Deep Drawing Mold Technical Guide: Material Selection, Blank Sizing, and Process Parameters

Introduction

Many practitioners find deep drawing molds intimidating because they involve numerous design considerations, and trial runs often require multiple die modifications before achieving ideal results. Accumulating practical experience through continuous practice is invaluable for deep drawing mold design.

1. Material Selection

1.1 Common Material Types

Cold-rolled steel sheets commonly used for deep drawing include:

• 08Al (aluminum-killed steel) - uniform performance, higher price
• No. 8 steel - most widely used
• 08F
• No. 10 steel
• No. 15 steel
• No. 20 steel

1.2 Rimmed Steel vs. Killed Steel

Rimmed steel is not suitable for parts with high stamping requirements or strict appearance standards. The representative killed steel grade is aluminum-killed 08Al.

TIP: When customer requirements are not extremely stringent and repeated trials fail to meet expectations, try switching to a different material.

1.3 International Reference Materials

Japanese SPCC-SD deep-drawing steel offers superior deep drawing performance compared to 08Al.

2. Blank Size Determination

2.1 Basic Principle

For simple rotationally symmetric drawn parts in non-thinning draws, although material thickness changes during forming, it remains very close to the original thickness. The blank diameter can be calculated based on the principle that blank area equals drawn part area (add trim allowance if needed).

2.2 Practical Approach: Trial Blanking

Since drawn part shapes and processes are often complex (sometimes involving thinning draws), and 3D software展开料 calculations cannot achieve 100% accuracy, the following procedure is recommended:

1. First perform unfold/blank calculation to get a general idea of blank shape and size
2. Determine overall dimensions of the blanking die
3. Do not machine punch/die cutting edges yet
4. Use wire EDM to cut blanks (for larger blanks, mill then manually finish)
5. Conduct repeated experiments through subsequent drawing operations to determine final blank size
6. Then machine the blanking die punch/die cutting edge dimensions

TIP: Use reverse scheduling - try the drawing die first, then machine the blank cutting edges afterward. This approach saves significant time and effort.

3. Drawing Coefficient (m)

The drawing coefficient is a key process parameter in drawing process calculations, typically used to determine drawing sequence and number of passes.

Factors Affecting Drawing Coefficient m:

• Material properties
• Relative thickness of material
• Drawing method (with or without blank holder)
• Number of drawing passes
• Drawing speed
• Punch and die corner radii
• Lubrication

TIP: Relative thickness, drawing method (blank holder or not), and number of passes are difficult to adjust during die modification - choose these parameters carefully! Have colleagues verify your drawing coefficient selection.

4. Lubrication and Film Protection

When encountering drawing cracks, take these measures:

• Apply lubricating oil on the die (NOT on the punch)
• Cover the die-contact side of the workpiece with 0.013-0.018mm plastic film

5. Workpiece Heat Treatment (Intermediate Annealing)

5.1 Cold Work Hardening Problem

During drawing, the workpiece undergoes cold plastic deformation causing cold work hardening, which results in:

• Reduced plasticity
• Increased deformation resistance
• Increased hardness

Combined with unreasonable mold design, intermediate annealing may be required to soften the metal and restore plasticity.

Note: Intermediate annealing is NOT always necessary in standard processes - weigh the cost of adding an annealing operation carefully before deciding.

5.2 Annealing Methods

Standard companies (nitrogen furnace / bright annealing): The workpiece looks almost unchanged in color after annealing.

Without specialized equipment (conventional annealing):

• Fill the furnace chamber as completely as possible during annealing
• If workpieces are few, co-anneal with other parts (ensure similar annealing parameters)
• Place workpieces in an iron box, weld-seal it before loading into furnace
• Perform pickling after annealing to remove oxide scale

5.3 Key Issues During Annealing

• Decarburization - reduces effective material thickness
• Oxidation (major concern) - oxide scale increases die wear and reduces effective wall thickness