Sheet Metal Design

Press WorkingAlso called as Chipless Manufacturing and Cold Stamping. Machine Used – PressPress Comprises of :•Frame supporting Ram & Bed•Mechanism to operate ram•Ram with Punch•Die Block

Operations:1.Cutting Operations(work piece stressed beyond ultimate strength)

- Blanking, Punching, Notching, Perforating, Trimming, Shaving, Slitting & Lancing

2.Forming Operations (work piece stressed below ultimate strength)- Bending, Drawing, redrawing & Squeezing

Types of PressesClassified on following:

• Source of Power:

Mechanical - The energy of flywheel is utilized which is transmitted to the work-piece by gears, cranks, eccentrics or levers. The flywheel rotates freely on the crank shaft and is driven from an electric motor through gears or V belts.

Hydraulic - In hydraulic press, the ram is actuated by oil pressure on apiston in a cylinder.

Press SelectionFactors to be considered while selecting a pressOverall work size, the stock thickness and material, kind of operation to be performed, power required and speed of operation.

For punching, blanking and trimming operations usually the crank or eccentric type mechanical press is used. This is due to their small working strokes and high production rates. In these operations, there is sudden release of load at the end of the cutting stroke. This sudden release of load is not advisable in hydraulicpresses. So, hydraulic presses are not preferred for these operations.If however these are inevitable, then some damping devices are incorporatedin the press design.

For coining and other squeezing operations, which require very large forces, knuckle joint mechanical press is ideally suited. Hydraulic presses, which are slower and more powerful, can also be used for these operations. Hydraulic presses are also better adapted to pressing, forming and operations, which are slower processes.

Press WorkingDefinitions of the main components of the die and press :

Bed - The bed is the lower part of a press frame, that serves as a table to which a bolster plate is mounted.

Bolster plate - This is a thick plate secured to the press bed, which is used for locating and supporting the die assembly. It is usually 5 to 12.5 cm thick.

Die set - It is unit assembly which incorporates a lower and upper shoe, two or more guideposts and guidepost bushings.

Die - The die may be defined as the female part of a complete tool for producing work in a press. It is also referred to a complete tool consisting of a pair of mating members for producing work in a press. Die block, is a block or a plate which contains a die cavity.

Lower Shoe - The lower shoe of a die set is generally mounted on the bolster plate of a press. The die block is mounted on the lower shoe. Also, the guide posts are mounted in it.

Press Working

Press WorkingPunch - This is the male component of the die assembly, which is directly or indirectly moved by and fastened to the press ram or slide.

Upper Shoe - This is the upper part of the die set which contains guidepost bushings.

Back up plate - Back up plate or pressure plate is placed so that the intensity of pressure does not become excessive on punch hold or the plate distributes the pressure over a wide area and the intensity of pressure on the punch holder is reduced to avoid crushing.

Stripper - It is a plate which is used to strip the metal strip from a cutting or non-cutting punch or die. It may also guide the sheet.

Knockout - It is a mechanism, usually connected to and operated by the press ram, for freeing a work-piece from a die.Pitman - It is a connecting rod which is used to transmit motion from the main drive shaft to the press slide.

Shut Height - It is the distance from top of the bed to the bottom of the slide, with its stroke down and adjustment up.

Cutting OperationsNotching: This is cutting operation by which metal pieces are cut from the edge of a sheet, strip or blank.Perforating: This is a process by which multiple holes which are very small and close together are cut in a flat work material.Trimming: This operation consists of cutting unwanted excess material from the periphery of a previously formed component.Shaving: Edges of a blanked part are generally rough, uneven and unsquare. Accurate dimensions of the part are obtained by removing a thin strip of metal along the edges. This operation is termed as Shaving.Slitting: It refers to the operation of making incomplete holes in a workpiece.Lancing: This is a cutting operation in which a hole is partially cut and then one side is bent down to form a sort of tab or louver. Since no metal is actually removed, there will be no scrap.Nibbling: The nibbling operation which is used for only small quantities of components, is designed for cutting out flat parts from sheet metal. The flat parts range from simple to complex contours. This operation is generally substituted for blanking. The part is usually moved and guided by hand as the continuously operating punch cuts away at the edge of the desired contour.

Forming OperationsBending - In this operation, the material in the form of flat sheet or strip, is uniformly strained around a linear axis which lies in the neutral plane and perpendicular to the lengthwise direction of the sheet or metal.

Drawing - This is a process of forming a flat workpiece in to a hollow shape by means of a punch which causes the blank to flow into a die cavity.

Squeezing - Under this operation, the metal is caused to flow to all portions of a die cavity under the action of compressive forces.

Press OperationsShearing - Shearing is a sheet metal cutting operation along a straight line between two cutting edges by means of a power shear.

