Profile Extrusion

To manufacture plastic pipe, industry uses a process known as Profile Extrusion. This process is used to manufacture plastic products with a continuous cross-section such as; drinking straws, plastic evestroughing, decorative molding, window trimming and a wide variety of other products polymer melt into the hollow mold cavity under high pressure.

The plastic is fed in pellet form into the machines hopper ( this machine is known as an Extruder ), the material is conveyed continuously forward by a rotating screw inside a heated barrel being softened by both friction and heat. The softened plastic is then forced out through a die and directly into cool water where the product solidifies. From here it is conveyed onwards into the take-off rollers, which actually do the pulling of the softened plastic from the die.

The die is a metal plate placed at the end of the extruder with a section cut out of its interior, this cutout, and the speed of the take-off rollers, determines the cross-section of the product being manufactured. A simple way to understand this concept is to consider squeezing a toothpaste tube, the product comes out in a solid rod because of the opening at the end of the tube, if that opening had a different cross-section than the product produced would take on that new cross-section.

Raw Materials
Most common thermoplastic polymers can be used for extrusion and the material choice is dependent on both the performance requirements and on the economic constraints. It is here that the designer should seek specialist advice from the extrusion company or material suppliers.

Typical Materials for Plastic Profiles:

• HDPE (High Density Polyethylene)
• LDPE (Low Density Polyethylene)
• LLDPE (Linear Low Density Polyethylene)
• PETG
• Flexible PVC
• Butyrate
• Polypropylene
• Polystyrene
• ABS

The most commonly used material for general purpose extrusions is PVC. The wide application of this material is due to cost, chemical resistance and its availability in various hardness and colours. The hardness of PVC can vary from the rigid type used for windows (Shore ‘A’ hardness of 100 or British Standard softness of 0) to the plasticized or soft version used for garden hoses (generally Shore ‘A’ 80 deg or BSS 38) and even down to very soft materials of Shore ‘A’ 60 deg (BSS 75) which have limited uses. The colour can be either matched to a colour sample or chosen from several hundred standard colours. PVC is a very versatile material but, as with all materials, there are limitations and again specialist advice should be sought for critical applications.

Tooling
Steel dies are typically made by a wire EDM process. Some "downstream" tooling may be necessary to ensure shape of profile.

Cost
Dies and parts are relatively inexpensive.

Tolerances
While plastics extrusions can be produced to consistent tolerances the designer must be aware that these are not the same as for machined parts or for metals extrusion and are generally greater. The tolerance bands applicable vary with the relevant dimension, the material used and with the manufacturer but in general BS 3734:1978 for extruded rubber products (Table 2 Class E 2) can be used as a guide. Specific tolerances for critical areas and non-critical tolerances must be discussed and agreed between customer and producer. Inevitably, the unit price increases with the number of tolerance dimensions and the tightness of the tolerances specified.

Advantages

• Equipment widely available in all geographical areas. Short lead times.
• Relatively low tooling costs
• Inexpensive process
• Product combinations possible
• Design freedom

Disadvantages
Design possibilities severely limited because of linear nature of process.

Examples of Applications

• Typical applications/design possibilities
  The following application examples have been chosen to illustrate possibilities and the same ideas and techniques can, obviously, be used in many fields such as:
• Window profiles
  The basic frame of the window is an extruded, un-plasticized PVC section. This section contains air gaps or chambers which are carefully designed to give the necessary thermal and sound insulation. The normal colour is white and the polymer is UV stabilized to prevent fading. New developments with co-extrusion and printing techniques allow the profile to be produced with wood-effect or coloured finishes. This basic profile is mitre cut and welded into a frame to fit the windows of the house exactly. Extrusions are also used to provide the essential sealing lips on the profile. By skilled design a system of extrusions is built up to provide outward opening windows, tilt and turn windows, patio doors, roller shutters and other elements of the glazing system of the house.
• Sealing sections
  Extrusions are applied in many sealing applications where the designer has considerable choice in fixing method. A co-extrusion of hard and soft materials will allow the hard material to be screwed, nailed, stapled or glued to one sealing face and the soft material will still provide the required seal. A single hardness soft extrusion can be punched or stapled but may need a reinforcing rod. Alternatively, it may be clipped into one sealing face using a groove in the face as a location/fixing area. The designer can choose between these varied options and the extrusion manufacturer can provide advice on the technology available. Typical application areas are refrigerator door seals (which incorporate a magnetic extrusion for an airtight seal), car door and boot seals, acoustic cabinet seals and the window seals described above.
• Modular drawer profiles
  Drawer systems utilizing extrusions are available both as 'Do-It-Yourself' and professional kits. These illustrate important options for the designer: the ability to use an extrusion to provide variable length and width and the use of injection moulded corner pieces to provide the necessary jointing. The requirements for light weight and easy assembly rule out the use of welding and the assembly is built up using the clip-in corner pieces which give rigidity and professional finish.
• Decorative trim
  The decorative trim strips seen on bedroom and other furniture are examples of two important techniques available. One is the ability to apply a foil to the extruded PVC to give a bright and attractive finish (an option which is often used in the automobile industry for trim and bumper strips although, in this case, special exterior foils and techniques are necessary). The other is the use of double sided tape for rapid and strong mounting of the profile. For fixing to smooth, flat surfaces i.e. furniture, a film tape is used but when the surface is not regular then a foam tape may be used to give the necessary surface conformance and adhesion.

