In precision injection production, thick-wall plastic parts with wall thickness over 3mm are high-incidence products for surface shrinkage depression, internal voids and cavities. Ordinary thin-wall products can be stably formed by conventional processes, while thick-wall parts present special molding characteristics such as external rapid cooling and internal slow solidification, surface crusting first, large core shrinkage, and difficult exhaust. Common defects include visible surface shrinkage pits, pinholes after polishing, internal hollow cavities after sectioning, insufficient product strength and assembly collapse. Most manufacturers simply increase holding pressure and extend cooling time, resulting in a contradictory phenomenon that shrinkage is improved but voids are aggravated, or voids are optimized while shrinkage becomes more serious. To completely solve these two common defects of thick-wall parts, it is necessary to understand the balance logic of shrinkage compensation and exhaust pressure relief, and implement comprehensive management from raw materials, product structure, mold and process.
1. Fundamental Causes of Shrinkage and VoidsThe biggest molding problem of thick-wall parts is extremely uneven cooling. The surface of the part contacts the mold and hardens rapidly to form a shell, while the cooling speed of internal melt is more than three times slower than the surface. The solidification and shrinkage volume of thick-wall positions is large. If the gate freezes in advance without sufficient melt replenishment, the surface will be pulled down by internal tensile force to form shrinkage. At the same time, the accumulated melt in thick-wall parts is large in volume. Air entrained during plasticization, water vapor evaporated from residual moisture in raw materials, and gas trapped in the cavity cannot be discharged quickly, and finally enclosed inside the product to form round voids and hollow cavities. In short, insufficient holding pressure leads to shrinkage, while excessive holding pressure and poor exhaust cause voids, which is the core contradiction difficult to debug for thick-wall injection parts.
2. Product Structure Optimization
The most effective way to eradicate defects is to reduce material thickness by hollowing design and control the solid wall thickness within 3mm. For column positions, bosses and thick-wall areas that cannot be thinned overall, hollowing, grooving and symmetric reinforcing ribs are adopted to replace solid structures. The wall thickness difference of products must be controlled within 0.5mm to avoid local heat accumulation causing concentrated shrinkage and gas trapping. Large fillets are adopted at all thickness transition positions to balance cooling, shrinkage and feeding compensation, eliminating local heat accumulation, gas trapping and depression caused by uneven shrinkage.
3. Raw Material Drying and Plasticization ControlThick-wall parts are extremely sensitive to raw material moisture. Trace moisture will continuously vaporize in high-temperature melt to form large voids. Conventional plastic raw materials must meet strict drying standards: ABS dried at 75～80℃ for 2～3 hours, PC dried at 100～120℃ for 3～4 hours, ensuring the moisture content is below 0.02% to completely eliminate water vapor voids. The screw back pressure during plasticization is controlled at 0.5～0.8MPa. Appropriate back pressure can compact melt and discharge entrained air. Too low back pressure leads to loose melt with air inclusion, while excessive back pressure causes shear overheating and decomposition flue gas voids. Medium and low screw speed avoids excessive air entrainment during high-speed plasticization, reducing gas sources fundamentally.
4. Mold Optimization SchemeMolds for thick-wall parts must ensure sufficient feeding compensation and strong exhaust. The gate thickness is made 50%～60% of the product wall thickness to ensure the gate freezing time is later than the product cooling time, allowing holding pressure to continuously compensate shrinkage and avoid shrinkage caused by early gate sealing. The runner size is kept at 6～8mm to reduce pressure loss along the path. Exhaust grooves must be arranged at thick-wall positions, dead corners and terminal positions with exhaust groove depth of 0.02～0.03mm to prevent gas trapping and voids. The distance between mold cooling water channels and plastic positions is 20～30mm to balance the internal and external cooling of thick-wall parts, reduce internal and external shrinkage difference, and avoid internal hollow cavities locked by rapid surface hardening.
5. Precise Debugging of Injection Molding ParametersThick-wall parts are forbidden to be filled at one time with high speed and high pressure. The injection speed adopts slow-fast-segmented filling: slow speed in the early stage to avoid air entrapment, stable filling in the middle stage, and slow down in the final stage for exhaust, preventing air wrapped in the cavity from forming voids. Multi-stage decreasing holding pressure is adopted: the first stage holding pressure is 60%～70% of the injection pressure for shrinkage compensation, and the second stage pressure is slightly reduced to release pressure, avoiding high pressure trapping gas to form internal voids. The holding time must cover the complete freezing time of the gate, with 8～12 seconds for 3mm wall thickness. Appropriately increase the mold temperature, ABS controlled at 65～75℃ to delay surface crusting speed, leaving a longer compensation window for internal melt and reducing surface shrinkage. The cooling time is set at 10 seconds per millimeter of wall thickness to ensure full solidification of thick-wall core and avoid invisible cavities formed by secondary shrinkage in the later stage.
6. Correction of Common On-site Misunderstandings
The failure of most factory debugging lies in single-parameter adjustment. Simply increasing holding pressure will press gas into the interior to form voids; only reducing speed for exhaust will lead to insufficient compensation and deeper shrinkage; merely extending cooling time will harden the product surface and lock internal shrinkage. The correct logic is exhaust first, then shrinkage compensation, and finally balanced cooling. Only synchronous adjustment of the three can completely solve the coexistence of shrinkage and voids in thick-wall parts.
7. SummaryShrinkage and voids of thick-wall injection parts are comprehensive defects caused by structural factors, thermal shrinkage and gas retention, which cannot be completely solved by single process parameter adjustment. Structurally, reduce material thickness and control uniform wall thickness to eliminate excessive shrinkage space; in terms of raw materials, thorough drying and stable back pressure plasticization prevent water vapor and air entrapment; mold design adopts enlarged gate for compensation, optimized exhaust and balanced water channel cooling to eliminate gas trapping and cooling difference; process debugging uses segmented injection speed, multi-stage holding pressure and matched cooling time to balance the contradiction between compensation and exhaust. The coordinated implementation of the whole scheme can thoroughly solve stubborn problems such as surface shrinkage, internal voids, insufficient strength and later collapse of thick-wall injection parts, and greatly improve the yield and dimensional stability of mass production.
