Reversibility and Irreversibility

If 100% efficiency is unattainable, what is the max possible efficiency which can be attained and what factors promote the attainment of this max value? In trying to answer these questions, thermodynamics has invented & used the concept of reversibility, absolute temperature and entropy.

Reversible Process:

-for a system is defined as a process which once having taken place, can be reversed and leaves no change in either the system or surroundings. Only ideal processes can do this and restore both system and surroundings to their initial states. Hence an ideal process must be a reversible process.

No real process is truly reversible but some processes may approach reversibility, to a close approximation.

Example:

1) Frictionless relative motion

2) Extension and compression of a spring

3) Frictionless adiabatic expansion or compression of fluid.

4) Polytropic expansion or compression etc.,

The conditions for a process to be reversible may be given as follows:

i) There should be no friction

ii) There should be no heat transfer across finite temperature difference.

iii) Both the system and surrounding be stored to original state after the process is reversed.

Any process which is not reversible is irreversible.

Example: Movement of solids with friction, A flow of viscous fluid in pipes and passages mixing of two different substances, A combustion process.

Every quasistatic process is reversible, because a quasistatic process is of an infinite succession of equilibrium states.

Comparison between work and heat:

Similarities:

· Both are path functions and inexact differentials.

· Both are boundary phenomenon i.e., both are recognized at the boundaries of the system as they cross them.

· Both represent transient phenomenon; these energy interactions occur only when a system undergoes change of state i.e., both are associated with a process, not a state. Unlike properties, work or heat has no meaning at a state.

· A system possesses energy, but not work or heat.

· Concepts of heat and work are associated not with a ‘store’ but with a ‘process’.

Dissimilarities:

· Heat is energy interaction due to temperature difference only; work is by reasons other than temperature difference.

· In a stable system, there cannot be work transfer; however there is no restriction for the transfer of heat.

· The sole effect external to the system could be reduced to rise of a weight but in the case of a heat transfer other effects are also observed.

· Heat is a low grade energy whereas work is a high grade energy.

Classification of Flows

1. Steady and unsteady flows:

A flow is said to be steady if the properties (P) of the fluid and flow do not change with time (t) at any section or point in a fluid flow.

A flow is said to be unsteady if the properties (P) of the fluid and flow change with time (t) at any section or point in a fluid flow.

Eg: Flow observed at a dam section during rainy season, wherein, there will be lot of inflow with which the flow properties like depth, velocity etc.. will change at the dam section over a period of time representing it as unsteady flow.

2. Uniform and non-uniform flows:

A flow is said to be uniform if the properties (P) of the fluid and flow do not change (with direction) over a length of flow considered along the flow at any instant.

A flow is said to be non-uniform if the properties (P) of the fluid and flow change (with direction) over a length of flow considered along the flow at any instant.

Eg: Flow observed at any instant, at the dam section during rainy season, wherein, the flow varies from the top of the overflow section to the foot of the dam and the flow properties like depth, velocity etc., will change at the dam section at any instant between two sections, representing it as non-uniform flow.

3. One, two and three dimensional flows:

Flow is said to be one-dimensional if the properties vary only along one axis / direction and will be constant with respect to other two directions of a three-dimensional axis system.

Flow is said to be two-dimensional if the properties vary only along two axes / directions and will be constant with respect to other direction of a three-dimensional axis system.

Flow is said to be three-dimensional if the properties vary along all the axes / directions of a three-dimensional axis system.

4. Laminar and Turbulent flows:

When the flow occurs like sheets or laminates and the fluid elements flowing in a layer does not mix with other layers, then the flow is said to be laminar. The Reynolds number (Re) for the flow will be less than 2000.

When the flow velocity increases, the sheet like flow gets mixed up and the fluid elements mix with other layers there by causing turbulence. There will be eddy currents generated and flow reversal takes place. This flow is said to be Turbulent. The Reynolds number for the flow will be greater than 4000.

For flows with Reynolds number between 2000 to 4000 is said to be transition flow.

