Tool Making - Quality requirements for STEEL mold :- PART-II

 In the previous PART-I we saw about  A) tool steel performance requirements in the work.
In the current PART-II we are going to see about :-

                                 B) die steel in the process performance requirements
                                 C) mold steel in metallurgical quality requirements






 B) die steel in the process performance requirements


(1)machinability:-
                                                       Including steel machinability by cutting both hot forging and cold, depending on the chemical composition of steel after heat treatment of internal organization and the quality of metallurgical production, in recent years, in order to improve the machinability of steel in add some steel in the steel cutting element or change in the distribution of the inclusions to improve the surface quality of die steel and reduce the die wear. In thermal processing, a number of high-carbon high-alloy tool steel, in particular, to improve the carbide morphology and distribution, grain size and degree of austenitic alloy is very important.

In addition should have good processability, but also has good electrical turning model of processing and processing of such impression.

(2) hardenability and hardenability:-
                                                                    Die under these two performance requirements are different emphases in different working conditions, and for the requirements of the hardness of the uniformity of the high cross-section of the mold, such as hammer forging die steel, its high hardenability more important, while only requires a small die with high hardness, such as blanking blanking die steel, even more emphasis on high hardenability.

(3) heat deformation:- 
                                                  Mold parts in the heat, require little deformation in all directions to have similar changes, and organizational stability. Quenching distortion is small, except with the quenching temperature] time and cooling medium and other factors, but it mainly depends on the composition of the steel uniform, metallurgical quality and organizational stability.

(4) decarburization sensitivity:-
                                                              Die steel forging, annealing or quenching, in the absence of heated under protective atmosphere, its surface will have defects such as oxidation and decarbonization, so that die down in durability. In addition to decarburization and heat treatment process, equipment related, in respect of material itself, depending on the chemical composition of steel, especially carbon, in the presence of high silicon, molybdenum and other elements, it will also increase decarburization.

In addition, the conditions should be based on the use of molds should be considered mirror polishing of the mold making, and so grinding and electrochemical properties.


   C) mold steel in metallurgical quality requirements

                                                          It can play a high metallurgical quality of the properties of the steel substrate, die within the metallurgical quality of steel and its basic performance with the same importance in the study of performance, we must study the quality of metallurgical factors. More frequently encountered in general internal and external quality of die steel in the following areas:

(1)the uniformity of chemical composition:-
                                                                                      Die steel usually contains many elements of the alloy steel, steel ingot mold from the liquid during solidification, crystallization of the separation were the reason, various elements in liquid steel solidification structure in the formation of uneven distribution of segregation, the chemical composition segregation will result in differences in microstructure and properties, which affect the quality of steel is one important factor. Reduce the segregation of steel, can effectively improve the performance of steel. In recent years, many of metallurgical plant and abroad to study in the production of uniform, grain refinement of steel.

(2) the content of harmful elements:-
                                                                        Sulfur and phosphorus in steel during solidification and the formation of phosphide and sulfide precipitation in the grain boundary, resulting in the intergranular brittleness of the plastic reduction of the steel, high S, P content, easy to make the ingot cracks in the rolling and may significantly reduce the mechanical properties of steel. Fortunately, one studies in Japan said that S, P content on the W (Cr) 5% hot work die steel (H13) of the impact toughness and thermal fatigue performance results show that, as the W (S, P) content of 0.025% from and 0.010% to W (P) 0.005%, and W (S) 0.001%, the thermal fatigue crack length and number will be reduced by half. Hitachi Metals will SKD61 steel in W (P) content from 0.03% to 0.001%, while steel can be 45HRC toughness increased by the 39.2J/cm2 to 127.5J/cm2. In addition, the reduction of steel in the S, P content also can increase the isotropic steel.

(3) non-metallic inclusions in steel:-
                                                                     Good quality steel chemical composition not only to comply with technical standards and non-metallic inclusions in steel content should be kept as small as non-metallic inclusions in steel volume occupied by the small, but for steel Performance is great. Reduce non-metallic inclusions in steel is the steel of the main tasks. Commonly referred to as non-metallic inclusions in steel, mainly refers to the iron and other alloying elements with oxygen, sulfur and nitrogen compounds formed by the role, such as FeO, MnO, Al2O3, SiO2, FeS, MnS, AlN, VN, etc. , as well as steel and pouring into the refractory, which elements are mainly Si, Al, Fe, Cr, Ca, Mg and other oxides. 


