Extrusion Process

The purpose of sheet extrusion is to transform thermoplastic resins, usually in pellet form, into roll or sheet stock through a combination of heat and pressure. The sheet may then be processed into shapes through thermoforming or pressure forming.

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".

Auxiliary Equipment

The proper auxiliary equipment is required to achieve the desired results in the sheet extrusion process:


Most feed to the extrusion hopper is made up of mixtures of resins, colors, and other additives. In order to provide a uniform mix in the sheet a blender is required before the hopper. This might be a batch ribbon blender, a conical blender, or in some cases, an automatic metering device which feeds multiple streams of material to the hopper in precise dosages.

Screen Changers

These are commonly used after the material exits the extruder barrel to filter the plastic melt and remove the occasional foreign particles and break up conglomerates of color. Changers are usually automatic so the process need not be shut down to change dirty screens for fresh ones.

Melt Pump

This device is used where a variety of different resin types are extruded. Each polymer would require a special screw for optimum throughput. The melt pump eliminates the need for fine tuning temperatures and reduces gauge variation in machine direction. In addition, as screws wear, their performance is affected. The melt pump overcomes these deficiencies and ensures positive uniform flow to the die.

Static Mixers

Used to improve distributive mixing in single-screw extruders, static mixers are well suited for color mixing and melt temperature homogenization of a broad range of thermoplastics. Additional benefits include achieving uniform product density, eliminating melt lines in the machine direction, and reducing the time needed to change colors. Add-on mixing units provide compounders a low cost means to increase extruder productivity, achieve higher yield from raw materials, and improve physical characteristics of the product.

Texture Rolls

When the melt leaves the die and enters the roll nip it is soft enough to take the finish of the rolls it is in contact with. Thus, textured or grain rolls will impart a mirror image of their finish to the sheet. This can give both functional and aesthetic qualities to the sheet. Though haircell, matte, and sandblast are among the most common textures, there are literally hundreds available on a custom basis.


One of the many advantages of thermoplastic sheet products is that they can be recycled and processed directly back into the sheet. The equipment that is used to prepare the plastic material for reprocessing is typically called a grinder or granulator. Since thermoplastics can be recovered and reused so easily, grinding equipment has become vital to the plastics industry.


Many resins are hygroscopic (they absorb moisture). Thus, they must be dried before being fed to the extruder. The most common dryer uses hot dehumidified air which passes through and around the resin particles in a tall cylindrical hopper.


Generally direct extrusion or roll fed thermoforming is limited to sheet under 0.100 thick. Cut sheet stock is usually cut in line at the time of extrusion to the desired dimensions for a particular forming application and molded in thicknesses from 0.030 to 0.450 inches depending on finished part design requirements.

Sheet stock can also include coextruded sheet, laminated sheet and foam core sheet. Coextruded sheet with a weatherable cap is used for outdoor applications requiring protection from UV exposure. Decorative and protective laminated films are used for such applications as luggage, office furniture and for applications needing abrasion resistance.

The following sheet properties have a significant influence on thermoformability and formed part quality:

  1. Dimensions (length, width, thickness, and flatness)
  2. Surface type and color
  3. Orientation
  4. Contamination
  5. Toughness
  6. Moisture content

Sheet Gauge

Uniformity is recommended to be +/- 1.0% or less both across the sheet and down the sheet in the extruded direction. For more critical commercial thermoforming applications gauge uniformity of 0.05% is required. This is often necessary when sheet gauge is less than 0.100 inches. It is difficult to obtain this close tolerance during extrusion, but the benefits in these cases are significant. Proper sheet gauge control as described above can translate to higher part output rates, less part-to-part thickness variations and less scrap.

Orientation (internal strain)

During extrusion, a thermoplastic resin can be stretched to cause the polymer molecules to line up more in the direction of stretch than in other directions. This is usually in the direction of extrusion and is described as uniaxial orientation. The amount of orientation can be determined by placing samples of the material in an oven at 325 degrees F for 30 minutes, between two thin sheets of aluminum dusted with talc. The amount of shrinkage represents the amount of orientation. Shrinkage of 10 to 15% in the extrusion direction is normal, though less would be more desirable. Shrinkage in the cross machine direction is usually lower, normally 5% or less. A large amount of orientation will cause differential drawing during forming. Resistance to draw is greater in the oriented direction than in unoriented direction. for sheet thicker than 0.175" high orientation (>15%) can cause sheet to pull free of the clamps during the heating process. For sheet less than 0.175" thick, high orientation (>25%) can cause the same phenomenon.


The impact resistance of a sheet is its ability to resist cracking when struck with an object. The toughness can influence the performance of the thermoformed part during assembly, shipment, or in its end use. Impact resistance is most commonly determined using a falling dart impact test (see Thermoforming Testing).

Moisture Content & Contamination

A frequent cause for rejecting thermoformed parts is moisture or contamination in the sheet stock. Hygroscopic materials absorb moisture into the sheet as well as on the surface, and this can cause surface defects during forming. For hygroscopic materials such as ABS, Styrene, and Polycarbonate, it can be helpful to wrap the extruded sheet in polyethylene film to minimize moisture absorption. This will also protect sheet from surface contamination and damage during storage and shipping. Thermoforming output rate can also be improved (and sheet moisture addressed) by preheating the sheet prior to forming.

Extruded sheet sometimes develops a static electrical charge. This condition causes the sheet to attract dust and foreign particles from the surrounding environment. Thermoformed parts made from dusty or dirty sheet will exhibit surface defects. A polyethylene cover (mask) should be used during any prolonged shutdown periods such as holidays and weekends. In many cases, contamination or moisture problems effect only the top and/or bottom sheets of a stack.

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