Forming and Molding Plastics


Reading Assignment

Recommended Reading

See also:  Mold Design



  • Thermoplastic sheet material is heated and then placed over a mold
  • A vacuum, pressure, or mechanical tool is applied to draw the material into the mold
  • The die can impart the dimensions and finish or texture on the final product
  • Typical products are thin-walled parts, plastic luggage, plastic trays, and panels for light fixtures

Thermoforming is one of the fastest growing methods of producing plastic packaging. Additionally, thermoforming is used to produce parts for many other industries including food, medical, appliance, signage, and automotive. Plastic thermoforming is used in both high and low volume production operations as well as for prototype applications. The principal advantages of the process are its low initial tooling cost and its low equipment costs when compared to plastic injection and plastic blow molding processes. In its basic operation, a flat thermoplastic sheet of specific size is clamped in place, heated to its softening temperature, then forced against the contours of a mold or form by either air or vacuum pressure or by mechanical means. Once cooled, the thermoformed part retains the shape of the mold or form. The part is then trimmed and separated from the remaining sheet material, which is recycled for reuse.

Thermoforming Materials

The most widely used plastic materials in thermoforming are the amorphous or non-crystalline types, with the most popular including:

  • ‘PVC’ or Polyvinyl Chloride
  • Polyethylene
  • Impact Modified Polystyrene
  • Acrylic
  • ‘ABS’ or Acrylonitrile Butadiene Styrene
  • Polycarbonate

Material stock for thermoforming is available in two different forms, cut sheet, which is primarily used for heavier gauge products requiring thicker wall sections, and thinner gauge roll-fed sheet, which is used in high-volume, light weight packaging applications. These materials often incorporate colors, anti-static agents, ultraviolet inhibitors, fire retardants, and other additives to enhance the final product use and packaging requirements. Additionally, thermoplastic sheets with multiple layers may be used. These can provide efficient barriers to oxygen and moisture and other attributes in critical packaging applications.

Material Clamping & Heating

There are two main types of clamping systems used to insure that the thermoplastic sheets are firmly held in place during the heating phase and the subsequent thermoforming operation:

  • Window style clamping frames, which consist of adjustable upper and lower sections that are hinged on one side. Precut sheets are placed between these sections, and the frame is closed securing the sheets.
  • Transport chain systems, which are used for high production roll-fed operations.

The continuous chains have teeth that pierce the edges of the rolled sheet and then drag it into position for heating and forming. Once the thermoplastic sheet is secured in the heating area, it is heated to forming temperature using either radiant electric, gas infrared, or direct contact heating systems. Forming temperature, depending upon specific material type, range from 250o to 700o Fahrenheit or 120o to 370o Celsius.

Thermoforming Molds

Typically, thermoforming molds have protruded, or convex surfaces, and are referred to as male, or positive, molds; or they have concave, cavity surfaces, and are referred to as female, or negative, molds. Molds can be further defined as being single cavity or ‘one-up’ molds for single or short run production, and multiple cavity, and or ‘family’ molds for volume production. Family molds are multiple cavity molds used to produce more than one part design simultaneously from a single sheet of plastic stock. Thermoforming molds for short run production or prototype work use molds made of wood, plastic, epoxy, or other relatively inexpensive material. These molds are not temperature controlled. High production thermoforming molds are always made of aluminum because of its lightweight, machine ability, and high thermal conductivity. Aluminum molds contain channels through which water, the primary cooling medium, is pumped. Cooling rate and temperature control affect the shrinkage and other attributes of the thermoformed part. To achieve part detail, molds must also be able to evacuate all air trapped between the plastic and mold surfaces. This is done through the use of a vacuum or providing vent holes at specific locations within the thermoforming mold.

