Abstract: Taking top cover of a 12-speed heavy-duty truck transmission as research object, this paper analyzes in depth causes of deformation in flat die castings. Study shows that insufficient clamping force of die casting machine, local deformation of mold base, and unreasonable casting pressure are main factors leading to casting deformation. A systematic solution is proposed, encompassing optimization of mold gating system, mold base inspection and surface repair, precise control of casting pressure, and design of automated straightening tooling. Effectiveness of all measures has been verified, significantly reducing casting deformation, improving product qualification rate and production efficiency. This provides theoretical and practical guidance for improving quality and reducing scrap rate of flat die castings.
Metal pressure casting (or die casting for short) refers to a process in which liquid or semi-liquid metal is filled into die casting mold cavity at a high speed under high pressure and solidified under pressure to obtain a casting. Due to their large surface area, flat die-cast parts exhibit varying cooling rates between center and edges, and across different wall thicknesses, leading to uneven thermal shrinkage and varying thermal stresses within casting, resulting in deformation. Secondly, during solidification, flat parts are constrained by mold core, inserts, or already solidified portions, generating tensile or compressive stresses. Stress release after demolding causes further deformation. Root cause of casting deformation lies in release of residual stress, and key factors influencing degree of deformation involve the entire process, including materials, design, manufacturing process, molds, and post-processing. Deformation affects dimensional accuracy of castings, causing black skin defects in subsequent machining processes, leading to assembly failure and scrap. This paper uses cover of a mechanical 12-speed heavy-duty truck transmission as an example to study influencing factors of deformation in flat die-cast parts, proposes a systematic method for controlling casting deformation, and provides practical guidance for solving deformation problem of die-cast flat parts.

12JS160T-1702015-YDC top cover (Figure 1) is a crucial component of heavy-duty truck's control system. It has a flat plate structure, dimensions of 455mm * 294mm * 70mm, an average wall thickness of 4mm, and is made of ADC12 aluminum alloy. It is manufactured using a Lijing DCC1600T die-casting machine.

 

Figure 1 Product Structure
Required dimension from flange face to bolt mounting boss end face is 15mm ± 0.5mm. Actual measurements range from 15.8-16.4mm. Measurements of multiple batches of blanks from different molds, combined with 3D scanning results (Figure 2), revealed that 100% of blanks exhibited dimensional deviations beyond acceptable limits. Deformation area was not fixed, with a maximum deformation of 1.4mm.
Approximately 5% of deformed blanks, after machining, showed black skin defects in different areas (Figure 3), failing to meet assembly requirements and resulting in scrap.

 

Die casting process is mainly achieved through a die casting machine. A necessary condition is calculation of required clamping force of die casting machine. Clamping force must be able to resist expansion force generated by high-pressure alloy liquid during die casting process, i.e.: F_clamping = KF_expansion
Where: F_clamping—clamping force, kN; F_expansion—expansion force, kN; K—safety factor, usually 1.2 for cold chamber die casting machines.
Formula for calculating expansion force is as follows: F_expansion = AP
Where: P—pressurization pressure, MPa; A—total projected area of all parts of casting on parting surface, cm².
Figure 4 shows design diagram of gating system for this product. Using area calculation function in NX software, the total projected area was measured to be A = 1991.84 cm². Taking casting pressure P = 80 MPa and safety factor K = 1.2, calculation yields F_lock = 1912 kN. Testing revealed that maximum clamping force of Lijing DCC1600T die-casting machine is 1600 kN. Clamping force required by gating system exceeds maximum clamping force of production equipment, resulting in significant flash during casting production (as shown in Figure 5), which is one of important causes of deformation.

A 3D scan of mold base for this product was performed (as shown in Figure 6). Results showed that both fixed and moving mold bases had multiple deformations. In particular, deformation in a localized area at contact surface between fixed mold base and mold core exceeded 0.52 mm, exceeding drawing requirement of ±0.05 mm. Maximum deformation on contact surface of mold base exceeds 2mm, with some areas of moving mold base exceeding 0.55mm. Maximum deformation on contact surface of mold base exceeds 2.5mm, deformation is also observed in pins and ejector pin holes.

 

Figure 6. 3D scanning results of mold base
Flatness of die-casting mold base is one of core factors affecting casting deformation, especially for flat castings. Deformation is directly and significantly influenced by two main paths: mold rigidity transmission and thermal imbalance.
(1) When flatness error of contact surface between mold base and mold core is ≥0.1mm/m², under high-speed clamping (usually 80-120MPa), a micro-gap of 0.05-0.2mm is generated at parting surface, resulting in flash in casting. Mold base deformation leads to a change in cavity projection shape; for example, a 0.1mm convex mold base results in a 0.08mm concavity in corresponding area of casting.
(2) In areas where flatness of contact surface between mold base and die-casting machine template exceeds tolerance, contact area between mold and equipment template decreases, local heat dissipation area decreases by 20%-40%, and mold temperature in poor contact area is 30-50℃ higher than normal area, exacerbating uneven cooling and causing casting deformation.
(3) Mold base deformation causes ejector plate guide post to tilt, ejection force deviates from design axis, casting is twisted and deformed under high temperature conditions.