                                                                   Non-metallic inclusions in steel on its source, can be divided into internal and external inclusions , the inclusions cents in the liquid and the solidification of steel in the process of formation of compounds.

Non-metallic inclusions in steel in the basic kinds of sense to stay a certain size as a crack, it destroys the continuity of the metal, causing stress concentration, the role of stress in the outside world, the crack extension is easy to develop and expand the lead to performance reduced. The presence of plastic inclusions, with the extension of forging deformation rolling process, resulting in anisotropy of steel produced. While inclusion of peeling polishing process to improve the surface roughness of the mold. Therefore, the mold is large and important to improve the purity of steel is very important.


(4)white dot:-
                                Hot rolled steel billet and white point is more common in large forging defects, is a breakdown of internal steel mold making. The presence of white steel with a very negative impact on performance, which mainly affects the mechanical properties of the steel mold design decreases when the forging hardening heat treatment cracking, or use to develop into more serious accident damage, so in any case Next, we can not use a white point of the forgings. White point of the different sensitivity of steel is different, generally considered prone to white spots are chrome steel, steel, chrome molybdenum steel, manganese steel, manganese steel, molybdenum, chromium nickel molybdenum steel, chromium tungsten steel. Which contains W (C) greater than 0.30%, W (Cr) greater than 1%, W (Ni) 2.5% of the earth martensitic chrome-nickel steel and chrome-nickel molybdenum steel, the sensitivity of the biggest white spot. The reason is the formation of white steel, precipitation of hydrogen in the precipitation accumulation, vertical section of the steel to form crystalline silver bright white thick round or oval spots. It tends to make forgings and billet material internal cracks. Die steel 5CrNiMo, 5CrMnMo white as the most likely to occur if the increase in carbon elements Cr, Mo, and V can reduce the white point after the sensitivity. In the production of such steel degassing must pay attention to and strengthen the great relief after forging or cold forging dehydrogenation annealing.

(5)the oxygen content:-
                                                 Generally we did not provide for die steel in steel gas content allowed. With the increase of oxygen content, oxide particles and volume increased, decreased fatigue properties of steel, are also prone to hot cracking. Some people have been tested 4Cr5MoSiV1 steel, the oxygen content of no more than 1.5 * 10-5, which provides Japan's Sanyo Special Steel Company of high purity oxygen content of steel is not greater than 1.0 * 10-5. Therefore, in recent years, in order to improve the quality of the mold making supplier. Gradually die steel at home and abroad to the low oxygen content in the direction of development.

(6) the carbide inhomogeneity:-
                                                            Carbide is the most essential components of die steel, in addition to the carbide can be dissolved in austenite, there will be some residual carbon is not soluble in austenite. Carbide size, shape, distribution of properties such as the use of die steel has a very important influence. On the carbide size, shape and distribution methods and steel smelting, steel ingot solidification conditions and heat processing of deformation conditions and so on. Eutectoid carbon steel may be formed at the grain boundaries or in the processing of the wind-shaped deformation of carbide carbide carbide is elongated to form zonal or both, ledeburite die steel, there is a carbide and secondary carbides in the hot deformation process, the mesh can be broken most of eutectic carbide, carbide be extended along the deformation direction, resulting in ribbon, with the increase of deformation, the carbide becomes uniform, fine . Carbide heterogeneity on quenching deformation, cracking, mechanical properties of steel greater.

(7) segregation:-
                                Segregation of the steel composition and organization of the performance of non-uniformity, which is in the mold steel macrostructure often test the existence of a defect. Is the ingot formed in the solidification process, and steel, chemical composition and pouring temperature and so on. Generally divided into dendritic segregation, square segregation, dotted segregation. Because the presence of dendritic segregation, so that all negative then the mechanical properties in different directions showed significant differences. Square ingot segregation is due to crystallization, the end of the columnar crystals and equiaxed ingot heart interval, bringing together more impurities and pores formed. Square severe segregation, the impact on the quality of steel is significant, especially cutting the amount of large parts or the force of the Department of heart mold parts. Segregation in addition to the mechanical properties of die steel isotropic, the polishing of the mold have an impact on performance. Therefore, foreign-related standards have strict requirements.

