Advance Manufacturing Notes

HOT MACHINING:

A MEANS OF LOWER COST TURNING AND MILLING OF HARD STEELS AND

The Electroforming Process:

Electroforming is a highly specialized process for fabricating a metal part by electrode position in a plating bath over a base form or mandrel which is subsequently removed.

The electroforming manufacturing method generates an exact copy via replication in metal (typically nickel or copper) of a master mandrel. Specific benefits of electroforming are low unit costs, exact atomic scale repeatability and excellent process control for quality. Often times the design engineer will find electroforming is preferred for many applications requiring a complex true geometric shape form factor, thin walls, and high repeatability.

The process is additive in that the geometry of the desired part is accomplished by deposition of metal instead of removal of material as in typical machine tooling. This mandrel is often three-dimensional, as in the case of a reflector (parabola, ellipse, conic etc.) or other complex shapes.

The mandrel is placed into a special electroforming tank where nickels electrodeposited over it under very tightly controlled conditions. Over a period of time (minutes to hours) the amount of nickel will increase in thickness, however the original mating surface against the mandrel remains perfect. This addition gives the product appropriate structural integrity.

During the process, pure nickel is deposited onto a stainless steel mandrel. This mandrel is precision machined, ground and polished in a precise negative of the desired form geometry. Other materials can be used for a master (high grade plastics, glass, aluminum, etc.) with special preparation, but due to their high attrition rate, they are not suited for high-volume, long production runs. Since the master is directly responsible for the electroformed mirror's geometry as well as for its surface specularity, the finished mirror is an extremely accurate Application of the master's surface.

When the electroforming process is finished, and the pre-determined thickness achieved, the mandrel is removed from the plating solution and the electroformed part is carefully separated from the mandrel. The mandrel is cleaned and prepared to make yet another cost effective exacting replica. The nickel replica is processed to remove any sharp edges and becomes a solid freestanding, exact replica of the mandrel surface. The replica is now ready to be coated with a spectral enhancement for its specific use.

Opti-forms have developed specialized fabrication techniques for the production of these masters as well as testing methods to insure the highest possible surface accuracies. Typically, our master geometry will not deviate from the specified theoretical geometry by more than +/- 2 arc minutes of surface slope. The final surface specularity of the polished master is typically 80-50 scratch and dig per Mil-O-13830A. Another advantage of stainless steel masters is their capability, in most cases to produce thousands of electroforms with only minimal interval polishing to preserve the original surface quality.

The advantage of the electroforming process is that it faithfully reproduces the form or mandrel exactly, to within one micron, without the shrinkage and distortion associated with other metal forming techniques such as casting, stamping or drawing. And, since the mandrel is machined as an outside surface, close dimensional tolerances and high surface finishes can be held and maintained on complex interior configurations.

Electroformed metal is extremely pure, with superior properties over wrought metal due to its refined crystal structure. Multiple layers of electroformed metal can be molecularity bonded together, or to different substrate materials to produce complex structures with "grown-on" flanges and bosses.

Electroforming should be thought of as a basic manufacturing process when considering alternatives best suited for making any particular item. Other basic metal forming processes including casting, forging, stamping, deep drawing, machining and fabricating may serve well for most applications, but can be pressed beyond their limits when requirements call for extreme tolerances, complexity or light weight. Electroforming is a real problem solver in these instances.

The diagram to the right shows the electroforming process at work. The positively charged electroformed metal source (anode) at the left is broken down (ionized) in the copper electrolyte solution and is attracted to the negative charged mandrel (cathode). Build-up is achieved over all mandrel surfaces at an approximate deposition rate of .001" per hour.

The electrodeposited copper is much stronger than would be expected of such thin material because the metal is effectively work-hardened during deposition by the introduction of stacking faults in to the fine-grained structure. The coil formers are therefore handle able.

Ø Electroforming always involves applying a coating, (of unlimited thickness), to a 3- dimensional shape. This enables items with very complex internal shapes, for instance tubing manifolds, bellows, and mold recesses, to be electroformed on to a machined or fabricated mandrel.

If it is geometrically possible to extract the mandrel from the finished electroform then materials such as stainless steel may be used.

A good example of this is the moulds used to make bars of chocolate. A single machined stainless steel master mandrel is used to produce a large number of electroformed moulds. These are linked to produce a continuous belt of moulds on the production line. The thin walls possible in an electroform allows good control over the cooling rate of the chocolate.

