Dec 04,2025
Microcellular foam injection molds: material and structural challenges in the era of lightweight
Microcellular foam injection molds: material and structural challenges in the era of lightweight
Microcellular foaming (MuCell) injection molding technology, in which supercritical fluid (SCF, such as N2 or CO2) are injected into the melt to form closed cells with a diameter of 10-100 microns in the cavity, is used to reduce the size of the cells, it is a revolutionary process to achieve lightweight products, low internal stress, high dimensional stability. This also puts forward the unprecedented new request to the mold design.
Special requirements of microcellular foaming process for mould
Extremely high clamping force utilization: the foaming process requires a high initial injection speed to trigger uniform nucleation, requires excellent mold rigidity, and the template can not produce microelasticity, otherwise it will produce flash.
Ultra-high-speed filling and venting: melt/gas mixture filling is extremely fast, which requires an extremely efficient venting system, otherwise trapped air will lead to cell coalescence, scorching or incomplete filling.
Precise control of mold temperature: mold temperature directly affects the cell growth and curing. A lower mold temperature (usually 40-60 °C) favors the formation of fine cells, but a more precise temperature control system is required to ensure uniformity.
Key Technical Points of die design
Gate design:
Preferred Fan Gate or film gate: provide wide entrance, avoid jet, ensure foam fluid smooth, uniform spread throughout the cavity.
Avoid Pin Gate: easy to lead to spray, the formation of uneven cell structure and surface gas marks.
Exhaust system upgrade:
Increase the depth and width of the exhaust slot: due to the filling of the melt/gas mixture with lower viscosity and surface tension, the depth of the exhaust slot can be appropriately increased to 0.03-0.06 mm.
Wide use of exhaust steel: the key to ensure success is to use exhaust steel inserts extensively at all air-trapping risk points such as the last filling area and the bottom of the reinforcement.
Vacuum exhaust: for complex structure, high surface requirements of the parts, vacuum exhaust is the most reliable protection.
Die steel and surface treatment:
High toughness steel: foam molding is usually accompanied by high cavity pressure peak, the use of pre-hardened steel (such as P20/718) or quenching steel (such as H13) to ensure the die life.
Corrosion resistant coating: supercritical fluid may be somewhat aggressive to steel and can be protected by PVD coatings such as CRN, which also improve mold release.
Mold solutions to typical defects
Silver Streak:
Cause of formation: the cell is sheared off during flow and crazes are formed on the surface.
Mold countermeasures: optimize the gate design, to avoid excessive flow shear; improve mold temperature, reduce the melt and die wall shear; strengthen exhaust.
Orange Peel:
Cause of formation: Bleb is broken in cuticular layer.
Mold countermeasures: improve mold temperature, delay the cooling of the skin layer, let the bubble grow in the interior; polishing the cavity with high-gloss mirror to reduce the resistance of gas breakthrough.
Warpage:
Cause of formation: the shrinkage behavior of foamed products is different from that of traditional injection molding, and the anisotropy is more obvious.
Die countermeasures: predict the deformation trend through CAE analysis, and carry out anti-deformation compensation (correction) in advance in the die design stage; optimize the cooling system to ensure uniform cooling.
Conclusion:
The design of microcellular foaming mold needs to break the thinking pattern of traditional injection mold. It requires engineers to have a deep understanding of the interplay between fluid dynamics, foaming nucleation mechanisms and material properties. Through targeted gate, exhaust and cooling design, and the selection of the right steel, the mold can become an enabler of this advanced process, rather than a bottleneck, and ultimately take the lead in the lightweight race.
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