5. Compressible and Incompressible flows:

Flow is said to be Incompressible if the fluid density does not change (constant) along the flow direction and is Compressible if the fluid density varies along the flow direction

r = Constant (incompressible) and r ¹ Constant (compressible)

6. Rotational and Irrotational flows:

Flow is said to be Rotational if the fluid elements does not rotate about their own axis as they move along the flow and is Rotational if the fluid elements rotate along their axis as they move along the flow direction

KINEMATICS OF FLUID FLOW

Fluid kinematics refers to the features of a fluid in motion. It only deals with the motion of fluid particles without taking into account the forces causing the motion. Considerations of velocity, acceleration, flow rate, nature of flow and flow visualization are taken up under fluid kinematics.

A fluid motion can be analyzed by one of the two alternative approaches, called Lagrangian and Eulerian.

In Lagrangian approach, a particle or a fluid element is identified and followed during the course of its motion with time.

Difficulty in tracing a fluid particle (s) makes it nearly impossible to apply the Lagrangian approach. The alternative approach, called Eulerian approach consists of observing the fluid by setting up fixed stations (sections) in the flow field.

Motion of the fluid is specified by velocity components as functions of space and time. This is considerably easier than the previous approach and is followed in Fluid Mechanics.

Eg: Observing the variation of flow properties in a channel like velocity, depth etc, at a section.

CATIA

CATIA (Computer Aided Three-dimensional Interactive Application) is a multi-platform CAD/CAM/CAE commercial software suite developed by the French company Dassault Systemes and marketed worldwide by IBM. Written in the C++ programming language, CATIA is the cornerstone of the Dassault Systemes product lifecycle management software suite.

The software was created in the late 1970s and early 1980s to develop Dassault's Mirage fighter jet, then was adopted in the aerospace, automotive, shipbuilding, and other industries.

CATIA competes in the CAD/CAM/CAE market with Siemens NX, Pro/ENGINEER, Autodesk Inventor and SolidEdge.

History

CATIA started as an in-house development in 1977 by French aircraft manufacturer Avions Marcel Dassault, at that time customer of the CADAM CAD software.

Initially named CATI (Conception Assistée Tridimensionnelle Interactive — French for Interactive Aided Three-dimensional Design ) — it was renamed CATIA in 1981, when Dassault created a subsidiary to develop and sell the software, and signed a non-exclusive distribution agreement with IBM.

In 1984, the Boeing Company chose CATIA as its main 3D CAD tool, becoming its largest customer.

In 1988, CATIA version 3 was ported from mainframe computers to UNIX.

In 1990, General Dynamics Electric Boat Corp chose CATIA as its main 3D CAD tool, to design the U.S. Navy's Virginia class submarine.

In 1992, CADAM was purchased from IBM and the next year CATIA CADAM V4 was published. In 1996, it was ported from one to four Unix operating systems, including IBM AIX, Silicon Graphics IRIX, Sun Microsystems SunOS and Hewlett-Packard HP-UX.

In 1998, an entirely rewritten version of CATIA, CATIA V5 was released, with support for UNIX, Windows NT and Windows XP since 2001.

In 2008, Dassault announced and released CATIA V6. While the server can run on Microsoft Windows, Linux or AIX, client support for any operating system other than Microsoft Windows is dropped.

Features

Commonly referred to as a 3D Product Lifecycle Management software suite, CATIA supports multiple stages of product development (CAx), from conceptualization, design (CAD), manufacturing (CAM), and engineering (CAE).

CATIA can be customized via application programming interfaces (API). V4 can be adapted in the Fortran and C programming languages under an API called CAA. V5 can be adapted via the Visual Basic and C++ programming languages, an API called CAA2 or CAA V5 that is a component object model (COM)-like interface.

Although later versions of CATIA V4 implemented NURBS, V4 principally used piecewise polynomial surfaces. CATIA V4 uses a non-manifold solid engine.

Catia V5 features a parametric solid/surface-based package which uses NURBS as the core surface representation and has several workbenches that provide KBE support.