MANDREL

A mandrel is the original from which a replica is electroformed. Most mandrels are made from stainless steel. Mandrels are either expendable or reusable. Reusable, or permanent, mandrels are often made of stainless and expendable mandrels of softer, easily machined materials like aluminum.

The expendable mandrel is destroyed in the separation process. They are commonly used when the mandrel has features preventing it from being mechanically separated from the electroform. Cold shields, by design, contain re entrant angles and must be electroformed on expendable aluminum mandrels that are chemically dissolved after electroforming as there is no other way to disassociate

The machined aluminum compressor rotor below is a mandrel used to produce a metal mould.

A thick deposit of copper is applied.

Then the aluminum is dissolved out of the electroform as shown below.

THE THEORY OF ADHESIVE BONDING

INTRODUCTION

There are a number of theories on how adhesives work and there is little common agreement as to which theory is the most relevant for any particular bonding case. It is actually quite important to know the mechanism of bonding because this has an impact on the surface preparation of the adherent surfaces and the materials being attached.

It is known that joints bonded with adhesives are generally stronger in compression, shear and tension than in peeling/tearing - it is much easier to break an adhesive joint by accessing an edge and peeling it away. It is also apparent that it is relatively difficult to ensure that an adhesive joint is in pure tension and if the tension load is of centre or is not normal to the joint there is a tendency for peeling. The best adhesive joints are designed for shear stresses with mechanical guidance and reinforcement e.g keys, corners, shoulders etc.

CONDITIONS FOR SATISFACTORY BONDING

There are accepted conditions which result in higher adhesive bond strengths as listed below

• Cleanliness of surfaces.. The bond surface is ideally cleaned of loose matter and also cleaned of surface oxides and adsorbed gases.


• The choice of adhesive should be such that it wets the adherent surface and also solidifies under an acceptable regime of time, temperature and pressure.

The adhesive should be selected to suit the service conditions of environment and temperature. It should be noted that the difference in coefficient of thermal expansion between the adhesive and adherent can have an important effect on the joint design working

Bonding Theories

There are a number of adhesive theories contributing to the overall study of bonding as listed below:

• The mechanical interlock theory


• The adsorption theory


• The chemisorptions theory


• The electrostatic theory


• The diffusion theory

The mechanical interlock theory

This is the simplest theory and is based on the factor that, at the microscopic level all surfaces are very rough consisting of crevices, cracks and pores. The adhesive penetrates these features and hardens such that it keys into the surfaces and forms a strong surface bond ( this is probably similar to velcro ). The adhesive thus is able to bond two surfaces together and ideally the only weakest part of the bonded joint is the adhesive strength.

The adsorption theory

This theory is based on the assumption that the adhesive "wets" the surface of the adherent surface (meaning that the adhesive applied to the adherent spreads spontaneously when the join is formed). This theory has resulted in adhesive materials being developed which have a lower surface tension than the adherent surfaces. Examples supporting this theory include epoxy resins which wet steel and result in a good bond - these resins do not wet PE or PTFE and result in a poor bond.

According to this theory, in the event of intimate contact between the adhesive and the adherent, the adhesive strength arises as a result of secondary intermolecular forces at the interface. These may include Van der Waals forces (dipole-dipole, dipole-induced dipole interactions and hydrogen bonds).

The chemisorptions theory

This is a variation on the adsorption theory in that stronger chemical bonds (ionic, covalent metallic) form across the joint interface. Ref Molecular Bonds . In this regard, introduction of molecular bonding between the adhesive and the adherent will obviously improve the adhesive bond strength. This can be attained by reactions at the surfaces, using proper surface treatments, or by using additional coupling agents.

The electrostatic theory

This theory states that an electrostatically charged double bond develops at the bond interface as a result of the interaction of the adhesive and and adherent which contributes significantly to the bond strength.

This is a controversial theory as many have doubted the actual significance of the forces involved. While this concept may be useful to explain some specific examples of adhesion, significant doubts have been cast regarding its overall value. These include improved adhesion strengths with lowering of temperature for a large number of adhesive system (lower temperatures should result in poorer electrostatic forces). Also it has been identified that virtually no changes in adhesion performance result with gross variations in the electronic character of the adhesives.