More examples are possible but the engineering designer is seeking to innovate and, hopefully, those examples outlined above can help this innovation through increased awareness of the process and its capabilities.

Sheet Extrusion

Sheet extrusion is a technique for making flat plastic sheets from a variety of resins. The thinner gauges are thermoformed into packaging applications such as drink cups, deli containers, produce trays, baby wipe containers and margarine tubs. Another market segment uses thick sheet for industrial and recreational applications like truck bed liners, pallets, automotive dunnage, playground equipment and boats. The third primary use for extruded sheet is in geomembranes, where flat sheet is welded into large containment systems for mining applications and municipal waste disposal.

Thermoplastic sheet production is a significant sector of plastics processing. Thermoplastic sheets are flat, plastic materials with a gauge of at least 250 microns and which include both flexible and rigid materials, as well as solid, foamed, and hollow materials.

General

Solid sheet extrusion units consist of at least one extruder and one sheet extrusion die. They are followed by the polishing stack, in general comprising 3 calenders, calibrating and cooling the sheet with their surfaces or calender nips. Behind this the roller conveyor and the draw-off rolls for air cooling are located. The sheet is finally cut and stored. Sheet extrusion characteristics:

• width in excess of 2 m
• thicknesses ranging from approx. 0.5 to 15 mm
• no limitations as to length
• setup as multilayer sheets with functional surfaces (colour, haptics, UV-protection ...)
• grain/structured surfaces
• easier forming possible (corrugated panels, folding, thermoforming ...)

Materials

Polystyrene continues to be the most common polymer for use in sheet extrusion. It is the dominant material for thermoformed packaging and competes with ABS and PP in technical markets. End use applications include tubs and pots for yogurt, margarine, and desserts. Thermoformed packaging is also used in many other applications in the food industry.

There are three primary techniques used to manufacture thermoplastic sheet. These are:

1. Extrusion through a flat die onto casting rolls.
2. Extrusion through an annular die onto a sizing mandrel. The pipe-like cross section that is extruded will be slit in one or more places and then flattened and handled as sheet.
3. Resins and additives will be plasticated between large rolls and then sized through a series of additional rolls into a flat sheet. This process is known as calendering.

Each of these methods has advantages and disadvantages depending on factors such as type of polymer being processed, thickness and width of sheet, and surface quality desired.

Single Layer Flat Sheet extrusion is the most common technique used in extruding plastic sheet for the thermoforming industry. The classic machinery components for this process can be described as follows:

Resin is fed into an extruder where it is plasticated into a melt.

The extruder, consisting of a heated barrel with an internal rotating screw, pumps the melted resin into a flat sheet die which sizes the sheet (thickness and width).

The sheet exits the die in a semi-viscous state and travels through a series of rolls to cool. These rolls also determine final sheet size, thickness, and width.

The flat sheet may then be wound onto continuous rolls, or "pre-sheared" into discrete lengths.

Coextrusion is a process that allows the combination of different materials and colors in a single sheet. This is done to achieve special properties which are specific to a certain polymer, or for aesthetic effects with color, or for economic reasons where an inexpensive material "sub-strata" is combined with a more expensive material "cap".

Applications

Within the building and construction industries, sheet extrusion is used for a variety of applications. One of the main uses of extruded PS sheet is for thermal insulation materials for walls, roofs, and under floors.

In the automotive industry, sheet is currently used to produce interior trim, panels, and dashboards. Foamed polyolefin sheet, both cross-linked and non-cross-linked, is also used in automotive applications.

