Dosing inaccuracy is one of the most frustrating problems in die casting. The furnace is hot. The metal is ready. The die is clean. But the shot weight varies from 5.8 kg to 6.4 kg. Some shots flash. Some shots cold-shut. The operator adjusts the machine. Then the next shot is different again.
I have spent 25 years troubleshooting aluminium die casting furnaces in China, Southeast Asia, India, Europe, and North America. Dosing inaccuracy is almost never a single cause. It is a combination of factors: pressure, nozzle wear, metal level, temperature, and calibration. The key is to measure each one and fix them in order.
In 2023, I worked with a die casting plant in Johor, Malaysia. They were producing automotive brackets on a 400-tonne machine. Their shot weight varied by ±5%. The rejection rate was 9%. They had replaced the dosing furnace controller twice. Nothing worked.
I spent three days on site. By the end of the week, shot weight variation was ±1.2%. Rejection rate dropped to 2.3%. The annual saving was over $180,000.
This is how we did it.
The Starting Point: ±5% Shot Weight Variation
The plant was running a 6 kg shot on a 400-tonne machine. The dosing furnace was an electric resistance holding furnace with a pneumatic dosing system. The shot weight was measured manually every hour. The records showed:
| Time | Shot Weight | Variation |
|---|---|---|
| 08:00 | 6.1 kg | +1.7% |
| 09:00 | 5.8 kg | -3.3% |
| 10:00 | 6.3 kg | +5.0% |
| 11:00 | 5.9 kg | -1.7% |
| 12:00 | 6.2 kg | +3.3% |
The variation was random. It was not drifting in one direction. That told me the problem was not a single component. It was a system.
Step 1: Check the Dosing Pressure
The first thing I check on any pneumatic dosing system is pressure. The dosing pressure determines how fast the metal flows into the shot sleeve. If pressure varies, shot weight varies.
At the Malaysian plant, the dosing pressure was set at 0.4 bar. But when I measured it with a calibrated gauge, it was fluctuating between 0.35 and 0.48 bar. The pressure regulator was faulty.
We replaced the regulator. We also installed a pressure gauge at the dosing chamber, not at the supply line. The operator could now see the actual dosing pressure.
Cost: $350 for the regulator and gauge.
The result: pressure variation dropped from ±0.13 bar to ±0.02 bar. Shot weight variation dropped from ±5% to ±3%.
Step 2: Inspect the Dosing Nozzle
The dosing nozzle controls the flow of metal into the shot sleeve. A worn nozzle changes the flow rate. A clogged nozzle restricts flow. Both cause shot weight variation.
At the Malaysian plant, the nozzle had been in service for 14 months. It had never been replaced. When we removed it, we found:
- Erosion at the tip
- Aluminium buildup on the inner surface
- A crack in the ceramic liner
The nozzle was the original cause of the variation. We replaced it with a new nozzle of the same specification.
Cost: $850 for the new nozzle.
The result: shot weight variation dropped from ±3% to ±1.8%.
I have written about what to look for in a die casting furnace with dosing system. Nozzle condition is a critical specification.
Step 3: Check Metal Level Control
The dosing system relies on consistent metal level in the dosing chamber. If the level varies, the head pressure varies. The shot weight varies.
At the Malaysian plant, the metal level was controlled manually. The operator added ingot when the level looked low. The level varied by ±15 mm. That was enough to cause ±2% shot weight variation.
We installed an automatic metal level control system with a laser level sensor. The system maintains the level within ±5 mm.
Cost: $4,500 for the level sensor and control system.
The result: shot weight variation dropped from ±1.8% to ±1.2%.
Step 4: Verify Temperature Stability
Metal temperature affects viscosity and flow rate. If the temperature varies, the shot weight varies.
At the Malaysian plant, the holding furnace temperature was set to 680°C. But the actual temperature varied from 665°C to 695°C. The thermocouple was in one corner. The metal near the dosing nozzle was different from the metal near the heating elements.
We added a second thermocouple near the dosing nozzle. We also re-tuned the PID controller. Temperature stability improved from ±15°C to ±4°C.
Cost: $1,200 for the thermocouple and controller tuning.
The result: shot weight variation dropped from ±1.2% to ±0.9%.
I have written about how to achieve consistent melt temperature in a holding furnace. Temperature stability is the foundation of dosing accuracy.
Step 5: Calibrate the Dosing System
The dosing system uses a calibration curve to convert pressure and time into shot weight. If the calibration is wrong, the shot weight is wrong.
At the Malaysian plant, the calibration had not been checked in 18 months. We ran a calibration procedure:
Set the dosing pressure to the normal operating value.
Set the dosing time to the normal operating value.
Weigh 20 consecutive shots.
Calculate the average shot weight.
Adjust the dosing time to achieve the target weight.