The diffusion theory

When an adhesive contains an adherent solvent the adhesive can diffuse into the adherent surface (substrate) with an interchange of molecules. The theory is is only

Really applicable to polymers where a movement and entanglement of long molecules can occur.

This can be viewed as a molecular interlock enabled adhesion. For plastics, the theory includes for effects of contact time, influence of time and temperature on bonding rate, and the influences of polymer molecular weight and polymer structure.
While the diffusion theory applies well for cases of self-adhesion or auto-adhesion, it does not fit well in providing an explanation for polymer-polymer adhesion. High molecular weight thermoplastic polymers often display very high melt viscosity and will not diffuse easily within the time scale of most bonding operations.

The weak boundary layer theory for most metals there is a surface layer such as a scaly oxide layer. For a successful bond this layer is ideally removed by surface treatments before a strong adhesive bond can be achieved. Aluminum has a strong coherent oxide layer which is suitable for bonding.

Natural adhesives

Natural adhesives are made from inorganic mineral sources, or biological sources such as vegetable matter, starch (dextrin), natural resins or from animals e.g. casein or animal glue. They are often referred to as bio adhesives. One example is a simple paste, made by mixing flour and water. Animal glues are traditionally used in bookbinding, wood joining, and many other areas but now are largely replaced by synthetic glues. Other form of natural adhesives is blood albumen (made from protein component of blood) and is used in plywood industry.

Synthetic adhesives

Elastomers, thermoplastics, Emulsion, and thermosetting adhesives are examples of synthetic adhesives.

Drying adhesives

These adhesives are a mixture of ingredients (typically polymers) dissolved in a solvent. White glue and rubber cements are members of the drying adhesive family. As the solvent evaporates, the adhesive hardens. Depending on the chemical composition of the adhesive, they will adhere to different materials to greater or lesser degrees. These adhesives are typically weak and are used for household applications.

Contact adhesives

Contact adhesives must be applied to both surfaces and allowed some time to dry before the two surfaces are pushed together. Some contact adhesives require as long as 24 hours to dry before the surfaces are to be held together.^[6] Once the surfaces are pushed together, the bond forms very quickly.^[7] It is usually not necessary to apply pressure for a long time, so there is no need to use clamps.

Natural rubber and polychloroprene (Neoprene) are commonly used contact adhesives. Both of these elastomers undergo strain crystallization. Contact adhesives are used in strong bonds with high sheer-resistance like laminates, such as bonding Formica to a wooden counter, and in footwear, such as attaching an outsole to an upper.

A glue gun, an example of a hot adhesive

Main article: Hot glue

Hot adhesives, also known as hot melt adhesives, are simply thermoplastics. They are thermoplastic materials applied in molten form (in the 65-180 C range), which solidify on cooling to form strong bonds between a wide range of materials. These adhesives have become popular for crafts because of their ease of use and the wide range of common materials to which they can adhere. A glue gun, pictured right, is one method of applying a hot adhesive. The glue gun melts the solid adhesive and then allows the liquid to pass through the "barrel" of the gun onto the material where it solidifies.

Paul E. Cope is reputed to have invented thermoplastic glue around 1940 while working for Procter & Gamble as a chemical and packaging engineer.^{[citation needed]} His invention solved a problem with water-based adhesives that were commonly used in packaging at that time. Water-based adhesives often released in humid climates, causing packages to open and become damaged.

Milky-white dispersions often based on polyvinyl acetate. Used extensively in the woodworking and packaging industries. Also used with fabrics and fabric-based components, and in engineered products such as loudspeaker cones.

Ultraviolet (UV) light curing adhesives, also known as light curing materials (LCM), have become popular within the manufacturing sector due to their rapid curing time and strong bond strength. Light curing adhesives can cure in as little as a second and many formulations can bond dissimilar substrates and withstand harsh temperatures. These qualities make UV curing adhesives essential to the manufacturing of items in many industrial markets such as electronics, telecommunications, medical, aerospace, glass, and optical. Unlike traditional adhesives, UV light curing adhesives not only bond materials together but they can also be used to seal and coat products.

Pressure sensitive adhesives (PSA) form a bond by the application of light pressure to marry the adhesive with the adherent. They are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow (i.e. "wet") the adherend. The bond has strength because the adhesive is hard enough to resist flow when stress is applied to the bond. Once the adhesive and the adherend are in close proximity, molecular interactions, such as Van der Waals forces, become involved in the bond, contributing significantly to its ultimate strength.