There are a number of other applications where thermoformed sheet plays a significant role. These include the manufacturing of luggage, refrigerator liners, and shower units.

Types of plastic extrusion

• Sheet Extrusion
  Sheet extrusion is a technique for making flat plastic sheets from a variety of resins. The thinner gauges are thermoformed into packaging applications such as drink cups, deli containers, produce trays, baby wipe containers and margarine tubs. Another market segment uses thick sheet for industrial and recreational applications like truck bed liners, pallets, automotive dunnage, playground equipment and boats. The third primary use for extruded sheet is in geomembranes, where flat sheet is welded into large containment systems for mining applications and municipal waste disposal.
• Profile Extrusion
  Rubber Profile Extrusion is accomplished by forcing uncured rubber through a die, under heat and pressure, to form a part with a uniform cross section. This uncured rubber is then run through a heating unit to initiate the chemical cross linking reaction that causes the rubber to cure.
• Pipe extrusion
  Pipe extrusion is defined as a process of forcing the polymer melt through a shaping die (in this case: circular). The extrudate from the die is sized, cooled and the formed pipe is pulled to the winder or a cut off device with the aid of haul off device.
• Co-extrusion
  The process of extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling.
• Blown Film Extrusion
  In film blowing a tubular cross-section is extruded through an annular die (usually a spiral die) and is drawn and inflated until the frost line is reached. The extruded tubular profile passes through one or two air rings to cool the material.
• Cast Film Extrusion
  The cast film process differs from the blown film process through the fast quench and virtual unidirectional orientation capabilities. These characteristics allow a cast film line to operate at higher production rates while producing amazing optics. Applications in food and retail packaging take advantage of these strengths.
• Foam Extrusion
  During the chemical foam extrusion process plastic resin and chemical foaming agents are mixed and melted. The chemical foaming agent decomposes liberating gas which is dispersed in the polymer melt and expands upon exiting the die. Typically foamed profile extrusions require more intense cooling than solid profiles due to the insulation properties of the foam structure.
• Pultrusion
  Similar to extrusion but with much higher Strengths- even used to make road bridges. Glass or other fibres are incorporated into the extrusion and so loadings of up to 60% glass can be achieved with very good fibre alignment. Materials are generally thermosetting type materials such as epoxy.
• Calendering
  Calendering is a process that usually uses four heated rolls rotating at slightly different speeds. Again the material is fed into the rolls, heated and melted, and then shaped into sheet or film. PVC is the most commonly calendered material.

Extrusion

Extrusion is a manufacturing process where a billet of material is pushed and/or drawn through a die to create a shaped rod, rail or pipe. The process usually creates long length of the final product and may be continuous or semi-continuous in nature. Some materials are hot drawn whilst other may be cold drawn.

Perhaps the most interesting of these processes is the manufacture of pipe where not only is the outside diameter controlled but also either a fixed or floating die is also used to set the internal diameter and hence the wall thickness.

Commonly extruded materials are copper (pipe for plumbing), aluminium (various extrusion profiles for tracks, frames, rails), steel (rod, track) and a multitude of plastics (pipes, rods, rails, seals).

It is common in the plastic extrusion process to use plastic chip, which is then melted and rather than drawing the material through the die to squeeze the plastic out of the die in a similar fashion to the extrusion of toothpaste from a tube.

Extrusion has found a great application in Food Processing. Various products like pastas, breakfast cereals, ready to eat snacks, fry-ums etc. are now manufactured by extrusion. Softer foods such as meringue have long been piped using pastry bags.

Food Extrusion was used as a shaping tool since time immemorial. In India, it has been used to shape products like chaklis and sev. In Italy, it was used for the manufacture of pastas. The first industrial extruders came into existence around 75 years ago (Mercier, Linko & Harper 1989). Initially used only for mixing and forming pasta and for the mincing of meat, they have morphed into high temperature short time bioreactors that transform raw ingredients into intermediate or final products.

The first industrial food extrusions involved the use of piston or ram type extruders to stuff casings in the manufacture of sausages and processed meats (Harper 1981). These were followed by meat choppers and mincers, which consisted of a screw forcing the meat out of a small die plate. These were the first twin screw extruders used in the food industry. The pasta industry became the second food industry to use extrusion with the development of hydraulically operated batch cylindrical ram macaroni presses around 1900. However, the application of the single screw extruder which revolutionized the industry was its use as a continuous pasta machine in the 1930s. The pasta press mixes semolina flour, water and other ingredients to form a uniform dough. The screw of the extruder works the dough and forces the mixture through specially designed dies to create the variety of shapes that pastas are available in now.

