Manufacturing Engineering - Press shop

The stamping of sheet metals can be defined as the process of changing the shape of the sheet metal blank into a useful shape in the plastic deformation state, using a die and a mechanical press. The stamping engineering efforts are not limited to production engineering but also include the development of the required tooling. Such tooling includes the die making in addition to the fixtures and the automation tools such as the transfer mechanisms typically equipped with suction or electromagnetic cups. For the die making process, stamping engineering starts with the desired panel shape provided by the designer in a CAD file, in addition to the sought panel mechanical properties such as dent resistance (i.e. yield strength). Then, the engineers start with the material selection, i.e. selecting the steel grade, thickness and heat – treatment from what is typically provided by the steel mill. Feasibility analyses follow for each selected material, which lead to a process plan (process settings). After that, the die surface design starts with Finite Element (FE) simulation and numerical trials, followed by the actual (experimental) testing. Successful die designs will then be constructed and validated through a series of try-outs in the die-maker facility and then at the stamping line, using different number of parts (prototypes) and dimensional validation strategies.

The science behind the steel:

The formability of sheet metal can be further analyzed from the metal flow patterns within the die cavity. In stamping, there are three main patterns of metal flow: elastic flow, plastic flow, and rigid movement. The rigid movement is when the position of sheet metal changes without actual deformation, while the elastic flow is the Hooke’s-law based deformation that is temporary and disappears once the load is removed. The plastic flow is the permanent deformation of the sheet that should be constrained. To control the metal flow patterns, stamping engineers manipulate the process variables such as the binder pressure and panel alignment within the die cavity. Not all the steel used in the production process is the same; it varies in thickness, width, strength and weight, according to how and where it will be employed. Some of the lightest steel is in fact the strongest, and it is used to both help the car withstand accident impacts and reduce bodyweight. The properties of each steel type determines the process and speed with which it is formed into parts. Before it can be used, the steel is subjected to an intense, 16-point scrutineering process in the raw materials laboratory to confirm its quality in the key areas of strength, flexibility and mechanical properties. Microscope examinations range all the way from 10x magnification to electron microscopy. Testing the steel to this level not only stops imperfections having an impact on quality but allows press shop to apply exact control over the way the steel reacts in the presses during the forming process.

Generally, the stamping process constitute following main operations:

Blanking (or blank preparation) : involves a cutting action about a closed shape that is the piece retained for future forming (i.e. the blank). The blank shape is composed of any number of straight and curved line segments. Other cutting operations exist in the automotive stamping for developed blanks, such processes include: (1) piercing which is the forming of a hole in sheet metal with a pointed punch without metal fallout; (2) lancing that creates an opening without completely separating the cut piece from the body of the sheet metal, such as the case for louvers; (3) trimming is the process of removing unwanted metal from the finished piece that was required for some previous stamping operation, such as binder areas, or was generated by a previous stamping operation, such as the earing zone on the top of a deep drawn cup; and (4) parting operations are used to separate two identical or mirror image stampings that were formed together (typically for the expediency of making two parts at one time or to balance the draw operation of a nonsymmetrical part).

First forming operations: which aim at forming the blank into a semi-developed blank that has the initial shape. First forming operations include bending and flanging through either a shrink flanging, where the length of the flange shrinks as it is formed, or stretch flanging where the material is stretched as it is flanged.

Drawing operations: most of the deformation modes are based on biaxial stretch over the punch or a bend-and-straighten from the flange. Drawing, sometimes known as cup drawing, radial drawing, or deep drawing, has a very specific set of conditions which differentiate it from other operations. The unique attribute of deep drawing is the deformation state of the flange. As the blank is pulled toward the die line, its circumference must be reduced. This reduction in the circumference generates a compressive stress in the circumferential direction, resulting in a radial elongation as the metal is extruded in the opposite direction.

Subsequent operations: most of the automotive panels require a sequence of forming steps because the degree of forming (flange angle, etc.) cannot be accomplished in a single step. Such operations include the re-strike step which comes after the metal has been stretched over a large radius punch (to avoid splitting), to spread the metal into the desired shape without any additional tension in the stamping line. Another typical subsequent forming operation is the redrawing. Limits are imposed on the blank diameter which can be drawn into a cup of a given diameter. Should a deeper cup be required, an intermediate diameter cup is drawn first; then the cup is redrawn in one or more subsequent stages to achieve the final diameter and height.

Assembling activities: these include variety of specialized cells for combining panels to form BiW components such as joining the door inners and outers. Additionally, other assembling might be done in die-joining strategies.

