours before installation to ensure uniform overall temperature and eliminate local differences. Control temperature variation across die zones within 20℃ during production to maintain stable metal flow.
3. Billet and Equipment Management
Strictly control billet quality Use billets qualified by homogenization treatment and control impurity content. Clean oxide scales and oil stains on billet surfaces before extrusion to prevent foreign matter from entering the die cavity. Strictly control sodium content in high-magnesium alloys to reduce aluminum pickup tendency.
Ensure equipment operation accuracy Regularly calibrate extruder spindle coaxiality and puller stability, and replace aged seals and guide rails. Use die support pads of appropriate stiffness to avoid die vibration during extrusion. Keep the extrusion container inner wall clean and regularly remove accumulated aluminum and impurities.
On-Site Prevention and Control Recommendations
Die line management focuses on prevention. It is recommended that enterprises establish a three-level control mechanism:
Pre-production confirmation: Check bearing surface condition, hardness and dimensions before die installation, and verify process parameters and billet quality. Production is prohibited if conditions are not met.
In-production inspection: Sample and inspect surface quality every 10–20 profiles during production. When die lines worsen, promptly identify the cause and take measures such as speed reduction, temperature adjustment or off-machine die repair.
Post-production review: Conduct root-cause analysis for batches with out-of-tolerance die lines, update die maintenance cycles and process parameters, and form a closed quality loop.
As a common issue in the aluminum extrusion industry, die lines cannot be completely eliminated, but can be stably controlled within standard limits through refined management. Systematic optimization of dies, processes and materials not only improves product surface quality and yield, but also extends die life and reduces production costs, making it a key way for aluminum profile manufacturers to enhance core competitiveness.
