La pressofusione a bassa pressione e la pressofusione per gravità sono due processi fondamentali per la formatura dei metalli, ciascuno costruito su principi meccanici distinti. Mentre entrambi modellano il metallo fuso in parti finite utilizzando stampi, i loro approcci al riempimento delle cavità, uno guidato dalla pressione controllata, l'altro dalla gravità naturale: crea forti differenze di qualità, efficienza, e costo. Per i produttori, choosing between them means balancing needs like part strength, volume di produzione, e bilancio. But what exactly sets their workflows apart? How do these differences impact end-product performance? And which process is right for your specific application? This article answers these questions with detailed comparisons and actionable guidance.
1. Core Principles: The Fundamental Contrast in Filling Mechanisms
The biggest divide between the two processes lies in how molten metal is delivered to the mold cavity—a difference that shapes every other aspect of their operation.
UN. Pressofusione a bassa pressione
- Driving Force: Usi dry compressed air or inert gas (per esempio., azoto) to apply controlled pressure (typically 0.02–0.15MPa) to a sealed crucible holding molten metal.
- Filling Process: Pressure forces metal up a vertical liquid lift tube and into the mold cavity from the bottom up. This creates a slow, steady “laminar flow” (no turbulent splashing), ensuring the cavity fills completely without trapping air.
- Solidificazione: The pressure is maintained during cooling (pressure-holding crystallization). This pushes remaining molten metal into shrinkage gaps, eliminating defects like pores or voids.
B. Pressofusione per gravità
- Driving Force: Relies solely on metal’s own weight (gravity)—no external pressure is applied.
- Filling Process: Molten metal is poured into a sprue (funnel-shaped inlet) at the top of the mold. It flows downward through runners and gates into the cavity via “natural flow”—filling speed depends on the mold’s design and metal’s fluidity.
- Solidificazione: Cooling happens passively, with no pressure to counteract shrinkage. This means thicker sections may develop small shrinkage pores, though the slow filling reduces gas entrainment compared to high-pressure methods.
2. Side-by-Side Process Comparison: Key Operational Differences
To understand how these processes perform in practice, let’s break down their workflows, parametri, and constraints in a clear, data-driven table:
| Process Aspect | Pressofusione a bassa pressione | Pressofusione per gravità |
| Filling Mode | Bottom-up laminar flow; pressure-controlled (0.02–0.15MPa). | Top-down gravity flow; no external pressure. |
| Filling Speed | Lento, uniform (typically 5–15cm/s); adjustable via pressure. | Variable (depends on mold design); faster than low-pressure but slower than high-pressure die casting. |
| Metal Fluidity Requirement | Da basso a medio; works with most non-ferrous alloys (alluminio, magnesio). | Alto; requires alloys with good flowability (per esempio., alluminio, rame) to fill cavities via gravity alone. |
| Mold Design Complexity | Alto; needs sealed crucibles, liquid lift tubes, and pressure ports. | Basso; simple sprue-runner-gate systems (no pressure-related components). |
| Tempo di ciclo | Più a lungo (60–120 seconds per part); includes pressure ramp-up and holding. | Moderare (45–90 seconds per part); faster than low-pressure but slower than high-pressure die casting. |
| Waste Rate | Basso (5–8%); no need for risers (extra metal to feed shrinkage). | Più alto (10–15%); may require risers for thick-walled parts, increasing material waste. |
