Die Casting
How Do Low-Pressure Die Casting and Gravity Die Casting Differ?
Low-pressure die casting and gravity die casting are two foundational metal-forming processes. Both shape molten metal into finished parts using molds, but their approaches to filling cavities create stark differences in quality, efficiency, and cost. One uses controlled pressure to push metal up from the bottom . The other relies on gravity to pour metal down from the top .
What Are the Core Principles of Each Process?
Low-pressure die casting
Driving force: Uses dry compressed air or inert gas like nitrogen to apply controlled pressure at 0.02-0.15 MPa 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 slow, steady laminar flow with no turbulent splashing, ensuring the cavity fills completely without trapping air.
Solidification: Pressure maintains during cooling—called pressure-holding crystallization. This pushes remaining molten metal into shrinkage gaps, eliminating defects like pores or voids.
Gravity die casting
Driving force: Relies solely on metal’s own weight—no external pressure applied.
Filling process: Molten metal pours into a sprue at the top of the mold. It flows downward through runners and gates via natural flow. Filling speed depends on mold design and metal’s fluidity.
Solidification: Cooling happens passively with no pressure to counteract shrinkage. Thicker sections may develop small shrinkage pores, though slow filling reduces gas entrainment compared to high-pressure methods.
| Principle | Low-Pressure | Gravity |
|---|---|---|
| Driving force | Compressed air/inert gas at 0.02-0.15 MPa | Gravity only |
| Filling direction | Bottom-up | Top-down |
| Flow type | Laminar, controlled | Natural, variable |
| Solidification | Under pressure | Passive |
How Do Their Processes Compare Step by Step?
Filling mode
Low-pressure: Bottom-up laminar flow, pressure-controlled at 0.02-0.15 MPa.
Gravity: Top-down gravity flow, no external pressure.
Filling speed
Low-pressure: Slow and uniform at 5-15 cm/s; adjustable via pressure.
Gravity: Variable depending on mold design. Faster than low-pressure but slower than high-pressure die casting.
Metal fluidity requirement
Low-pressure: Low to medium. Works with most non-ferrous alloys—aluminum, magnesium.
Gravity: High. Requires alloys with good flowability like aluminum or copper to fill cavities via gravity alone.
Mold design complexity
Low-pressure: High. Needs sealed crucibles, liquid lift tubes, and pressure ports.
Gravity: Low. Simple sprue-runner-gate systems with no pressure-related components.
Cycle time
Low-pressure: Longer at 60-120 seconds per part; includes pressure ramp-up and holding.
Gravity: Moderate at 45-90 seconds per part. Faster than low-pressure but slower than high-pressure die casting.
Waste rate
Low-pressure: Low at 5-8%. No need for risers—extra metal to feed shrinkage.
Gravity: Higher at 10-15%. May require risers for thick-walled parts, increasing material waste.
| Process Aspect | Low-Pressure | Gravity |
|---|---|---|
| Filling mode | Bottom-up laminar | Top-down gravity |
| Filling speed | 5-15 cm/s, adjustable | Variable |
| Fluidity requirement | Low-medium | High |
| Mold complexity | High (sealed crucible, tubes) | Low (sprue-runner-gate) |
| Cycle time | 60-120 seconds | 45-90 seconds |
| Waste rate | 5-8% | 10-15% |
What Performance and Quality Differences Exist?
Part density
Low-pressure: High at ≥ 99.5% theoretical density. Pressure eliminates shrinkage pores.
Gravity: Moderate at 98-99%. Small pores may form in thick sections.
Tensile strength
Low-pressure: Superior—280-320 MPa for aluminum alloys. Dense structure boosts strength.
Gravity: Good—240-280 MPa for aluminum alloys. Slightly lower due to minor porosity.
Surface finish
Low-pressure: Excellent at Ra 1.6-3.2 μm. Smooth filling avoids surface defects like cold shuts.
Gravity: Moderate at Ra 3.2-6.3 μm. May have minor surface irregularities from uneven flow.
Defect rate
Low-pressure: Low at 2-5% scrap rate. Minimal oxidation or gas defects.
