Design and calculation method of steel die pressin

2022-08-21
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Abstract: on the basis of literature [1], the design and calculation methods, contents and steps of metal powder steel die pressed blank and corresponding pressing die structure are described and summarized; The design and calculation results of a typical powder part are introduced

key words: metal powder; Steel mold pressing forming; sinter; Pressing die

metal powder forming is similar to the traditional metal plastic processing technology. For a specific metal powder part, after determining its steel mold pressing method (see literature [1]), it is necessary to according to the relevant technical requirements and original data of the part production, including the part drawing, powder composition and its process performance, processing process flow, pressing method and batch size, etc, Design the blank and die

I. process characteristics and material performance parameters of metal powder and calculation of compacted billet

1 Process characteristics and material performance parameters of metal powder

when formulating the metal powder pressing forming process, its basic characteristic parameters include: (1) powder loose (or shake) density, that is, the average density of metal powder in the die before pressing ρ 0 (2) The compressibility of the powder shall be determined according to the experimental data or relevant standards; (3) The deformability of the powder shall be determined according to the experimental data or relevant standards; (4) Powder flow stress

when designing the press forming die, the following parameters need to be known: (1) the density of the powder pressed blank given in advance ρ p; (2) The springback of the workpiece line after pressing (and subsequent plastic processing), using the relative value α p( α RP) or absolute quantity LP (LRP); (3) The linear shrinkage of the workpiece after sintering, using the relative value β Or absolute quantity ε express; (4) Mass loss caused by sintering oxidation or other reasons, using relative value ζ express; (5) The increase in the density of the workpiece caused by subsequent plastic processing, using the relative value τ express. The relative values of the above rebound and shrinkage can be converted into absolute values according to the following formula: (1)

Where a - the line size of the workpiece

α p, α rp, β Values are given in advance

2. Determine the pressing forming method

after having the above technical data and data, you can select the type of pressing equipment and the corresponding pressing method according to the actual production conditions, and select the pressing direction. For details, see literature [1]

3. Calculation of pressed blank

the essence of calculation of pressed blank is to determine the volume, quality and relevant dimensions of pressed blank according to the processed powder parts, so as to determine the size of pressed concave mold cavity and test the dimensional accuracy of pressed blank. See Table 1 for details. Table 1 Calculation of blank formed by metal powder pressing

definition of calculation content or calculation formula relevant description of blank area FP (cm2) projected area of blank in the plane perpendicular to the pressing direction - Blank volume VP (cm3) VP = (1+ ξ/100)Ve; Ve - Part volume ξ— Relative mass loss pressed billet mass GP (kg) GP = ρ pVp ρ P - average density powder loading mass of pressed parts G0 (kg) G0 = cmgp; Cm = 1.02 ~ 1.05 coefficient cm takes into account the mass loss during powder loading and pressing, and the height of pressed billet HP (mm) HP = h-lp+ ε+Δ m; Δ M - machining allowance, H - powder part size LP, ε See the above for the meaning of other line sizes. The same method is used to determine the powder loading height H0 (mm) H0 = K ρ Hp; K ρ=ρ p/ρ 0K ρ— Powder compaction coefficient

II. Working dimensions of die and mandrel

the determined dimensions include the total height of die, the working dimensions of die cavity and mandrel, as follows:

1 The total height of the die

as described in document [1], the charging height of each section of the die should be proportional to the corresponding height of each section in the workpiece, so as to ensure the uniformity of the density distribution of the pressed blank. The total height of the die mainly depends on its loading height, and at the same time, it is also necessary to consider the upper and lower punches entering the guide part of the die, as shown in Figure 1a

Figure 1 Schematic diagram for determining the total height of the die

hd = h0+hh+hl (2)

where HD - the total height of the die

h0 - the total loading height

hh - the introduction length of the upper punch HL - the introduction length of the lower punch

for the fixed structure, because the die itself has the function of distributing powder along the height direction, the introduction parts of the upper and lower punches can be ignored (see Figure 1b), then

hd = H0 (3)

