Stack Mould

Stack Mould: the mould is primary used for mass production of null and shallow types. In this type of moulding more than one cavity is locked and filled at time. A multi level injection mould in which two sets of cavities are filled simultaneously. The stack mould reduces the clamping force required by multi-cavities mould. Typically multiple cavities are oriented on a single parting line and the requirement clamping force is the sum of the clamp needed by each cavities, die on two or more stacked parting line. The injection force exited on the plate rerating parting lines, so the resulting clamp force is the same as for just one part line.

Stack mould provided more parts fn cycle then would otherwise be possible in a given size moulding process. The stacked injection mould is just what the name implies. A multiple two plate mould is placed one on top of the other.

This construction can be used with three plate moulds and hot runner mould. A stacked two plate mould construction doubles the output from a single process and reduces the clamping pressure required to one half as compared to a mould of the same number of cavities in a two-plate mould. This method is sometimes called “two-level moulding”.

                                               OR

Stack Molds are a series of molding faces “stacked” together to create multiple faces or levels for molding. Each level or face is a parting line and produces molded product. The benefit of stack molding is to increase the output of a given molding machine and operation.

                                               OR

The defining characteristic of a stack mold are the two (or more) mold parting surfaces or mold split lines. A stack mold does not require much more clamp force than a single face mold because the projected part surface areas of the cavities on both sides of the center block cancel out each others force. A rule of thumb for a clamp force estimate is to take the projected part surface area times the melt pressure and multiply it by a factor of 1.1.

Gate-to-Gate Melt Transfer

Increasingly more popular is the melt transfer through the first mold parting surface without a sprue bar because it eliminates the sprue bar as an obstacle. The basic principle of a “sprue-bar-less” melt transfer is a nozzle-to-nozzle melt transfer.

As the mold parting surfaces close, two nozzle front ends establish a melt passage. These nozzles separate when the mold opens. A leak free connection or seal at 2000 bar or 30,000 psi is accomplished with the specially designed manifold system for stack molds from Mold Hotrunner Solutions. During mold opening, the transfer nozzles provide a dry surface with absolutely no stringing or drooling of the melt. 

A fully automated 24/7 molding operation with stack molds requires a functional melt transfer which operates over millions of cycles.

Sequentially controlled stack mold valve gate manifold with a four nozzle valve gate to valve gate melt transfer through the mold parting line featuring cooling free Black Box™ actuators.

Generally, flat parts are more suitable for stack mold consideration than are deep core parts because of the machine opening stroke and the increased mold height of a stack mold. It is also important to check the shot capacity of the machine barrel ensuring that the injection unit can supply enough melt volume cycle for cycle and for all cavities.

MHS stack molds using Rheo-Pro® hot runner systems have a long track record for application success in thin wall packaging, caps and closures, containers and lids, collapsible crates, trays, panels, grills and large surface parts for appliances.

Tandem Molding: Tandem molding is a type of stack molding process in which the parts are injected in alternating cycles. While the molding machine opens to demold one parting line, the other is held together by a locking system. The technology can be employed in almost any standard injection molding machine and offers all of the benefits of stack molding as well as extended cooling times.

Fill analysis for Tandem molded washer components with metal insert molding, valve-to-valve melt transfer through part center hole. First part weight 3.6kg, second part 1.0kg

                    Cycle 1                               Cycle 2

                    Part A Cooling                   Part A Filling

                    Part B Filling                      Part B Cooling

Compact Valve Gate Stack Molds
The Rheo-Pro is an internal actuated valve gate system with no external cyclinder and no elastomeric seals. It requires no cooling. iVG™ represents the latest and most innovative valve gate technology in the industry and is ideal for compact, reliable stack mold solutions.

Cooling-Free Valve Gating
Black Box™ and iVG™ hot runners represent a new generation of valve gate systems that does not require maintenance during the life of the mold. These systems eliminate all quality issues related to cooling water such as corrosion and contamination. Less plumbing also simplifies the design and assembly of the mold. New cooling-free technology can benefit all valve gate applications, including high temperature plastics such as PEEK, LCP, PSU, PEI, and PPS, known for their special properties. Processing these materials requires high melt temperatures of up to 450°C in the hot runner and mold temperatures of over 200°C. Black Box™ and iVG™ hot runner systems deliver uninterrupted performance under extreme conditions without cooling or wear. The result is a significant increase in reliability and mold uptime.