PSAs are designed for either permanent or removable applications. Examples of permanent applications include safety labels for power equipment, foil tape for HVAC duct work, automotive interior trim assembly, and sound/vibration damping films. Some high performance permanent PSAs exhibit high adhesion values and can support kilograms of weight per square centimeter of contact area, even at elevated temperature. Permanent PSA's may be initially removable (for example to recover mislabeled goods) and build adhesion to a permanent bond after several hours or days.

Removable adhesives are designed to form a temporary bond, and ideally can be removed after months or years without leaving residue on the adherend. Removable adhesives are used in applications such as surface protection films, masking tapes, bookmark and note papers, price marking labels, promotional graphics materials, and for skin contact (wound care dressings, EKG electrodes, athletic tape, analgesic and transdermal drug patches, etc.). Some removable adhesives are designed to repeatedly stick and unstick. They have low adhesion and generally can not support much weight.

Pressure sensitive adhesives are manufactured with either a liquid carrier or in 100% solid form. Articles are made from liquid PSAs by coating the adhesive and drying off the solvent or water carrier. They may be further heated to initiate a cross-linking reaction and increase molecular weight. 100% solid PSAs may be low viscosity polymers that are coated and then reacted with radiation to increase molecular weight and form the adhesive; or they may be high viscosity materials that are heated to reduce viscosity enough to allow coating, and then cooled to their final form.

The strength of attachment, or adhesion, between an adhesive and its substrate depends on many factors, including the means by which this occurs. Adhesion may occur either by mechanical means, in which the adhesive works its way into small pores of the substrate, or by one of several chemical mechanisms.

In some cases an actual chemical bond occurs between adhesive and substrate. In others electrostatic forces, as in static electricity, hold the substances together. A third mechanism involves the van der Waals forces that develop between molecules. A fourth means involves the moisture-aided diffusion of the glue into the substrate, followed by hardening.

Failure of the adhesive joint can occur in different locations

There are several factors that could contribute to the failure of two adhered surfaces. Sunlight and heat may weaken the adhesive. Solvents can deteriorate or dissolve adhesive. And physical stresses may cause the separation of surfaces. When subjected to loading, debonding may occur at different locations in the adhesive joint. The major fracture types are the following:

Cohesive fracture

is obtained if a crack propagates in the bulk polymer which constitutes the adhesive. In this case the surfaces of both adherents after debonding will be covered by fractured adhesive. The crack may propagate in the center of the layer or near an interface. For this last case, the cohesive fracture can be said to be “cohesive near the interface”. Most quality control standards consider a good adhesive bond to be cohesive.

The fracture is adhesive or inter-facial when debonding occurs between the adhesive and the adherent. In most cases, the occurrence of interfacial fracture for a given adhesive goes along with smaller fracture toughness. The interfacial character of a fracture surface is usually to identify the precise location of the crack path in the interphase.

Other types of fracture include:

• The mixed fracture type which occurs if the crack propagates at some spots in a cohesive and in others in an interfacial manner. Mixed fracture surfaces can be characterized by a certain percentage of adhesive and cohesive areas.


• The alternating crack path fracture type which occurs if the cracks jumps from one interface to the other. This type of fracture appears in the presence of tensile pre-stresses in the adhesive layer.


• Fracture can also occur in the adherent if the adhesive is tougher than the adherent. In this case the adhesive remains intact and is still bonded to one substrate and the remnants of the other. For example, when one removes a price label, adhesive usually remains on the label and the surface. This is cohesive failure. If, however, a layer of paper remains stuck to the surface, the adhesive has not failed. Another example is when someone tries to pull apart Oreo cookies and all the filling remains on one side. The goal in this case is an adhesive failure, rather than a cohesive failure.

Modes of failure

A general design rule is a relation of the type is that the material properties are greater than the function required (i.e. geometry, loads, etc.). The engineering work will consist of having a good model to evaluate the function. For most adhesive joints, this can be achieved using fracture mechanics. Concepts such as the stress concentration factor and the strain energy release rate can be used to predict failure. In such models, the behavior of the adhesive layer itself is neglected and only the adherents are considered.