The main stamping defects analyzed are as follows:

1. Splitting in the stamped panel: local necking rupturing in the stamped panel away from the edge.
2. Splitting at the edge of the panel: rupturing near the edge of the stamping due to the lower deformation capacity at the edge due to the shear zones (edge burrs and cracks).
3. Wrinkling: surface waviness resulting from compressive plastic instability.
4. Shape change: this is the elastic recovery within the panel caused by distortion and spring-back.
5. Low stretch: causing a lower work hardening performance of the formed panel, thus affecting its dent resistance.
6. Surface soft or low oil canning load ability: typically caused by the residual stresses from the different loadings in sequential stamping.

The materials:

Generally, the automotive stamping focuses on two modes of stamping: deep drawing and stretch forming. The deep drawing mode is when the sheet metal is formed (drawn) from the binder by the punch. In other words, if the clamping pressure of the holding down ring is high enough to avoid buckling and wrinkling in the flange(the part of the test – piece clamped between the holding down ring and the die), then the punch stroke will force the material down into the die, thereby pulling the material through the flange and into the die cavity.The stretch forming mode is when the clamping pressure of the holding down ring is very high, such that the friction forces in the flange are high enough to allow the material to flow in. The punch stroke will then force the material in the cup wall and on the bottom of the cup to deform instead. In this case, there will be very little deformation in the flange, but considerable deformation in the cup wall or cup bottom. Automotive body panels are made from steel sheets and extrusions, aluminum sheets (as in the hoods in some vehicle models) and extrusions, in addition to injection-molded plastic parts (as in the fenders in some models). This text will focus on the different steel and aluminum grades and alloys that constitute the vehicle body’s members and panels. The motivation behind selecting such materials for automotive applications is mainly due to their advantageous performance in terms of their dent resistance, energy absorption, strength (and their retained strength at different operating temperatures), and their stiffness. Additionally, steel and aluminum have favorable manufacturability characteristics i.e. their formability, weldability (and hemming ability), and finally their paintability. Other materials such as titanium might exhibit better performance in certain areas such as corrosion resistance and strength, however, the material cost (around 20 – 30 times that of steel), in addition to its processing difficulty (due to temperature sensitivity), limits its wide applicability in vehicle bodies. The steel grades are classified into three main categories: (1) the hot – rolled, cold – rolled low carbon steel; (2) the conventional higher strength steel (HSS); and (3) the advanced (or ultra) high strength steel (AHSS). The HSS and AHSS strength is achieved through special chemical composition, or special alloying elements, in addition to special mill processing. These steels are distinguished based on their strength values, with strength value > 600 MPa being the AHSS threshold. The low carbon formable steel grades are typically described from a formability point of view as in draw quality (DQ) and extra deep draw quality (EDDQ) grades because formability is the prime consideration in making a panel; whereas in high strength steels the yield and tensile strength level are the prime considerations. Hence such grades are classified accordingly. Higher strength steels are desirable for dent resistance, increased load-carrying capability, improved crash energy management, or for mass reduction through a reduction in sheet metal thickness, or gauge.

The main stamping presses:

The automotive presses are typically of two types: the mechanical presses and the hydraulic ones. Mechanical presses use the energy stored in their flywheel which is then translated into a reciprocating motion that forces the press slide through a crankshaft, eccentric shaft or eccentric gear. Such presses come in two different configurations: presses with a gap frame and with a straight side. The gap frame presses come in a wide range of tonnage capacities from about 20 to 600 tons, with speeds ranging from about 20 to 800 strokes per minute (SPM). On the other hand, the hydraulic presses control their force by configuring their hydraulic pressure which then moves one or more rams in a predetermined sequence. The hydraulic presses cover a wide range of applications because of their large bed sizes and their ability to provide their full tonnage at any point in the stroke, which is not the case for the mechanical presses. Hydraulic presses are widely used for deep drawing and for the short runs with frequent die changes. The automotive presses are typically designated based on the following system: SU4-1000-144-96 where: 1000 stands for the tonnage, 144 is the bed size, i.e. 144 inches from left to right, and 96 is the bed size from front to back, i.e. 96 inches. The letters describe the following possibilities: SS is a straight side press, G is a gap press, SE is straight side eccentric, SC is straight side crank, and DU is the Danly under-drive press, S is a single action press, and T is a transfer press. So SU translates into single action press with under-drive. Other automotive presses include the hemming presses used to join the doors’ inner and outer panels; such presses rely on single or multi-stage flanging dies. The automotive presses are arranged in different line configurations according to their action on the part, and the specific part design; these configurations include:

• Tandem press line: where the different presses are arranged without any intermediate material storage in between. For example, a double action press can be accompanied by three or four other single action presses to achieve the deep drawn shape and then the trimming, piercing, flanging and restrikes actions by the single ction presses. The panel flow between these presses in a tandem line can be manual or automated. The tandem press arrangement is typically used for low to medium volume panels, with around six strokes per minute work sequence. An automated arrangement of a tandem press line is shown in Figure 2.38 .

