Co-Injection Moulding

This is a process that creates a skin and core material arrangement in a molded part. The skin material is injected first into the mold cavity, and is immediately followed by a core material. As the skin material flows into the cavity, the material next to the cavity walls freezes and material flows down a center channel. When the core material enters it displaces the skin material in the center of the channel by pushing the skin ahead. As it flows ahead it continues to freeze on the walls producing the skin layer

Machine Based Co-Injection
The Co-Injection process requires two injection/processing units. The units generally inject material through a manifold located at the end of the injection barrels. The manifold ports the two melt streams into a centrally located nozzle. The machine controls the injection units to achieve a skin-core-skin flow sequence through the manifold into the mold. Last skin flow is needed to clear the short nozzle section of core material and to seal the gate area with skin. This arrangement can be used on any single or multiple cavity, conventional cold runner mold.

Mold Based Co-Injection
This same process can be achieved on a hot runner mold by utilizing a hot runner system from Incoe Corporation of Troy Michigan. This system, sometimes identified as "Mold Based Co-Injection", still utilizes two injection units. The two melt streams are directed into the mod via separate channels. These two channels remain separate until they reach the gate area of the part. At this point they flow through a nozzle arrangement similar to the normal co-injection manifold.

Co-Injection Benefits

• Lower Cost Parts
• Higher Strength Core
• Sound Absorption Core
• Reduced Cooling Time for Lower Temperature Core
• Improved Aesthetic Qualities
• Combined Property Characteristics

Co-Injection Features of a Milacron Machine

• Independent Co-Injection Manifold
• Easy Purging Of Individual Barrels and Manifold.
• Reversible Manifold for Optimizing Barrel Capacity with Core %.
• Active Split Screens^TM for Viewing Multiple Screens At Once.
• Control Sequences for Utilizing in Mold Co-Injection Manifolds.
• Flexible Manifold Design for Machine or Mold Based Co-Injection.

Applications:

• Foamed Core for reduced weight and noise transmission
• Glass filled cores for improved physical properties
• Low cost core for cost savings.
• High gloss skin material over structural core material for combination of aesthetic and structural properties.
• Post Consumer Recycled material in core. Environmental friendly.
• Post Industrial recycled Material in core
• Reground painted parts recycled into core.

Thermoforming

Thermoforming is a manufacturing process for thermoplastic sheet or film. The sheet or film is heated between infrared, natural gas, or other heaters to its forming temperature. Then it is stretched over or into a temperature-controlled, single-surface mold. Cast or machined aluminum is the most common mold material, although epoxy, wood and structural foam tooling are sometime used for low volume production. The sheet is held against the mold surface unit until cooled. The formed part is then trimmed from the sheet. The trimmed material is usually reground, mixed with virgin plastic, and reprocessed into usable sheet ^[1]. There are several categories of thermoforming, including vacuum forming, pressure forming, twin-sheet forming, drape forming, free blowing, and simple sheet bending.

Process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to render it soft and flowable. Then applying differential pressure to make the sheet conform to the shape of a mold or die positioned below the frame.

• Thermoforming process
• Choosing The proper forming parameters
• Thermoforming tool design guidelines

Thermoforming process

The thermoforming process involves the following steps:

• Extrusion of sheet
• Place the sheet on a mould
• Draw the sheet into the shape of the mould by heat and negative force (vacuum).

Raw Materials
Most thermoplastics are usable. Must be in sheet form.

Tooling
Generally, machined aluminum is used, although poured composites and even wood can be used for short runs.

Cost
Tooling costs are generally low and piece prices are strictly dependent upon the speed of the machinery.

Examples of Application
Covers, displays, blister packaging, trays, drinking cups & food packaging.

Some of the possible advantages of thermoforming over injection molding include

• Large surface-area parts formed on inexpensive molds andmachines, due to low pressure and temperature requirements
• Easy formation of very thin-walled parts that are difficult to make by other techniques.
• Very high rates of production of thin-walled parts atrelatively low capital investment.

Advantages

• Extremely adaptive to customer design needs
• Rapid prototyping development
• Material and process is optimized for cost effectiveness
• High-speed production allows for just-in-time shipments
• Flexible tooling design offers a competitive advantage
• On-the-fly product enhancements with low additional costs
• Visually pleasing appearance
• Weight savings for consumer and manufacturer
• Wider design scope
• Lower tooling costs
• No anticorrosion spray necessary
• Paintable and colored plastic availability
• Fully integrated process with limitless flexibility for small to large product designs

Disadvantages

• High initial equipment investment
• High startup and running costs possible
• Part must be designed for effective molding
• Accurate cost prediction for molding job is difficult

Choosing The proper forming parametres

Optimum forming conditions depend on part and molddesign, part draw ratio, host polymer, sheet thicknessand thermoforming method.

Sheet storage
To maintain the properties of Stat-Rite sheet, Noveonwraps and seals the rolls in heavy polyethylene with adesiccant to help prevent damage and moisture pick-upduring shipping or storage. Handling guidelines follow:

• Rolls should be used within 6 months after receipt.
• Do not remove the wrapping until you are ready to use the roll.
• Rolls should be opened only in a controlled humidity andtemperature environment.
• Rolls should be used as soon as possible after removal ofprotective wrapping
• Stat-Rite sheet should be stored under controlledtemperature of 60°F to 80°F and low humidity conditions.
• Sheets stored for any length of time should be pre-driedbefore forming.

Forming temperature
Sheet temperature should be determined with aninfrared pyrometer. For best results, the infrared deviceshould be mounted through the oven wall. Unlike theirhost polymers, Stat-Rite alloys show little sheet sag attheir optimum forming temperature. Heat transfer inthermoforming depends on heater radiation, air convec-tion, and conduction through the plastic. With infraredtemperature measurement and heating cycle time control,sheet temperature can be controlled to within +/- 10°F or +/- 5°C.