Failure will also very much depend on the opening mode of the joint.

• Mode I is an opening or tensile mode where the loadings are normal to the crack.


• Mode II is a sliding or in-plane shear mode where the crack surfaces slide over one another in direction perpendicular to the leading edge of the crack. This is typically the mode for which the adhesive exhibits the higher resistance to fracture.


• Mode III is a tearing or antiplane shear mode.

As the loads are usually fixed, an acceptable design will result from combination of a material selection procedure and geometry modifications, if possible. In adhesively bonded structures, the global geometry and loads are fixed by structural considerations and the design procedure focuses on the material properties of the adhesive and on local changes on the geometry.

Increasing the joint resistance is usually obtained by designing its geometry so that:

• The bonded zone is large


• It is mainly loaded in mode II


• Stable crack propagation will follow the appearance of a local failure.

Adhesive bonding: Science, technology and applications

Edited by R D Adams, formerly University of Bristol, UK

- Essential information for all those concerned with the industrial application of adhesives
- this important collection examines adhesives and adhesive bonding for load-bearing applications
- arranged in a user-friendly format with three main sections: fundamentals, generic uses and industry specific applications

- written by a leading international team of authors

This important collection reviews key research on adhesive behavior and applications in sectors as diverse as construction and automotive engineering. The book is divided into three main parts: fundamentals, mechanical properties and applications. Part 1 focuses on the basic properties of adhesives, surface assessment and treatment. Part 2 concentrates on understanding how adhesives perform under stress and the factors affecting fatigue and failure. The final part of the book reviews industry specific applications in areas such as building and construction, transport and electrical engineering.
With its distinguished editor and international team of contributors, Adhesive bonding is a standard reference for all those concerned with the industrial application of adhesives.

FUNDAMENTALS OF ADHESIVE BONDING

History of adhesive bonding.What is adhesives and sealants and how do they work?
Introduction
- Adhesives which harden by loss of solvent

- Adhesives which harden by loss of water

- Adhesives which harden by cooling

- Adhesives which harden by chemical reaction

- Adhesives which do not harden - pressure sensitive adhesives

- Adhesion by physical adsorption

- Adhesion by chemical bonding

- The electrostatic theory of adhesion

- Mechanical interlocking

- Adhesion by interdiffusion

- Weak boundary layers

- Pressure sensitive adhesion

- Future trends

- Sources of information

- Introduction
- Surface topography

- Surface thermodynamics

- Surface chemical analysis

- Concluding remarks

- Acknowledgements

- Pretreatments for metals

- Pretreatments for inorganic materials

- Pretreatments for plastics

- Pretreatments for elastomers

- Summary and future trends

Definition of Composite Materials

A composite is a structural material that consists of two or more combined constituents that are combined at a macroscopic level and are not soluble in each other Reinforcing phase: fibers, particles, or flakes Matrix phase: polymers, metals, ceramics High specific strength and modulus, as well as high fatigue strength and fatigue damage tolerance Anisotropic Designable or tailorable materials for both microstructure and properties Production of both material and structure or component in a single

Operation - manufacturing flexible, net-shape, complex geometry

Corrosion resistance and durable Other unique functional properties - damping, low CTE (coefficient of thermal expansion)

Role of Matrices in Composites

• Transfer stresses between the fibers

• Provide a barrier against an adverse environment

• Protect the surface of the fibers from mechanical abrasion

• Determine inter-laminar shear shear strength

• Determine damage tolerance of composites

• Determine in-plane shear strength

• Determine the processibility of composites

• Determine heat resistance of composites

MODEL OF UNIDIRECTIONAL COMPOSITE

Definition of Composite Materials

A composite is a structural material that consists of two or more combined constituents that are combined at a macroscopic level and are not soluble in each other Reinforcing phase: fibers, particles, or flakes Matrix phase: polymers, metals, ceramics

Characteristics of Composite Materials

High specific strength and modulus, as well as high fatigue strength and fatigue damage tolerance Anisotropic Designable or tailorable materials for both microstructure and

Properties Production of both material and structure or component in a single operation - manufacturing flexible, net-shape, complex geometry Corrosion resistance and durable other unique functional properties - damping, low CTE (coefficient of

History of Plastics and Polymers

Plastics are polymers. What is a polymer? The most simple definition of a polymer is something made of many units. Think of a polymer as a chain. Each link of the chain is the "mer" or basic unit that is made of carbon, hydrogen, oxygen, and/or silicon. To make the chain, many links or "mers" are hooked or polymerized together. Polymerization can be demonstrated by linking strips of construction paper together to make paper garlands or hooking together hundreds of paper clips to form chains.