3. Prestazione & Qualità: Which Delivers Better Results?
The filling mechanism directly impacts part quality, proprietà meccaniche, and defect rates. Below is a detailed comparison of key performance metrics:
| Performance Metric | Pressofusione a bassa pressione | Pressofusione per gravità |
| Densità della parte | Alto (≥99.5% theoretical density); pressure eliminates shrinkage pores. | Moderare (98–99%); small pores may form in thick sections. |
| Resistenza alla trazione | Superiore (per esempio., 280–320MPa for aluminum alloys); dense structure boosts strength. | Bene (per esempio., 240–280MPa for aluminum alloys); slightly lower due to minor porosity. |
| Finitura superficiale | Eccellente (Ra 1.6–3.2μm); smooth filling avoids surface defects like cold shuts. | Moderare (Ra 3.2–6.3μm); may have minor surface irregularities from uneven flow. |
| Tasso di difetti | Basso (2–5% scrap rate); minimal oxidation or gas defects. | Moderare (5–10% scrap rate); risks include cold shuts (from slow flow) or shrinkage pores. |
| Heat Treatment Compatibility | Eccellente; uniform structure resists deformation during heat treatment (per esempio., T6). | Bene; can be heat-treated but may require pre-inspection to avoid pore expansion. |
4. Applicazioni: Matching Process to Part Needs
Each process excels in specific scenarios, based on part design, materiale, e requisiti prestazionali. The table below maps processes to ideal use cases:
| Part Requirement | Prefer Low-Pressure Die Casting | Prefer Gravity Die Casting |
| Alta resistenza, Safety-Critical Parts | Ruote automobilistiche, engine blocks/covers (need density and strength to withstand loads). | None—safety parts require the higher density of low-pressure casting. |
| Large Thin-Walled Components | Missile housings, aircraft structural parts (laminar flow prevents thin-section voids). | Not suitable—gravity flow can’t fill thin walls (≤3mm) uniformly. |
| Thick-Walled, Simple Shapes | None—overkill for low-performance thick parts. | Rail transit accessories (per esempio., bogie brackets), robot structural parts, ship propeller hubs (thick sections, low complexity). |
| Cost-Sensitive, Medium-Volume Parts | None—higher equipment costs make it uneconomical for low-margin parts. | Beni di consumo (per esempio., large cookware), valvole industriali (simple design, medium volume). |
| Non-Ferrous Alloy Parts | Ideal for aluminum, magnesio, and copper alloys (controllable flow suits these materials). | Best for aluminum and copper alloys (buona scorrevolezza); less suitable for low-fluidity alloys. |
5. Cost Analysis: Upfront Investment vs. Long-Term Savings
Cost is a critical factor for manufacturers. Below is a breakdown of equipment, muffa, and per-part costs (based on aluminum alloy parts, 10,000-part batch):
| Cost Category | Pressofusione a bassa pressione | Pressofusione per gravità |
| Equipment Investment | Alto (\(150,000–)300,000); includes sealed crucibles, pressure control systems, and lift tubes. | Basso (\(50,000–)100,000); simple melting furnaces and open molds. |
| Mold Cost | Alto (\(15,000–)40,000); complex designs with sealed cavities and lift tubes. | Basso (\(5,000–)15,000); simple, open designs with minimal features. |
| Per-Part Material Cost | Basso (\(0.4–)0.6/kg); no risers reduce waste. | Moderare (\(0.5–)0.7/kg); risers increase material usage. |
| Labor Cost | Moderare; requires trained operators to monitor pressure (1–2 operators/line). | Basso; simple process needs minimal supervision (1 operator/2 lines). |
| Total Batch Cost | ~\(30,000–)60,000 | ~\(15,000–)30,000 |
6. Yigu Technology’s Perspective on the Two Processes
Alla tecnologia Yigu, we see low-pressure and gravity die casting as complementary tools—each solving unique manufacturing challenges. For automotive clients needing high-strength wheels, our low-pressure casting lines (equipped with real-time pressure monitoring) deliver parts with 99.8% density and <3% scrap rates, meeting IATF 16949 standard. For rail transit clients, our gravity casting solutions cut upfront costs by 50% while producing durable brackets that pass 100,000-cycle load tests.
We’re optimizing both processes: 1) Adding AI to low-pressure systems to auto-adjust pressure for different part thicknesses (reducing cycle time by 15%); 2) Developing modular gravity molds that switch between part designs in 30 minuti (ideal for small-batch production). Our goal is to tailor each process to the client’s “pain points”—whether it’s quality, costo, or flexibility—instead of forcing a one-size-fits-all solution.
Domande frequenti
- Can gravity die casting be used for thin-walled parts (per esempio., 2mm di spessore)?
No—gravity-driven flow can’t fill thin walls uniformly, leading to voids or incomplete filling. Thin-walled parts (≤3mm) require low-pressure die casting, which uses controlled pressure to push metal into narrow cavities without gaps.
- Is low-pressure die casting worth the higher upfront cost for medium-volume production (per esempio., 5,000 parts/year)?
It depends on part value: Per componenti di alto valore (per esempio., ruote automobilistiche, where defects cost $100+/part), yes—lower scrap rates and better quality offset equipment costs. For low-value parts (per esempio., simple brackets), gravity casting is more economical, even with higher material waste.
- Which process is better for magnesium alloys—low-pressure or gravity die casting?
Low-pressure casting is better. Magnesium is prone to oxidation, and low-pressure’s sealed crucible and inert gas protection reduce oxidation by 80% compared to gravity casting (which exposes molten magnesium to air during pouring). This ensures magnesium parts meet corrosion-resistance standards (per esempio., ASTM B94).