Metal sheet is held on top of hardened die, shearing blade cuts downward, usually driven by hydraulic or electric force.

Punching & Blanking - Blanking and punching are similar sheet metal cutting operations that involve cutting the sheet metal along a closed outline. If the part that is cut out is the desired product, the operation is called blanking and the product is called blank.

If the remaining stock is the desired part, the operation is called punching.

Both operations are illustrated on the example of producing a washer:

Blanking & Punching

Cutting of sheet metal is accomplished by a shearing action between two sharp edges. The shearing action is illustrated in the figure:

Punching & Blanking

Blanking & PunchingClearance:Clearance c is the distance between the punch and die. The correct clearance depends on sheet-metal type and thickness t:

c = at (Typically 2 to 10% of thickness)where a is the allowance (a = 0.075 for steels and 0.060 for aluminum alloys)

Blanking & Punching

The usual clearances per side of the die, for various metals, are givenbelow in terms of the stock thickness, t :

For brass and soft steel, c = 5% of tFor medium steel, c = 6% of tFor hard steel, c = 7% of tFor aluminium, c = 10% of t

Blanking & PunchingCutting forces -

Cutting force in all shearing operations is determined byF=StL

where S is the shear strength of material, for approximate solutions,S=0.7UTS

T is thickness of sheet L is the length of the cut edge

Strip LayoutPreparation of Blanking Layout

It is a layout, of position of the work-pieces in the strip and their orientation with respect to one another.

Major considerations are• Economy of material• Direction of material grain • Scrap twisting & wedging

The strip layout with maximum material saving may not be the best strip layout, as the die construction may become more complex which would offsetthe savings due to material economy unless a large number of parts are to be produced.

Forming OperationsSome Design Tips:• Dimension the part in a single direction wherever possible

Sequential nature of the forming process and introduction of dimensional variation at each bend

It is in line with the process and helps to control tolerance accumulation

• Allow a more generous bend tolerance (+/- .007”) as tighter tolerances, while achievable, will result in higher costs

• Use consistent bend radius for all bends per part ; it minimize setup changes

• Dimensioning should be done from a feature to an edge. Avoid feature-to-feature

dimensions over two or more planes. Feature-to-bend dimensions may require special fixtures or gauging.

• This also means that tolerances in the title block of a drawing may be unnecessarily restrictive for certain dimensions and angles, while very appropriate for others.

Forming OperationsForming Near Holes – When a bend is made too close to a hole, the hole may become deformed. Figure "A" shows a hole that has become teardrop shaped because of this problem. To save the cost of punching or drilling in a secondary operation the following formulas can be used to calculate the minimum distance required:

For a hole < 1" in diameter the minimum distance "D" = 2T + R (see fig. "B")For a slot or hole > 1" diameter then the minimum distance "D" = 2.5T + R (see fig. "C“)

Forming OperationsEdge Distortion –

An exaggerated example of edge deformation is pictured in figure "A" below.The overhang caused by this distortion can be as large as ½ the material thickness. As material thickness increases and bend radius decreases the overhang becomes more severe. In situations where an overhang is unacceptable the part can be relieved as in figure "B".

Forming OperationsFeature placement restrictions – When placing formed features next to one another, care should be taken to allow clearance between features. If the station does not clear a form alreadyplaced in the part, the form could be flattened out. An example of good vs. bad placement is illustrated below. Relieving the stripper can overcome this problem in some cases.

Forming OperationsCounter sinks – A counter sink can be put in sheet metal by both machining and/or punching.Each of these methods give the finished part different characteristics. The cross sections of the features are shown below, they are listed from least to most expensive (tooling cost not included).• Formed – Low Cost – Used for thin gages, 18 gage and lighter.• Punched – Low Cost – Most common, used for anything heavier than 18 gage.• Punched and Machined – Medium Cost – Used for harder materials that cannot be formed with a punch, e.g. heavy gage stainless.• Machined Complete – High Cost – Rarely used, only for high tolerance applications or materials too thick to be punched.

Forming OperationsCutoffs:There are three kinds of cutoffs in blanking: straight/square, half round or partial radius and full radius. The square cutoff is the most economical. The full radius is not recommended as it leaves an unavoidable “feather edge” burr along the outside material edge.

Piercing:Holes-Minimum diameter of holes should be equal or greater than 1.2 X material thickness, and 2X material thickness for stainless steel or high tensile materials.

Edge-to-hole- Allow 2x material thickness (“web”) to prevent bulging of material

Bending Operations

Bending is defined as the straining of the sheet metal around a straight edge.

Bending induces plastic deformation in the material, so material retains its shape after releasing the force.