For thin-gauge thermoforming, where the sheet thicknessis typically less than 0.060" (1.5 mm) conduction throughthe sheet is usually less important than radiation and convection to the sheet surface. Sheets thicker than about0.010" (0.25 mm) should usually be heated on both sides.

When determining the best forming temperatureremember:

• Electrical properties will be affected by excessive heating.
• Maximum thermoforming temperature for Stat-Rite is 374°F(190°C) for acrylic, 338°F (170°C) for PETG (see Table 1).
• Forming at lower sheet temperatures yields the best hotstrength, minimum spot thinning, and shorter forming andcooling cycle times.
• Forming at higher sheet temperatures yields lower internalstress levels, better mold surface replication, deeper draws,longer cooling times, higher formed part shrinkage, morenonuniform part wall thickness, and vacuum hole nipples.
• Plug assist forming often produces improved parts.
• The heater temperatures should be selected to meet thedesired heating time and overall cycle time.

Heating Time
The time required to heat a sheet to its proper formingtemperature depends on sheet thickness, surface finish,material color, heater temperature, and the type ofheaters used. Generally, excessive sheet heating leadsto color shift, discoloration, surface blistering, delamina-tion, and loss of both physical and electrical properties.For thin-gauge thermoforming, the time to form andcool the sheet against the mold surface must equalthe time to heat the sheet to the forming temperature. Cooling begins the instant the sheet is transferred fromthe oven to the forming station. It is recommended thatthe average sheet temperature drop not exceed 10°F(5°C) during this transfer. Thus, transfer time should beonly a few seconds

Heating source
SInfrared heating elements are the most commonly usedheating source. Ceramic and quartz tube heaters arereplacing older metal rod heaters, since they are farmore energy efficient and more easily controlled. Theintensity of the heating source, usually given in Watt/in2or kW/m2, is usually controlled by the power and thefraction of time the heater is on. Ovens should bedesigned to provide even heat distribution over theentire sheet surface. In certain instances, screens orheat shields can be used to shadow local areas on thesheet to aid in improving wall thickness distribution

Cooling time
The formed part should be cooled to a temperaturebelow its distortion or set temperature. Cooling timesdepend on mold temperature, mold material heat transferproperties, coolant type, part wall thickness, part design,sheet temperature and ambient temperature.

Vacuum
A good vacuum system with the capacity to quicklyevacuate the volume of the mold is essential. A goodrule of thumb is that the volume of the vacuum tankshould be at least four times the volume of the moldcavity. And the vacuum developed by the vacuum pumpshould be 28.5 inches of mercury or 35 Torr.

Mold design
Machined or cast aluminum molds are recommended forcommercial Stat-Rite thermoforming. Water coolingchannels are recommended for mold temperature uniformity and cooling cycle control. Highly polishedmolds are not needed or recommended for vacuum forming. Matte part surface is achieved by grit blastingor chemical etching. In certain instances, polyfluorocarbon-impregnated aluminum surfaces are used to allow thesheet to locally slide during forming. This can yield apart with more uniform wall thickness.Syntactic foam and epoxy are recommended for plugs.For deep draw parts, plugs should be bull-nosed andpolyfluorocarbon-coated, to provide local slip and moreuniform wall thickness.

Thermoforming tool design guidelines

Modern developments in tooling, thermoforming machines and techniques, together with improved thermoformable polymers, have made thermoforming one of the most rapidly growing polymer processing areas.

As with all processes, there are processing limitations. The following are guidelines to assist you with designingyour product for optimum strength, appearance and performance.

• Minimum draft angle should be 2° to 5° on male molds ormale portions of female molds and 1/2° to 1° on female molds. For textured mold surfaces, the draft angle should beincreased 1o per 0.2 thousands of an inch or 5 µm of texture.
• To minimize nipple height, the diameter of any vacuum hole should not exceed the local sheet thickness. For very thin sheets, alternate means of air evacuation, such as slot vents or porous plugs, should be considered to avoid nipple formation. If the rate of air evacuation is too low, the sheet will not fully form against the mold. This indicates that there are an insufficient number of vacuum holes.
• Undercuts should be avoided. If undercuts are necessary, they should be discontinuous around the periphery and should be shallow. If deep or continuous undercuts are required, breakaway portions of the mold will be needed to affect part removal without scuffing.
• Molds must be oversized to allow for polymer shrinkage. On male molds and male portions of female molds, 0.3% to0.5% shrinkage allowance is recommended. On female molds, 0.5% to 0.8% shrinkage allowance is recommended. The polymer grade, coefficient of thermal expansion, part geometry, mold temperature, initial sheet temperature, initial sheet thickness, and forming cycle all affect polymer shrinkage.• Radii on ribs and fillets should not be less than the localsheet thickness. The radii should be as much as four times the local wall thickness in areas where high loading is encountered or good stiffness is required.
• The draw ratio is given as the surface area of the formed part divided by the surface area of the sheet used to form the part. The average thickness reduction is the reciprocal of the a real draw ratio. Often, the depth-to-width ratio, viz,H:D, is used for axisymmetric parts but it is not accurate for rectangular parts since it ignores the effects of the length dimension. • In vacuum or drape forming, the depth of draw is usually limited to the narrowest width of the part, viz, H:D <1.greater>
• The best part-to-part dimensional tolerance is achieved by forming against a heated mold. However, cooling cycletimes increase with increased mold temperature.

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

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.

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.

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.

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

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.

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.

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.

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

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.

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.