Internal Valve Gate

Clean, Compact and Durable: Its world's first and only internal valve gate nozzle. It is the only pneumatic valve gate hot runner that operates entirely without elastomeric seals, lubricants, or cylinder cooling. Unlike electric actuators, the valve can operate at extreme temperatures of up to 400°C (750°F). Its patented design completely rewrites the rules of valve gating technology and creates endless new possibilities, from high cavitation to single drop hot runner systems.



Stack Molds for All Part Sizes
The versatile range of iVG™ nozzles opens up new possibilities for cost effective production with all stack mold and tandem mold applications. In addition to back-to-back valve gating, the new technology can also be used inside a stack mold as a simple and reliable way to transfer melt from the stationary mold to the first parting line. The larger iVG30 nozzle can be extended and comes with melt bore diameters up to 20 mm. This is an easy and effective way to double production of large and heavy weight parts.

Stack Molds for All Part Sizes

The versatile range of nozzles opens up new possibilities for cost effective production with all stack mold and tandem mold applications. In addition to back-to-back valve gating, the new technology can also be used inside a stack mold as a simple and reliable way to transfer melt from the stationary mold to the first parting line. The larger iVG30 nozzle can be extended and comes with melt bore diameters up to 20 mm. This is an easy and effective way to double production of large and heavy weight parts.

Advantages

1. Doubled Part Output: Instead of increasing the mold size by adding more cavities, a stack mold maintains mold size and machine size by adding a second layer of cavities parallel to the first layer. The fill, pack and cooling time remain the same for a stack mold and only the mold open and mold close time will add slightly to the cycle time.  

2. Reduced Part Price: The part price is determined by the machine hour rate, which is directly related to the machine clamp tonnage. A stack mold requires only about half the clamp tonnage than a single-face mold with the same number of cavities.

3. Efficiency and Improved Automation: Stack molds can produce multi-component assemblies in one shot and in one machine using the same parameters. By comparison, single-face molds would require the production synchronization between multiple machines, complicating post-molding operations.

Shrinkage

Shrinkage:

The shrinkage of plastics signifies the volume contraction of polymers during the cooling step of the processing of polymers. A small amount of shrinkage occurs after ejection as the part continues to cool and after that the part may continue to shrink very slightly until the temperature and moisture content stabilize.

                                                             OR

Plastic injection molding shrinkage is the contraction of a plastic molded part as it cools after injection. Most of the part shrinkage occurs in the mold while cooling, but a small amount of shrinkage occurs after ejection, as the part continues to cool (especially for Delrin or POM).

                                                             OR

What is shrinkage and warpage?

Processing and design parameters that affect part shrinkage.  Warpage.  Warpage is adistortion where the surfaces of the molded part do not follow the intended shape of the design.  Part warpage results from molded - in residual stresses, which, in turn, is caused by differential shrinkage of material in the molded part

The shrinkage factor will depends on:

Plastics material

1. Processing condition

2. Product design

Mould design: Shrinkage value is higher for Crystalline material than Amorphous material.

Plastic Shrinkage: Material shrinkage during and after manufacturing plays an important role in why injection molded plastic parts warp.  Before we dive into parts warpage, it's important to understand how and why plastic materials shrink.  To do that, we need to start at the molecular level with a close look at what happens when plastics melt and cool.  For the most part, the melting and cooling characteristics depend on the type of polymer and whether any filler or fiber reinforcement is present.  

1. Amorphous Materials One polymer type is referred to asamorphous, which includes materials such as ABS, polystyrene, and polycarbonate, among others.  They have a random and entangled molecular orientation in their natural state, much like a bowl of spaghetti.  As these materials melt, the forces between molecules weaken and they move away from each other.  In addition, the shear experienced during the injection phase (which is similar to friction) causes individual molecules to uncoil and align to the direction of flow.  When flow stops, the molecules relax and return to a state of random orientation.  The intermelecular forces pull them closer together until the temperature drops enough to freeze them in place.  These forces result in uniform shrinkage, but the relaxation effect causes more shrinkage in the direction of flow.

2. Semi - crystalline materials: Unlike amorphous materials, semi - crystalline materials have regions of highly ordered, tightly bundled molecular structures.  When they melt, the crystalline structures loosen and the molecules align to the direction of flow, much like amorphous polymers.  But when the materials cool, they don 'trelax.  Instead, they maintain their orientation in the direction of flow and the molecules begin to recrystallize, resulting in significantly higher shrinkage rates.  In this case, however, the effect is much greater in the direction perpendicular to flow.  .  3. Fiber - reinforced materials: Fibers are often combined into a polymer material to add strength and other properties.  When fibers are introduced into the plastic, they may counteract shrinkage effects due to molecular orientation described above.  Fibers do not expand or contract as temperature changes, so fiber - filled materials will typically experience reduced shrinkage in the direction of their orientation.