• Transfer press line: where several forming steps or tooling are combined in one press, with all in press panel handling using automation fixtures (tooling) that utilizes suction cups to transport the panel from one forming station to the next. The transfer press line consolidates the stamping presses, however, it works in serial mode, where any stoppage in one station leads to stoppages in all other stations within the same transfer line. The transfer press line can use a tri-axis or cross-bar transportation system to transfer the panels from one station to the next.

The stamping die functions in four different ways:

1. The die position and locate the blank correctly within its cavity according to datum point or line.
2. The die clamps the blank using its binder to hold the blank in place while stamping.
3. The die deforms the blank in three different actions: it cuts (trims, pierces, and shears), it folds (flange), and it stretches and draws the metal.
4. The die releases the formed panel using its mechanisms of cams and rejection pins.

Stamping dies are classified according to different categories:

1. Manufacturing processes: blanking, punching, bending, deep drawing, etc.Forming dies in automotive stamping, are of two main types: (a) Toggle die: runs in double action, with a binder on outer ram (slide) to hold the blank, and a punch mounted on inner ram to form the blank; and (b)Lower cushion die: nitrogen, air pressurized cushions to control force. Typical die schematics are shown in Figure 2.39 .
2. Number of operations: Single-operation (simple), and multi-operation (or combination) dies.
3. Number of stations: Single station and progressive dies. Single station dies may be: (a) combination: a die in which both cutting and non-cutting operations are done at one press stroke); or (b) compound : a die in which two or more cutting operations are done at every press stroke.

Production quantities: high, medium, and low, which can be further categorized into:

• Class A: these dies are used for high production only. The best of materials are used, and all easily worn items or delicate sections are carefully designed for easy replacement. A combination of long die life, constant required accuracy throughout the life, and ease in maintenance are prime considerations, regardless of tool cost.
• Class B: these dies are applicable to medium production quantities and are designed to produce designated quantities only. Die cost relative to total production volume becomes an important consideration. Cheaper materials may be used, provided they are capable of producing the full quantity, and less consideration is given to ease of maintenance.
• Class C: these dies represent the cheapest usable tools that can be built andare suitable for low – volume production.
• Temporary dies: these dies are used for small production and represent the lowest cost tools that will produce the part.

Hot stamping:

In the last decade new technologies have been developed, one of these is called “hot-stamping”. This procedure is slower than cold forming and allows the maximum production of three pieces per minute. The sheet metal is brought to a temperature of 900 ° by an electric oven, during transport from the oven to the mold is cooled to a temperature of about 650 ° – 750 ° at molding. At the end of the stroke the punch is held for a few seconds in order to cool the plate to allow it to pass from an austenitic to a martensitic structure. Martensitic structures are obtained through cooling fast called “quenching”, in order to “freeze” a phase that is stable at high temperature but room temperature is unstable. Usually the tempering is obtained by immersion, or thanks to the exchange intense thermal between the material and a cooling fluid that laps it externally. The remarkable reticular deformations, which hinder the movement of dislocations, are the cause before hardening.

 The martensitic structure is macroscopically fragile and highly tensioned, therefore a treatment is often followed at temper of discovery (the combination of the two treatments is called reclamation) in order to reach a good compromise between hardness, strength and toughness of the metal. In the case of steel the unstable starting phase at room temperature is austenite. This variation of the internal structure limits its elastic returns (Spring-back) and gives considerable hardness and resistance to sheet allowing the elimination of reinforcements in some areas with the relative decrease in the final weight. This increase in resistance makes the following difficult workings that many times can not be performed by classic machinery, but need laser cutting especially in the critical areas. Despite the excellent results obtained this type of procedure is very slow and expensive, in fact it is used for a number limited of components such as those of structure as underbody beam, pillar reinforcement, front and central uprights reinforcements.

Taylor welded:

The tailor-welded blanks, coils, and tubes (TWB/C/T) open the door to customizing the sheet metal thickness and grade at different locations within the same sheet or coil, and to varying the wall thicknesses along tube shapes. The main objective behind the TW technology is to allow for mass savings in the automobile body, by customizing the thickness or grade at specific locations. For example, the door inner panel has to satisfy a stiffness requirement only at the hinge area, so earlier applications used a thicker sheet to stamp the inner, while a TWB allows the OEM to have a thicker portion in the hinge area and the rest of the sheet can be made from a thinner gauge to reduce the overall weight of the panel. Also, TW technology enables improvements in the vehicle structure performance and might lead to cost reduction.