Bending Operations

Bend Allowance:

This is the stretching length that occurs during bending. It must be accounted to determine the length of the blank,

Lb =Σ L + Σ BA

where Lb is the length of the blank, L are the lengths of the straight parts of the blank, BA is the bend allowance,

where A is the bend angle; t is the sheet thickness;R is the bend radius; Kba is a factor to estimate stretching, defined as follows:for R < 2t , Kba = 0.33for R ≥ 2t, Kba = 0.50

Bending Operations

Deep Drawing Operations

Guerin ProcessThe Guerin process involves the use of a thick rubber pad to form sheet metal over a positive form block:

Guerin Process

Advantages: small cost of tooling

Limitations: for relatively shallow shapes

Area of application: small-quantity production

Hydroforming Process

It is similar to Guerin process but instead of rubber pad a rubber diaphragm filled with fluid is used in this process.

(1) start-up, no fluid in the cavity; (2) press closed, cavity pressurized with hydraulic fluid;(3) punch pressed into work to form part. Symbols: v - velocity, F – appliedforce, and p - hydraulic pressure

Advantages: small cost of tooling

Limitations: simple shapes

Area of application: small-quantity production

Stretch Forming ProcessIn stretch forming process, the sheet metal is stretched and bent to achieve the desired shape.

(1) start of the process(2) form die is pressed intothe work causing it to stretched

and bent over the form. Symbols: v - velocityFdie - applied force

Advantages: small cost of tooling, large parts

Limitations: simple shapes

Area of application: small-quantity production

Spinning ProcessSpinning is a metal forming process in which an axially symmetric part is gradually shaped over a mandrel by means of a rounded tool or roller.

Flat circular blanks are often formed into hollow shapes such as photographic reflectors.In a lathe, tool is forced against a rotating disk, gradually forcing the metal over the chuck to conform to its shape.Chucks and follow blocks areusually made of wood for this process

Advantages: small cost of tooling, large parts (up to 5 m or more)

Limitations: only axially symmetric parts

Area of application: small-quantity production

High Energy Rate FormingThese are metal forming processes in which large amount of energy is applied in a very short time. Some of the most important HREF operations include:

Explosive formingIt involves the use of an explosive charge placed in water to form sheet into the die cavity.

High Energy Rate Forming

Advantages: small cost of tooling, large parts

Limitations: skilled and experienced labor

Area of application: large parts typical of the aerospace industry

Electrohydraulic FormingThis is a HREF process in which a shock wave to deform the work into a die cavity is generated by the discharge of electrical energy between two electrodes submerged in water. Similar to explosive forming, but applied only to small part sizes.

Electromagnetic Forming

The sheet metal is deformed by the mechanical force of an electromagnetic field induced in the work-piece by a coil.

Advantages: can produce shapes, which cannot be produced easily by the other processes

Limitations: suitable for magnetic materials

Area of application: most widely used HERF process to form tubular parts

Plastic Part Design

Thermosets

A chemical change occurs in these plastics during thecuring process and cross linking of the molecular chainsoccurs. This irreversible condition is usually inducedby mixing a resin with an activator and normally coincides with moldings or forming of the component into its final shape.

Thermosets cannot be re-formed after manufacture. Any subsequent shaping is carried out using traditional wood or metalworking type tools.

Thermoplastics

Thermoplastics soften or melt when they are heated, rather like wax, and regain their rigidity when cool.Various ‘Thermoforming’ processes have been developed for changing the shape of thermoplastic sheets by exploiting this property, including:•Line Bending Vacuum Forming•Dip Coating Dome Blowing•Blow Moulding Rotational Moulding•Injection Moulding ExtrusionUnlike thermosets, no chemical change, or cross linking of molecules, occurs during heating and cooling so that when a thermoformed object is re-heated, it will soften again and can be re-formed or left to return to its original shape.

Wall Sections

Parts should be designed with a minimum wall thickness consistent with part function and mold filling considerations. The thinner the wall the faster the part cools, and the cycle times are short, resulting in the lowest possible part costs.

Also, thinner parts weight less, which results in smaller amounts of the plastic used per part which also results in lower part costs.

The wall thicknesses of an injection-molded part generally range from 2 mm to 4 mm (0.080 inch to 0.160 inch). Thin wall injection molding can produce walls as thin as 0.5 mm (0.020 inch).

Wall Sections

Parts should be designed with a minimum wall thickness consistent with part function and mold filling considerations. The thinner the wall the faster the part cools, and the cycle times are short, resulting in the lowest possible part costs.

Also, thinner parts weight less, which results in smaller amounts of the plastic used per part which also results in lower part costs.

The wall thicknesses of an injection-molded part generally range from 2 mm to 4 mm (0.080 inch to 0.160 inch). Thin wall injection molding can produce walls as thin as 0.5 mm (0.020 inch).