Forming Forces & Thermoforming Processes

The force needed to form the part within the mold can be either vacuum (negative pressure) or positive air pressure, or a combination of both. The type or types of force used depends upon the material type, size and thickness, the mold material & design, product aesthetics, final product size, and the annual production volume. The most common methods of thermoforming include:

  • Drape Thermoforming, in which the plastic sheet is stretched over a positive mold. Once the plastic seals against the mold edges, a vacuum is introduced pulling the material tightly against the mold contour.
  • Cavity Thermoforming, in which a heated sheet of plastic is laid over a negative or concave mold. Once the materials seals at the mold edges, it is subjected to a vacuum pulling the material tightly into the mold.
  • Pressure Thermoforming, in which material is positioned between a pressure plate and a mold. Air pressure is then introduced through the pressure plate forcing the plastic against the mold surface. Pressure thermoforming is used for finely detailed parts and requires strongly made molds.
  • Plug Assist Thermoforming, which is similar to cavity forming but with a male plug forcing the material partially into the mold cavity. A vacuuming completes the thermoforming and is sometimes aided by positive air pressure. This thermoforming method is particularly fast and helps maintain consistent wall thicknesses.
  • Twin Sheet Thermoforming, which is used to produce hollow parts. Typically two preheated thermoplastic sheets are positioned between mold halves. These mold halves are brought into position with their respective preheated sheets, sealing their top edges. A vacuum is then applied, forming the two individual part halves. Before the thermoformed sheets cool, the mold halves are brought together welding the halves into a hollow construction. Additional air pressure may be used to complete the twin sheet thermoforming operation.


Thermoformed Parts Trimming

Once parts are thermoformed, they must be trimmed from the initial thermoplastic sheet. This may be performed in-cycle within the thermoforming system, or as a post thermoforming operation. The leftover sheet material is then typically recycled or reground for reuse in future production. Trimming methods include:

  • Hand Trimming: With hand trimming, parts are held in a jig or fixture while knives, saws, or routers are used to remove the excess material.
  • Punch & Die Sets: Punch and die sets are used to trim the entire perimeter of thermoformed parts and for high precision shaped openings that are difficult to produce using hand tools.
  • Steel Rule Dies: Steel rule dies consist of hardened, sharpened, knife-edged steel strips assembled into a form that matches the shape of the required trimming. Typically, the thermoformed product is located on the steel rule die assembly, and pressure is applied to push the part through the steel rule die, trimming the plastic.
  • Machining: Part trimming using ‘CNC’, or computer numerical control, machining is accomplished using semi-automatic and fully automatic machine tools, and robots. The cutting edges or teeth of the CNC tools must be of a special design compatible for use with plastic materials.
  • Laser Trimming: Using a high-powered laser beam, very accurate and polished finished edges are produced. The most common type of laser used is the Neodymium-Doped Yttrium-Aluminum Garnet, or ‘YAG’ solid-state laser.


Used to laminate two or more layers of sheet material.


  • Used for long plastic products with a uniform cross-section
  • Pellets or powders are fed through a hopper and then into a chamber with a large screw
  • The screw rotates and propels the material through a preheating section where it is heated, homogenized, and compressed
  • To preserve its shape, the material is cooled by jets of air or water spraying


Blown Film Extrusion (Film Blowing)

Blown film extrusion is the process by which many plastic films are made. The film blowing process basically consists of a extruding a tube of molten thermoplastic and continuously inflating it to several times initial diameter, to form a thin tubular product that can be used directly, or slit to form a flat film.

The Process
Plastic melt is extruded through an annular slit die, usually vertically, to form a thin walled tube. Air is introduced via a hole in the centre of the die to blow up the tube like a balloon. Mounted on top of the die, a high-speed air ring blows onto the hot film to cool it. The tube of film then continues upwards, continually cooling, until it passes through nip rolls where the tube is flattened to create what is known as a ‘ lay-flat’ tube of film. This lay-flat or collapsed tube is then taken back down the extrusion ‘ tower’ via more rollers. On higher output lines, the air inside the bubble is also exchanged. This is known as IBS (Internal Bubble Cooling).

The lay-flat film is then either kept as such or the edges of the lay-flat are slit off to produce two flat film sheets and wound up onto reels. If kept as lay-flat, the tube of film is made into bags by sealing across the width of film and cutting or perforating to make each bag. This is done either in line with the blown film process or at a later stage.

Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film which is drawn down along the extrusion direction only.