2 Working dimensions of the cavity and mandrel

the working dimensions of the cavity are mainly determined by the external dimensions of the powder parts. At the same time, it is necessary to consider the changes of the external dimensions of the powder in the processes of pressing, sintering and subsequent fine pressing, as well as the maximum allowable wear of the cavity; The working size of the mandrel mainly depends on the internal size of the part. Similarly, the change of its internal size in each process should also be considered, and it should be determined according to the upper limit deviation of the part, so as to ensure that the mandrel has the maximum wear allowance. The specific calculation is shown in Table 2. Table 2 working dimensions of concave mold cavity and mandrel of metal powder pressing forming die

- working dimensions of concave mold cavity (mm) working dimensions of mandrel (mm) dimension name calculation formula and description dimension name calculation formula and description nominal dimensions of pressing die cavity DN = Dmin LP ± ε ± η Rp

dmin - minimum allowable size of parts

η RP - subsequent fine pressing allowance mandrel nominal

size DN = Dmax LP ± ε ± η RP ± LRP

Dmax - maximum size of mandrel, maximum cavity

allowable size dn '= Dmax LP ± ε ± η Rp

dmax - maximum allowable size of part mandrel maximum

allowable size dn '= Dmin LP ± ε ± η rp±lrp

dmin=d+ δ L - minimum allowable size of mandrel

d - Part aperture; δ 1 - deviation cavity wear under aperture

allowance Δ D=Dn′-Dn- δ A

δ A - actual deviation of cavity size, mandrel wear allowance Δ d=dn′-dn- δ a

δ A - actual deviation of mandrel size nominal size of subsequent fine pressing cavity drpmin = Dmin LRP diameter of subsequent fine pressing mandrel (hole) DRP = Dmax ± LRP

"+" - when the aperture is reduced by rebound after fine pressing

"-" - maximum allowable size of cavity when the aperture is increased by rebound after fine pressing drpmax = external dimension of Dmax LRP blank sintering blank DS = Dmin ± η Rp

"+" - for positive deviation fine pressing

"-" - for negative deviation fine pressing, the internal dimension of sintered billet DS = DRP ± η Rp

reduce the error

"+" - when it is negative deviation fine pressing

"-" - when it is positive deviation fine pressing, the external dimension of the pressed blank DP = DS ± ε

"+" - when the powder shrinks after sintering

"-" - when the powder expands after sintering, the internal dimension of the pressed billet DP = DS ± ε

"+" - when the internal size decreases after sintering

"-" - when the internal size increases after sintering

note: where there is a "±" in the formula, it indicates the actual increase or decrease of the size after this process

III. pressing force and the size of female mold pre-stressed ring

1 The pressing force, jacking force and equipment tonnage

the pressing force of metal powder is usually determined by its unit flow pressure:

P = NF

where - the average unit flow pressure of metal powder, which is determined by the pressing experimental curve or actual experience

F - the cross-sectional area of the pressing billet

n - the number of medium cavities in the pressing mold (pressing multiple pieces at a time)

after the pressing force is determined, Then you can choose the tonnage of the press: Pt = CPP (5)

, where CP - tonnage margin coefficient of the press, CP = 1.25 ~ 1.30

and the jacking force of the pressed parts is required by the side pressure PC, the surface area of the powder side: temperature 10 ℃ ~ 40 ℃, humidity 20% RH ~ 70% rhfc and the friction coefficient between the powder and the mold wall μ (= 0.1 ~ 0.2) confirm: pout = μ Fcpc (6)

