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常州工学院毕业设计（论文）
外文文献
学 院 机电学院
专业班级 10成型一
学生姓名 高丽君
指导教师 陈怀国
A. Mold Componets
Molds used in injection molding consist of two halves; one stationary and one movable. The stationary half is fastened directly to the stationary platen and is in direct contact with the nozzle of the injection unit during operation. The movable half of the mold is secured to the movable platen and usually contains the ejector mechanism. There are many possible mold designa, including multiple piece molds for complicated parts. On production molding equipment many articles may be shot at the same time by the use of multiple cavity molds. The use of a balanced runner system carries the plastic from the sprue to each individual cavity. At this poin the material passes through a gate into the cavity. The gate is a restriction, smaller then the runner, to provide for even filling of the mold cavity and to allow the products to be easily removed form the runner system. With most injection molding system, the articles can be snapped away from the runner or sprue without additional trimming. Prouducts that have been injection molded can usually be identified by finding where the gate was broken off. The gate will usually be located at the edge or parting line of an object or in the center of cylindrical product.
Molds are expensive, as are the machines. Yet, once the product has been designed, molds made, and production stared, articles can be produced in quantity at low cost. Virtually all thermoplastics can be injection molded through variations in mold and machine design.
Mold (and die) parts that are mass-produced and standardized in shape and dimension are referred to as “standards” (or “standard parts”). Specialized operators of milling machines, lathes, lathes, electronic discharge machining (EDN) equipment and grinders produce mold components independently of each other, following detaied mold part drawings. Finally, all these items come together with the standard mold base and hardware and are assembled by the mold maker. Today, standard components for the moldmaking industry are marketed by a number of companies. Fig.3.1.1 illustrate the standard components for Molds.
Table 3.1.1 Status of standardization (1998) components for
Compression, Injection, and Die-Cast Mold
Pos. No.
Denomination
Standard
DIN
ISO
1
Plate, plain
16760-1
6753-2
2
Plat, drilled
V 16760-2
/
3
Support pillar
DIN ISO 10073
10073
4
Centring sleeve
16759
9449
5
Locating guide pillar
16761
8017
6
Locating guide busch
16716
8018
7
Ejector pin, cylindrical head
1530-A
6751
8
Shouldered ejector pin, cylindrical head
1530-C
8694
9
Ejector pin, conical head
1530-D
/
10
Flat ejector pin
1530-F
8693
11
Ejector sleeve
16756
8405
12
Sprue puller insert
16757
/
13
Sprue bushing
16752-1
10072
14
Angle pin
/
8404
15
Locating unit, round and falt
/
8406
16
Locating ring
16763
10907-1
10907-2
17
Thermal insulating sheet
16713
15600
18
Cooling connectors
16766
/
B Mold Construction
The construction of the mold for injection molding begins with the working drawing. From it the tequirements for the mold can be specified. These would include the material from which the mold should be made, the availability of equipment for machining the mold, and the mold capacity of the die set on the machine.
Cold rolled steel is an ideal material for laboratory molds, since it machines well, is fairly inexpensive, and holds up well for nozzle pressure and wear. Its major disadvantage is that it will rust quickly unless protected by mold telease or wax during storage. Complicated mold cavities need specialized machining and polishing, therefore, circular cavities which can be turned and polished on the lathe require less equipment and machining skill.
Similar molds may also be machined from aluminum, and they have the advantage of not rusting. Excessive wear develops on the sprue due to the high nozzle pressure on the soft aluminum, but this can be overcome by the use of a steel cover plate on the top of the mold.
Another method of mold construction is by the casting process using an aluminum filled epoxy resin. This type of mold is particularly suited to products of intricate design and products that are difficult to machine. The cast epoxy is strong and gives good surface detail, however, it is brittle and should have a steel top plate attached to absorb the wear of the nozzle. A pattern of the product must be secured or made and placed on a mold plate. The drag of a small steel flask is placed around the pattern and the epoxy resin is poured to fill the mold half. When this half of the mold has been cured, the cope is placed over it and the remainder of the mold poured. Upon curing, the flask is removed, all surfaces machined smooth, dowel pinholes drilled, and dowels inserted. A steel cap plate should be bolted to the top halves and the sprue, runners, and gates machined. Instructions for mixing, pouring, and curing the aluminum filled epoxy should be followed according to the manufacturer’s specifications.