Polymers have been with us since the beginning of time. Natural polymers include such things as tar and shellac, tortoise shell and horns, as well as tree saps that produce amber and latex. These polymers were processed with heat and pressure into useful articles like hair ornaments and jewelry. Natural polymers began to be chemically modified during the 1800s to produce many materials. The most famous of these were vulcanized rubber, gun cotton, and celluloid. The first semi-synthetic polymer produced was Bakelite in 1909 and was soon followed by the first synthetic fiber, rayon, which was developed in 1911.

Extrusion

is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes is its ability to create very complex cross-sections and work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms finished parts with an excellent surface finish.^[1]

Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). The extrusion process can be done with the material hot or cold.

Hot extrusion

Hot extrusion is done at an elevated temperature to keep the material from work hardening and to make it easier to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses that range from 250 to 12,000 tons. Pressures range from 30 to 700 MPa (4,400 to 102,000 psi), therefore lubrication is required, which can be oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions. The biggest disadvantage of this process is its cost for machinery and its upkeep

Hot extrusion temperature for various metals[1]
Material	Temperature [°C (°F)]
Magnesium	350-450 (650-850)
Aluminium	350-500 (650-900)
Copper	600-1100 (1200-2000)
Steel	1200-1300 (2200-2400)
Titanium	700-1200 (1300-2100)
Nickel	1000-1200 (1900-2200)
Refractory alloys	up to 2000 (4000)

The extrusion process is generally economical when producing between several kilograms (pounds) and many tons, depending on the material being extruded. There is a crossover point where rolling becomes more economical. For instance, some steels become more economical to roll if producing more than 20,000 kg (50,000 lb).

Cold extrusion is done at room temperature or near room temperature. The advantages of this over hot extrusion are the lack of oxidation, higher strength due to cold working, closer tolerances,

There are many different variations of extrusion equipment. They vary by four major characteristics:

1. Movement of the extrusion with relation to the ram. If the die is held stationary and the ram moves towards it then its called "direct extrusion". If the ram is held stationary and the die moves towards the ram it’s called "indirect extrusion".

2. The position of the press, either vertical or horizontal.

3. The type of drive, either hydraulic or mechanical.

4. The type of load applied, either conventional (variable) or hydrostatic.

A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure (often used for steel and titanium alloys), oil pressure (for aluminum), or in other specialized processes such as rollers inside a perforated drum for the production of many simultaneous streams of material.

There are several methods for forming internal cavities in extrusions. One way is to use a hollow billet and then use a fixed or floating mandrel. A fixed mandrel, also known as a German type, means it is integrated into the dummy block and stem. A floating mandrel, also known as a French type, floats in slots in the dummy block and aligns itself in the die when extruding. If a solid billet is used as the feed material then it must first be pierced by the mandrel before extruding through the die. A special press is used in order to control the mandrel independently from the ram.^[1] The solid billet could also be used with a spider die, porthole die or bridge dies. All of these types of dies incorporate the mandrel in the die and have "legs" that hold the mandrel in place. During extrusion the metal divides and flows around the legs, leaving weld lines in the final product.^[4]

Typical extrusion presses cost more than $100,000, whereas dies can cost up to $2000.

Direct extrusion

Plot of forces required by various extrusion processes.

Direct extrusion, also known as forward extrusion, is the most common extrusion process. It works by placing the billet in a heavy walled container. The billet is pushed through the die by a ram or screw. There is a reusable dummy block between the ram and the billet to keep them separated. The major disadvantage of this process is that the force required to extrude the billet is greater than that need in the indirect extrusion process because of the frictional forces introduced by the need for the billet to travel the entire length of the container. Because of this the greatest force required is at the beginning of process and slowly decreases as the billet is used up. At the end of the billet the force greatly increases because the billet is thin and the material must flow radially to exit the die. The end of the billet, called the butt end, is not used for this reason.^[5]

INDIRECT EXTRUSION

In indirect extrusion, also known as backwards extrusion, the billet and container move together while the die is stationary. The die is held in place by a "stem" which has to be longer than the container length. The maximum length of the extrusion is ultimately dictated by the column strength of the stem. Because the billet moves with the container the frictional forces are eliminated. This leads to the following advantages

• A 25 to 30% reduction of friction, which allows for extruding larger billets, increasing speed, and an increased ability to extrude smaller cross-sections


• There is less of a tendency for extrusions to crack because there is no heat formed from friction


• The container liner will last longer due to less wear


• The billet is used more uniformly so extrusion defects and coarse grained peripherals zones are less likely.