Shrinkage is very complex in injection moulding and influenced by:

1. Hold-on pressure

2. Mould temperature

3. Hold-on time

4. Distance from the gate

5. Degree of crystallanity

6. Melt temperature

7. Injection rate

8. Wall thickness of the moulding

9. Kind and amount of fillers in polymer.

How do youprevent plastic shrinkage?  

By changing temperatures, pressures, and packing and cooling times, it is possible to mitigate shrinkage.  By applying pressure to a liquid plastic, you can compress the molecules into a smaller volume and then inject more material into the mold to compensate for shrinkage.

How do you calculate material shrinkage?  

Divide the amount of shrinkage by the original size to find the shrinkage rate.  In the example, divide 2 by 8 to get 0.  25.  Multiply the shrinkage rate by 100 to find the shrinkage as a percentage.  In the example, multiply 0.  25 by 100 to get 25 percent.

Why Variations Happen 

While it is clear that varying shrinkage rates can cause warpage, it's also important to understand why these differences occur in the first place.  Here are five of the most common reasons: 

1. Cooling rates: With any semi-crystalline material, a higher cooling rate results in less time for the crystalline structures to form.  This effect decreases total volumetric shrinkage.  The same effect applies to amorphous materials, but because there is less overall shrinkage the degree to which high cooling rates reduce shrinkage is lessened.  

2. Orientation Due to Filling: Initially, the orientation of long, stringy polymer molecules is caused by shear stress during flow when the polymer is still at a high temperature and shear stress is removed, the orientation will relax.  (Orientation is locked - in only when shearing and freezing occur simultaneously.) When this relaxation occurs in amorphous materials, there is generally more shrinkage parallel to flow.  Because the molecules of crystalline materials are aligned in the direction of flow, most crystallization will occur perpendicular to flow, causing more shrinkage in that direction.

3. Mold Restraint: While the part is in the mold, it can't shrink within the plane of its surface but it can shrink in the direction of its thickness.  This has two effects.  First, there is more shrinkage in the thickness direction.  Second, the polymer accumulates stresses in the plane of its surface. After ejection, these stresses may.  relax as the part continues to cool, causing warpage.  The higher the mold temperature, the lower the cooling rate, and the more stresses relax from the part. Mold restraint is also material dependent.  Materials that resist creep (and relax more slowly) have higher linear shrinkage, while materials that relax more quickly have lower linear shrinkage.  

4. Temperature Differences Through the Thickness: When the mold temperature on one side of the cross - section is |  different from the other, shrinkage will not be uniform from side to side.  In essence, the plane on one side of the part will shrink more, causing it to be smaller than the other side creating a bending moment that can lead to warpage.

5. Thickness variations and uneven packing: when there are varying thicknesses of the part, thick areas will take longer to cool, which can lead to higher shrinkage.  A similar effect occurs with areas that are far from the gate.  If a constant packing profile is used, areas closer to the gate will be denser and cool at a different rate than areas further from the gate, causing shrinkage variance.

Polysulphone (PSU)

POLYSULPHONE (PSU):

PROPERTIES

1. Good dimensional stability

2. Good thermal stability

3. Excellent creep resistance

4. Self extinguishing

5. Resistance to mineral acids, alkalies, salt solution, detergents and oils.

6. Good oxidative stability at elevated temperature

7. Can be used continuously in steam at temperature upto 150 ºC

8. Good electrical properties

9. Transparent in nature

APPLICATIONS

Appliances

Dish washer parts, Microwave oven parts, Beverage dispensers, coffee pot / makers, steam table pans and bowls.

Electrical/Electronics

Connectors, fuse bodies, switch housings, coil bobbins, TV components and computer parts

Medical

Chemotherapy devices, electro surgical and cryogenic surgical tools, membranes for renal dialysis, inhalers, pharmaceutical filtering devices, surgical tray, sterlizing equipments parts.

Poly Oxy Methylenes (POM)

POLY OXY METHYLENES (POM): 

- Polyether polymers are the most diverse of the non vinyl polymer in the structural variety that have achieved commercial prominence.They includes polyacetals and PPO. 

- The polyacetal is synthesised by chain-reaction polymerisation. Sometime this ether is also prepared by ring-opening polymerisation.

- PPO (Polyphenylene oxide) is prepared by step-reaction polymerisation.

                                                                 OR

Polyoxymethylene (POM), also known as acetal,[2] polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic polymers, it is produced by different chemical firms with slightly different formulas and sold variously by such names as Delrin, Ultraform, Celcon, Ramtal, Duracon, Kepital, Polypenco, and Hostaform.