V5 can work with other applications, including Enovia, Smarteam, and various CAE Analysis applications.

Supported operating systems and platforms

CATIA V6 runs only on Microsoft Windows.

CATIA V5 runs on Microsoft Windows (both 32-bit and 64-bit), and as of Release 18 Service Pack 4 on Windows Vista 64. IBM AIX, Hewlett Packard HP-UX and Sun Microsystems Solaris are supported.

CATIA V4 is supported for those Unixes and IBM MVS and VM/CMS mainframe platforms up to release 1.7.

CATIA V3 and earlier run on the mainframe platforms.

Notable industries using CATIA

CATIA is widely used throughout the engineering industry, especially in the automotive and aerospace sectors. CATIA V4, CATIA V5, Pro/ENGINEER, NX (formerly Unigraphics), and SolidWorks are the dominant systems.

Aerospace

The Boeing Company used CATIA V3 to develop its 777 airliner, and is currently using CATIA V5 for the 787 series aircraft. They have employed the full range of Dassault Systemes' 3D PLM products — CATIA, DELMIA, and ENOVIA LCA — supplemented by Boeing developed applications.

European aerospace giant Airbus has been using CATIA since 2001.

Canadian aircraft maker Bombardier Aerospace has done all of its aircraft design on CATIA.

Automotive

Many automotive companies use CATIA to varying degrees, including BMW, Porsche, Daimler Chrysler, Audi, Volkswagen, Bentley Motors Limited, Volvo, Fiat, Benteler AG, PSA Peugeot Citroën, Renault, Toyota, Ford, Scania, Hyundai, Škoda Auto, Tesla Motors, Proton, Tata motors and Mahindra & Mahindra Limited. Goodyear uses it in making tires for automotive and aerospace and also uses a customized CATIA for its design and development. Many automotive companies use CATIA for car structures — door beams, IP supports, bumper beams, roof rails, side rails, body components — because CATIA is very good in surface creation and Computer representation of surfaces.

Shipbuilding

Dassault Systems has begun serving shipbuilders with CATIA V5 release 8, which includes special features useful to shipbuilders. GD Electric Boat used CATIA to design the latest fast attack submarine class for the United States Navy, the Virginia class. Northrop Grumman Newport News also used CATIA to design the Gerald R. Ford class of supercarriers for the US Navy.

Other

Architect Frank Gehry has used the software, through the C-Cubed Virtual Architecture company, now Virtual Build Team, to design his award-winning curvilinear buildings. His technology arm, Gehry Technologies, has been developing software based on CATIA V5 named Digital Project. Digital Project has been used to design buildings and has successfully completed a handful of projects.

CATIA V4 to V5/V6 Conversion

CATIA V5 and V6 can directly use the CATIA V4 models, but changes in the CATIA data structure requires data conversion from CATIA V4 to V5/V6. This is due to both a change in geometric kernel between CATIA V4 and CATIA V5, and changes in the CAD data structure between CATIA V5 and CATIA V6.

Dassault Systemes provides utilities to convert CATIA V4 data to CATIA V5 with a one-to-one mapping. Still, cases show that there can be issues in the data conversion from CATIA V4 to V5, from either differences in the geometric kernel between CATIA V4 and CATIA V5, or by the modelling methods employed by end users. Experiment results show that there can be data loss during the conversion (from 0% to 90%). The percentage loss can be minimized by using the appropriate pre-conversion clean-up, choosing the appropriate conversion options, and clean-up activities after conversion.

Engineering service providers have solutions, but mostly they are unique to a particular company and its processes / standard of modeling method. A common solution for 100% data conversion has yet to be devised. It is important to note that ANY change from one modeling kernel to another would cause similar problems; this issue is not unique to CATIA.

SolidWorks

SolidWorks is a 3D mechanical CAD (computer-aided design) program that runs on Microsoft Windows and was developed by Dassault Systèmes SolidWorks Corp., a subsidiary of Dassault Systèmes, S. A. (Vélizy, France). SolidWorks is currently used by over 3.4 million engineers and designers at more than 100,000 companies worldwide.