The disadvantages are:^[6]

• Impurities and defects on the surface of the billet affect the surface of the extrusion. These defects ruin the piece if it needs to be anodized or the aesthetics are important. In order to get around this the billets may be wire brushed, machined or chemically cleaned before being used.


• This process isn't as versatile as direct extrusions because the cross-sectional area is limited by the maximum size of the stem.

Hydrostatic extrusion

In the hydrostatic extrusion process the billet is completely surrounded by a pressurized liquid, except where the billet contacts the die. This process can be done hot, warm, or cold, however the temperature is limited by the stability of the fluid used. The fluid can be pressurized two ways:^[6]

1. Constant-rate extrusion: A ram or plunger is used to pressurize the fluid inside the container.

2. Constant-rate extrusion: A pump is used, possibly with a pressure intensifier, to pressurize the fluid, which is then pumped to the container.

The advantages of this process include:^[6]

• No friction between the container and the billet reduces force requirements. This ultimately allows for faster speeds, higher reduction ratios, and lower billet temperatures.


• Usually the ductility of the material increases when high pressures are applied.


• An even flow of material.


• Large billets and large cross-sections can be extruded.


• No billet residue is left on the container walls.

The disadvantages are:^[6]

• The billets must be prepared by tapering one end to match the die entry angle. This is needed to form a seal at the beginning of the cycle. Usually the entire billet needs to be machined to remove any surface defects.


• Containing the fluid under high pressures can be difficult.

EXTRUSION MATERIALS

Metals that are commonly extruded include:^[9]

• Aluminum is the most commonly extruded material. Aluminum can be hot or cold extruded. If it is hot extruded it is heated to 575 to 1100 °F (300 to 600 °C). Examples of products include profiles for tracks, frames, rails, mullions, and heat sinks.


• Copper (1100 to 1825 °F (600 to 1000 °C)) pipe, wire, rods, bars, tubes, and welding electrodes. Often more than 100 ksi (690 MPa) is required to extrude copper.


• Lead and tin (maximum 575 °F (300 °C)) pipes, wire, tubes, and cable sheathing. Molten lead may also be used in place of billets on vertical extrusion presses.


• Magnesium (575 to 1100 °F (300 to 600 °C)) aircraft parts and nuclear industry parts. Magnesium is about as extrudable as aluminum.


• Zinc (400 to 650 °F (200 to 350 °C)) rods, bar, tubes, hardware components, fitting, and handrails.


• Steel (1825 to 2375 °F (1000 to 1300 °C)) rods and tracks. Usually plain carbon steel is extruded, but alloy steel and stainless steel can also be extruded.


• Titanium (1100 to 1825 °F (600 to 1000 °C)) aircraft components including seat tracks, engine rings, and other structural parts.

Magnesium and aluminum alloys usually have a 0.75 μm (30 μin). RMS or better surface finish. Titanium and steel can achieve a 3 μm (125 μin). RMS.^[1]

In 1950, Ugine Séjournet, of France, invented a process which uses glass as a lubricant for extruding steel.^[10] The Ugine-Sejournet, or Sejournet, process is now used for other materials that have melting temperatures higher than steel or that requires a narrow range of temperatures to extrude. The process starts by heating the materials to the extruding temperature and then rolling it in glass powder. The glass melts and forms a thin film, 20 to 30 mils (0.5 to 0.75 mm), in order to separate it from chamber walls and allow it to act as a lubricant. A thick solid glass ring that is 0.25 to 0.75 in (6 to 18 mm) thick is placed in the chamber on the die to lubricate the extrusion as it is forced through the die. A second advantage of this glass ring is its ability to insulate the heat of the billet from the die. The extrusion will have a 1 mil thick layer of glass, which can be easily removed once it cools.^[3]