- The material was produced by the polymerisation of formaldehyde, which was isolated by first Butlerv in 1859 in polymeric form. It was not commercially available until 1952. 

- The first commercially available acetal resin was marketed by Du Pont in 1952 under the trade name of Derlin. 

- The Du Pont monopoly was usually shortlived as Celcon, as acetal copolymer produced by the Celanese Corporation in 1960. It was commercialized in 1962. In the same year it was also available as Hoste form from Farb Werke Hoechst Co. Germany.

- In 1963, it was also available from Danippon celluloid Co., of Osaka, Japan and Imperial Chemical industries (ICI), Britan when Celanese joined them. In the early 1970’s ultra form GmBH (A Joint venture of BASF and Degussa) introduced copolymer under the name of ultra form. 

- In the early 1970’s Japanese Co., Ashai chemicals introduced the homopolymer under the name Tenal.

- By the late 1990’s the main manufacturers where the American based Du Pont, the Japanese based PolyPlastics and the European based Ticone. Among atleast 8 plants in Asia those of Mitsubishi Gas and Ashai were significant as was also that of BASF.

POM is characterized by its high strength, hardness and rigidity to −40 °C. POM is intrinsically opaque white, due to its high crystalline composition, but it is available in all colors. POM has a density of 1.410–1.420 g/cm³.

Typical applications for injection-molded POM include high-performance engineering components such as small gear wheels, eyeglass frames, ball bearings, ski bindings, fasteners, guns, knife handles, and lock systems. The material is widely used in the automotive and consumer electronics industry.

Monomer Ingredients for Polyacetal:

- Polyacetal is made from formaldehyde (HCHO).

- Copolymer of polyacetal is made from trioxane, the cyclic trimer of formaldehyde.

Chemistry of  Preparation of Polyacetal

Formaldehyde polymerise in the following ways,

1. The cyclic trimer (trioxane) and tetramer are obtained by a trace or sulphuric acid on hot formaldehyde vapor (type i)

2. Linear polymer with degrees of polymerisation of about 50 and  a terminal hydroxy group are obtained by evaporation of aqueous solution of formaldehyde (type ii)

3. In the presence of strong acid, the average chain length may be doubled. Evaporation leads to products (type iii)

4. In the presence of lime water more complex reactions occur, leading to the formation of aldose and hexose (type iv)

Relations of Structure and Properties of POM

- Due to structural similarity properties of acetal polymers are compared with those of polyethylene. 

- Both polymers are linear with a flexible chain backbone and are thus both thermoplastic.

- Both the structures are regular and since there is no question of tacticity arising both polymers are capable of crystallization. 

- In the case of both materials polymerization conditions may lead to structures which slightly impede crystallization; with the polyethylene, this is due to a branching mechanism, whilst with the polyacetals this may be due to co-polymerization.

- The acetal polymer molecules have a shorter backbone (-C-O-) bond and they pack more closely together than those of polyethylene. The resultant polymer is thus harder and has a higher melting point. 

Characteristics of POM (for identification)

The characteristics of POM are,

- The material is semicrystalline and maintain high dimensional stability and it is sensitive to UV light

- It is opaque

- It is identified by the strong smell of formaldehyde, when burned, faint color flame, melt and drips

- Its melting point is 165-175°C

- Its short term and long term service temperatures are respectively 160 - 140°C and 90 - 100°C.

Characteristics of POM

- Good appearance

- Homopolymer is resistant to mid acids and bases

- Good electrical properties but affected by moisture

- Stiff and rigid

- Good toughness

- Notch sensitive

- Excellent fatigue resistance under repeated load

- Excellent creep resistance under continuous load

- Low coefficient of friction

- Good abrasion resistance

- Maintains the mechanical, chemical and electrical properties over broad temperature range and time

- High resistance to thermal and oxidative degradation

- Very good resistance to stress relaxation 

- Excellent dimensional stability

- Good processability

- Copolymers have better thermal stability 

- Burn slowly without smoke generation

- Susceptible to UV degradation

- Attacked by phenol and aniline

- Difficult to electroplate 

- Degradation at high processing temperature and liberate formaldehyde

Properties of Polyacetals (special features)

The principal features of acetal resins leading to commercial application may be summarized as follows.

- Stiffness

- Fatigue endurance

- Resistance to creep 

- Low co-efficient of friction

- Good appearance

Mechanical Properties

The stress- Strain behaviour of polyacetal is such that it could be used to replace metallic materials in many precision engineering applications.

