12 Common Problems Affecting Plastic Hanger Molds

What is the primary factor affecting the machinability of a material?

The chemical composition of steel is very important. The higher the alloy composition of the steel, the more difficult it is to machine. When the carbon content increases, the metal cutting performance decreases.

The structure of the steel is also very important to the metal cutting performance. Different structures include: forged, cast, extruded, rolled and machined. Forgings and castings have surfaces that are very difficult to machine.

Hardness is an important factor affecting metal cutting performance. The general rule is that the harder the steel, the harder it is to machine. High-speed steel (HSS) can be used to process materials with a hardness of up to 330-400HB; high-speed steel + titanium nitride (TiN) coating can process materials with a hardness of up to 45HRC; and for materials with a hardness of 65-70HRC, it must be Carbide, ceramic, cermet and cubic boron nitride (CBN) are used.

Non-metallic inclusions generally have an adverse effect on tool life. For example, Al2O3 (alumina), which is a pure ceramic, is highly abrasive.

The last one is residual stress, which can cause metal cutting performance problems. A stress relief operation is often recommended after rough machining.

2) What are the cutting characteristics of cast iron?

Generally speaking, it is:

The higher the hardness and strength of cast iron, the lower the metal cutting performance and the lower the life expectancy from inserts and tools. Cast iron used in metal cutting production generally performs well for most types of metal cutting. The metal cutting performance is related to the structure, and the harder pearlitic cast iron is more difficult to process. Flake graphite cast iron and malleable cast iron have excellent cutting properties, while ductile iron is quite poor.

The main types of wear encountered when machining cast iron are: abrasive, adhesive and diffusion wear. Abrasion is mainly caused by carbides, sand inclusions and hard casting skins. Bonded wear with built-up edge occurs at low cutting temperatures and cutting speeds. The ferritic part of cast iron is easiest to weld to the insert, but this can be overcome by increasing cutting speed and temperature.

On the other hand, diffusion wear is temperature-dependent and occurs at high cutting speeds, especially when using high-strength cast iron grades. These grades have a high resistance to deformation, resulting in high temperatures. This wear is related to the interaction between the cast iron and the tool, which makes some cast irons need to be machined at high speeds with ceramic or cubic boron nitride (CBN) tools for good tool life and surface quality.

The typical tool properties generally required for machining cast iron are: high thermal hardness and chemical stability, but also related to process, workpiece and cutting conditions; cutting edge toughness, thermal fatigue wear and edge strength are required. Satisfaction in cutting cast iron depends on how the wear of the cutting edge develops: rapid dulling means hot cracks and nicks leading to premature cutting edge fracture, workpiece breakage, poor surface quality, excessive waviness, etc. Normal flank wear, balanced and sharp cutting edges are just what generally takes effort.

3) What are the main and common processing procedures in mold manufacturing?

The cutting process should be divided into at least 3 process types:

Roughing, semi-finishing, finishing, and sometimes even super-finishing (mostly high-speed cutting applications). Residual milling is of course prepared for finishing after the semi-finishing operation. It is very important that in each operation an effort should be made to leave an evenly distributed allowance for the next operation. Tool life may be extended and more predictable if there are few rapid changes in toolpath orientation and workload. If possible, finishing operations should be carried out on dedicated machine tools. This improves the geometric accuracy and quality of the molds in shorter commissioning and assembly times.

(4) What kind of tools should be mainly used in these different processes?

Roughing process: round insert milling cutter, ball end milling cutter and end milling cutter with large nose radius.

Semi-finishing process: round insert milling cutter (round insert milling cutter with a diameter range of 10-25mm), ball end mill.

Finishing process: round insert milling cutter, ball end mill.

Residual milling process: round insert milling cutter, ball end milling cutter, vertical milling cutter.

It is very important to optimize the cutting process by selecting specialized tool size, geometry and grade combinations, as well as cutting parameters and suitable milling strategies.

See Catalog C-1102:1 for Mold Making for the high productivity tools that can be used

(5) Is there one of the most important factors in the cutting process?

One of the most important goals in the cutting process is to create an evenly distributed machining allowance for each tool in each operation. This means that tools of different diameters (large to small) must be used, especially in roughing and semi-finishing operations. The main criterion at all times should be to be as close as possible to the final shape of the mold in each process.

The evenly distributed machining allowance for each tool ensures constant and high productivity and a safe cutting process. When ap/ae (axial depth of cut/radial depth of cut) is constant, cutting speed and feed rate can also be kept at a constant high level. In this way, there is less variation in mechanical action and working load on the cutting edge, resulting in less heat and fatigue, resulting in improved tool life. If the subsequent operations are some semi-finishing operations, especially all finishing operations, unmanned or partially unmanned processing can be carried out. A constant material allowance is also a basic criterion for high-speed cutting applications.

Another beneficial effect of a constant machining allowance is the low detrimental effect on the machine tool - guideways, ball screws and spindle bearings.

