Cutting by Spark Erosion with Travelling-Wire Electrodes

This is a very economical process for cutting through-holes of arbitrary geometry in workpieces. The walls of the openings may be inclined to the plate surface. Thanks to the considerable efficiency of this process, low cavities are increasingly being cut directly into mold plates.

Cutting by spark erosion is based on the same principle of thermal erosion that has been used in EDM for some time. The metal is removed by an electrical discharge without contact or mechanical action between the workpiece and a thin wire electrode. The electrode is numerically controlled and move through the metal like a jig or band saw. Deionized water is the dielectric fluid, and is fed to the cutting area through coaxial nozzles. It is subsequently cleaned and regenerated in separate equipment. Modern equipment has 5-axis CNC controls with high-precision positioning systems.

Deionized water has several advantages over hydrocarbons. Il creates a wider spark gap, which improves flushing and the whole process; the debris is lower, there are no solid decomposition products and no arc is generated that would inevitably result in a wire break. In addition, there is a lower risk of emissions

Figure below depicts the principle of cutting by spark erosion.

Standard equipment can handle complicated openings and difficult contours with cutting heights up to 600 mm. The width of the gap depends on the diameter of the wire electrode and is determined by the task at hand. It is common practice to use wire with a diameter of 0.03 to 0.3 mm. The wire is constantly replaced by winding from a reel. Abrasion and tension would otherwise cause the wire to break. Furthermore, the cuts would not be accurate as the wire diameter would become progressively shorter The maximum cutting speed of modem machines is roughly 350 With the aid of so-called multi-cut technology (principal cut and several follow-up cuts), surfaces with a roughness height of 0.15 pm can be achieved. As with conventional EDM, the workpiece is subjected to thermal load that can lead to structural changes in the layers near the surface. Mechanical finishing of the eroded surfaces may be advisable in such cases.

Product Shape

Every product has its own characteristic shape. The “shape” may be a simple, flat strip or disk, or a complicated, three-dimensional shape incorporating many features which are desired in the product. Certain rules must be kept in mind by the product designer with respect to product shape:

1. The shape must accomplish all that is expected of the product and provide adequate strength for its use.

This is a very important stipulation that is often overlooked the first time around and which often, unfortunately, requires redesign shortly after the product is “out in the field” (after being seriously tested by the end users). Only too often, strength is thought to be increased by simply indiscriminately adding more plastic to the thickness. However, this can result in a serious waste of plastic.As discussed earlier, the cost of plastic can represent a very large portion of the cost of any molded product; also，the thicker the plastic, the slower is the molding cycle (and the production), thus adding to the cost of molding (and the product).

2. The shape must be so designed that it can be produced readily (and economically) by the method intended for its manufacture.

For example, for various design reasons, it may be desirable to make a hollow product by injection molding. However, for certain plastics and products, this may not be possible at all, or else certain complicated, slow, and costly methods would be required: the hollow form could be assembled, by using more than one molded piece joined together; collapsible cores might be used, resulting in a delicate and costly mold; or possibly lost-core molding, a very expensive molding technology, might be employed.

3. The shape must be designed for appearance so that the product has sales appeal and is pleasing to the end user, where applicable. From past design experience, it can be said that… a good-looking shape is also a well-functioning shape…
4. The product shape should include as many features and functions as possible to make the product attractive for use, easy (and at low cost) to mold, and to reduce the costs of any further steps in the manufacturing process where the molded product will be used.

Typical features appearing in the molded product are:

• Openings (inlets, outlets, passages for electrical devices, openings in walls to reduce total mass, where possible, ventilation slots and “louvers”)；
• bosses;
• provisions for methods of assembling other components or hardware;
• provisions for assembling other plastic components (covers，etc.) by snap fit, or by sliding into place;
• handles and grips for easy handling of the product by the end user, stops to restrict motions, etc.;
• studs for mounting other parts;
• registers;
• lettering and logos;

• “knock-outs (provisions in the product wall for future openings, to be broken through by user);
• springs;
• dowels and other locating devices, and many more.

In the remainder of this section, we will discuss some principles and guidelines to consider when designing the shape of a product, which usually consists of a solid, homogeneous outer layer of molded plastic, often referred to as the “wall.” Note that new technologies such as co-injection allow the wall to consist of more than one layer of plastic to enable the use of some low-cost, possibly reground, plastics together with higher cost virgin or even completely different materials with different physical and other properties. There are also occasions where, using insert molding techniques, the outer wall is entirely or in part some other material, such as steel, printed cardboard, etc., and the plastic is used just to create some or all of the joints or to add some of the features usually associated with all-plastic products. Typical features such as openings, louvers, holes, reinforcements such as ribs and gussets, provisions for mounting such as studs and hubs.

HOW TO SELECT HEATER SIZE:

• Establish maximum temperature at which part will operate.
• Calculate total wattage needed to produce this temperature.
• Establish the diameter and length of the Heater that is best suited to the dimensions of your part.
• Estimate the number of HEATERS needed for even heat. Divide total wattage required by the number, of Heaters to determine wattage rating of each unit.
• Determine the watt density of the size selected.
• Determine fit.
• Use graph below to make certain that the watt density you have established does not exceed the maximum allowable watt density of the cartridge. Read along the part temperature curve to the horizontal line representing the fit you have established. From this intersection point read down to the Watt Density Scale. This is the maximum watt density you should use. A higher rating would shorten cartridge life. A lower rating prolongs cartridge life.
• If you find that watt density is excessive you can correct in three ways: (1) Use a tighter fit (2) Use more or larger heaters (3) Use lower wattage. (In this ⑶se allow for longer heat up time.)
• Tight fits are achieved by grinding OD of cartridge and reaming the hole size accurately.