Thermal Properties

- Acetals have a heat distortion temperature in excess of 110°C and can be used in applications upto this temperature intermittently.

- However, acetal can loose strength and toughness after long exposure to hot environments. 

- Homopolymers resist deterioration upto one and a half years at 82°C in air while the copolymers may be used continuously at temperature upto 104°C in air. 

- Mouldings of acetal remain dimensionally stable over the recommended use temperature range.

Electrical Properties

- The electrical insulation properties of the acetal resins may be described as good but not particularly outstanding.

- However, application where impact toughness and rigidity are required along with good electrical insulation characteristics they be used.

Water absorption

The water absorption of polyacetals is low, 15 mg after immersion for 24 hours and 30 mg after 96 hours at 200° C.

Optical properties

- Polyacetal moldings are translucent to white. 

- The light transmission of 2mm thick injection molded panels is 50%, the refractive index is 1.48. 

- The gloss of the moldings depends on the surface finish of the mold.

Permeability to gases and vapours

- The permeability of polyacetal is very low compared with that of other plastics.

- This applies to both aliphatic and halogenated hydrocarbons. 

- Polyacetal is resistant to fuel gases and is therefore suitable for use in gas fittings and aerosol containers.

Chemical properties

- Polyacetals are resistance to weak acids, weakly alkaline solutions (strongly alkaline solutions only for copolymers), gasoline, benzene, alcohols, oils, grease, halogenated hydrocarbons, water, detergents. They are not resistance to strong acids and oxidising agents.

- Polyacetals are not susceptible to stress cracking.

Weathering resistance 

- Polyacetals are damaged by UV radiation. Resultant changes in properties occur more rapidly with smaller wall thicknesses. Degradation can be delayed by light stabilizers.

- Active carbon black has proved to be the most effective stabilizer. Less effective are organic light stabilizers used for natural or colored material. 

- Some pigmented grades exhibit good weathering resistance with added UV absorber. 

- Resistance to high energy radiation:- polyacetal molding should be used in situation where the total radiation dosage exceeds approx. 3.104 kg-1 /3mrad. Yellowing and embrittlement occur at higher dosages.

Flammability

- As polymerization products of formaldehyde, polyacetals are flammable. 

- They burn with a weak bluish flame and drip. 

- After extinguishing or incomplete combustion there is a choking smell of formaldehyde.

Toxicity and Sterilization 

Toxicity

- Polyacetals are free of smell and taste.

Sterilization

- Items made of plastics are usually sterilized using a dosage of 25.104 Jkg-1 /2.5 Mrad. This cause some degradation  which is associated with a decrease in toughness. 

Properties

1. High tensile, impact, and stiffness 

2. Outstanding fatigue endurance 

3. Excellent resistance chemicals 

4. Excellent dimensional stability 

5. Good electrical insulating characteristics 

6. Good resilience and resistance to creep

7. Good abrasion resistance 

8. Natural lubricity 

9. Wide end-use temperature range

Applications of POM

1. Gears, Rollers, Pulleys, bolts, nuts, shelf support brackets, detergent pumps, spray nozzles, mixing blades

2. Sprinklers, pump housings, impellers, pistons

3. Cooling fans, Filter bodies & valves, water meters, tool holders.

Polyphenylene Sulphide (PPS)

POLYPHENYLENE SULPHIDE(PPS):

Polyphenylene sulfide (PPS) is a semi crystalline polymers, high temperature engineering thermoplastic. It is rigid and opaque polymer with a high melting point (280°C). It consists of para-phenylene units alternating with sulfide linkages..

The first commercial process for PPS was developed by Edmonds and Hill (US patent 3 354 129, Yr. 1967) while working at Philips Petroleum under the brand name Ryton.

Today, all commercial processes use improved versions of this method. PPS is produced by reaction of sodium sulphide and dichlorobenzene in a polar solvent such as N-methylpyrrolidone and at higher temperature [at about 250° C (480° F)].

Benzene rings and sulfur atoms form the backbone of the symmetrical macromolecule and characterize the properties:

- high strength, stiffness and hardness,

- high heat distortion temperature,

- low moisture absorption,

- favorable flow properties,

- high dimensional stability,

- high chemical resistance,

- high weathering & radiation resistance 

- low flammability (without additives).

Structure and General Properties

-  PPS has a symmetrical structure and is only slightly branched

-  So it is highly crystalline; degree of crystallinity, and thus the properties of moldings, depend on the thermal history to a very great extent.