In the late 1930s General Mills used the extruder in the manufacture of ready to eat cereals. Extruded corn collets were developed around the same time. However, the concept was not commercially developed till 1946. The desire to precook animal feeds to improve digestibility and palatability led to the development of the cooking extruder late in the 1940s, which has greatly expanded the application of extruders in the food industry.

Cooking extruders come in a variety of sizes and shapes and provide the capability to vary the screw, barrel, and die configurations as required by the product. Temperature is controlled by direct steam injection or heating through external barrels. Preconditioning of the feed in an atmospheric or pressurized chamber allows ingredients to be partially cooked and uniformly moistened before extrusion.

Modern food extruders can be designed to combine a range of unit operations into one process which does not require much pre or post processing. They can carry out one or more of the following in one step: transport, grinding, hydration, shearing, homogenization, mixing, compression, degassing, cooking with partial melting and plasticization of the mix, starch gelatinization, protein denaturation, destruction of microorganisms and anti-nutritional factors, pumping, shaping, expansion, formation of porous and fibrous texture and partial dehydration. Depending on their design, they can be used to make a variety of products including pastas, breakfast cereals, puffed snacks (corn puffs/collets, kurkure, cheese balls etc.), meat substitutes like soya nuggets, fry ums, breading substitutes, modified starches, soft-moist and dry pet foods and confections.

The second revolution in food extrusion came with the use of variable pitch single screw extruders. These extruders further improved the mixing versatility of the extruder. The most recent advance for the food extrusion industry has been the use of twin screw extruders. The screws either rotate in the same direction (co-current) or in opposite direction (counter-current) to each other. These extruders, while more complex than single screw extrudes, offer better control over residence time distribution and internal control of shear for thermolabile materials. They are also more versatile in that they accept lower moisture feeds and are self cleaning due to the wiping effect of the screws.

Food extruders today are all screw extruders and the early ram and piston type extruders have disappeared from the industry. The various components of an extruder are a drive, feed assembly, extrusion screw, extruder barrel and an extruder discharge. The drive consists of a support / stand, a drive motor, a set of gears for variation of speed, a gear transmission (to reduce speed and increase torque) and a thrust bearing (to support and centre the screw and absorb its thrust).

The type of feeder section depends on the material to be fed. Different feeders are available for dry, wet and slurry like materials. For solids and dry materials hoppers / bins, vibratory feeders, variable speed screw conveyers and weigh belts are used. Water wheels, positive displacement pumps, variable orifices and variable head feeding devices are available for liquid or slurry like feeds. These feeders can be batch or continuous feeders as per requirements. Often the raw materials are fed with such feeders into a preconditioner from where they are fed into the screw section.

Strech 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

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.

Twin Sheet Forming

Twin sheet forming

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. 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

• Increased Structural Integrity and Rigidity
• Enclosed Cross-Section Capability
• Low Tooling Cost
• Internal Reinforcement Options: Structural Member, Rigid Foam, Etc.
• The process has some distinct advantages over blow-molding and rotomolding.

The process has some distinct advantages over blow-molding and rotomolding.

Compared to blow-molding, the twin-sheet process:

• Tooling and machines are more cost-competitive for small to modest run sizes.
• Each sheet may be a different thickness, material or color.
• More flexibility with parting line structure is possible.

Compared to Rotomolding, the twin-sheet process:

• Many more resin types are available in twin-sheeting.
• More structural beams can be created in twin-sheeting
• Much higher production rates

Match Die Forming

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

Pressure Forming

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.

Steps Material is heated to proper temp then moves over the mold. Platens close and lock. Vacuum and air pressure are applied. 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.

Drape Forming

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.

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. 1) 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. 2) 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.

Vacuum Forming

Vacuum-snap back is an excellent and often used process for forming deep draw products with uniform wall thickness. Vacuum is used to pre-stretch the hot plastic before the mold makes contact with the sheet. Vacuum snap-back, while more complex than plug assist, can produce deeper drawn products with better wall uniformity and less mark-off. A vacuum pre-stretch box is required. The pre-stretch box is sealed against the hot sheet and vacuum is applied. The plastic is drawn into the box as a hemisphere with the height of the hemisphere usually controlled by a photocell. Other methods can be used to control the hemisphere height, but a photocell works well.