(8) loose:-
                         Loose density of steel is not the performance. Most loose in the upper and central parts of ingots, because in these places more focus on the result of impurities and gases. Because osteoporosis defects, reducing the strength and toughness of steel, but also seriously affected the surface roughness after processing, in general the impact of mold steel is not particularly large, but if cold roll, a large block, punch and  mold and others have more stringent requirements. Such as deep cavity of the forging die and punch requires loose no more than 1 or 2, for the dial or light pieces of plastic mold steel, requires no more than one loose.

Tool Making - Quality requirements for STEEL mold :- PART-I

                                                         Mold growing in the modern manufacturing industry occupies an important place and, in particular the automotive and electrical industry more than 70% of the parts in the mold manufacturing process used. However, INDIA imported a large number of high-quality molds to analyze the main reason for our high quality steel is not steel-making level, but do not realize that improving the quality of the die steel is a system control process.
                                

                                      




                                                         In addition to metallurgical quality, making the process of forging processing, preparation heat treatment, machining and final heat treatment will affect the internal organization of the mold and the stress state to determine the final performance of mold. According to all in the mold manufacturing process, the die life and made of precision, quality, surface properties, except with the mold design, manufacturing, machine tools and precision as well as other conditions of operation, but with the mold materials and heat treatment process are closely related. 


                                                          According to the statistics show that the mold material selection due to early failure and internal defects caused by improper accounting for about 10%, caused by the improper heat treatment, about 50%, so the correct choice of mold steel with excellent quality and the correct treatment is of great significance.

The properties of die steel including the use of performance, process performance and the three aspects of metallurgical quality:-
                         A) tool steel performance requirements in the work
(1) hardness:-
                       Die at work is a complex stress state, such as hot die temperature is usually in exchange to withstand alternating stress, so it should have good resistance to softening or plastic deformation ability of the state, despite the long-term work environment to maintain the shape and size precision molds. Mold steel hardness is one of the students to the performance. The hardness of cold-working die generally choose to 58HRC more, hot die, especially for high resistance to thermal fatigue properties of the mold, usually of about the hardness of 45HRC. On the common use of plastic molds, usually around 35HRC hardness required.

(2) strength and toughness:-
                                                  Part in the forming of the die under enormous impact, distortion and other loads, especially in modern high-speed stamping, high precision forging and liquid molding techniques and the development of a forming mold to bear a bigger load, often due to the strength of steel and toughness is not enough, resulting in marginal or partial collapse of the cavity, chipping or breaking and early failure, so the mold heat treatment should have high hardness and toughness.

(3) wear:-
                         Parts forming the mold cavity surface materials and the relative motion, so that the wear surface of the cavity, which makes the dimensional accuracy of mold, shape and surface roughness change and failure. Wear is a complex process, affecting many factors, in addition to the role of the mold depends on external conditions, also depends largely on the uneven use of the chemical composition of steel, organizational status, mechanical properties.

(4)fatigue:-
                           Mold work suffer mechanical shock and thermal shock alternating stress, hot work die in the process, thermal alternating stress more clearly lead to mold thermal cracking. By the impact of stress and temperature gradient caused by cracks, often in small shallow cavity surface cracks, which led to the rapid spread and expansion of die failure. In addition, the steel chemical composition and organization of the uneven presence in steel metallurgical defects such as inclusions, pores, micro cracks can lead to reduced fatigue strength of steel, because the action of alternating stress, first These weak areas and the development of fatigue cracks as fatigue failure.

(5) adhesion:-
                              The surface of the mold parts Picks of the two metal atoms with each other or single-phase diffusion function, often there will be some adhesion of the metal to be processed, especially the cutting, cutting tools and stamping tools, or scarring of the surface will produce the phenomenon of adhesion This will affect the degree of edge sharpness and local organizations, the chemical composition changes to the edge part of the crack or scratch off adhesive metal mold, so that the surface rough. So good anti-adhesion is also very important.