History

SolidWorks was introduced in 1995 as a competitor to CAD programs such as Pro/ENGINEER, I-DEAS, Unigraphics, and CATIA. SolidWorks Corporation was founded in 1993 by Jon Hirschtick, who recruited a team of engineers to build a company that developed 3D CAD software, with its headquarters at Concord, Massachusetts, and released its first product, SolidWorks 95, in 1995. In 1997 Dassault Systèmes, best known for its CATIA CAD software, acquired the company and currently owns 100% of its shares. SolidWorks was headed by John McEleney from 2001 to July 2007, and is now headed by Jeff Ray.

Modeling Methodology

SolidWorks is a parasolid-based solid modeler, and utilizes a parametric feature-based approach to create models and assemblies.

Parameters refer to constraints whose values determine the shape or geometry of the model or assembly. Parameters can be either numeric parameters, such as line lengths or circle diameters, or geometric parameters, such as tangent, parallel, concentric, horizontal or vertical, etc. Numeric parameters can be associated with each other through the use of relations, which allows them to capture design intent.

Design intent is how the creator of the part wants it to respond to changes and updates. For example, you would want the hole at the top of a beverage can to stay at the top surface, regardless of the height or size of the can. SolidWorks allows you to specify that the hole is a feature on the top surface, and will then honor your design intent no matter what the height you later gave to the can

Features refer to the building blocks of the part. They are the shapes and operations that construct the part. Shape-based features typically begin with a 2D or 3D sketch of shapes such as bosses, holes, slots, etc. This shape is then extruded or cut to add or remove material from the part. Operation-based features are not sketch-based, and include features such fillets, chamfers, shells, applying draft to the faces of a part, etc.

Building a model in SolidWorks usually starts with a 2D sketch (although 3D sketches are available for power users). The sketch consists of geometry such as points, lines, arcs, conics (except the hyperbola), and splines. Dimensions are added to the sketch to define the size and location of the geometry. Relations are used to define attributes such as tangency, parallelism, perpendicularity, and concentricity. The parametric nature of SolidWorks means that the dimensions and relations drive the geometry, not the other way around. The dimensions in the sketch can be controlled independently, or by relationships to other parameters inside or outside of the sketch.

SolidWorks pioneered the ability of a user to roll back through the history of the part in order to make changes, add additional features, or change the sequence in which operations are performed. Later feature-based solid modeling software has copied this idea.

In an assembly, the analog to sketch relations are mates. Just as sketch relations define conditions such as tangency, parallelism, and concentricity with respect to sketch geometry, assembly mates define equivalent relations with respect to the individual parts or components, allowing the easy construction of assemblies. SolidWorks also includes additional advanced mating features such as gear and cam follower mates, which allow modeled gear assemblies to accurately reproduce the rotational movement of an actual gear train.

Finally, drawings can be created either from parts or assemblies. Views are automatically generated from the solid model, and notes, dimensions and tolerances can then be easily added to the drawing as needed. The drawing module includes most paper sizes and standards (ANSI, ISO, DIN, GOST, JIS, BSI and GB).

Market

Solidworks Corp. has sold over a million licenses of Solidworks worldwide. The Sheffield Telegraph comments that Solidworks is the world's most popular CAD software. Its user base ranges from individuals to large companies, and covers a very wide cross-section of manufacturing market segments. Commercial sales are made through an indirect channel, which includes dealers and partners throughout the world. Directly competitive products to SolidWorks include Pro/ENGINEER, Solid Edge, and Autodesk Inventor.

Fluid Mechanics Basic Definitions

• Mechanics : Deals with action of forces on bodies at rest or in motion.

• Velocity and Speed: Rate of displacement is called velocity and Rate and distance traveled is called Speed.

Unit: m/s

• Acceleration: Rate of change of velocity is called acceleration. Negative acceleration is called retardation.

• Momentum: The capacity of a body to impart motion to other bodies is called momentum.

