Low pressure die casting (LPDC) uses 0.06-0.15 MPa of air pressure to push molten aluminum upward into a die, filling smoothly and solidifying under pressure. The process suits structural parts such as subframes and battery housings. But no matter how good the process is, a poor design still produces defective parts. This article is written for engineers designing LPDC parts: it covers how to set wall thickness, tolerances, and draft angles, which design rules are unique to LPDC, and how to avoid porosity and shrinkage at the design stage. A design checklist is included for drawing reviews.
Qu'est-ce que le moulage sous pression ?
Low pressure die casting mounts the die above a sealed furnace, applies 0.06-0.15 MPa of compressed air, and pushes metal up a riser tube to fill the cavity under pressure. The key difference from high pressure die casting is filling behavior: LPDC fills smoothly with less gas entrapment, and parts can be heat treated. Compared with gravity casting, the pressure controls filling speed and feeds better, so the structure is denser. LPDC suits aluminum structural parts in runs of a few thousand pieces and up.
How Low Pressure Die Casting Works
The LPDC cycle has six steps: close and preheat the die, pressurize, fill, hold under pressure to solidify, release pressure, and open the die. The die is preheated to 150-300°C, compressed air enters the sealed furnace, metal rises through the riser tube into the cavity, pressure is held until solidification is complete, then the die opens. The pressure curve and die temperature control fill quality and feeding throughout the cycle, which is why LPDC demands tight process control.
Key Design Parameters for LPDC Parts
| Paramètres | Recommended Value | Notes |
|---|---|---|
| Wall thickness | ≥3 mm, as uniform as possible | Thin walls resist filling; abrupt thickness changes create hot spots |
| Linear tolerance | ISO 8062 CT8 | Wall thickness CT9; check the table for part size |
| Draft angle | 1° external, 2-3° internal, range 0.5-3° | Adjust for feature depth and surface texture |
| Wall transitions | Gradual | Avoid abrupt changes and isolated thick sections to reduce shrinkage |
| Corners | Filleted | Avoid sharp corners that concentrate stress and restrict filling |
| Ribs | Used for stiffness | Use ribs instead of thicker walls to control weight and hot spots |
Start with wall thickness and draft angle, then check transitions and fillets. Uniform wall thickness is the first rule of LPDC design: sections that change thickness suddenly solidify at different rates and shrink most often.
Design Rules Unique to LPDC
LPDC differs most from high pressure and gravity casting in filling direction: metal rises from the bottom through the riser tube. That creates three rules you will not find in other processes.
Bottom gating and feed path. LPDC must fill from the bottom, and the gate position defines the feed path through the riser tube. Every thick section must sit on that feed path so it keeps receiving metal during solidification; a hot spot far from the gate will shrink.
Review gate and riser positions early. Gate and riser placement has no direct equivalent in other casting processes and is where LPDC defects most often start. Settle it before cutting tooling, not after trial runs.
Avoid isolated thick sections. Thick sections that cannot be fed are shrinkage hotspots. Design sections so they connect to the feed path, or compensate with chills and die temperature control.
These three rules are the first things a foundry checks when reviewing LPDC drawings.
Design Defects and Prevention
Three defects dominate LPDC parts, and most can be prevented at the design stage:
- Porosité: entrained gas comes from unstable filling; avoid thin walls and complex sections that cause turbulence. Dissolved gas comes from the melt and is controlled by degassing.
- Rétrécissement: comes from insufficient feeding; keep walls uniform, avoid isolated hot spots, and rely on risers and cooling design.
- Cold shuts: metal streams meet without fusing; avoid over-thin walls and long fill paths, and raise pouring temperature when needed.
Checking these three at the design stage costs far less than reworking after trial runs. That is why the standard flow is design review, simulation, then trial.
Materials for Low Pressure Die Casting
LPDC uses heat-treatable aluminum casting alloys, most commonly A356 (AlSi7Mg0.3), which flows well and responds to T6 heat treatment for structural parts. ZL101A is the Chinese equivalent with similar properties. Choose material by two questions: does the part need heat treatment, and does the service environment demand temperature or corrosion resistance?
LPDC Design Checklist
Run through these six checks before drawing review:
- Is wall thickness ≥3 mm and reasonably uniform?
- Are transitions gradual, with no isolated hot spots?
- Are draft angles 1° external and 2-3° internal?
- Are corners filleted?
- Do thick sections sit on the feed path (reachable by bottom filling)?
- Has the gate and riser position been reviewed?
Pass all six and the design is ready for simulation. Fix design issues before cutting tooling.
Questions fréquemment posées
What is the minimum wall thickness for LPDC? The design standard is ≥3 mm. Smooth filling can go thinner, but below 2 mm filling resistance rises and needs finer pressure-curve and die-temperature control, adding cost and risk.
What tolerance can LPDC hold? Linear dimensions typically follow ISO 8062 CT8, with wall thickness at CT9. Check the table for part size; tighter fits can be machined after casting.
What draft angle does LPDC need? 1° minimum on external surfaces and 2-3° on internal surfaces, adjusted for feature depth and surface texture. Too little draft pulls and scratches the surface during ejection; too much wastes material.
What defects are common in LPDC? Porosity, shrinkage, and cold shuts. Uniform walls, no isolated hot spots, and a clear feed path prevent most of them at the design stage.
Is simulation necessary before tooling? Recommended. LPDC filling paths and solidification feeding are complex; simulation predicts defect distribution before tooling, then trial runs confirm it, saving significant trial-and-error cost.
Conclusion
LPDC design comes down to three things: uniform walls, correct draft, and an open feed path. Remember the numbers: wall thickness ≥3 mm, tolerance CT8, draft 1-3°. Remember the LPDC rule: it fills from the bottom, so thick sections must connect to the feed path. Run these checks at the design stage and trial-run and production defects drop sharply. If your part needs a design review, send the drawing to the MinHe engineering team. We check wall thickness, tolerances, and feed path point by point and return manufacturability feedback with a process proposal.