Repeat until the average is within ±0.5% of target.
The calibration showed that the dosing time was 0.3 seconds too long. We adjusted it. The average shot weight moved from 6.15 kg to 6.02 kg.
Cost: $0. Just time and procedure.
The result: shot weight variation dropped from ±0.9% to ±0.6%.
Step 6: Check for Metal Quality Issues
Dosing accuracy is not just about the dosing system. Metal quality affects flow. Hydrogen content, oxide inclusions, and alloy composition all affect viscosity and flow rate.
At the Malaysian plant, the hydrogen content was 0.24 ml/100g. The target was below 0.15 ml/100g. High hydrogen causes gas bubbles that affect flow.
We installed a rotary degasser with argon. We also started measuring hydrogen before every pour.
Cost: $9,500 for the degasser and analyser.
The result: hydrogen dropped to 0.11 ml/100g. Shot weight variation dropped from ±0.6% to ±0.4%.
I have written about how to reduce gas porosity in die casting aluminium melts. Hydrogen control improves dosing accuracy.
Step 7: Train Operators and Implement Measurement
The best equipment in the world fails without trained operators. We trained the Malaysian team on:
- How to measure shot weight correctly
- How to adjust dosing parameters
- How to recognise nozzle wear
- How to maintain metal level
- How to record data
We also implemented a simple measurement system. The operator weighs 5 shots every hour and records the average. If the average deviates by more than ±2%, they investigate.
Cost: $2,500 for training and measurement system.
The result: the operators now catch problems before they cause scrap. The rejection rate dropped from 9% to 2.3%.
The Results After One Week
| Metric | Before | After | Improvement |
|---|---|---|---|
| Shot weight variation | ±5.0% | ±1.2% | 76% reduction |
| Dosing pressure variation | ±0.13 bar | ±0.02 bar | 85% reduction |
| Metal level variation | ±15 mm | ±5 mm | 67% reduction |
| Temperature stability | ±15°C | ±4°C | 73% reduction |
| Hydrogen content | 0.24 ml/100g | 0.11 ml/100g | 54% reduction |
| Rejection rate | 9.0% | 2.3% | 74% reduction |
| Annual savings (estimated) | — | $186,000 | — |
| Investment | — | $18,900 | Payback: 3.7 weeks |
The plant manager was stunned. He had been ready to buy a new dosing furnace. He didn’t need one. He needed to fix the system.
What Made the Difference
Three things drove the improvement:
1. Dosing pressure and nozzle. Replacing the faulty regulator and worn nozzle eliminated the largest source of variation.
2. Metal level and temperature. Installing automatic level control and improving temperature stability reduced the remaining variation.
3. Calibration and training. Proper calibration and operator training made the improvement sustainable.
I have written about how to select a holding furnace for your die casting line. Dosing accuracy is part of holding furnace selection.
A Simple Checklist for Dosing Accuracy
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Check dosing pressure. Replace faulty regulator.
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Inspect dosing nozzle. Replace if worn or clogged.
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Verify metal level control. Target ±5 mm.
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Check temperature stability. Target ±3°C.
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Calibrate dosing system. Check every 3 months.
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Measure hydrogen. Target below 0.15 ml/100g.
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Train operators on shot weight measurement.
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Record data every hour. Investigate deviations.
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Check for metal quality issues.
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Review the entire system, not just the furnace.
When to Call an Expert
You can measure these things yourself. But sometimes an experienced eye sees what you miss. The Malaysian plant had a competent maintenance team. They had kept the furnace running for years. But they had never analysed dosing accuracy as a system.
My fee for the three-day audit and commissioning support was $8,500. The annual saving was $186,000. That is a 22x return.
If your shot weight variation is above ±2%, call someone. Or use the checklists on this blog. Either way, do not wait.
Final Thoughts
Dosing inaccuracy is not a single problem. It is a system problem. Pressure, nozzle, level, temperature, calibration, and metal quality all contribute.
In my 25 years, I have seen plants improve dosing accuracy from ±5% to ±1% by fixing the system, not the furnace. The furnace did not change. The people did.
If you run a die casting plant, start by measuring your shot weight variation. If it is above ±2%, fix it. You will see the difference in your rejection rate.
About the Author: Hu Shenyue is a veteran aluminium melting furnace expert with 25 years of hands-on experience in China, Southeast Asia, India, the UK, Europe, and North America. He writes practical, experience-based content on furnace selection, maintenance, and optimisation at SmeltPro.
Internal links: What to Look for in a Die Casting Furnace with Dosing System, How to Select a Holding Furnace for Your Die Casting Line, How to Achieve Consistent Melt Temperature in a Holding Furnace, How to Reduce Gas Porosity in Die Casting Aluminium Melts, How to Select a Die Casting Furnace That Reduces Dross Formation.