(6)polishing and etching performance:-
                                                                     With the mold, in particular the widespread use of plastic mold, low surface roughness values ​​(sometimes even the extent of the mirror) has been very nature of the necessary, low surface roughness effects to mold the life and production efficiency and product quality. High surface quality can reduce corrosion (especially local pitting); reduce the risk of cracking, polished steel, chemical composition, structure, hardness and carbide distribution must be uniform. Large carbides and in particular their segregation into the strip, the polishing of the surface is extremely harmful. Particularly important that the steel can not contain no large deformation of oxide inclusions, or segregation, and thus must be strictly controlled, and oxygen smelting process. Vacuum arc remelting, electroslag remelting good effect, this process has become the main advanced plastic mold steel production. Even the simple vacuum degassing will also help eliminate a large oxide inclusions, the smelting process can not only reduce the oxide content, and oxides can be more fine, uniform, while controlling the process of smelting and oxygen, can also change the inclusions type, so that the plastic softening and has good toughness and improve the polishing performance.


                                                                 Steel, the hole is not closed for any would affect the polishing performance, so hot lamination process in the metallurgical defects such as loose and tight to keep the organization is necessary, which can be formed by the modern processing technologies. For example, repeatedly upsetting and stretching techniques, rotating forging technology, high temperature static and repression can be refined the original cast, dendrites in the gap. Remelting, vacuum arc remelting refining process, the uniformity of the steel is also very beneficial. Hardened by the heat treatment or surface defects caused should avoid decarburization causes uneven hardness. Composition of these measures with a reasonable design and control, we can produce steel mirror finish are excellent.
In addition, the mold should be based on differences in working conditions and environment, consider the use of tool steel should have good thermal conductivity, corrosion resistance, oxidation resistance and permeability and so on.

Tool Making - Cutting Tool Selection a new approach...

Selecting the best cutting tool is a critical activity to your business success. It’s a re-occurring job that directly impacts cycle time and your shop’s competitiveness.

                                                    So many choices, how best to choose.


                                     Tool selection is about picking tools that fit the machine and cut the part features at the highest quality with the shortest cycle time and at the lowest cost.  Here we will review some of the macro considerations in metal cutting tool selection.

The Most Important Tool.  If any one tool stops working, production stops. But it does not follow that each tool deserves equal attention.  The tool with the longest cut time will have the most influence on cycle time, so all things being equal this tool should command your greatest attention.  Give extra attention to the tool that cuts critical part features and the tightest tolerance. Also, tools that are most likely to produce poor chips (e.g., drilling, grooving and threading) deserve more attention. Bad chips cause downtime.

Fit for the Machine. Tools come in hands—right and left—so it’s important to pick the correct hand.  Normally right-hand tools fit machines running CCW (as viewed looking into the spindle) and left-hand tools fit machines running CW.  If you have several lathes—some that use left-hand tools and others able to use right or left hand—choose left-hand tools. For mills there is a tendency to use a tool with too much reach. While extended-reach tools will cover more parts, you rapidly give up rigidity, increase tool deflection, reduce cutting rates, and they are more likely to chatter.  Toolchangers have limits on size and weight.  If you paid for coolant thru the spindle, select tools that have this capability.

Fit for the Material. Most machined materials are carbon steel, so most cutting tools are designed to best cut carbon steel.  Grade selection is dependent upon the material cut.  For nonsteels, tool manufacturers offer lines of toolholders and matching insert geometry designed for these materials—such as high-temperature alloys, titanium, aluminum, composites, plastics and pure metals, etc. If you cut any of these, ask for information about tools designed specifically for those materials.

Size of Tool. A common mistake is selecting a lathe tool that is too small and a mill that is too large. Large lathe tools are more rigid, while larger milling tools are higher priced and spend more time cutting air. In general, larger tools cost more than smaller tools.

Indexable versus Regrindable. This one is simple: avoid grinding tools. With the few exceptions of some drills and end mills, you should select an insertable or indexable design, when offered. You will save labor costs and get more consistent results.

Tool Material and Grades. This is a function of the material you are cutting and the machine’s speed and feed limits. Select the most universal grade for a material class, which will most likely be a coated grade. Use the manufacturer’s grade selection charts. A common mistake is to use a brand grade cross reference chart to solve a tool life problem. If what you have isn’t working and you select the closest like brand you are likely to get similarly poor results. Focus on the mode of failure.