Gravity: Moderate at 5-10% scrap rate. Risks include cold shuts from slow flow or shrinkage pores.
Heat treatment compatibility
Low-pressure: Excellent. Uniform structure resists deformation during heat treatment like T6.
Gravity: Good. Can be heat-treated but may require pre-inspection to avoid pore expansion.
| Performance Metric | Low-Pressure | Gravity |
|---|---|---|
| Part density | ≥99.5% | 98-99% |
| Tensile strength (Al) | 280-320 MPa | 240-280 MPa |
| Surface finish | Ra 1.6-3.2 μm | Ra 3.2-6.3 μm |
| Defect rate | 2-5% | 5-10% |
| Heat treatment | Excellent | Good (pre-inspection needed) |
What Are the Ideal Applications for Each?
Choose low-pressure die casting for
High-strength, safety-critical parts: Automotive wheels, engine blocks, engine covers. Need density and strength to withstand loads.
Large thin-walled components: Missile housings, aircraft structural parts. Laminar flow prevents thin-section voids. Gravity can’t fill thin walls under 3mm uniformly.
Non-ferrous alloy parts: Ideal for aluminum, magnesium, and copper alloys. Controllable flow suits these materials.
Choose gravity die casting for
Thick-walled, simple shapes: Rail transit accessories like bogie brackets, robot structural parts, ship propeller hubs. Thick sections, low complexity. Low-pressure would be overkill.
Cost-sensitive, medium-volume parts: Consumer goods like large cookware, industrial valves. Simple design, medium volume. Higher equipment costs of low-pressure make it uneconomical for low-margin parts.
Non-ferrous alloy parts with good flowability: Best for aluminum and copper alloys. Less suitable for low-fluidity alloys.
| Part Requirement | Low-Pressure | Gravity |
|---|---|---|
| High-strength, safety-critical | ✓ Automotive wheels, engine blocks | ✗ |
| Large thin-walled under 3mm | ✓ Missile housings, aircraft parts | ✗ |
| Thick-walled, simple shapes | ✗ Overkill | ✓ Bogie brackets, propeller hubs |
| Cost-sensitive, medium-volume | ✗ Higher equipment cost | ✓ Cookware, industrial valves |
| Non-ferrous alloys | ✓ Al, Mg, Cu | ✓ Al, Cu (good flowability) |
What Are the Cost Differences?
Based on aluminum alloy parts, 10,000-part batch:
| Cost Category | Low-Pressure | Gravity |
|---|---|---|
| Equipment investment | $150,000-$300,000—sealed crucibles, pressure control, lift tubes | $50,000-$100,000—simple melting furnaces, open molds |
| Mold cost | $15,000-$40,000—complex sealed cavities, lift tubes | $5,000-$15,000—simple open designs |
| Per-part material cost | $0.40-$0.60/kg—no risers reduce waste | $0.50-$0.70/kg—risers increase material usage |
| Labor cost | Moderate—trained operators for pressure, 1-2 operators/line | Low—simple process, 1 operator/2 lines |
| Total batch cost | ~$30,000-$60,000 | ~$15,000-$30,000 |
Low-pressure has higher upfront investment—equipment at $150,000-$300,000, molds at $15,000-$40,000. But per-part material cost is lower at $0.40-$0.60/kg because no risers reduce waste.
Gravity has lower upfront investment—equipment at $50,000-$100,000, molds at $5,000-$15,000. But per-part material cost is higher at $0.50-$0.70/kg because risers increase waste.
For 10,000 parts, gravity total batch cost is roughly half— $15,000-$30,000 vs $30,000-$60,000.
How to write a spec that gets priced correctly
A drawing is a contract. If it leaves a number open, the shop fills that number in with its own default. The default is rarely the one you had in mind. Put these five lines on every RFQ and most of the back-and-forth disappears before it starts.
- 3D model with the parting line marked
- Alloy grade and the standard it meets
- Wall thickness and draft on every face
- Machining allowance on the faces that will be cut
- Pressure tightness or porosity class
The point is not paperwork. Each line removes a decision the shop would otherwise make for you. A complete RFQ comes back priced. An incomplete one comes back guessed.