(6) Why is the circular insert milling cutter the first choice for mold roughing?

If a square shoulder milling cutter is used for rough milling of the pocket, a large amount of stepped cutting allowance must be removed in the semi-finishing process. This will change the cutting force and bend the tool. The result is an uneven machining allowance for finishing, which affects the geometric accuracy of the mold. Unpredictable cutting effects can occur if a shoulder mill (with a triangular insert) with a weaker nose is used. Triangular or diamond-shaped inserts also generate higher radial cutting forces and are less economical roughing tools due to the lower number of insert cutting edges.

Round inserts, on the other hand, can be milled in a variety of materials and in all directions, and if used, there are smoother transitions between adjacent toolpaths, and can also leave a smaller and more uniform finish for semi-finishing margin. One of the characteristics of round inserts is that the chip thickness they produce is variable. This allows them to use higher feed rates than most other inserts.

The entering angle of the round insert changes from almost zero (very shallow cuts) to 90 degrees, and the cutting action is very smooth. At the maximum depth of cut, the entering angle is 45 degrees, and when profiling along a straight wall with an outer circle, the entering angle is 90 degrees. This also explains why the strength of the round insert tool is great - the cutting load is gradually increased. Roughing and semi-roughing should always be done with a round insert cutter such as the CoroMill 200 (see Tooling Catalog C-1102:1) as the first choice. In 5-axis cutting, the round insert is very suitable, especially it does not have any restrictions.

With good programming, round insert mills can largely replace ball nose end mills. Round inserts with low runout combined with finely ground, positive rake and light cutting geometries can also be used for semi-finishing and some finishing operations.

(7) What is the effective cutting speed (ve) and why it is always very important to the basic calculation of the effective cutting speed on the effective diameter of high productivity.

Since the table feed amount depends on the rotation speed at a certain cutting speed, if the effective speed is not calculated, the table table feed amount will be incorrectly calculated.

If the nominal diameter value (Dc) of the tool is used when calculating the cutting speed, when the depth of cut is shallow, the effective or actual cutting speed is much lower than the calculated speed. Tools such as round insert CoroMill200 cutters (especially in the small diameter range), ball nose end mills, large nose radius end mills and CoroMill390 end mills (for these tools see Sandvik Coromant's mould making Sample C-1102:1). As a result, the calculated feed rate is also much lower, which severely reduces productivity. What's more, the cutting conditions of the tool are below its capabilities and recommended application range.

When 3D cutting is performed, the diameter of the cutting changes, which is related to the geometry of the die. One solution to this problem is to define areas of the mold with steep walls and areas of the part with shallow geometry. Good compromises and results can be achieved if dedicated CAM programs and cutting parameters are programmed for each area.

(8) What are the important application parameters for successful hardened die steel milling?

One of the main factors to observe when finishing hardened die steels with high speed milling is the use of shallow cuts. The depth of cut should not exceed 0.2/0.2mm (ap/ae: axial depth of cut/radial depth of cut). This is to avoid excessive bending of the shank/cutting tool and to keep the die being machined with tight tolerances and high precision.

It is also very important to choose a rigid clamping system and tool. When using solid carbide tools, it is important to use the tool with the largest core diameter (maximum flexural rigidity). A rule of thumb is that if you increase the diameter of the tool by 20%, say from 10mm to 12mm, the bending of the tool will decrease by 50%. It can also be said that if the tool overhang/projection is shortened by 20%, the bending of the tool will be reduced by 50%. Large diameter and tapered holders further increase stiffness. When using ball nose end mills with indexable inserts (see Tooling Catalog C-1102:1), the flexural rigidity can be increased by a factor of 3-4 if the shank is made of solid carbide.

The selection of special geometries and grades is also very important when finishing hardened die steels with high-speed milling. It is also very important to choose a coating with high hot hardness like TiAlN.

(9) When should down milling be used and when should up milling be used?

The main advice is: use climb milling as much as possible.

When the cutting edge is just cutting, in climb milling, the chip thickness can reach its maximum value. In up-cut milling, it is the minimum value. In general, the tool life is shorter in up-cut milling than in climb milling, because the heat generated in up-cut milling is significantly higher than in down-cut milling. As the chip thickness increases from zero to maximum in up-cut milling, more heat is generated because the cutting edge experiences stronger friction than in down-cut milling. Radial forces are also significantly higher in up-cut milling, which has a detrimental effect on the spindle bearings.

In climb milling, the cutting edge is mainly subjected to compressive stress, which is much more favorable for carbide inserts or solid carbide tools than the tensile force generated in up milling. of course there are exceptions. When using solid carbide end mills (see tools in Die Catalog C-1102:1) for side milling (finishing), especially in hardened materials, up milling is the first choice. This makes it easier to achieve tighter tolerance wall straightness and better 90 degree angles. If there is a mismatch between different axial passes, the tool marks are also very small. This is mainly because of the direction of the cutting force. If a very sharp cutting edge is used in cutting, the cutting forces tend to "pull" the knife towards the material. Another example where up-cut milling can be used is milling with older hand mills that have large lead screws. Up-cut milling produces clearance-eliminating cutting forces for a smoother milling action.

