Die Casting Intensification Pressure and Holding Time The Two-Phase Secret to Shrinkage Prevention

Content Menu

● Introduction

● Understanding Shrinkage in Die Casting

● The Role of Intensification Pressure

● The Importance of Holding Time

● The Two-Phase Approach: Combining Pressure and Time

● Real-World Examples and Case Studies

● Challenges and Solutions in Implementation

● Conclusion

● Q&A

 

Introduction

Shrinkage remains one of the most persistent challenges in high-pressure die casting. Engineers working with aluminum alloys regularly deal with porosity, sink marks, and internal voids that compromise part integrity and lead to scrap. The good news is that two closely linked process parameters—intensification pressure and holding time—can be adjusted together to control shrinkage effectively. When applied in sequence during the solidification phase, they create a reliable way to feed the casting and maintain density.

In typical high-pressure die casting cycles, molten metal fills the die cavity under high speed, then a boost of pressure is applied to compensate for the volume contraction that occurs as the metal solidifies. This boost is called intensification pressure. The duration for which that pressure is maintained is the holding time. Together they form a two-phase feeding mechanism that directly addresses the root cause of shrinkage: insufficient liquid metal supply to shrinking regions during cooling.

Over the past decade, research on alloys such as EN-AC 46000 and A356 has shown that small changes to these parameters can produce measurable improvements in density, microstructure, and mechanical properties. For example, studies using ProCAST simulations and metallographic analysis have demonstrated how higher intensification pressure combined with optimized holding time reduces dendritic cell size and limits pore formation in both thin and thick sections. Similar findings appear in rheo-diecasting experiments where slurry holding time influences primary and secondary solidification phases.

This article examines the mechanics behind these parameters, reviews supporting evidence from published work, and provides practical examples from production environments. The goal is to give manufacturing engineers a clearer understanding of how to use intensification pressure and holding time to achieve consistently low-shrinkage castings.

Understanding Shrinkage in Die Casting

Shrinkage in die casting arises from the 5–7% volume contraction that occurs when aluminum alloys transition from liquid to solid. The contraction is uneven because cooling rates vary across the casting. Thin sections solidify quickly against the die walls, while thicker regions remain molten longer. This temperature gradient causes liquid metal in the core to feed the outer layers as they contract, but if feeding paths freeze prematurely, voids form.

There are two primary forms of shrinkage defects. Macro-shrinkage appears as visible sinks on the surface or large internal cavities. Micro-shrinkage consists of small, dispersed pores that reduce tensile strength and fatigue life. Both types are more pronounced in alloys with wide freezing ranges, such as hypoeutectic Al-Si compositions.

Experimental work on EN-AC 46000 alloy has shown that cooling rates directly influence the size of dendritic cells and the distribution of eutectic silicon. In thick sections (around 11 mm), slower cooling leads to larger cells and coarser silicon particles, increasing the risk of shrinkage pores. In contrast, faster cooling in thin walls (3 mm) produces finer structures and fewer defects.

Rheo-diecasting studies with A356 alloy reveal another aspect of shrinkage behavior. During semisolid processing, primary α-Al particles form first, followed by secondary solidification of the liquid phase. If the slurry is held for too long or too short, the secondary phase grows unevenly, leading to constitutional undercooling and larger pores.

High-pressure die casting reviews also highlight how large pores from gas entrapment or semi-solid deformation contribute to ductility scatter. Controlling these defects requires precise management of the solidification environment, where intensification pressure and holding time play central roles.

The Role of Intensification Pressure

Intensification pressure is the additional force applied after the die cavity is filled, typically in the range of 100–200 bar. Its main function is to push more molten metal into the casting to compensate for the volume lost during solidification.

Without sufficient intensification, the metal contracts away from the die walls, creating gaps that become shrinkage pores. When the pressure is applied correctly, it maintains contact between the solidifying metal and the die, allowing continuous feeding.

Research on EN-AC 46000 alloy compared three plunger velocity settings combined with different compression pressures. Higher intensification pressure resulted in faster cooling rates and smaller dendritic cell sizes. In one case, cell size dropped from approximately 20 μm to 10 μm in a 3 mm wall, with a corresponding reduction in porosity.

In another study, A356 rheo-diecastings showed that intensification pressure influences the refinement of both primary α1 and secondary α2 phases. When combined with appropriate holding time, the eutectic spacing was reduced significantly, leading to denser structures.