2. Hot Runner Systems
Hot runners are classified according as they are heated: insulated-runner systems (it is not described in this article) and genuine hot-runner systems.
The latter can be further sub-classified according to the type of heating: internal heating, and external heating.
Heating is basically performed electrically by cartridge heaters, heating rods, band heaters, heating pipes and coils, etc. To ensure uniform flow and distribution of the melt, usually a relatively elaborate aontrol system comprising several heating circuits and an appropriate number of sensors is needed. The operating voltage is usually 220 V to 240 V, but small nozzles frequently have a low voltage of 5 V, and also 15 V and 24 V operating voltage.
Runner systems in conventional molds have the same temperature level as the rest of the mold because they are in the same mold block. If, however, the runner system is located in a special manifold that is heated to the temperature of the melt, all the advantages listed below accrue. Runner manifolds heated to melt temperature have the task of distributing the malt as far as the gates without damage. They are used for all injectionmolded thermoplastics as well as for crosslinking plastics, such as elastomers and thermosets.
In the case of thermoplastics, these manifolds are usually referred to as the hot-runner system, the hot manifold, or simply as hot runners. For crosslinking plastics, they are known as cold runners.
A. Hot-Runner Systems
Hot-runner systems have more or less become established for highly-automated production of molded thermoplatic parts that are produced in large numbers. The decision to use them is almost always based on economics, i. e. production size. Quality considerations, which played a major role in the past, are very rare now because thermoplastics employed today are almost all so that they can be processed without difficulty with hot-tunner systems that have been adapted accordingly.
Hot-tunner systems are available as standard units and it is hardly worthwhile having them made. The relevant suppliers offer not only proven parts but also complete systems tailored to specific needs. The choice of individual parts is large.
B. Economic Advantages and Disadvantages of Hot-Runner Systems
1. Economic Advantages
Savings in materials and costs for regrind.
Shorter cycles; cooling time no longer determined by the slowly solidifying runners; no nozzle retraction required.
Machines can be smaller because the shot volume-around the runners-is reduced, and the clamping forces are smaller because the runners do not generate reactive forces since the blocks and the manifold block are closed.
2. Economic Disadvantages
Much more complicated and considerably more expensive.
More work involved in running the mold for the first time.
More susceptible to breakdowns, higher maintenance costs (leakage, failure of heating elements, and wear caused by filled materials).
3. Technological Advantages
Process can be automated (demolding) because do not need to be demolded.
Gates at the best position; thanks to uniform, precisely controlled cooling of the gate system, long tlow paths are possible.
Pressure losses minimized, since the diameter of the runners is not restricted.
Artificial balancing of the gate system; balancing can be performed during running production by means of temperature control or special mechanical system (e. g. adjustment of the gap in a ring-shaped die or use of plates in flow channel. Natural balancing is better).
Selective influencing of mold filling; needle valve nozzles and selective actuation of them pave the way for new technology (cascade gate system: avoidance of flow lines, in-mold decoration).
Shorter opening stroke needed compared with competing, conventional three-platen molds.
Longer holding pressure, which leads to less shrinkage.
4. Technological Disadvantages
Risk of thermal damage to sensitive materials because of long flow paths and dwell times, especially on long cycles.
Elaborate temperature control required because non-uniform temperature control would cause different melt temperatures and thus non-uniform filling.
C. Design of a Hot-Runner System and its Components
Hot-runner molds are ambitious systems in a technological sense that involve high technical and financial outlay for meeting their main function of conveying melt to the gate without damage to the material.
D. Externally/Internally Heated Systems
The major advantages and disadvantages of the two types .
