Aluminum can forming process and mold

Aluminum can forming process and mold

Abstract: The can body stretching process, the thinning stretching process and the bottom forming process were analyzed, and some key technologies in the design and manufacture of the molds related to these processes were studied.
Key words: cans; forming process; mold; thinning and stretching
1 Introduction
Aluminum cans occupy a considerable proportion in beverage packaging containers. The manufacture of cans combines advanced technologies in many industries such as metallurgy, chemicals, machinery, electronics, food, etc., and becomes a microcosm of aluminum deep processing. With the increasing competition in the beverage packaging market, how to minimize the thickness of the sheet in the can production, reduce the quality of the single can, improve the material utilization rate, and reduce the production cost is the important pursuit of the company. aims. To this end, technological transformation and technological innovation characterized by light-weighting are quietly emerging. The lightweighting of cans involves many key technologies, of which tank forming and mold technology are important aspects.
2 tank manufacturing process and technology
2.1 tank manufacturing process
The main manufacturing process of CCB-1A can body is as follows: coil conveying → coil lubrication → blanking, stretching → tank forming → trimming → cleaning / drying → stacking / unloading → coating color → baking Dry → color printing → primer → drying → internal spraying → internal drying → tank lubrication → necking → spinning neck.
In the process flow, blanking, drawing, tank forming, trimming, shrinking, and spinning/flanging processes require mold processing, among which blanking, drawing, and can forming processes and molds are the most critical. The level of technology and the level of mold design and manufacturing directly affect the quality and production cost of cans.
2.2 Tank manufacturing process analysis
(1) Blanking-stretching compounding process. When stretching, the material of the edge of the blank forms a cup along the radial direction, so the unit body in the plastic flow region is a bidirectionally stressed, one-way tensioned three-direction stress state, as shown in FIG. Due to the action of the arc of the punch and the arc of the drawing die, the wall thickness of the lower part of the cup is reduced by about 10%, and the thickness of the cup is increased by about 25%. The size of the arc at the corner of the cup has a great influence on the subsequent process (formation of the can body). If the control is not good, the can is easily broken. Therefore, the blanking process must consider the following factors: cup diameter and draw ratio, punch arc, stretch die arc, convex, die gap, mechanical properties of aluminum, friction properties of the mold surface, material Surface lubrication, stretching speed, lug rate, etc. The production of the lugs is mainly determined by two factors: one is the performance of the metal material, and the other is the design of the tensile mold. The lugs appear at the highest point of the cup and are also the thinnest point, which will affect the formation of the can, resulting in incomplete trimming and increased scrap rate.
Based on the above analysis, it was determined that the draw ratio selected by the drawing process was m = 36.55%, the blank diameter Dp was 140.20 ± 0.01 mm, and the cup diameter Dc was 88.95 mm.

(2) Tank forming process.
Thinning and stretching process analysis. The typical aluminum can stretching, thinning and stretching process is shown in Figure 2, and the stress state during the thinning and stretching process is shown in Figure 3. During the stretching process, the metal concentrated in the conical portion of the die mouth is the deformation zone, and the force transmission zone is the cylinder wall and the bottom of the casing after passing through the die. In the deformation zone, the material is in a three-direction stress state of axial tension, tangential compression and radial compression. Under the action of three-direction stress, the grain refines and the strength increases, accompanied by work hardening. In the force transmission area, the stress conditions of the materials in different parts are different. The metal in the rounded area of ​​the punch is the most severely stressed. It is axially and tangentially pulled in both directions, and is radially pressed. The trend of thinning is severe, and the metal is easily broken from here, resulting in failure of stretching. Comparing the stress state of the metal in the deformation zone and the force transmission zone, it can be known that whether the thinning and drawing process can smoothly proceed mainly depends on the tensile stress of the metal of the rounded portion of the tensile punch, and when the tensile stress exceeds the limit of the material strength, It will cause breakage, otherwise the stretching process can proceed smoothly. Therefore, reducing the tensile stress during the stretching process is the key to ensuring smooth progress of the stretching.