The momentum of a moving body is measured by the product of mass and velocity the moving body

Momentum = Mass x Velocity

Unit: Kg m/s

• Newton’s first law of motion: Every body continues to be in its state of rest or uniform motion unless compelled by an external agency.

• Inertia: It is the inherent property the body to retain its state of rest or uniform motion.

• Force: It is an external agency which overcomes or tends to overcome the inertia of a body.

• Newton’s second law of motion: The rate of change of momentum of a body is directly proportional to the magnitudes of the applied force and takes place in the direction of the applied force.

• Mass: Measure of amount of matter contained by the body it is a scale of quantity.

Unit: Kg.

• Weight: Gravitational force on the body. It is a vector quantity.

F = ma

W = mg

Unit: newton (N) g = 9.81 m/s2

• Volume: Measure of space occupied by the body.

Unit: m3

1 m3 = 1000 litres

• Work: Work done = Force x Displacement  Linear motion.

Work done = Torques x Angular displacement  Rotatory motion.

Unit: Nm or J

• Energy: Capacity of doing work is called energy.

Unit: Nm or J

Potential energy = mgh

Kinetic energy = ½ mv2 or ½ mrw2

Power: Rate of doing work is called Power.

• Matter: Anything which possess mass and requires space to occupy is called matter.

• States of matter:

Matter can exist in the following states

• Solid state.

• Fluid state.

• Solid state: In case of solids intermolecular force is very large and hence molecules are not free to move. Solids exhibit definite shape and volume. Solids undergo certain amount of deformation and then attain state of equilibrium when subjected to tensile, compressive and shear forces.

• Fluid State: Liquids and gases together are called fluids. Incase of liquids Intermolecular force is comparatively small. Therefore liquids exhibit definite volume. But they assume the shape of the container Liquids offer very little resistance against tensile force. Liquids offer maximum resistance against compressive forces. Therefore, liquids are also called incompressible fluids. Liquids undergo continuous or prolonged angular deformation or shear strain when subjected to tangential force or shear force. This property of the liquid is called flow of liquid. Any substance which exhibits the property of flow is called fluid. Therefore liquids are considered as fluids.

In case of gases intermolecular force is very small. Therefore the molecules are free to move along any direction. Therefore gases will occupy or assume the shape as well as the volume of the container.

Gases offer little resistance against compressive forces. Therefore gases are called compressible fluids. When subjected to shear force gases undergo continuous or prolonged angular deformation or shear strain. This property of gas is called flow of gases. Any substance which exhibits the property of flow is called fluid. Therefore gases are considered as fluids.

Twin sheet thermoforming

Twin-sheet thermoforming is a process of vacuum or pressure forming two sheets of plastic essentially simultaneously, with a separate mold on the top and bottom platens. Once the plastic has been molded, it remains in the molds, and while still at its forming temperature the two molds are brought together under high pressures, and the two sheets are welded wherever the molds dictate a weld.

Process

The process creates 3 dimensional parts with formed features on both sides. The parts are typically very strong, stiff, and quite light-weight.

Step 1: Two preheated thermoplastic sheets are simultaneously heated between the two molds till they are are entirely plasticized.

Step 2: On reaching the the specific temperature the two molds move together. The two Sheets are deep-drawn and tightly molded to each other in one step. No adhesives are used. There are neither resulting pressure nor strain.

Step 3: To achieve a high level of detail or precision, the forming can be supported by high pressure or (and) vacuum

Step 4: ... and the hollow body, without solvent, molding additives and adhesives, is finished. Furthermore, there are no inner strains.

Materials

• H.M.W. HD Polyethylene
• ABS
• PC/ABS
• Polycarbonate

Typical applications are

pallets, industrial dunnage, portable toilets, medical housings, surfboards, fuel tanks, air/ventilation ducts, electrical enclosures, recreational boats, cases, toys, marine products, doors, tables, spine boards and numerous transportation-related products.