What actually drives the price
Buyers usually push on material because it is the line they can see. In practice the material is rarely the biggest number on a quote. These five lines decide most of it:
- die steel and cavity count
- part weight and wall section
- draft and slide count
- porosity requirement
- machining allowance
Ask for the quote split by line. A supplier who cannot break it down is guessing, and you will pay for the guess. The line you can usually renegotiate is the tolerance band, not the raw material.
Problems you will meet, and the fix
Every process has a failure mode it prefers. The same four show up again and again on this kind of work, and none of them are mysteries once you know what to look for.
- Cold shuts where two flow fronts meet
- Porosity that shows up only after machining
- Soldering on a hot spot in the die
- Shrinkage voids in a heavy section
Catch the fault on the first part, not the hundredth. A first-article check costs an hour. A bad batch costs the whole order. Write the check into the purchase order so nobody has to remember it.
What to check on the first article
The first article is the cheapest inspection you will ever buy. Use it properly and the rest of the run becomes boring, which is exactly what you want.
- Every dimension on the drawing, with the measured value written beside it.
- The material certificate, matched to the heat or lot number on the parts.
- The finish against the sample you approved, under the same light.
- Fit against the mating part, assembled rather than measured.
- Packaging, because parts that arrive damaged were never in tolerance.
Check in this order: datums, then mating features, then cosmetic faces. If the datums are wrong, every number after them is noise. Keep the report on file. When a problem shows up in month six, that document is the only thing that tells you what changed.
How to compare two quotes fairly
Two numbers are not comparable unless the scope matches. Line them up before you decide, because a low number with three gaps is not a low number.
Same drawing, same material, same finish, same inspection. Anything else is a different job wearing the same part number. Then look at what each quote leaves out. Freight, packaging, certificates and tooling are the usual gaps.
Ask each supplier to state their assumptions in writing. The one who does is usually the one who has run the process before.
Scaling from one part to a batch
The first part teaches you the process. The hundredth teaches you the process cost.
Watch three things as volume grows: setup amortisation, tool wear, and how much hand work the finish still needs. If the unit price barely moves between ten and a hundred, the cost sits in material or in hand work, not in setup. Ask which, because only one of them is negotiable.
Lead time behaves the same way. A one-off is scheduled around other jobs. A repeat order is scheduled around your forecast. Give the shop a forecast and the lead time gets shorter, not longer.
How to read a supplier's quote
A quote is a short document with a long shadow. Four things in it decide whether the job runs smoothly, and only one of them is the price.
- The scope line. It should name the process, the material and the finish, not just the part.
- The tolerance basis. General tolerance or the drawing, and which standard it follows.
- The exclusions. Tooling, freight, certificates and inspection are the usual omissions.
- The validity date. Material prices move, and an open-ended quote is a promise nobody can keep.
Read the exclusions before you read the total. A quote that leaves out tooling is not cheaper. It is a quote for a different job.
What to send with the first order
The first order sets the pattern for every order after it. Send a complete package once and the shop files it, and the second order takes a fraction of the time.
- A STEP or native model plus a PDF drawing carrying the same revision letter.
- The material callout by standard number, not by trade name.
- The finish and the exact faces it applies to.
- Any test the part must pass: fit, seal, drop, salt spray, dielectric.
- A contact who can answer a question inside one working day.
Then write down what was agreed. An email that confirms the material, the finish and the date is worth more than a longer purchase order that leaves them open.
When this is the wrong choice
A good supplier tells you when the process does not fit. These are the cases where we say so.
- The volume is so low that setup costs more than the parts are worth.
- The tolerance is tighter than the process can hold without a second operation.
- The geometry cannot be made without a tool feature that will not pay back.
- The material you need is not available in the form the process requires.
- The schedule is shorter than the tooling or setup time, and nobody has said so yet.
None of these are refusals. They are reasons to look at a different route, and the earlier that conversation happens the cheaper it is. The most expensive mistake in manufacturing is a process chosen before the question was understood.