-  Melting temperature is 285°C

-  Toughness of the brittle basic material is improved by the addition of glass fiber and mineral fibers.

In the original process developed by Philips, the product obtained has a low molecular weight and can be used in this form for coating applications. To produce molding grades, PPS is cured (chain extended or crosslinked) around the melting point of the polymer in the presence of a small amount of air. This curing process results in:

1. Increase in molecular weight

2. Increased toughness

3. Loss of solubility

4. Decrease in melt flow

5. Decrease in crystallinity

A darkening in color (a brownish color in contrast to this linear PPS grades are off-white)

Over the period of time, modification to the process have been reported to eliminate curing stage & develop products with improved mechanical strength.

Regular PPS is an off-white, linear polymeric material of modest molecular weight and mechanical strength. When heated above its glass transition temperature (Tg ~85°C), it crystallizes rapidly. Main three types of PPS include:

Linear PPS

The MW of this polymer is nearly double as compared to regular PPS.

The increased molecular chain length results in high tenacity, elongation and impact strength

Cured PPS

Obtained from heating of regular PPS in the presence of air (O2)

Curing results in molecular chain extension & formation of some molecular chain branches increases the MW and provides some thermoset-like characteristics

Branched PPS

Has higher MW than regular PPS

The backbone of the extended molecule has extended polymer chin branched from it

Branched PPS has improved mechanical properties, tenacity and ductility.

PROPERTIES

1. Good fatigue, creep and dimensional stability

2. Continuous use at 240 Deg.C.

3. Non-burning

4. Very good chemical resistance even upto 190-200℃

5. Low water absorption

6. Good radiation resistance

APPLICATIONS

1.Electrical & Electronics

2. Connectors, sockets, coil formers, floppy disc heads, bush holders, insulating plates

3. Automotive

4. Valves, carburator parts, lamp sockets Industrial Pump components, metering equipment, medical/dental equipments.

Cutting Tools

Cutting Tools:

1. One of most important components in machining process

2. Performance will determine efficiency of operation

3. Two basic types (excluding abrasives)

     - Single point and multi point

4. Must have rake and clearance angles ground or formed on them

Cutting-Tool Materials:

1. Lathe toolbits generally made of five materials

     - High-speed steel

     - Cast alloys (such as stellite)

     - Cemented carbides

     - Ceramics

     - Cermets

2. More exotic finding wide use

     - Borazon and polycrystalline diamond

Lathe Toolbit Properties

1. Hard

2. Wear-resistant

3. Capable of maintaining a red hardness during machining operation

     Red hardness: ability of cutting tool to maintain sharp cutting edge even when turns red because of high heat during cutting

4. Able to withstand shock during cutting

5. Shaped so edge can penetrate work

High-Speed Steel Toolbits

1. May contain combinations of tungsten, chromium, vanadium, molybdenum, cobalt

2. Can take heavy cuts, withstand shock and maintain sharp cutting edge under red heat

3. Generally two types (general purpose)

     - Molybdenum-base (Group M)

     - Tungsten-base (Group T)

4. Cobalt added if more red hardness desired

Cemented-Carbide Toolbits

1. Capable of cutting speeds 3 to 4 times high-speed steel toolbits

2. Low toughness but high hardness and excellent red-hardness

3. Consist of tungsten carbide sintered in cobalt matrix

4. Straight tungsten used to machine cast iron and nonferrous materials (crater easily)

5. Different grades for different work

Cemented-Carbide Applications

1. Used extensively in manufacture of metal-cutting tools

     - Extreme hardness and good wear-resistance

2. First used in machining operations as lathe cutting tools

3. Majority are single-point cutting tools used on lathes and milling machines

Types of Carbide Lathe Cutting Tools

1. Brazed-tip type

     - Cemented-carbide tips brazed to steel shanks

     - Wide variety of styles and sizes.

2. Indexable insert type

     - Throwaway inserts

     - Wide variety of shapes: triangular, square, diamond, and round

     - Triangular: has three cutting edges

     - Inserts held mechanically in special holder

Grades of Cemented Carbides

1. Two main groups of carbides

     - Straight tungsten carbide

          - Contains only tungsten carbide and cobalt

          - Strongest and most wear-resistant

          - Used for machining cast iron and nonmetals

     - Crater-resistant 

           - Contain titanium carbide and tantalum carbide in addition to tungsten carbide and cobalt

          - Used for machining most steels

Coated Carbide Toolbits

1. Made by depositing thin layer of wear-resistant titanium nitride, titanium carbide or aluminum oxide on cutting edge of tool

     - Fused layer increases lubricity, improves cutting edge wear resistance by 200%-500%