Main Advantages

1. Increased Structural Integrity and Rigidity
2. Enclosed Cross-Section Capability
3. Low Tooling Cost
4. Internal Reinforcement Options: Structural Member, Rigid Foam, Etc.
5. The process has some distinct advantages over blow-molding and rotomolding.
6. The process has some distinct advantages over blow-molding and rotomolding.
7. Compared to blow-molding, the twin-sheet process:
8. Tooling and machines are more cost-competitive for small to modest run sizes.
9. Each sheet may be a different thickness, material or color.
10. More flexibility with parting line structure is possible.
11. Compared to Rotomolding, the twin-sheet process:
12. Many more resin types are available in twin-sheeting.
13. More structural beams can be created in twin-sheeting
14. Much higher production rates

Inline thermoforming

The in-line thermoforming process is designed to take advantage of the hot sheet coming off the extruder. The sheet is mechanically conveyed directly from the extruder through the oven to maintain the sheet at a forming temperature and then to the forming station.

The forming step must be synchronized with the extruder take-off speed. This type of thermoforming is usually limited to sheet 0 125" or thinner and applications that do not require critical thermoforming. i.e.. optimum material distribution and close tolerances.

This process Is more difficult to control than other thermoforming processes· The major disadvantage is that with the extruder and former being tied directly together an upset In one causes a shutdown in both. The majority of roll-fed machines or in-line machines are commonly used for the production of thin-walled products such as cups, trays, lids. internal packaging, and other finished products with a finished wall of 0.003 to 0.060+ in· in thickness. Because of the speed of these machines, secondary operations are incorporated within the unit. These may consist of printing, filling, sealing, die cutting, scrap cutting, or automated removal and stacking of finished product. The normal roll-fed machines consist of the roll station, upper and lower heating banks. form Station, cooling station, and trim station.

Matched die forming

In this process, both halves of the part are formed by molds with no vacuum or air pressure. The sheet is heated until it is soft, and then both mold halves clamp together to form the part. Used with parts that do not have large draws.

Advantages

• excellent definition and dimensional contol on both sides.
• complexity
• high tolerances

Stretch forming

Stretch Forming A plastic sheet forming technique in which the heated thermoplastic sheet is stretched over a mold and subsequently cooled. It is quick, efficient, and has a high degree of repeatability.

Advanced pre-stretch forming or mechanical assisted forming techniques require thermoforming equipment with both top and bottom platens. Automatic machine sequence control is also usually required.

The only real advantage of this process is that only a male form is needed. The disadvantages are many, and include requiring the male form to beconstructed strong enough to resist thelarge forces exerted by the mechanicalequipment that is needed to stretchmany lineal inches of plastic in onedirection. That force may reach manytons.Additionally, minor dirt particles on,or minor deviations of the molds exteri-or surface, will show up as opticaldefects (mark-off) on the concave innersurface of the part. Such defects arevery difficult to remove by later effortsusing abrasives and polishing products.Other problems are excessive thin-ning of the plastic at the deepest por-tion of the formed part and the intro-duction of severe internal stresses thatusually result in early failure when thepart is exposed to sunlight

Drape forming

Drape forming is similar to straight vacuum forming except that after the sheet is framed and heated, it is mechanically stretched, and a pressure differential is then applied to form the sheet over a male mould. In this case, however, the sheet touching the mould remains close to its original thickness. It is possible to drape-form items with a depth-to-diameter ratio of approximately 4 to 1; however, the technique is more complex than straight vacuum forming. Male moulds are easier to build and generally cost less than female moulds; however, male moulds are more easily damaged. Drape forming can also be used with gravitational force alone. For multi-cavity forming, such as tote trays, female moulds are preferred because they do not require as much spacing as male moulds.

Processing steps

Step 1. The plastic sheet is clamped in a frame and heated. Heating can be timed or electronic sensors a can be use to measure sheet temperature or sheet sag.

Step 2. Drawn over the mold - either by pulling it over the mold and creating a seal to the frame, or by forcing the mold into the sheet and creating a seal. The platen can be driven pneumatically or with electric drive. In some very small machines the platen can be manually moved up or the clamped sheet can be manually pushed over the mold.

