Mould/die manufacture Flashcards
What are the basic functions of the movable mould?
Push out the moulded part through the pin ejector.
Form with the stationary part the entire part cavity and melt running system.
What are the basic functions of a stationary mould?
Enable control temperature within the mould cavity.
Allow injection of melt through sprue bushing.
Locate the injection unit through the locating ring.
Locate the moving mould through guiding pins.
Guide melt through sprue and runners.
How are moulds or dies manufactured?
Moulds are manufactured with a high surface finish in hard steel alloys.
The cavities can be made via classical machining (milling, drilling), high speed milling, or electro-discharge machining.
To get the surface finishes many cavities then get finishing operations such as abrasive polishing or grinding, honing, and electro-chemical machining.
What are the characteristics of conventional machining mdm?
Drilling or milling to chip away the material.
Machine from solid unhardened steel for roughing and semi-finishing. Then heat treat the cavity to the required hardness. Automatic or manual polishing.
Low feed speed, low cutting speed, low rpm, big depth of cut, poor surface finish.
What is electro discharge machining?
The removal of material through rapid recurring electrical discharges between an electrode, which is the cutting tool (cathode), and the workpiece material (anode), all within the presence of a dielectric fluid. This is a very slow process.
The electrical discharge generates intense heat on the surface of the material. This results in a recast white layer on the surface that has changed in strength and results in micro-cracking. This white layer is removed via grinding or polishing.
The dielectric fluid is used to wash away the vaporised material and cools down the cutting area.
Disadvantages: expensive and time consuming secondary polishing operations
What are the seven stages involved in electro discharge machining?
- As the voltage rises, an electrical field builds up at the point of least resistance, which is usually in small opposing humps where the gap is narrowest in the surface roughness, or where there is the largest mount of Conductivity of the fkuid and suspended debris particles.
- A bridge of negatively charged particles forms between the electrode and the workpiece. The voltage begins to stabilise and the current remains at 0. Partial ionisation of the fluid occurs.
- The first stage of the discharge commences as dielectric fluid stops insulating, and a current begins to flow whilst the voltage falls.
- Negatively and positively charged particles are attracted to anode and cathode accordingly, as electric current flows at a higher rate, as voltage falls. A tunnel of vapour begins to form and a small area of the metal begins to melt.
- The discharge tunnel continues to build up, the current begins to stabilise, and there is a large increase in temperature and pressure in the vapour pocket.
- The spark is now the strongest and the temperature is the hottest, because the vapour bubble is expanding rapidly. At the end the power is switched off.
- The sudden current drop means that no further heat is generated. The number of electrically charged particles crossing the gap rapidly declines, collapsing the tunnel, and the molten metal evaporates explosively away from the workpiece surface. The molten and vaporised metal solidify quickly in the dielectric fluid. Hugh temperatures can cause the hydrocarbon dielectric fluid to crack and release a small amount of elemental carbon. Each discharge leaves a tiny crater in the workpiece.
What is high speed milling?
Hsm is performed on hardened materials at high speeds and feeds, around 4-6 times faster than conventional cutting. Hsm can be used for roughing to finishing, and in finishing to super finishing components of any size. The heat generated during machining is mainly dispersed into the chip, and very little into the material or tool. Thus, the tool lasts longer, and the surface of the die can have an excellent surface finish.
A ball nosed end mill can be used for Hsm.
Advantages: save costs and time, less post processing, low temperature, low cutting forces, machine very thin walls, shallow cuts, improved fatigue performance of the dies
Disadvantages: high acceleration and deceleration or machine table and emergency stop not possible, low tool overhangs, low tool runout, needs special machine tools and tool holders with reduced runouts
Edmund can still be used to make tiny features.
What are the three stages involved in high speed milling?
- Face milling is used to flatten the surfaces. Cutting is based on a linear tool path.
- Roughing the cavity is done to create the rough shape of the cavity. Cutting can be based on ramping cutting, spiral plunge milling (preferable), or on a linear tool path.
- Finishing the cavity to final specifications. Cutting strategy is based on mould contour tool path. It is better to finish downwards, rather than upwards.
When finishing in an upwards direction, there is an increased chatter tendency, lower resultant cutting force, higher values of cavity surface roughness, and lower tool life. If finish in a downwards direction, all the opposite.
What is the best way to cool the insert and cavity surface during Hsm?
The truth is that it is best to not use coolant to avoid the thermal shock on the insert, which means it is likely to break.
Yet, as the workpiece needs to be cooled, the best way is to use compreseed air that is supplied through the spindle to blow chips away.
The second best way is to use oil supplied through the spindle at high pressure.
Lastly, high pressure fluid could be supplied through the spindle.