The steps of vacuum snap-back are:

• After the plastic sheet is heated and the sheet cart returns to the forming station, the bottom platen moves up sealing the vacuum pre-stretch box against the hot sheet. Vacuum is then applied. When the stretching plastic crosses the photocell beam, vacuum is turn off.
• The mold is moved into the formed hemisphere. When the mold is sealed against the hot plastic, vacuum is applied to the mold and vacuum is released from the pre-stretch box causing the plastic to snap to the contours of the mold.
• The pre-stretch box is then lowered and cooling air is blown against the hot plastic. After the plastic cools, the mold vacuum is released, air eject is applied through the mold and then the mold is removed from the formed plastic part.
• Placing the mold on the bottom platen and the pre-stretch box on the top platen will also work for vacuum snap-back.

Advantages

• well controlled part thickness

though longer cycle times

Free Forming

This method of thermoforming does not use a mold. Instead, an acrylic sheet is clamped in a frame and either a vacuum or compressed air draws the material to a desired depth. An electric eye determines when the proper depth has been reached and cuts off the pressure. Since only air touches the sheet of material, there is no markoff. Free forming is used to create windshields for planes, skylights, or anything where optical quality is necessary.

Advantage is achieving high clarity.

Billow Forming

Billow Forming - a method of thermoforming sheet plastic in which the heated sheet is clamped over a billow chamber. Air pressure in the chamber is increased causing the sheet to billow upward against a descending male mold.

Similar to vacuum snapback except the heated sheet is blown upward into a bubble shape and then the plug or mold is driven into the pre-stretched sheet from the top plate.

Using compressed air produces greater stretching forces but requires a much stronger pre-stretch box.

Plug Assisted Forming

Plug assist forming is a widely used forming technique and requires the use of a female (cavity) mold. The limited depth of draw of female molds is improved by the use of plug assist. With plug assist the plastic sheet is mechanically pre-stretched by a plug that is pushed into the hot plastic before the application of vacuum to the mold. The plug has a geometry that is usually 10 - 30 percent smaller than the interior of the female mold cavity. The plug is constructed of materials with low thermal conductivity or is heated. Low thermal conductivity plugs or heated plugs must be used to keep the plastic sheet from cooling when the sheet comes in contact with it. Materials such as wood, syntactic foam, and cast thermoset plastics can be used to make a low thermally conductive plug. This insulator type plug can be covered with felt to reduce mark-off. Aluminum with temperature controlled electric heaters can also be used. Aluminum plugs produce excellent results but are usually more costly than insulator type plugs. Different wall and bottom thickness can be produced by controlling how deep the plug goes into the mold and by controlling and varying plug temperatures.

The steps in plug assist forming are: After the sheet is heated and the sheet cart moves back to the forming area, the bottom platen moves up to the plastic sheet and seals. The top platen with the plug moves down pushing the plug into the hot plastic. After the plug reaches the required depth, vacuum is applied to the female mold forming the plastic to the contours of the mold. The top platen moves back up and cooling fans cool the plastic covering the inside of the female mold.

Advantages

• better wall thickness uniformity especially for cup or box shapes
• reduces stretching or thinning of material during forming.

Vacuum Forming

Vacuum forming, commonly known as vacuforming, is a simplified version of thermoforming, whereby a sheet of plastic is heated to a forming temperature, stretched onto or into a single-surface mold, and held against the mold by applying vacuum between the mold surface and the sheet.

The vacuum forming process can be used to make product packaging, speaker casings and even car dashboards.

Normally, draft angles must be present in the design on the mold (a recommended minimum of 3°), otherwise release of the formed plastic and the mold is very difficult.

Vacuum forming is usually – but not always – restricted to forming plastic parts that are rather shallow in depth. A thin sheet is formed into rigid cavities for unit doses of pharmaceuticals and for loose objects that are carded or presented as point-of-purchase items. Thick sheet is formed into permanent objects such as turnpike signs and protective covers.

Relatively deep parts can be formed if the form-able sheet is mechanically or pneumatically stretched prior to bringing it in contact with the mold surface and before vacuum is applied ^[1].

Suitable materials for use in vacuum forming are conventionally thermoplastics, the most common and easiest being High Impact Polystyrene Sheeting (HIPS). This is molded around a wood, structural foam or cast/machined aluminum mold and can form to almost any shape. Vacuum forming is also appropriate for transparent materials such as acrylic which are widely used in applications for aerospace such as PCW (passenger cabin windows) canopies for military fixed wing aircraft and "bubbles" for rotary wing aircraft.