Step 3. Then vacuum is applied through the mold, pulling the plastic tight to the mold surface. A fan can be used to decrease sheet cooling time.

Step 4. After the plastic sheet has cooled, the vacuum is turned off and compressed air is sent to the mold to help free it from the plastic. The platen then moves down pulling the mold from the formed part. The formed sheet is unclamped, removed, and a new cycle is ready to start.

Main techniques

differing by the position of the mold during the first stage.

1st Method: The sheet (without masking) is placed on top of the mold in its basic, flat state. Both sheet and mold are then slid into a hot-air circulating oven and heated to about 150-155?°C (300-312?°F). When the sheet (and mold) reaches the required temperature it sags and drapes over the heated mold. Both are then pulled out of the oven and quickly helped, by gloved hands, to conform more precisely to the mold. It is then allowed to cool down.

2nd Method: The sheet is placed into a hot-air circulating oven (without masking), and heated to about 150-155?°C (300-312?°F). When the sheet reaches the required temperature it is quickly pulled out of the oven and placed on top of the mold. there the sheet sags, aided quickly by the gloved helping hands, and takes the accurate shape of the mold. For better results we recommend pre-heating the mold to about 80-100?°C (175-210?°F) before putting the heated sheet on top. Then it is, likewise, allowed to cool down.

Advantages

• better part dimensional control on inside of part
• lower mold costs
• ability to grain surface (tubs, showers, counter tops, etc.)
• faster cycle times.
• Disadvantage is more scrap due to larger clamps and trim area

Applications

Drape forming is widely used for large panels that require retaining a simple non-flat shape as in a curved display wall. Another useful application of this process is for the construction of wide sections of odd-shaped walls that will still retain overall even material thickness.

Pressure forming

Pressure forming is a variation of vacuum forming that utilizes both vacuum and compressed air to force the plastic sheet against the mold. As the platens are closed, the vacuum pulls on one side of the sheet and compressed air pushes on the other. Specially shaped tooling is used to match the top and bottom halves of the mold creating a seal to maintain pressures of up to 500 psi, therefore, the platens must be locked together. This compressed air pressure reduces the cycle time and makes it possible to run at lower temperatures, it also improves the distribution of the material creating a more even wall thickness and enhances the detail of the part to a nearly-injection-molded quality. After the part has been formed, the platens unlock and one of the platens moves out of the way to speed up the cooling process.

The increased air pressure will require a stronger mold and a locking device for the platens so consequently a higher tooling expense will be incurred.

Materials

Theoretically, any thermoplastic material can be pressure formed. However, some materials are more difficult to work with than others. Polyethylene, for instance, flows easily and causes few problems for pressure formers. With vacuum alone, polyethylene can be intricately formed. On the other hand, polycarbonate, which chills quickly, can cause manufacturers to be concerned about tool design and plug assists.

Medical device manufacturers usually specify that their products should be formed of a material that passes the Underwriters Laboratories (UL) 94 V0 or 94 5V tests for flammability. The resins most commonly used in pressure-formed medical products are flame- retardant grades of acrylonitrile butadiene styrene (ABS).

In many cases, assists are used to help distribute material evenly and to coin it into sharp or narrow corners. Depending on its complexity, the design of a product's tooling may require the former to use matched heated molds and assists; otherwise, assists can be made of low-heat-transferring materials such as wood.

Advantages

• Sharp, crisp lines and details
• Low tooling costs
• Short lead time
• Textured surfaces and molded in colors
• Formed in undercuts
• Ideal for short runs
• Zero degree draft on sidewalls
• Embossed and Debossed areas
• Highly detailed openings
• Superior, uniform tolerance control

Applications

Use pressure forming when the part will be the "face" of the product expecting a long life. You can pressure form a company logo or model designation with styling lines, surface texture or other features in a light weight and durable part. Use pressure forming when you have undercuts or rims and a greater depth of draw.

Processing Steps

Material is heated to proper temp then moves over the mold.

Platens close and lock.

Vacuum and air pressure are applied.