Polyethylenes (HDPE, LDPE and LLDPE) are the most common resins in use, but a wide variety of other materials can be used as blends with these resins or as single layers in a multi-layer film structure. these include pp, pa, evoh. In some cases, these materials do not gel together, so a multi-layer film would delaminate. To overcome this, small layers of special adhesive resins are used in between. These are known as “tie layers”.


  • Produce tubing (both flat and gussetted) in a single operation
  • Regulation of film width and thichness by control of the volume of air in the bubble, the output of the extruder and the speed of the haul-off
  • Eliminate end effects such as edge bead trim and non uniform temperature that can result from flat die film extrusion
  • capability of biaxial orientation (allowing uniformity of mechanical properties)
  • Very high productivity
  • Permits the combination of a number of different materials and properties

Blown film can be used either in tube form (e.g. for plastic bags and sacks) or the tube can be slit to form a sheet.
Typical applications include Industry packaging (e.g. shrink film, stretch film, bag film or container liners), Consumer packaging (e.g. packaging film for frozen products, shrink film for transport packaging, food wrap film, packaging bags, or form, fill and seal packaging film), Laminating film (e.g. laminating of aluminium or paper used for packaging for example milk or coffee), Barrier film (e.g. film made of raw materials such as polyamides and EVOH acting as an aroma or oxygen barrier used for packaging food, e. g. cold meats and cheese), films for the packaging of medical products, Agricultural film (e.g. greenhouse film, crop forcing film, silage film, silage stretch film).

Blow Molding

These processes represent the most popular way of producing hollow products
such as bottles, drums, and other vessels out of thermoplastic materials.
This modern industrial technology has evolved from the ancient art of glass

Process Summary

  • Thermoplastics can be converted to hollow-shape containers such as bottles
  • The preform is heated and placed between the two mold halves
  • The mold closes and the preform is expanded from air or gas pressure
  • The mold is then cooled, halves separated, and the product is removed
  • Flash, extra material, is trimmed from the part and recycled

Among the many types of resins used are:

• various densities of polyethylene
• polyethylene terephthalate polypropylene
• polyvinyl chloride
• thermoplastic elastomers
• polystyrene
• fluoropolymers, and many others

The principle process is “extrusion blow molding.” Others include injection
blow molding, biaxial stretch blow molding, and co-extrusion blow molding.
All of which utilize elements of either extrusion or injection, or both. All
of the processes share distinct production stages:

• plasticizing or the melting of resin
• parison production which refers to most blow molding operations; or preform
production when referring to biaxial stretch blow molding
• inflation and cooling phases in the mold
• ejection from the mold

A fifth stage required in extrusion blow molding involves trimming the final

Process Operation

The same blowing technique is common to all the process variations and is
accomplished through either a blow pin, needle, stuffer, or a core rod.
The process begins with applications of heat and pressure to create the
“melt.” The melt is then processed through a reciprocating screw and ram
assembly that pushes the material through a die to produce the “parison.”
This production of the parison may be continuous or intermittent and is
similar to the injection molding process. The reciprocal screw, which heats
and moves the resin, has feed, compression, and metering zones. Once the
proper amount of melt is available, a ramming action delivers the material to
the die and forms the parison. In the case where very large parisons need to
be formed, an accumulator type of machine is used. This is reservoir system,
which allows a melt delivery rate independent of the screw and ramming

In the continuous extrusion form of blow molding. The screw does not
reciprocate, but continues turning and thus continuously delivers melt to the
head and die assemblies, forming a continuous parison. Most extruder head and
die assemblies are known as the “cross head” type which divert the flow of
the resin from horizontal to vertical. Crossheads may either be center-feed
or side-feed. The center-feed design produces a uniform flow downward around
the tip of conical core or mandrel and results in a straight flow all around
the mandrel. Side-feed assemblies force the resin around the perimeter of the
mandrel and then extruded through the die as a parison with varying wall
thickness’. To control the parison’s temperature and wall thickness, a
programmer is used.

