Processing of the coldest extrusion plunger

2022-08-08
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Cold extrusion plunger processing

Abstract: a new cold extrusion process for motorcycle carburetor plunger is proposed, and the selection of part materials, die design, machining method, surface hard anodizing process are introduced in detail. Compared with the die-casting blank and machining forming process commonly used for such parts in China, the quality and service life of the parts are greatly improved

key words: plunger; Cold extrusion; Cold extrusion die

gasoline engine is the heart of motorcycle. Some people call carburetor the heart of motorcycle engine, and plunger is the heart of plunger carburetor. Therefore, as the heart of the carburetor, the choice of material, process method and surface treatment level of the plunger have a direct impact on the performance of the carburetor assembly. This paper is a simple summary of the research results of a new process method - cold extrusion plunger, and discusses it with colleagues

1 development of cold extrusion plunger

in Japan with developed industry, the plunger of motorcycle carburetor has long adopted cold extrusion blank and surface hard anodizing process. The chemical composition of the blank material is shown in Table 1. After surface treatment, the hardness HV > 440

to improve the design level of domestic carburetor plunger, we have explored and studied the selection of raw materials, forming methods, machining and surface treatment processes

1.1 the technological process required for the finished plunger

as the plunger is a key part in the carburetor assembly, its shape is relatively complex and its precision is required to be high. Therefore, its production process is long, and its general process is: raw material selection  cold extrusion blank  machining  surface treatment

1.2 selection of materials

in order to speed up the trial production process, according to the chemical composition of Japanese plunger materials, we selected two similar forged aluminum for trial production, and its chemical composition is shown in Table 2. These two kinds of wrought aluminum have small deformation resistance and good plasticity. We design the mold together with the aluminum factory and specially prepare it to make the surface quality and dimensional accuracy of the blank materials meet the design requirements

1.3 cold extrusion blank

the cold extrusion process of plunger is a new process in China's carburetor industry, and there is no successful precedent. Therefore, the success or failure of the cold extrusion process of the plunger directly determines the success or failure of the development of the cold extrusion plunger

The process flow of cold extrusion blank is shown in Figure 1

Figure 1 process flow of cold extrusion blank

1.3.1 inspection of materials

chemical composition, surface quality and size of each batch of ordered materials should be spot checked. Only qualified raw materials can be put into production

1.3.2 cutting

since the blank is made of solid bars, the blank length shall be calculated before cutting. The volume of the blank is calculated according to the constant volume condition, and the trimming amount should be added, that is,

v0=vp+vs (1)

where: V0 is the volume of the blank, mm3; VP extrusion volume, mm3; Vs is the volume of trimming, mm3. Generally, vs is 3% - 5% of VP

the cutting length of the blank is

h0=v0/a0 (2)

where: H0 is the blank length, mm; A0 is the cross-sectional area of the blank, mm2

1.3.3 deburring

remove the burrs generated during blank cutting

1.3.4 softening treatment

in order to reduce the deformation resistance of the blank, improve the plasticity, improve the structure, refine the grain, and make the blank easy to cold extrusion, softening treatment shall be carried out before cold extrusion. The specific annealing specification is: 410 ℃± 10 ℃, holding for 4h, and cooling to 150 ℃ with the furnace. Test the hardness results before and after softening treatment: the hardness of the blank before treatment is hb68 ~ 72; The hardness of the treated blank is hb38 ~ 40

1.3.5 apply lubricant

when cold extrusion, the unit extrusion pressure is very large. Applying lubricant can reduce the extrusion pressure,

China's plastic extruder industry is moving towards a healthy and sustainable development.

it makes the metal fluidity good, the extrusion wall thickness uniform, the unloading force small, and improves the surface finish of cold extrusion parts. The lubricant used is zinc stearate powder. Usage: clean the blank and roll it in the roller with powdered zinc stearate powder for 15min, so that the blank is firmly and evenly coated with a layer of zinc stearate

1.3.6 cold extrusion blank

cold extrusion is that the metal blank is put into the mold cavity, and under the action of strong pressure and a certain speed, the metal is forced to extrude from the mold cavity, so as to obtain the extrusion parts with the required shape, size and certain mechanical properties. Obviously, cold extrusion depends on the mold to control the metal flow, and depends on the massive transfer of metal volume to form the part blank

using the cold extrusion process can reduce the consumption of raw materials, and the material utilization rate is as high as 80% - 90%; Cold extrusion is carried out on a press, which can complete the forming of simple parts in one stroke. Therefore, compared with cutting, the productivity can be greatly improved and the production cost can be greatly reduced. In the process of cold extrusion, the metal material is under the action of three-dimensional compressive stress, and the grain structure of the extruded metal material will be finer and denser; The metal streamline is not cut off, but continuously distributed along the contour of the extrusion (as shown in Figure 2); At the same time, because cold extrusion uses the work hardening characteristics of metal materials after cold working, the strength of cold extruded parts is greatly improved

Figure 2 metal streamline diagram during cold extrusion

1.3.6.1 design of cold extrusion blank diagram

according to the design requirements of product drawing (as shown in Figure 3), at the same time, consider the principle of minimizing machining allowance and no machining

Figure 3 Diagram of trial products

because the shape of the plunger is not too complex, and it is cold extruded with plastic forged aluminum, it is designed according to the scheme of one-time extrusion. See Figure 4 for the specific size and shape of the blank drawing

Figure 4 trial production blank figure

check calculation section reduction rate:

ε A=(A0-A1)/A0 × 100%=d12/d02 × 100%=

132/17.72 × 100%=54%

does not exceed 75% - 90% of the allowable section reduction rate, so it can be formed at one time

the inner cavity of the upper half of the part can be directly guaranteed by the shape and dimensional accuracy of the punch without cutting. The inner cavity and outer circle of the lower half of the part require high accuracy, so allowance must be left, which is finally guaranteed by cutting. The variation range of metal flow should also be given in the length direction, and the precision requirements of finished products should be achieved by cutting

1.3.6.2 design of cold extrusion die

the unit pressure during cold extrusion is very large. The die must withstand static high pressure, impact, friction between the blank and the die contact surface, and fatigue at the same time. Therefore, in fact, it may not be enough to mine a corresponding amount of minerals. In order to ensure the normal work and service life of the die, the cold extrusion die should have the following characteristics

a. the materials of the working part should have high strength, high hardness, high wear resistance, certain toughness, good thermal stress, thermal stability, heat fatigue resistance and other properties

b. smooth fillet transition shall be adopted at the transition of the working part of the die to prevent large stress concentration and cracking, resulting in early failure of the die

c. there must be thick and hardened pressure pads between the working part of the die and the upper and lower baseplates to ease the ultra-high pressure transmitted from the punch and concave die and prevent crushing the upper and lower baseplates

d. in order to improve the strength of the working part of the die, the cold extrusion die generally does not adopt the integral structure, but adopts the combined structure with prestress

e. the lower bottom plate is forged from medium carbon steel with sufficient thickness to ensure that the mold has high strength and rigidity

The structural design of the mold is shown in Figure 5. The working part is composed of die sleeve (female die), upper punch (punch) and lower punch (reverse punch)

Figure 5 cold extrusion die figure

die sleeve (concave die): determines the accuracy of the excircle dimension of the blank, and the inner surface must be smooth (Ra0.4 μ m) In order to facilitate stamping forming and facilitate the flow of blank materials. At the same time, it can prevent the material from being stuck when returning

upper punch (punch): the shape of the inner cavity of the plunger (not machined) is completely guaranteed by the upper punch. During design and manufacture, it is necessary to ensure smoothness and sufficient strength to prevent fracture during stamping

lower punch (reverse punch): it mainly plays the role of forming and returning the lower end of the plunger. The surface should also be smooth and have sufficient strength

the above three key parts have to bear huge impact during stamping, so their manufacturing accuracy and strength are very important. The material is die steel Cr12MoV, the quenching hardness is HRC60 ~ 62, and the surface is smooth by polishing after fine grinding

the cold extrusion die shown in Figure 5 belongs to the cold extrusion die without guiding device, and the guiding accuracy of the punch is guaranteed by the guide rail accuracy of the press. During trial production, design a special die sleeve, carefully adjust and coincide the center of the upper punch and the female die, and then fasten the lower base plate on the workbench. This kind of structure adjustment is difficult, and the accuracy of machine tool guide affects the consistency of extrusion wall thickness. In order to overcome this defect, during the trial production, we plan to design a cold extrusion die guided by guide post and guide sleeve for several other products. As shown in Figure 6

Figure 6 guide post cold extrusion die figure

1 Die handle 2 Punch 3 Punch retaining ring 4 Pressure ring 5 Upper pressure plate 6 Retaining ring

7 Upper bottom plate 8 Guide sleeve 9 Guide post 10 Unloading ring 11 Unloading plate 12 Bushing

13 Lower bottom plate 14 Pull rod bushing 15 Pull rod 16 External stiffening ring 17 Middle stiffening ring

18 Inner die 19 F pressure plate 20 Ejector rod 21 Ejector rod 22 Top plate 23 Reverse pull rod

1.3.6.3 selection of press

a. calculate the extrusion force

it is difficult to obtain the exact value of the unit extrusion force during cold extrusion by theoretical calculation. At present, only approximate solutions can be found. In production practice, the following data are generally used to estimate the unit extrusion force. Duralumin extrusion: 1000 ~ 1500MPa. Due to section reduction ε A=54%, and the deformation degree is above the middle, so the unit extrusion pressure is taken as p=1400mpa, then the extrusion pressure is

f=pa=p × π/4D2=

1400 × π/4 × 17.72=344.5 (KN)

b. press is selected

according to the calculation results and considering the full safety factor, the company's existing 630kn punch can be directly used without new investment, and there is energy reserve

for the press used in cold extrusion, the following special requirements should be put forward: large energy, good rigidity, high guiding accuracy (especially for the mold without guidance), ejection mechanism (which can be used together with the mold), reliable overload protection device, and suitable extrusion speed. It is recommended to wait and see temporarily. The 630kn punch press of the standard model now adopted basically meets the above requirements and has met the normal use needs in the trial production

1.3.7 heat treatment

in order to reduce the internal stress of cold extruded parts and create conditions for the machining of the next process, the following heat treatment process is adopted

a. heat 520 ℃± 5 ℃, keep it for 3 ~ 3.5H, and put it into 50 ℃~ 80 ℃ water for cooling, and the cooling time is within 20s

b. heat 160 ℃± 5 ℃, and keep the temperature for 8 ~ 8.5H

2 machining

the main process flow is: rough grinding the excircle → flat turning one end face and chamfer → flat turning the other end face and concave hole → special machine for plunger machining (including milling long guide groove, short inclined plane, arch height surface, oil pinhole, etc.) → fine grinding the excircle → shot blasting and deburring. The key processes are as follows

a. grinding excircle: grinding instead of turning improves productivity and processing quality (using centerless grinder). Because the surface of aluminum is relatively soft, it is easy to stick to the grinding wheel during grinding and affect the surface

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