Casting pressure is core process lever for controlling deformation of die-cast parts. Its influence is two-sided: insufficient pressure leads to shrinkage deformation, while excessive pressure causes stress deformation. When holding pressure is <80 bar, shrinkage rate at far end of casting can reach 2-3 times normal value. For this product, when casting pressure was set to 60 bar, flatness deviation was 0.8 mm; after increasing to 80 bar, flatness deviation decreased to 0.3 mm. When casting pressure is >120 bar, flash in casting increases, ejection resistance increases, and risk of ejection deformation increases by 40%.

 

Figure 7. Flash and 3D scanning results of castings after casting pressure exceeded limit
According to principle of die-casting machine, casting pressure is provided by accumulator. When nitrogen pressure of product is set to 120 bar, casting pressure exceeds 1000 bar. Calculations show that clamping force required for casting at this point is 2340 kN, far exceeding clamping force of die-casting machine. A large amount of flash exists during casting production process, and 3D scanning results show severe bulging of casting, as shown in Figure 7.

Based on analysis of product processing results, and while ensuring stable casting quality, six slag pockets on both sides of casting were removed. Projected area of casting decreased from 1991.84 cm² to 1607.27 cm², and required clamping force decreased from 1912 kN to 1543 kN. Improved DCC1600T die-casting machine met clamping force requirements for this product, and flash was significantly reduced during casting production (as shown in Figure 8).

 

Figure 8. Optimization effect diagram of gating system

Based on 3D scanning results of mold base, mold base underwent repair, as shown in Figure 9.

 

Figure 9. Effect diagram of mold base after repair
Contact surfaces between mold base and die-casting machine template are surface-ground, requiring over 90% of area to be polished to a smooth finish with a flatness ≤0.05mm. Bottom contact surfaces of mold base and mold core, as well as mating surfaces of moving and fixed mold bases, are machined by 0.5mm, requiring a flatness ≤0.05mm. Based on actual grinding and surface reduction, mating surfaces of moving mold rear limit post, sprue bushing, and mold core are repaired. After repair, mold base is reassembled and inspected with red lead. Areas with poor fit are ground.

By reducing accumulator nitrogen pressure from 120bar to 95bar and casting pressure from 1000bar to 750-800bar, required clamping force for casting is 1447-1543kN. Equipment fully meets clamping force requirements, and flash generated in casting is significantly reduced.

For areas of significant casting deformation, straightening is necessary to ensure casting's flatness meets requirements. Conventional method involves placing casting on a flatness gauge, measuring with a feeler gauge, and manually correcting areas with excessive flatness using a copper hammer to meet drawing requirements. However, manual straightening is inefficient, labor-intensive, and requires two people per machine to meet production cycle demands.

 

1. Top plate; 2. Spring rod assembly; 3. Push plate; 4. Upper template; 5. Guide sleeve assembly; 6. Guide post assembly; 7. Upper mold; 8. Cutting tool assembly; 9. Straightening assembly; 10. Lower mold; 11. Base; 12. Clamping assembly.
Figure 10: Schematic diagram of cutting and straightening assembly.
To improve straightening efficiency and reduce labor intensity, an automated edge-trimming straightening assembly was designed (as shown in Figure 10). Upper mold features a conformal clamping component to maximize clamping of casting and ensure uniform distribution of clamping force. Lower mold has 10 adjustable support components that adjust their height according to casting deformation, enabling automatic straightening of casting during removal of gating system and ensuring casting's flatness requirements are met.

After implementing above measures, dimensions from flange face to bolt mounting boss end face of blanks produced in different shifts were measured. Required dimension was 15mm ± 0.5mm, and actual measured dimensions were 14.8-15.4mm, all conforming to drawing requirements. A 3D scan comparison of castings before and after improvement was performed. Results (Figure 11) show that maximum deformation of casting decreased from 0.74mm to 0.46mm, demonstrating a significant improvement effect.

 

Figure 11 Comparison of 3D scanning results of castings before and after improvement.
A total of 13,937 blanks were produced after improvement for machining verification (Figure 12). After machining, 100% of parts conformed to drawing requirements, with zero black skin defects. Black skin scrap rate decreased from 5% to 0%, demonstrating a significant improvement effect.

 

Figure 12 Comparison of 3D scanning results of castings before and after improvement.

Taking top cover of a 12-speed heavy-duty truck transmission as research object, this paper analyzes in depth factors affecting deformation of flat die-cast parts. Main issues are: unreasonable design of gating system leading to an increase in projected area of casting; insufficient clamping force of die-casting machine to meet clamping force requirements for casting production, resulting in bulging of casting; deformation of mold base after a certain number of uses, leading to excessive flatness of casting; and excessive casting pressure, increasing flash and obstructing ejection process, resulting in warping deformation of casting. A systematic solution is proposed, encompassing optimization of mold gating system, mold base inspection and surface repair, precise control of casting pressure, automated edge trimming and straightening tooling. 3D scanning and processing verification of improved sample verified effectiveness of improvement measures, significantly reducing deformation of product and solving black skin defect in product processing.