2. Size of pre-stressed ring of female die

in order to improve the service life of the press forming die and ensure the dimensional accuracy of the pressed parts, the female die often adopts the structure of pre-stressed ring, as shown in Figure 2. Its size depends on the pressure on the inner wall of the die during pressing. For iron powder compaction, the lateral pressure can be estimated according to the empirical formula: PC = CC ρ MP (7)

where CC = 0.00725, M = 6.8

Figure 2. The size relationship of the pre-stressed ring of the female mold

and the size of the pre-stressed ring must ensure that the female mold has the maximum stiffness. If the inner radius of the female mold cylinder is R1, then there are:

when PC ≤ 200MPa, R2 = 2r1, R3 = 4r1 (8a)

when PC> 200MPa: (8B)

at this time, the interference between the female mold and the pre-stressed ring is:

where e - modulus of elasticity of die material

according to the above one 2、 According to the calculation results of the three sections, the pressing blank diagram, sintering blank diagram, subsequent fine pressing blank diagram, pressing concave model cavity diagram and subsequent fine pressing concave model cavity diagram can be drawn

IV. structural design and strength check of pressing die

1 Mold structure form

after determining the basic dimensions of the die, punch and mandrel, and selecting the model and specification of the press, the mold structure can be designed according to the production plan. There are usually three types of die structures:

(1) the combined type is suitable for the pressing and forming of metal powder parts in small batches, and the powder loading height of each section in the female die is adjusted by the corresponding punch cushion block

(2) the fixed type is applicable to the pressing and forming of small batches of metal powder parts on the general press, and the powder loading height of each section in the female die is adjusted by the punch on the floating die base

(3) the structural type used for the pressing and forming of large quantities of metal powder parts on the fixed special press

2. Mold closing height

as shown in Figure 3, when the fixed mold structure is adopted, the upper punch can be fixed or floating, and its total height can be determined as:

lh = hht+h1+h2 (10)

Figure 3 Schematic diagram of determining the mold closing height

similarly, the total height of the lower punch is:

ll = hlt+h3+h4 (11)

where HHT, HLT - upper The design of the host and auxiliary equipment of the lower punch fixed plate experimental machine draws lessons from the advanced technology of Shimadzu, Japan

h1, H4 - the pressing stroke of the upper punch and the ejection stroke of the lower punch respectively

h2 - the distance between the upper end face of the die and the lower end face of the upper punch fixed plate when the press is at the bottom dead center

h3 - the length of the lower punch into the die barrel

3 Strength check of the stressed parts of the die

as the press forming die works under the cyclic high load, the strength check of the main stressed parts must be carried out

(1) the punch is generally in the state of compressive stress, and its strength check formula is:

σ= P/Fmin≤[ σ] (12)

where p - pressing force of metal powder

fmin - minimum sectional area of punch

[ σ]— The allowable compressive stress of hardened steel

(2) when the length diameter ratio l/d of slender punch (mostly lower punch) and mandrel is ≥ 3, the bending strength needs to be checked. The specific steps are as follows:

(a) calculate the critical load of longitudinal bending instability (13)

where l - the length of the free part of the punch, Equal to the middle position and distance from the fixed plate of the punch to the part of the punch leading into the female die

jmin - the minimum sectional moment of the punch

e - the elastic modulus of the tool steel material, generally e = 2.15 × 105mpa

pcr - critical load producing longitudinal instability

(b) select the bending safety factor of the punch: for hardened steel punch, generally take

p/pcr ≤ n = 2 ~ 3 (14)

(c) determine the maximum allowable length of the punch: (15)

(3) check the compressive strength of the supporting die base and fixed plate: machine operation

σ= P/F≤[ σ] (16)

generally, the thickness of the intermediate fixing plate equivalent to the projected area of the punch die base can be taken as 5 ~ 8mm

(4) spring check: cylindrical compression coil springs are mainly used in the metal powder parts pressing and forming dies. Its selection and verification can be carried out according to relevant manuals

v. brief description of design examples

Figure 4 iron powder parts figure

for the iron powder parts with internal and external flanges shown in Figure 4, the final average bulk density is required to reach 6.25g/cm3, with an annual output of 500000 pieces. The specific design calculation is briefly described as follows:

1 Workpiece processor

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