E. Externally Heated System
1. Advantage
Large flow channel cause low flow rare and uniform temperature distribution.
2. Disadvantage
The temperatures required for external heating have to be very much higher. For PA 66, for example, the mold temperature is approximately 100℃ and the manifold temperature is at a temperature difference of approximately 170℃ from the mold block, which means.
Special measures required for fixing the hot-runner nozzles to the gates because of the considerable themal expansion.
Risk of disruption if this is not adepantely resolved.
Higher heating power (over 500 W per 100 mm line for a typical cross-section measuring 40·7mm2).
Insulation from the mold block.
Large ,unsupported ateas and therefore, with large-surface molds, risk of bowing of the mold platen on the feed side if this has not been designed thick enough and thus, as a direct consequence, the mold becomes very heavy.
F. Internally Heated System
A frozen layer of plastic forms on the inner surface of the channel and functions as an insulation layer.
The heat requirement of the system is much lower (toughly 55 W per 100 mm length of inside tube).
The temperature differences between mold and manifold blocks are negligible; therefore measures that would have been necessary for large heat expansion are not needed.
The hot manifold of an internally heated system if a compact block that is bolted tightly to mold. Consequently, the mold is very rigid and no measures are required for centering the nozzles and gates. This also allows the plate on the machine side to be manufactured as one block consisting of fixed mold with inbuilt manifold and corresponding rigidity.
The melt volume is small and so the dwell times of the flowing melt are short. On the other hand, the flow rates are very much greater and this can damage the material. It is not advisable to use internally heated systems for sensitive materials. When deciding on a certain system, advice can be obtained from suppliers.
3.Forming Theory
The confidence level in successfully forming a sheetmetal stamping increases as the simplicity of the part's topography increases. The goal of forming with stamping technologies is to produce stampings with complexgeometric surfaces that are dimensionally accurate and repeatable with a certain straindistribution, yet free from wrinkles and splits. Stampings have one or more forming modes that create the desired geometries. These modes are bending, stretch forming and drawing. Stretching the sheetmetal forms depressions or embossments. Drawing compresses material circumferentially to create stampings such as beer cans.As the surfaces of the stamping become more complex, more than one mode of forming
will be required. In fact, many stampings have bend, stretch and draw features produced in the form die. The common types of dies that shape material are solid form, stretch form and draw.
Solid Form The most basic type of die used to shape material is the solid form die. This tool simply displaces material via a solid punch "crashing" the material into a solid die steel on the press downstroke. The result is a stamping with uncontrolled material flow in terms of strain distribution. Since "loose metal" is present on the stamping, caused by uncontrolled material flow, the part tends to be dimensionally and structurally unstable.
Stretch Form Forming operations that provide for material flow control do so with a blankholder. The blankholder is a pressurized device that is guided and retained within the die set. Stampings formed with a blankholder may be
described as having three parts, shown in Fig. 1. Theyaretheproductsurface(shown in red), blankholder surface (flat area shown in blue) and a wall that bridges the two together. The theoretical corner on the wall at the punch is called the punch break. The punch opening is the theoretical intersection at the bottom of the draw wall with the blankholder. The male punch is housed inside the punch opening, whereas the blankholder is located around the punch outside the punch opening. These tools have a one-piece upper member that contacts both the b- lankholder and punch surfaces. A blank or strip of material is fed onto the blankholder and into location gauges. On the press downstroke, the upper die member contacts the sheet and forms a lock step or bead around the outside perimeter of the punch opening on the blankholder surface to prevent material flow off the blankholder into the punch. The blankholder then begins to collapse and material stretches and compresses until it takes the shape of the lower punch. The die actions reverse on the press upstroke, and the formed stamping is removed from the die.
Draw The draw die has earned its name not from the mode of deformation, but from the fact that the material runs in or draws off the blankholder surface and into the punch. Although the draw mode of deformation is present in draw dies, some degree of the stretch forming and bending modes generally
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