Fillets, Draft & Ribs

• General Rules• Fillets should be min ¼ X Wall thickness• High stress parts, ¾ x Wall thickness• Draft min of 1 deg. Finish affects draft• Ribs should be 1/2 to 2/3 of the nominal wallthickness and less than 3 times thickness in height.Taper of 1 deg. is typical. Note: excess thicknesspromotes shrinkage. Excess rib height combinedwith taper will produce thin sections requiring extrafill time at the mold.

Draft & Rounds

Round corners and edges wherever possible

For easy release of the part from the mold, add a slight taper to the sides (typically ~ 2 deg) - especially for textured walls and walls higher than 0.25".

Rib Design

Boss DesignBosses are used for the purpose of registration of mating parts or for attaching fasteners such as screws or accepting threaded inserts (molded-in, press-fitted, ultrasonically or thermally inserted).

The wall thicknesses should be less than 60 % of nominal wall to minimize sinking. However, if the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws. The base radius should be a minimum of 0.25 x thickness

Inserts

Molded-in Inserts - The inserts are placed in the mold prior to the injection of plastic. The injection of the plastic completely encases the insert on the outer diameter and provides very good retention. In fact retention of such inserts is the best compared to other process (barb/knurl design being the same). This process slows down the operation of the mold, since the inserts have to be manually placed inside the mold. Inserts can be automatically placed in a mold, but this greatly increases the complexity and cost of the mold. This can only be justified if the volume of production is very high to offset the cost savings in shorter cycle times.

Press Fitting - The inserts are designed with barbs (straight knurls that are interrupted) and can be press fitted inside the hole in the boss. This process is fast and requires no special tooling. However, high hoop stresses are generated, so the boss design has to be robust. Also, the retention is strictly based on press fitting and the small amount of material that flows inside the barbs. Thus retention is not very high.

Holes

Through Hole (shown with draft)

L= 4D for D <3/16" dia. core pinsL= 6D for D >3/16" dia. core pins

Snaps• Snaps allow an easy method of assembly and disassembly of plastic parts. Snaps consist of a cantilever beam with a bump that deflects and snaps into a groove or a slot in the mating part.                                                             • Snaps can have a uniform cross-section or a tapered cross section (with decreasing section height). The tapered cross-section results in a smaller strain compared to the uniform cross-section. Here we consider the general case of a beam tapering in both directions.                                       

Snaps

Some common problems of using snaps:

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Too high a deflection causing plastic deformation (set) of the latch (the moving member). Care has to be taken that the latch does not take a set. Otherwise, the amount of latch engagement could reduce, reducing the force to disassemble. If the set is bad enough the engagement might even fail. 

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The moving arm could break at the pivot point due to too high a bending stress. This can be avoided by adhering to the design principles and not exceed the yield strength of the material-in fact it should be kept well below the yield strength depending on the safety factor used. 

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Too much over travel leads to a sloppy fit between mating parts resulting in loose assemblies that can rattle.

Living HingeLiving hinges are thin sections of plastic that connect two segments of a part to keep them together and allow the part to be opened and closed. Typically these are used in containers that are used in high volume applications such as toolboxes, fish tackle boxes, CD boxes etc.

Living Hinge

Warpage

Shrinkage

It is an unsurprising fact that the material shrinks while cooling down inside and outside of the cavity, which on principle is not a problem. What is critical is uneven shrinkage in the part, as it results in deformation or even fracture.

As thermoplastics have an amorphous crystalline structure, their density at the end of the cooling process depends on how much time the molecules have to restructure before the material solidifies.

As a conclusion from this, the faster the molecules are “frozen”, the lower the amount of shrinkage. A low wall temperature will consequently reduce shrinkage. A high pressure within the cavity will also reduce shrinkage as it impedes the ordering processes of the molecules.

Sink MarkSink marks are localized depressions in the surface of injection molded parts caused by a non uniform shrinking of the plastic during the cooling process. In cosmetically critical parts, they can be a serious problem. Sink marks are dependent on part geometry and material shrink rates. Sink marks often come as a surprise when the tool is complete. If the sink marks are objectionable to the customer, the molder is often tasked with "eliminating them". Since the tool is complete, the molder does not have the luxury of changing the part geometry as shown in the example below. At that point, changing the material to one with a lower shrink factor is usually not an option due to sizing issues with other parts.  At this stage of the game, the only option left to the molder is to adjust the processing conditions to try to eliminate the sink. This usually means running the molding machine at the extremes of the process window, which can lengthen cycle times and increase the amount of molded in stresses.

Design Approach

Radius

Sharp corners greatly increase the stress concentration. This high amount of stress concentration can often lead to failure of plastic parts.

Sharp corners can come about in non-obvious places. Examples of this are a boss attached to a surface, or a strengthening rib. These corners need to be radiused just like all other corners. The stress concentration factor varies with radius, for a given thickness.

Typical Problems

Intended shape and typical deformation on a box with floor plate