The selection of the thinning stretching ratio is: re-stretching: 25.7%, the first thinning stretching: 20% to 25%, the second thinning stretching: 23% to 28%, the third Sub-thinning stretching: 35% to 40%.
During the forming process, there are many factors that affect the amount of tensile stress inside the metal, including the concave mold cone angle. The value is directly related to the flow characteristics of the metal in the deformation zone, which in turn affects the amount of forming force required for stretching. Therefore, whether the value is reasonable or not has an important influence on the implementation of the process. When α is small, the range of the deformation zone is relatively large, the metal is easy to flow, and the distortion of the mesh is small. As α increases, the range of the deformation zone decreases, the deformation of the metal concentrates, the flow resistance increases, and the grid is severely deficient. Moreover, as the taper angle of the die increases, the strain of the material in the deformed zone increases correspondingly. This indicates that when the taper angle of the die is large, not only the deformation range of the metal is concentrated, but also the amount of deformation increases rapidly, thus processing the metal in the deformed zone. The hardening phenomenon is exacerbated, causing an increase in the stress inside the metal, which adversely affects the stretching. On the other hand, when α is too large or too small, the tensile force is increased. The reason is that when α is too large, the metal flows sharply, the work hardening effect of the material is remarkable, and as the taper angle increases, the concave The force component of the mold cone portion that hinders the flow of the metal is increased, and thus the required tensile force is increased; After too little, although the metal flow has a small turning point, the total frictional resistance on the tapered surface is large due to the long contact surface between the metal and the concave surface in the deformation zone, so that the mesh distortion is small and the total tensile force is increased.
It can be seen that the reasonable determination of the cone angle of the concave mold should consider the deformation characteristics of the material in the deformation zone and the friction between the mold and the workpiece. The reasonable range of the cone angle of the concave mold has a direct influence on the stretching process. The process test shows that for the aluminum alloy 3104H19 of CCB-1A type, the reasonable value of the concave mold taper angle is suitable for α=5°~8°.
Bottom forming process analysis. The bottom forming of the can occurs at the end of the punch stroke, using a reverse re-stretching process. Fig. 4 is a schematic view showing the state of the forming of the bottom of the can, and the forming force of the bottom mainly depends on the nature of the friction force and the magnitude of the blanking force. Generally, the thickness and strength of the material are a contradiction. The thinner the material, the lower the strength. Therefore, the lightweight technology requires reducing the diameter of the bottom of the tank and designing a special shape of the bottom of the tank. The process test shows that if the angle of the outer wall of the tank bottom groove is greater than 40°, the pressure resistance of the tank bottom will be greatly reduced. Considering the formability of the metal, the punch arc R cannot be less than 3 times the thickness of the material. But if R is too large, it will reduce the strength. The inner surface arc R1 of the spherical surface and the bottom of the tank bottom is at least three times thicker, and R1 is usually 4 to 5 times thicker. Reducing the angle α2 of the inner wall of the tank bottom groove will increase the strength, and most of the production uses 10° or less.

There are two failure points at the bottom of the tank: one is the bottom spherical surface; the other is the bottom arc R of the tank connecting the spherical surface and the side wall. The strength of the spherical surface of the tank bottom depends on several factors: the modulus of elasticity of the material, the diameter of the bottom, the strength of the material, the radius of the sphere, and the degree of thinning of the metal as it forms. The spherical radius of the tank bottom is usually determined by the formula R ball = d1/0.77, and the actual R ball = 45.72mm
3 mold design and manufacturing
3.1 tank stretching die
The can body stretching process is actually a stretching process of the cylindrical member. During the stretching process, the flange portion of the material is easily unstable under the action of compressive stress, resulting in wrinkling, so it is necessary to consider setting the crimping preventing wrinkles. Device. When the material passes through the die, the fillet portion of the die is a transition zone, and its deformation is complicated. In addition to the radial and tangential compression, it is also subjected to bending, so the selection of the die fillet is particularly important. After the material is rounded through the die, it is in a stretched state. Since the tensile force comes from the punch pressure, it is transmitted through the corner of the punch. The material at the rounded corner of the punch is the most severe, and it becomes the most vulnerable. Dangerous section.
The blanking-stretching combined die structure is shown in Fig. 5.