Another breakthrough in lubrication is the use of phosphate coatings. With this process, in conjunction with glass lubrication, steel can be cold extruded. The phosphate coat absorbs the liquid glass to offer even better lubricating properties.^[3]

COMPRESSION MOLDING

Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or performs. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand. The advantage of compression molding is its ability to mold large, fairly intricate parts. Also, it is one of the lowest cost molding methods compared with other methods such as transfer molding and injection molding; moreover it wastes relatively little material, giving it an advantage when working with expensive compounds. However, compression molding often provides poor product consistency and difficulty in controlling flashing, and it is not suitable for some types of parts. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding. Compression-molding is also suitable for ultra-large basic shape production in sizes beyond the capacity of extrusion techniques. Materials that are typically manufactured .Compression molding was first developed to manufacture composite parts for metal replacement applications, compression molding is typically used to make larger flat or moderately curved parts. This method of molding is greatly used in manufacturing automotive parts such as hoods, fenders, scoops, spoilers, as well as smaller more intricate parts. The material to be molded is positioned in the mold cavity and the heated platens are closed by a hydraulic ram. Bulk molding compound (BMC) or sheet molding compound (SMC), are conformed to the mold form by the applied pressure and heated until the curing reaction occurs. SMC feed material usually is cut to conform to the surface area of the mold. The mold is then cooled and the part removed. Materials may be loaded into the mold either in the form of pellets or sheet, or the mold may be loaded from a plasticating extruder. Materials are heated above their melting points, formed and cooled. The more evenly the feed material is distributed over the mold surface, the less flow orientation occurs during the compression stage.

Thermoplastic matricies are common place in mass production industries eg. automotive applications where the leading technologies are Long Fibre reinforced Thermoplastics (LFT) and Glass fibre Mat reinforced Thermoplastics (GMT).

In compression molding there are six important considerations that an engineer should bear in mind

• Determining the proper amount of material.


• Determining the minimum amount of energy required to heat the material.


• Determining the minimum time required to heat the material.


• Determining the appropriate heating technique.


• Predicting the required force, to ensure that shot attains the proper shape.


• Designing the mold for rapid cooling after the material has been compressed into the mold.

Process definition

Compression molding is a forming process in which a plastic material is placed directly into a heated metal mold, then is softened by the heat, and forced to conform to the shape of the mold as the mold closes.

Process characteristics

The use of thermoset plastic compounds characterizes this molding process from many of the other molding processes. These thermosets can be in either preform or granule shapes. Unlike some of the other processes we find that the materials are usually preheated and measured before molding. This helps to reduce excess flash. Inserts, usually metallic, can also be molded with the plastic. As a side note, remember not to allow any undercuts on the shape, it will make ejection especially difficult. Thermoplastic matrices with an inherent indefinite shelf-life and shorter cycle moulding times are widely used and examples are shown in Ref 3.

Process schematic

The compression molding starts, with an allotted amount of plastic or gelatin placed over or inserted into a mold. Afterward the material is heated to a pliable state in and by the mold. Shortly there after the hydraulic press compresses the pliable plastic against the mold, resulting in a perfectly molded piece, retaining the shape of the inside surface of the mold. After the hydraulic press releases, an ejector pin in the bottom of the mold quickly ejects the finish piece out of the mold and then the process is finished. Also depending on the type of plunger used in the press there will or won't be excess material on the mold.

Work piece geometry

This process is commonly used for manufacturing electrical parts, dinnerware, and gears. This process is also used to produce buttons, buckles, knobs, handles, appliance housing, radio cases, and large containers. Common commercial examples are shown in Ref 3.

Setup and equipment

Compression mold presses are manufactured in a wide variety of sizes. Most presses utilize a hydraulic ram in order to produce sufficient force during the molding operation. The tools consist of a male mold plunger and a female mold.

Typical tools and geometry produced

Three types of molds used are the flash plunger-type, straight plunger-type, and the "landed" plunger-type molds. The flash type mold must have an accurate charge of plastic and produces a horizontal flash (this is excess material that protrudes out of the mold). The straight plunger-type mold allows for some inaccuracy in the charge of plastic and produces a vertical flash. The landed plunger type mold must have an accurate charge of plastic, and no flash in produced. Further details are explained in Ref 3.

Compression molded rubber boots before the flashes are removed.