Injection molding of thermoset and thermoplastics is the fastest growing element of the molding industry. New materials are being developed continuously; these and modified materials greatly enlarge the market for new plastics products. Molding machinery, mold engineering, product design, methods engineering, and automation have also been developed at a fast pace to keep up with materials developments. The mold designer must follow all of these developments in order to expand his understanding and experience.

INJECTION EQUIPMENT

The reciprocating-screw injection machine has made-the original plunger-type press obsolete in most cases, and it has been a tremendous help in expediting the growth of injection molding and the use of molded products.

The most widely used contemporary injection molding equipment includes the following basic types:

•  The plunger-injection press utilizes a heating chamber and a plunger operation to force material into the mold .
•  The reciprocating-screw press, often identified as an in-line press, utilizes a reciprocating screw to move and melt the granules of material as they are milled by the screw and passed through the heated injection cylinder. Most of the melting is achieved by mechanical working of the molding compound. Upon melting, the material builds up in front of the screw, forcing it to retract. At this point the screw stops and becomes the plunger, moving ahead to push the plasticized material into the mold.
• The two-stage screw press in most cases uses a fixed screw to plasticize the plastic granules and force the molten compound into a holding chamber from which it is transferred by a plunger into the mold.
• The rotating spreader is driven by a shaft cause it to revolve within the heating chamber, independent of the injection ram and thus to melt the plastic granules. Final filling of the mold is accomplished through the continuing movement of the injection ram.

An Election press consists of the clamping or movable end of the press which is moved by a hydraulic system, or by a toggle clamp arrangement actuated by a hydraulic system; the stationary end of the press provides nozzle protrusion and retention of the fixed half of the mold plus the plasticizing and material feed units.

Presses’ ate available with horizontal or vertical movement. Vertical movement presses are particularly desirable for insert or loose coring types of molding, The moving half of the press contains ejector or knockout systems for the most commonly used mold operations.

In addition to the mechanical motions, damping, and plasticizing functions, the press is equipped with numerous valves, timers, heating controls, and safety features for semiautomatic and completely automatic molding. It is designed and constructed to work continuously on fast cycles.

Mold designs must be achieved that meet the press requirements, material flow characteristics, speed, and cooling needs of the desired cycle.

Capacities of injection presses arc rated in ounces of polystyrene molded per cycle, or in cubic inches of material displacement. Caution must be used in converting the rated machine capacity to equivalents for the material that is to be used. This is a density differential correction.

Electroforming

This is a very similar process to (hut not the same as) electroplating. Whereas with plating a deposit of about 25 [im is the norm, electroforming can he millimetres thick.

Nickel or cobalt—nickel is deposited onto a former, which is made from an inert material, often acrylic. Other former materials may be used but if they are not electrically conductive they have to be made conductive by coating with chemically reduced silver.

The process consists of depositing a layer of cobalt—nickel up to 5 mm deep onto the former. Next a further layer of copper is deposited to increase the cavity wall thickness. At the end of the procedure the former is withdrawn and the composite cavity is inserted into a steel chase for support.

The advantage of this process is that a component accuracy of 1 micron can be achieved and there is no shrinkage involved, unlike in casting. The great disadvantage of this process is the time scale involved, which can he up to 10—12 weeks.

Cavity Corrosion and Erosion

When abrasive or corrosive materials such as glass-filled Nylon or PVC are being used, there is a danger that erosion or corrosion of the cavity will occur. These problems can severely damage the tool, which may then need expensive repairs.

The answer for both problems is to hard chromium-plate or nickel-plate the cavities. This gives a good level of protection to the cavity against both problems. When the plating begins to wear away it may he stripped off and the surfaces re-plated.

It should be remembered that，when using corrosive or abrasive materials, not only the cavity should be protected but also the runner and sprue bush as well.

Gassing and Burning

When the melt enters the cavity, it has to displace the air in front of it. Often this presents no problem, as the air will escape through the split line or ejector pins or down the sides of core pins.

There are some situations, however，where the air cannot escape easily. This often occurs with blind cavities or when high injection speeds are used. In these cases the incoming melt will compress the air in front of it, causing the material to burn. The problem is worse when large volumes of air have to be displaced when using high injection speeds.

To overcome this, vents have to be included to provide an easier path for the air to escape, as shown above. These vent channels have to be very shallow, usually 0.015 to 0.025 mm to avoid the possibility of flashing. The land length of the channels has to be kept short, to allow the air to expand and cool as quickly as possible.

For more minor gassing and burning problems, venting can be provided by grinding very small flats on the sides of ejector pins. Alternatively, special venting pins can be located at trouble spots.

Maximum Metal Conditions

It is good engineering practice to make sure that mould tool cavities can be adjusted if necessary after the first sampling trials. Critical snap-fit features or features that have to mate with other parts are examples of where such adjustment may be necessary. Failure to ensure this can result in very expensive changes or replacements in the tool.

It makes sense to dimension the tool cavities and cores so that small amounts can be machined away from them if the moulding dimensions are incorrect. In fact, it is better to systematically make sure that critical sizes are slightly out of tolerance from the start, so that they be can adjusted after moulding trials, thus eliminating the possibility of remakes being necessary. This method is called using maximum metal conditions (MMC).