Vacuum forming is a plastic thermoforming process that involves forming thermoplastic sheets into three-dimensional shapes through the application of heat and pressure. In general terms, vacuum forming refers to all sheet forming methods, including drape forming, which is one of the most popular. Basically during vacuum forming processes, plastic material is heated until it becomes pliable, and then it is placed over a mold and drawn in by a vacuum until it takes on the desired shape. Vacuum thermoforming is a great method for producing plastic parts that have sharp details and fit nicely to specific products.

During the vacuum forming process, a sheet of heated plastic material is placed over a male or female mold. The mold then moves towards the sheet and presses against it to create a seal. Next, the application of a vacuum draws out the air between the mold and the sheet so that the plastic conforms to the mold exactly. This is accomplished through venting holes in the mold that are joined to vacuum lines. The mold also has a water cooling system integrated into it that brings the temperature of the plastic to the set temperature needed. When the curing temperature is reached and the piece is formed, air blows back into the mold and separates the new part from the mold.

Vacuum forming produces plastic parts for various industries, such as the food, cosmetic, medical, electronics, entertainment, household products, toys, athletic equipment, appliance, automotive, office supplies and clothing industries. One of the most important industries that thermoforming serves, however, is packaging. Products like blister packs, inserts, trays and clamshells are used to house other products and are important for both preservation of the items they hold and the aesthetic designs they can provide. Consumer product manufacturers often use vacuum forming to produce plastic trays and glasses. Another interesting use for vacuum formed plastic is the creation of signs for gas stations and convenience stores.

The greatest advantage to vacuum forming is that it involves less parts and tooling than injection molding, and therefore is more cost-effective. It is an economical choice that can be used for small and medium production runs, with low cost tool modifications. There is great design flexibility available, from a variety of prototypes to custom made designs that can be used to cover almost any product. Most manufacturers also offer a wide variety of trim and other decoration options that can prove quite a visual advantage. Time of production is generally short, which frees up time to do more detail-oriented aspects of production. Sharp, precise detail is available for many products, which makes vacuum formed plastics an attractive alternative to other molding processes.

Advantages

• Economical for small to medium production runs
• Low tooling costs
• Quick startup
• High strength to weight ratio
• Efficient prototyping
• No need for painting; the color and texture are formed in

Types of Thermoforming

Types of Thermoforming

• Vacuum Forming
  Vacuum forming is a plastic thermoforming process that involves forming thermoplastic sheets into three-dimensional shapes through the application of heat and pressure. In general terms, vacuum forming refers to all sheet forming methods, including drape forming, which is one of the most popular. Basically during vacuum forming processes, plastic material is heated until it becomes pliable, and then it is placed over a mold and drawn in by a vacuum until it takes on the desired shape.
• Plug assist forming
  Plug assist forming is a widely used forming technique and requires the use of a female (cavity) mold. The limited depth of draw of female molds is improved by the use of plug assist.
• Vacuum snapback
  Vacuum-snap back is an excellent and often used process for forming deep draw products with uniform wall thickness. Vacuum is used to pre-stretch the hot plastic before the mold makes contact with the sheet. Vacuum snap-back, while more complex than plug assist, can produce deeper drawn products with better wall uniformity and less mark-off. A vacuum pre-stretch box is required.
• Billow Forming
  A method of thermoforming sheet plastic in which the heated sheet is clamped over a billow chamber. Air pressure in the chamber is increased causing the sheet to billow upward against a descending male mold.
• Free Forming
  This method of thermoforming does not use a mold. Instead, an acrylic sheet is clamped in a frame and either a vacuum or compressed air draws the material to a desired depth. An electric eye determines when the proper depth has been reached and cuts off the pressure. Since only air touches the sheet of material, there is no markoff.
• Pressure Forming
  This process is similar to vacuum forming, except with the addition of pressure, which pushes the sheet into the shape of the mold. This process is mainly used for parts that require styling and aesthetic qualities because pressure forming creates greater detail, allowing for textured surfaces, undercuts and sharp corners, which are not as easily created with vacuum forming.
• Drape Forming
  In drape forming, a sheet of plastic is heated and stretched down, generally over a male mold. Next, depending on the shape of the mold, gravity alone will pull the material to the mold or commonly, a vacuum is applied to draw the sheet to the mold which will more detail to the inside of the part.
• 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.
• 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.
• Inline thermoforming
  In this process the plastic film moves from a roll onto the inline equipment and through the heating section. The heated material advances into the forming section where pressure and/or vacuum force the plastic onto a mold. It then proceeds to another station where formed parts are die-cut.
• Twin sheet forming
  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.