Industry Experience: Choosing the Right Process
An automotive wheel manufacturer needed high-strength wheels that could withstand 10,000 cycles of load testing. Gravity cast wheels had porosity in thick sections, failing at 8,000 cycles. Switching to low-pressure die casting with pressure-holding crystallization eliminated porosity. Density reached 99.7%. Tensile strength hit 300 MPa. Wheels passed 12,000 cycles. Scrap rate dropped from 10% to 3%.
A rail transit supplier produced bogie brackets—thick-walled, simple shapes—at 5,000 units per year. Low-pressure would require $200,000 equipment investment, overkill for the application. Gravity die casting with simple molds cost $60,000 total. Brackets passed 100,000-cycle load tests. Cost per part $8 vs $15 if low-pressure used.
A consumer goods company needed large aluminum cookware at 8,000 units per batch. Gravity die casting with risers increased material waste to 12%, but total batch cost was $24,000. Low-pressure would cut waste to 6% but increase equipment and mold costs to $45,000 —not worth it for low-margin cookware.
Low-pressure and gravity die casting serve different manufacturing needs. Low-pressure uses controlled pressure at 0.02-0.15 MPa for bottom-up laminar filling. It delivers ≥99.5% density, 280-320 MPa tensile strength, Ra 1.6-3.2 μm finish, and 2-5% scrap rates. Ideal for high-strength safety-critical parts like automotive wheels and large thin-walled components under 3mm. Higher upfront investment— $150,000-$300,000 equipment, $15,000-$40,000 molds—but lower per-part material cost at $0.40-$0.60/kg.
Gravity relies on natural flow from top-down pouring. It delivers 98-99% density, 240-280 MPa strength, Ra 3.2-6.3 μm finish, and 5-10% scrap rates. Ideal for thick-walled simple shapes like rail brackets and cost-sensitive medium-volume parts. Lower upfront investment— $50,000-$100,000 equipment, $5,000-$15,000 molds—but higher per-part material cost at $0.50-$0.70/kg.
Choose based on part requirements. For strength-critical thin-walled parts where quality justifies cost, low-pressure wins. For simple shapes where cost matters most, gravity delivers.
Our solution for Low-Pressure Die Casting
YiguProto runs die casting in house, from a single prototype to a production batch. We quote the process, the material and the finish as separate lines, so you can see where the money goes instead of arguing with one number.
Send a STEP file or a drawing and you get a DFM report in one to two business days. It flags the features that add cost and the features that will not hold tolerance on the process you picked.
Every order ships with an inspection report on the features you call out. If a part misses, we make it again. Ready to price a part? Start with a free quote and DFM review, or read more about our die casting. You can also talk to an engineer before you commit to a drawing.
FAQ about Low-Pressure Die Casting
What files do you need in order to quote?
A STEP file plus a PDF drawing that carries the same revision letter. The model carries the geometry and the drawing carries the intent. When the two disagree, the shop picks one, and it is never the one you meant.
Is there a minimum order quantity?
No hard minimum. One part is fine. What changes with volume is which costs dominate, so a single unit is priced mostly on setup and a thousand units are priced mostly on cycle time and material.
What is the lead time for Low-Pressure Die Casting?
Expect 20 to 30 business days including die build. Tooling, material stock and the finishing queue all move that number, so we confirm the date in writing before the run starts rather than after.
What material should I specify?
Specify by standard, not by trade name. A trade name can change under you between two orders; a standard number cannot. Then add the one property that actually decides the part, whether that is strength, heat resistance or chemical exposure.
How much does Low-Pressure Die Casting cost?
Price comes from five lines: material, machine or tool time, setup, finishing and inspection. The material itself is usually the smallest of them. Send a drawing and we will split the quote so you can see which line is doing the damage.
What should I check before approving the parts?
Datums first, then the features that mate, then the cosmetic faces. Ask for a dimensional report, a material certificate and a finish sample. A part can be inside every tolerance and still not fit, so assemble one before you sign.
The short version
Low-Pressure Die Casting rewards the buyer who reads the data sheet and the drawing together. Pick the option for the property that matters, not for the price per kilo.
Tighten only what mates. Test a sample before the run. Then let the process do the rest.