     - Lowers breakage resistance up to 20%

     - Provides longer life and increased cutting speeds

2. Titanium-coated offer wear resistance at low speeds, ceramic coated for higher speeds

Ceramic Toolbits

1. Permit higher cutting speeds, increased tool life and better surface finish than carbide

     - Weaker than carbide used in shock-free or low-shock situation

2. Ceramic

     - Heat-resistant material produced without metallic bonding agent such as cobalt

     - Aluminum oxide most popular additive

     - Titanium oxide or Titanium carbide can be added

Diamond Toolbits

1. Used mainly to machine nonferrous metals and abrasive nonmetallics

2. Single-crystal natural diamonds

     - High-wear but low shock-resistant factors

3. Polycrystalline diamonds

     - Tiny manufactured diamonds fused together and bonded to suitable carbide substrate

Carbide Cutting Tools

1. First used in Germany during WW II as substitute for diamonds

2. Various types of cemented (sintered) carbides developed to suit different materials and machining operations

     - Good wear resistance

     - Operate at speeds ranging 150 to 1200 sf/min

3. Can machine metals at speeds that cause cutting edge to become red hot without loosing harness

Cutting-Tool Nomenclature

Cutting edge: leading edge of that does cutting

Face: surface against which chip bears as it is separated from work

Nose: Tip of cutting tool formed by junction of cutting edge and front face

Base: Bottom surface of tool shank

Flank: surface of tool adjacent to and below cutting edge

Shank: body of toolbit or part held in toolholder

Nose radius: radius to which nose is ground

Size of radius will affect finish

Rough turning: small nose radius (.015in)

Finish cuts: larger radius (.060 to .125 in.)

Point: end of tool that has been ground for cutting purposes

Lathe Toolbit Angles and Clearances

Lathe Cutting-tool Angles

Positive Rake Angle

1. Considered best for efficient removal of metal

     - Creates large shear angle at shear zone

     - Reduces friction and heat

     - Allows chip to flow freely along chip-tool interface

2. Generally used for continuous cuts on ductile materials not too hard or abrasive

Factors When Choosing Type and Rake Angle for Cutting Tool

1. Hardness of metal to be cut

2. Type of cutting operation

     - Continuous or interrupted

3. Material and shape of cutting tool

4. Strength of cutting edge

Shape of Chip

1. Altered in number of ways to improve cutting action and reduce amount of power required

2. Continuous straight ribbon chip can be changed to continuous curled ribbon

     - Changing angle of the keeness

     - Included angle produced by grinding side rake

     - Grinding chip breaker behind cutting edge of toolbit

Factors Affecting the Life of a Cutting Tool

1. Type of material being cut

2. Microstructure of material

3. Hardness of material

4. Type of surface on metal (smooth or scaly)

5. Material of cutting tool

6. Profile of cutting tool

7. Type of machining operation being performed

8. Speed, feed, and depth of cut

Turning:

Assume cutting machine steel:  If rake and relief clearanceangles correct and proper speed and feed used, a continuouschip should be formed.

Nomenclature of an End Mill

Operating Conditions

1. Three operating variables influence metal-removal rate and tool life

     - Cutting speed

     - Feed rate

     - Depth of cut

General Operating Condition Rules

1. Proper cutting speed most critical factor to consider establishing optimum conditions

     - Too slow:  Fewer parts produced, built-up edge

     - Too fast: Tool breaks down quickly

2. Optimum cutting speed should balance metal-removal rate and cutting-tool life

3. Choose heaviest depth of cut and feed rate possible

Blending

1. Five types of powders

     - Tungsten carbide, titanium carbide, cobalt, tantalum carbide, niobium carbide

2. One or combination blended in different proportions depending on grade desired

3. Powder mixed in alcohol (24 to 190 h)

4. Alcohol drained off

5. Paraffin added to simplify pressing operation

Compaction

1. Must be molded to shape and size

2. Five different methods to compact powder

     - Extrusion process

     - Hot press

     - Isostatic press

     - Ingot press

     - Pill press

3. Green (pressed) compacts soft, must be presintered to dissolve paraffin

Presintering

1. Green compacts heated to about 1500º F in furnace under protective atmosphere of hydrogen

2. Carbide blanks have consistency of chalk

3. May be machined to required shape

     - 40% oversize to allow for shrinkage that occurs during final sintering

Sintering

1. Last step in process

2. Converts presintered machine blanks into cemented carbide

3. Carried out in either hydrogen atmosphere or vacuum

     - Temperatures between 2550º and 2730º F

4. Binder (cobalt) unites and cements carbide powders into dense structure of extremely hard carbide crystals

Cutting Speeds and Feeds

1. Important factors that influence speeds, feeds, and depth of cut

     - Type and hardness of work material

     - Grade and shape of cutting tool

     - Rigidity of cutting tool

     - Rigidity of work and machine

     - Power rating of machine

Milling

1. Face milling

     - Ring-type distributor recommended to flood cutter completely

     - Keeps each tooth of cutter immersed in cutting fluid at all times

2. Slab milling

     - Fluid directing to both sides of cutter by fan-shaped nozzles ¾ width of cutter.