Practical applications include automotive brackets and housings. In one production example, intensification at 160 bar produced castings with average density of 2.70 g/cm³ and minimal voids, as confirmed by scanning electron microscopy.

The key is matching the pressure level to the alloy’s solidification characteristics. Too low, and feeding is inadequate; too high, and flash or die wear increases.

Key Factors Influencing Intensification Pressure

Alloy composition affects fluidity and shrinkage volume. Hypoeutectic Al-Si alloys benefit from higher pressure to overcome their tendency toward porosity. Die temperature is another factor—maintaining around 175 °C ensures uniform filling and solidification.

Plunger velocity during the filling phase also interacts with intensification. Faster filling can reduce turbulence but may require higher pressure to compensate for trapped air.

The Importance of Holding Time

Holding time is the duration that intensification pressure is maintained. It allows the casting to solidify under pressure, ensuring that feeding channels remain open until the metal is fully solid.

If the hold is too short, pressure drops before solidification is complete, and shrinkage pores form. If too long, cycle time increases and the risk of soldering rises.

In A356 rheo-diecasting, holding the slurry for 3 minutes before injection produced optimal primary particle size and stable secondary growth. Longer holds led to excessive grain growth, while shorter holds caused unstable solidification.

Production examples include pump housings in EN-AC 46000 alloy. A 5-second holding time under intensification pressure reduced porosity in 6 mm sections, with optical microscopy showing average particle sizes around 6 μm.

Thicker sections generally require longer holds to allow complete solidification, while thinner sections solidify quickly and need shorter holds to avoid unnecessary cycle time.

The Two-Phase Approach: Combining Pressure and Time

The real effectiveness comes from coordinating intensification pressure and holding time as a single strategy. The first phase applies the pressure boost to initiate feeding. The second phase maintains that pressure long enough for solidification to finish.

This sequence keeps feeding paths open and prevents premature freezing. In one study on automotive gearbox casings, a combination of high plunger speed, 150 bar intensification, and 4-second hold produced uniform density across 3–11 mm wall thicknesses.

In rheo-diecast A356 wheels, 3-minute slurry hold followed by high-pressure injection and 5-second intensification hold resulted in globular particles and low shrinkage.

These examples demonstrate that the two-phase method reduces ductility variability by controlling pore size and distribution.

Real-World Examples and Case Studies

In a European foundry producing EN-AC 46000 parts, process simulations showed that higher intensification pressure and adjusted holding time reduced scrap rates by approximately 30 %. Thick sections benefited most from longer holds.

In China, rheo-diecasting trials with A356 wheels found that a 3-minute hold produced the most uniform microstructures across the casting.

A UK research project on high-pressure die casting used intensive shearing and pressure-hold combinations to minimize inclusions and improve ductility consistency.

In electric vehicle motor housing production, controlled intensification and holding ensured dense structures and high conductivity in aluminum alloys.

Challenges and Solutions in Implementation

Common challenges include die wear from high pressure, gas entrapment from poor timing, and alloy-specific solidification behaviors.

Solutions involve simulation tools like ProCAST to predict optimal settings, intensive melt shearing to disperse nuclei, and automated monitoring to maintain consistency.

Conclusion

Intensification pressure and holding time are the two-phase mechanism that effectively prevents shrinkage in high-pressure die casting. When properly coordinated, they ensure adequate feeding during solidification, resulting in denser castings with fewer defects.

Evidence from EN-AC 46000 and A356 alloys shows clear improvements in cooling rates, microstructure refinement, and mechanical properties. Real-world applications in automotive and electric vehicle components demonstrate the practical value of these adjustments.

As manufacturing continues to demand higher quality and lighter parts, mastering these two parameters will remain essential for producing reliable die castings.

Q&A

Q1: How does intensification pressure reduce porosity in aluminum die castings?
A1: It supplies additional molten metal to compensate for contraction during solidification, filling potential voids.

Q2: What is the typical holding time for A356 slurry in rheo-diecasting?
A2: Approximately 3 minutes, which balances primary particle size and secondary solidification stability.

Q3: Does plunger velocity affect the two-phase process?
A3: Yes. Higher velocity promotes faster filling and can improve the effectiveness of pressure and hold.

Q4: How do section thicknesses influence holding time?
A4: Thicker sections need longer holds to complete solidification, while thinner sections require shorter holds.

Q5: What is the benefit of simulation tools in setting these parameters?
A5: They predict cooling rates and shrinkage locations, allowing precise optimization of pressure and time.