Power Requirements. Here the guiding principle is use what you paid for.  If you bought a 20-hp mill, then you should select tools and cutting data that uses 80 percent of the available power, when part and fixtures allow.  Pay special attention to the power/rpm charts in the machine owner’s manual.  Select tools designed to best cut in the efficient zone of the machine’s power band.

Number of Edges. The rule here is the more, the better.  A lathe tool with twice the number of cutting edges normally doesn’t cost twice as much. During the last decade, advances in the design of grooving and cutoff, and some milling inserts, doubled the number of cutting edges per insert.  It’s not uncommon to replace an old-design milling cutter having four cutting edges with a modern design having up to 16 cutting edges. The number of effective teeth has a direct effect on table feedrate and productivity.

One Piece Design versus Modular Construction. Small tools are best suited to one-piece designs, while it’s more appropriate to select a modular design for larger tools. For larger tools, when a tool fails you want that part to be the smaller, least expensive part. This is especially true for grooving and boring tools.

Single-Purpose versus Multi-Function. As a general rule, the smaller the part, the more appropriate a multifunction tool is. For example, a single multi-function tool that is able to drill, turn, bore, thread and chamfer. More complex parts are better suited for multi-function tools.  A machine makes you money when the tool is in the cut, not when it is being indexed.

Standard or Special. With the introduction of CNC, it was commonly believed that you put the part’s shape in the program, and not in the tool, thus specials were dead. Yet today about 15 percent of high-volume cutting tools are special. Why? Exact size requirements, station reduction and they save time.  If you have a large lot size, consider a special tool’s contribution to cycle time reduction.  Advances in custom on-demand manufacturing are shrinking the cost and leadtime between standards and specials.

Chip Control. Never forget your aim is to make parts, not chips. Clearly the chip tells how the tool is working, but in general there is too often a preoccupation with the chip. Most people are not trained to read the chip. Focus on the rule: a good chip doesn’t interfere with production and a bad chip does. Inserts are offered with chipbreaker and chipformers designed around the feedrate—from light finishing to heavy roughing. The smaller the chip, the more difficult it is to break.  Chip control is more of a problem in ductile materials. You can’t change the material, but you can change your tool selection, speed, feed, depth-of-cut, radius, etc. This is best covered by a discussion on the application at hand.

Programming. Once you have a tool, a part and a CNC machine, you need to define the toolpath. Ideally, you have an understanding of basic machine coding and an advanced CAM package. Toolpath must take into consideration the tool features (e.g., ramp angles, directions of cut, feed and speed). For each kind of metalworking tool there are programming techniques that can shorten cycle time, improve chips and reduce cutting force. A good CAM package is a great labor saver and productivity enhancer.

Latest Innovation versus Commodity Design. At the present pace of advancement, cutting tool productivity doubles about every decade.  If you check a 10-year-old cutting tool recommendation, you’ll find today’s tools make chips twice as fast and with 30-percent less power. Newer tools are made from stronger and tougher grades of carbide, and for higher speeds and lower cutting force. Newer tools have chipbreakers and grades that are less application-sensitive and more universal.  Newer tools are increasingly multi-function capable and modular—both features reduce inventory requirements while expanding capabilities.  Tools have been developed for new approaches to machining—such as plunge and turn tools, high-feed milling tools, high-speed machining, minimum-quantity lubricates and hard turning, etc. For these and other reasons, you need to keep up with the best practices and the latest advancements in cutting tool technology, or you risk obsolescence.

Price. Price is important, but not nearly as important as the cost of the tool.  Price is what you pay for the tool. A tool’s true cost is a function of its productivity.  Frequently, the most costly tool is the one with the lowest price. The cutting tool’s price as a percentage of a part’s cost, is a mere 3 percent.  Focus primarily on a tool’s productivity, not its purchase price.

Tool Design - Powdered Metal Technology (PMT)..a intro...

 A view of cavity detail in a powdered metal technology (PMT) cavity insert.