Cutting tool materials

1. Selection of cutting tool materials is very important

2. What properties should cutting tools have

     - Hardness at elevated temperatures

     - Toughness so that impact forces on the tool can be taken

     - Wear resistance

     - Chemical stability

Types of tool materials

1. Carbon  steel

2. High speed steel (HSS)

3. Cemented Carbides

4. Cast  alloys

5. Ceramics

6. Cubic boron nitride (CBN)

7. Diamond

Carbon Steel

1. Oldest of tool materials

2. Used for drills taps,broaches ,reamers

3. Inexpensive ,easily shaped ,sharpened

4. No sufficient hardness and wear resistance

5. Limited to low cutting speed operation

High speed steel

1. Retains  its  hardness  at  high  temperature

2. Red  hardness….

3. Relatively  good wear resistance

Tungsten Carbide

1. Composite material consisting of tungsten-carbide particles bonded together 

2. Alternate name is cemented carbides

3. Manufactured with powder metallurgy techniques  p335  Fig. 2

4. Small  particles  are pressed & sintered to desired shape 

5. Amount of cobalt present affects properties of carbide tools

6. As cobalt content increases – the  tougher  the  tool

Making tungsten carbides

Tungsten  &  carbon  mixed  then  heated  to  give  tungsten  carbide

Mix  tungsten  carbide  powder  with  binder

Usually  cobolt

                   Pressing  to  shape

                   Sintered

Cast alloys

1. Commonly known as stellite tools

2. Composition ranges –  38% - 53 % cobalt

                   30%- 33% chromium

                   10%-20%tungsten

3. Good wear resistance ( higher hardness)

4. Less tough than high-speed steels and sensitive to impact forces

5. Less suitable than high-speed steels for interrupted cutting operations

6. Continuous roughing cuts – relatively high g=feeds & speeds

7. Finishing cuts are at lower feed and depth of cut

Inserts

1. Individual cutting tool with severed cutting points

2. Clamped on tool shanks with locking mechanisms

3. Inserts also brazed to the tools 

4. Clamping is preferred method for securing an insert

5. Carbide Inserts available in various shapes-Square, Triangle, Diamond and round

6. Strength depends on the shape 

7. Inserts honed, chamfered or produced with negative land to improve edge strength

Insert Attachment

Fig : Methods of attaching inserts to toolholders : (a) Clamping and (b) Wing lockpins. (c) Examples of inserts attached to toolholders with threadless lockpins, which are secured with side screws.

Ceramics

1. Used as grinding wheels. 

2. As cutting tool inserts. These are used in a similar way to cemented carbide inserts.

3. They can withstand extremely high machining temperatures. 

4. They also have a high resistance to abrasion.

5. Ceramic cutting tools can he used to machine ‘difficult’ materials at really high cutting speeds — sometimes over 2000 m/min. 6. Compare this with the cutting speed for carbon steel cutting tools — 6 m/min. 

7. Ceramic cutting tools are very brittle. 

8. They can be used only on machines which are extremely rigid and free of vibration.

Cubic boron Nitride ( CBN ):

Made by bonding ( 0.5-1.0 mm ( 0.02-0.04-in)

Layer of poly crystalline cubic boron nitride to a carbide substrate by sintering under pressure

While carbide provides shock resistance CBN layer provides high resistance and cutting edge strength

Cubic boron nitride tools are made in small sizes without substrate 

Fig : (a) Construction of a polycrystalline cubic boron nitride or a diamond layer on a tungsten-carbide insert. (b) Inserts with polycrystalline cubic boron nitride tips (top row) and solid polycrystalline CBN inserts (bottom row).

Diamond:

1. Hardest known substance 

2. Low friction, high wear resistance

3. Ability to maintain sharp cutting edge

4. Single crystal diamond of various carats used for special applications

5. Machining copper—front precision optical mirrors for ( SDI)

6. Diamond is brittle , tool shape & sharpened is important

7. Low rake angle used for string cutting edge.