(1) Mold material: The material of the cemented carbide is selected for both the convex and concave molds.
(2) Deformation amount: In the can industry, the tensile ratio δ is generally used to indicate the deformation amount, δn=(dn-1―dn)/dn-1×100%. According to this formula, the calculation is as follows:
For the first stretch, δ1 = (d0 - d1) / d0 × 100% = (140.2001 - 88.951) / 140.2004 × 100% = 36.6%.
Further stretching takes δ2 = (d1 - d2) / d1 × 100% = (88.951 - 66.015) / 88.951 × 100% = 25.8%.
It is generally required that the total stretch ratio δ ≤ 64%, δ1 ≥ δ2 ≥ ... ≥ δn, δ1 ≤ 40%.
(3) Blanking device: The corrugated ring is used, and 0.2-0.3 MPa compressed air is used as the power source.
(4) Drawing die working part parameters:
Fillet radius:
The corner radius rA of the drawing die is 3.556 mm, and the radius rA of the drawing die is 1.78 mm.
The bending radius rB of the drawing punch is 2.921 mm, and the radius of the bending of the punch is taken to be rB2.286 mm.
gap:
The tensile die and the concave die have a large gap of Z/2, so the friction is small, and the tensile force can be reduced, but the gap is large, and the precision is difficult to control; the tensile mode convex and the concave die have a small gap of Z/2, and the friction is small. Large, increase the tensile force.
The unilateral clearance Z/2 can be calculated as follows:
Z/2=tmax+Kt
Where tmax - the maximum material thickness, take 0.285 + 0.005mm
t - nominal material thickness, take 0.285mm
K - coefficient, when t < 0.4mm, take 0.08
Then Z/2=0.290+0.08×0.285=0.313mm.

3.2 thinning tensile mode
The can body formation is actually a combined process of combining re-stretching and three-way thinning stretching. The design of the thinned tensile die is now introduced as follows:
(1) Mold material. Punch: The base material is alloy tool steel, the punch material is M2, the heat treatment hardness is 60~62HRC, and TiN is plated. Die (thinning stretch ring): The base material is alloy tool steel, and the die material is cemented carbide (brand name is VALENITEVCID-H.L.D or KE-84KENNAMETAL).
(2) The amount of deformation. The formula for calculating the thinning draw ratio is: δ=(tn-tn-1)/tn×100%, where tn and tn-1 are n times and n-1 times thinning and the side wall thickness of the part after stretching , calculated: δ1 = (0.285 - 0.225) / 0.285 × 100% = 21.05%; δ2 = (0.225 - 0.170) / 0.225 × 100% = 24.44 %; δ3 = (0.170 - 0.106) / 0.170 × 100% = 37.65%.
Canning factories often prepare matching tables for tensile rings and punches for a given material thickness, tank thickness, thin wall requirements, tensile ring and punch size, tensile machine accuracy, etc. for technicians, mold repairs Personnel and operators are equipped with punches and tabs.
(3) The working part parameters of the mold. Punch: the convex arc R1.016±0.025mm, and then the convex arc R2.286mm, and the outer side wall arc of the tank bottom groove R10.478±0.013mm. Thinned stretch ring: concave die cone angle α=5°, working belt width h=0.38+0.25 mm.
3.3 can bottom forming die
The structure of the bottom forming die is shown in Fig. 6.

The material of the tank bottom punch is made of alloy tool steel Crl2MoV, and the heat treatment hardness is 60~64HRC. The contour shape should be consistent with the tank design. The bottom pressure die material is made of alloy tool steel Cr5MoV, and the heat treatment hardness is 58~60HRC. The contour shape should match the punch.
4 Conclusion
(1) The important factors considered in the stretching process are: draw ratio, convex, concave arc radius, convex, concave die clearance, aluminum mechanical properties, lubrication, and operating parameters.
(2) The concave mold taper angle in the thinning and stretching process. The size is related to the flow properties of the metal in the deformation zone, the magnitude of the stress and the force of the mold. The reasonable range is α=5°-8°.
(3) The proper tank design is the key to the implementation of lightweight technology. The research shows that for the CCB-1A type tank, the design parameters are selected: the angle of the outer wall of the bottom ditch is α1=32°, the angle of the inner wall of the bottom groove is α2=5°, the arc of the punch is R=1.016mm, the spherical surface and the bottom of the tank The inner wall arc R1=1.524mm, the spherical radius of the bottom of the tank R ball = 45.72mm, can greatly increase the strength of the tank.