Powdered metal technology (PMT) combines the technology of plastic injection molding and powder metallurgy to create a new process.  The powdered metal technology process involves metal powders being mixed with a binder that is granulated to feed into an injection molding machine. A mold is designed and manufactured to create the cavity part for the injection mold.  Green parts are molded and after a multistep process to separate the binder from the metal, cavities are ready for any post molding work.

These parts, which are typically made from H-13 tool steel, can then be heat treated.  After heat treating, PMT cavities can be machined like any other cavity from hardened tool steel.  Machining processes such as EDM, grinding, etc., are no issue and this ability allows for engineering changes at a later date, if necessary.  Since PMT cavities are produced from a mold, any engineering changes can be made to the mold that produces the PMT cavities and new PMT cavities can be molded and machined for new part revisions.

This technology can enhance the capabilities of any moldmaker and help reduce the cost and increase the delivery for high cavitation injection molds. Since this process allows for PMT cavity duplication from a master mold, the greatest advantage of using this technology is that it guarantees that all cavities are precisely the same, regardless of volume or date of manufacture.  Extremely precise PMT cavity reproduction is possible.  Other key features of PMT include:

• Cavities are H-13 tool steel, with other steels available depending upon the application
• Can be heat treated to standard Rockwell hardnesses
• Can achieve surface finishes of up to Rmax 2
• Cavities can be coated for corrosion resistance (TiN, nickel plate)
• Short pull ahead times for duplicate cavities, including spares

From a moldmaker and end user standpoint, PMT allows them to produce higher cavitation molds that may have been too difficult or too costly with making cavities conventionally. PMT offers an excellent economy of scale, suitable for high cavitation molds—typically 16 cavities and higher.  Since a master mold must be designed and manufactured to create the PMT insert, the cost of this mold must be taken into account when producing cavities.  The cost of the mold can be amortized into the piece price of the individual PMT cavities or the mold can be paid for separately, lowering the individual PMT cavity piece price.  A moldmaker has to consider the cost and benefit for the PMT technology based on cavitation.  This is best for higher cavitation molds since the higher the number of cavities, the lower the cost to produce each cavity.

Depending upon the size and configuration of the cavity detail to be produced, feature tolerances of up to 15 microns are achievable.  In addition to repeatability of the cavities, high cavity definition can be achieved with PMT—fine details and sharp corners are easily produced.  This is possible because the impression of the master mold for the PMT cavity takes the green part shrinkage into account.  The part features in the master mold will be larger than those in the finished part, making them easier to machine.  Also, the mold impression will be opposite of the finished parts.  A recess in the final cavity is produced by a standing feature in the mold.  This external, standing feature can be machined with sharp corners.  This will translate into a sharp corner feature in the finished cavity, unlike conventional machining on a cavity, which can be limited based on machine cutter radius or EDM overburn.

Figure 1 shows a magnified view of cavity detail in a PMT cavity insert.  Also using PMT, if a special design for the cavity is needed for the mold design, these cavities can be produced with unique shapes that would be difficult or impossible to produce with other metalworking processes.

However, not all cavities can be made using PMT.  There are limitations as with other technologies.  Because of the process used to make PMT cavities, cooling in the injection mold of the finished cavities must be accomplished externally.  Waterlines cannot be designed into these cavities.  Typically, PMT cavities are inserted in plates, and then these plates are cooled from the mold base that contain water lines.  Even with this limitation, this process has allowed molding cycle times on PMT cavities as short as 3.2 seconds on small plastic parts.

Another limitation is the maximum size of the cavity.  The maximum PMT cavity size is 25mm x 30mm x 70mm, but can change based on the complexity of the cavity to be made.  Figure 2 shows a range of different shapes and sizes of finished PMT cavities.

                      Range of different shapes and sizes of finished powdered metal technology (PMT) cavities..

                                      For moldmakers who are thinking about designing and making high cavitation molds, PMT mold cavities takes some of the issues, cost and leadtime to make cavities out of the equation.  For moldmakers that are already manufacturing high cavitation systems, this would also be an excellent option to employ when making these molds to help cut costs and leadtime.  This technology can help lower individual cavity cost and can also reduce labor and machine time and cost.  The additional labor and machine time that is not used machining cavities can be used toward other parts of the project to reduce overall leadtime.  With the high level of repeatability and precision, and a lower cost per unit that PMT offers, this is a great technology to use for this type of work.