(10) Profile milling or contour cutting?

In pocket milling, the best way to ensure a successful climb milling toolpath is to use a contour milling path. Milling cutters (such as ball nose end mills, see Die Manufacturing Catalog C-1102:1) contour milling often results in high productivity because at larger tool diameters more teeth are cutting . Contour milling will help maintain cutting speeds and feed rates if the rotational speed of the machine spindle is limited. With this toolpath, there is also little variation in workload and orientation. This is especially important in high-speed milling applications and machining of hardened materials. This is because, if cutting speeds and feeds are high, the cutting edge and cutting process are more susceptible to adverse effects from changes in workload and orientation, which can cause changes in cutting forces and tool bending. Profile milling along steep walls should be avoided as much as possible. In down copy milling, the chip thickness is large at low cutting speeds. In the center of the ball-end knife, there is also the danger of the cutting edge breaking. If the control is poor, or the machine tool does not have a pre-reading function, it cannot decelerate fast enough, and the danger of edge chipping in the center is most likely to occur. Top profiling along steep walls is better for the cutting process because the chip thickness is at its maximum at favorable chip speeds.

In order to get the longest tool life, the cutting edge should be kept in continuous cutting for as long as possible during the milling process. If the tool enters and exits too frequently, tool life can be significantly shortened. This exacerbates thermal stress and thermal fatigue on the cutting edge. A uniform and high temperature in the cutting area is more beneficial to modern carbide tools than a large fluctuation. Profile milling paths are often a mix of up and down milling (zigzag), which means frequent engagements and retractions during the cut. This toolpath also has a bad effect on mold quality. Every time a knife is eaten, it means that the knife is bent, and there is a raised mark on the surface. When the tool is withdrawn, the cutting force and the bending of the tool are reduced, and there is a slight "overcut" of material at the exit.

(11) Why must some milling cutters have different pitches?

Milling cutters are multi-edge tools, and the number of teeth (z) can be changed, there are factors that can help determine the pitch or number of teeth used for different types of machining. Materials, workpiece size, overall stability, overhang size, surface quality requirements, and available power are all machining-related factors. Factors related to the tool include adequate feed per tooth, at least two teeth cutting at the same time, and the chip capacity of the tool, to name just a few.

The pitch (u) of a milling cutter is the distance from a point on the cutting edge of the insert to the same point on the next cutting edge. Milling cutters are divided into sparse, dense and ultra-dense pitch milling cutters. Most Coromant milling cutters have these 3 options, see Mold Manufacturing Catalog C-1102:1. A dense pitch means that there are more teeth and proper chip space, allowing cutting with high metal removal rates. Generally used for medium duty milling of cast iron and steel. The fine pitch is the first choice for general purpose milling cutters and is recommended for mixed production.

Sparse pitch means that there are fewer teeth and a large chip space on the circumference of the milling cutter. The sparse pitch is often used for roughing to finishing of steel, where vibration has a great influence on the machining results. The sparse pitch is a really effective solution to the problem and is the first choice for milling with long overhangs, low power machines or other applications where cutting forces must be reduced.

The chip space of the ultra-fine pitch tool is very small, and a higher table feed can be used. These tools are suitable for cutting interrupted cast iron surfaces, roughing cast iron and small stock cuts in steel, such as side milling. They are also suitable for applications where low cutting speeds must be maintained. Milling cutters can also have uniform or unequal pitches. The latter refers to the unequal spacing of the teeth on the tool, which is also an effective way to solve vibration problems.

When there is a vibration problem, it is recommended to use a sparse and unequal pitch milling cutter as much as possible. With fewer blades, there is less chance of increased vibration. Small tool diameters can also improve this situation. A combination of geometry and grade that works well should be used - a combination of sharp cutting edges and tough grades.

(12) How should the milling cutter be positioned for optimum performance?

The cutting length is affected by the position of the milling cutter. Tool life is often related to the length of cut that the cutting edge has to undertake. A milling cutter positioned in the center of the workpiece has a short cutting length. If the milling cutter is offset from the centerline in either direction, the cutting arc is long. Remember that there is a compromise that must be reached in how cutting forces work. With the tool positioned in the center of the workpiece, the direction of the radial cutting force changes as the insert cutting edge enters or exits the cut. The backlash of the machine tool spindle also exacerbates the vibration, causing the insert to vibrate.

By de-centering the tool, a constant and favorable direction of the cutting force is obtained. The longer the overhang, the more important it is to overcome all possible vibrations.

The information comes from the plastic hanger network