Aluminium Casting Product Design Optimization for Good Casting Quality
In die casting industry, product design of a die casting part plays a vital role in the final product quality. The various factors affecting the manufacturability of a die casting part are general casting wall thickness and variation in thickness, material flow length in the part or critical characteristics and quality requirements of the component.
Most of the die casters produce the Aluminum die casting parts to the print i.e. as per customer drawing. The detailed analysis of the product design before developing the HPDC die of the part is not done, which is very crucial to determine the achievable casting quality and satisfy the end customer.
Today, with the available simulation tools and techniques, we can predict the probable casting defects in the parts before we make the die. This along with strong partnership with the end customer can help to optimize the product design and achieve desired quality of the part.
Most of the casting defects in High Pressure Die Casting parts are due to:
- The product design related issues
- Constraints at the end user to meet the product assembly and functional requirements
- Awareness about the manufacturability and production method of the part
- Design of feeding system
- Thermal balancing of the die
Although some of the defects can be controlled during die design of the part, still majority of the defects remains unresolved due to constraints in die design and lead to high scrap rates on the shop floor. Some of such defects are:
- Leakage from the casting
- Cold shut
- Non filling
- Shrinkage porosity
- Casting Crack
- Surface finish due to soldering
Product Design Optimization
While analyzing the product design of a Die Casting Part, certain guidelines need to be followed in relation to manufacturability of the part thru HPDC process and achieve good casting quality:
- Draft Analysis of the part as per die construction
- No sharp edges in the casting, good filleting is required
- Thickness analysis – no sudden change in cross section
- Parting line construction as per casting quality and tool design requirements
Most of the OEMs do not involve the Tier 1 suppliers during their initial design stages. During development stage when the supplier suggests some changes to improve the casting quality, they are reluctant to accept the design changes as their product design is freeze. This leads to increased development time, poor quality of the product, dissatisfied customer and more scrap rate at the foundry
Virtual Simulation of HPDC process is used as an effective tool to predict the casting defects due to product design and optimize the product design accordingly. Some of the examples are described in next paragraphs.
Change in Part Geometry to Minimize the Sudden Change in Cross Section
In this example the shape of a mounting boss is changed from round shape to plus sign shape to optimize the section modulus and minimize the sudden change in the thickness. This is done without effecting the function and strength of the part. Solidification and Porosity results are compared and analyzed to conclude the change in product design. The net effect of this change in geometry is reduced porosity in the mounting boss.
Change in Parting Lines to Optimize the Product Design for HPDC Process
The following example indicates the effect of Air Pressure in the cavity before (~3.8 bar) and after changing the parting line (~1.8 bar) in the product design. The customer requirement was porosity level 1 as per ASTM E 505, which could not be achieved without changing the product design.
Change in Product Design by Providing Additional Ribs to Improve the Material Flow
The following example indicates how the addition of ribs change the filling pattern and sudden change in cross section to improve the material filling and reduce the air pressure in the cavity. This has helped to reduce the cold fill and shrink porosity defect in the casting at this particular location.
Change in Fillet to Improve the Material Flow and Reduce Air Entrapment
The following example indicates how the change in the corner fillets in the product design helped to change the filling pattern and reduce the air entrapment in the part from 12% to 8% after the change in fillet. The end result is better casting quality and good surface finish.
Conversion from GDC to HPDC
Using these guidelines and learning, there are a lot of opportunities to convert the parts produced thru GDC process to HPDC process. One such example is Fly Wheel Housing for commercial vehicles.
The key challenges were:
- Assembly features of Fly Wheel Housing in the Engine assembly should remain unchanged
- All functional requirements should be met as per the part produced thru GDC process
- Reduce the weight of the casting to make it viable to produce thru HPDC route
- Mechanical properties and strength of the part should withstand the static and dynamic load on the part while in Engine assembly.
- Quality requirements in terms of dimensional tolerance, porosity specs should be met.
To meet the above challenges, the complete part was re-modeled considering:
- The HPDC process requirements
- Tool design requirements and
- Simulation feedback
To finalize the product design, total 4 iterations were done on the product design in co-ordination with OEM team, two iterations of the FEA analysis done by the OEM team and 12 iterations done on the Magma Simulation.
The tool was designed and manufactured with 8 complex slides, Centre Feeding, high yield of 65.3% and proved “First Time Right”.
The First trial samples were validated and approved by the customer and the part was productionized.
The results achieved are:
- Part production process changed from GDC to HPDC without any changes in the engine assembly line and child parts
- Weight of the casting reduced from 9.49 kg to 6.31 kg (~33% saving)
- Machining areas reduced as better finish and tolerances achieved by HPDC process
- All customer quality requirements in terms of strength, porosity, and dimensional tolerances are met
- Productivity increased from average 50 parts per day thru GDC process to 600 parts per day thru HPDC process
Above examples indicates how the joint working between the Customer and Die Casting supplier helps to optimize the product design to meet the customer requirements, optimize the production process, reduce the weight of the part and scrap rate at the foundry. Simulation analysis as a supporting tool gives us the indication of the probable casting defects in the part and helps us to optimize the product design and justify the changes in product design.
Author of this article
Author of this article is Rajesh R Aggarwal - Director TechSense Engineering Services & Member of Technical Committee – ALUCAST 2017 - Pune, India.