While the intermittent extrusion system is able to produce a wide range of
products, the continuous system uses several process variations that widen
the product range even further. These include the shuttle or reciprocating
blow molding system and the rotary wheel blow molding system.

The shuttle system uses multiple molds and so requires multiple parisons. To
accomplish this a manifold is used to distribute the melt to several dies at
once as the parisons arrive at the molds blow position. A cutting device
separates the required portion of the continuous parison, a blow pin or
needle is inserted in the parison and with a jet of air the product is blown
into shape. For high volume production, 20 or more split molds can be mounted
on a horizontal turntable or vertical rotary wheel for continuous molding.
Injection blow molding utilizes elements of conventional thermoplastic
injection molding. This is more economical than the extrusion process and
generally is used for large production quantities of smaller containers of
less than liter size. Basically, the systems include an injection station, a
blow station, and a strip or eject station.

Biaxial Stretch and Co-Extrusion Blow Molding

These advanced processes have developed in response to market demands and the
newer resins available. Biaxial blow molding uses polyethylene terephthalate
or PET. and is the second most widely used blow molding process and produces
a container by stretching either a preform or parison in both axial and
radial directions. This stretching increases the materials strength clarity
and barrier properties while reducing material consumption. This method can
be used with several resin types and produces the bulk of soft drink
containers. The single stage version of the process produces the container
within one machine with multiple stations. As a two-stage operation, it is
similar to the single stage method except that a preform is produced by a
injection molding machine independent of the stretch blow molding machine.

Co-extrusion blow molding produces containers with multiple wall layers.
These layers can be clear, colored, virgin, or recycled material. The
different layers are extruded together and simultaneously in the head and die
assembly before their extrusion as a parison. Products produced may have from
just two to as many as seven layers. Typical products include fuel tanks,
motor oil containers and condiment bottles. (SME FMP Video Notes)


Compression Molding

  • Solid granules or preformed tablets of unpolymerized plastic are placed into an open, heated cavity
  • A heated plunger applies pressure to the plastics, melting it and making it turn into a fluid
  • The pressure in the cavity is maintained until the material is set


Advantages of Compression Molding

  • Material only moves a short distance, not through gates and sprues
  • Low mold cost
  • Low waste

Disadvantages of Compression Molding

  • Time to heat and cure makes it slow

Transfer Molding

  • Reduces turbulence and uneven flow that occurs often in high pressure, hot-compression molding
  • The material is first heated until molten and then is forced into the cavity by a plunger
  • The temperature and pressure are maintained until the thermosetting resin has cured


Injection Molding

  • Used for high-volume production of complex thermoplastic parts
  • Granules of a raw material are fed through a hopper into a cavity that is ahead of a plunger
  • The plunger moves forward and the material is heated
  • In the torpedo section, the material is mixed, melted, and superheated
  • The fluid then flows through a nozzle that is against the mold
  • Sprues and runners are used in the same way as in metal casting


This is the most common method of producing parts made of plastic. The
process includes the injection or forcing of heated molten plastic into a
mold which is in the form of the part to be made. Upon cooling and
solidification, the part is ejected and the process continues.

The injection molding process is capable of producing an infinite variety of
part designs containing an equally infinite variety of details such as
threads, springs, and hinges, and all in a single molding operation.

Thermoplastics are used primarily in injection molding.

Advantages of Injection Molding

  • Economical for large quantities
  • Often produces finished or nearly finished part from the mold
  • Rapid production rates
  • Little waste
  • Automation possible

Types of Injection Molding Machines

  • Standard Plunger
  • Two-Stage Plunger Preplasticator
  • Screw Preplasticator
  • Reciprocating Screw

Reaction Injection Molding

  • Two or more liquid reactants are mixed under pressure
  • The mixture then flows through a pressure-reducing chamber and into a mold
  • Exothermic reaction causes the thermosets to polymerize
  • Curing times are typically less than a minute
  • Low processing temperatures and low injection pressures
  • Typical for casting large parts

 Dealing with Undercuts


Rotational Molding

  • Produces hollow, seamless products
  • Typical products are tanks, bins, refuse containers, doll parts, footballs, helmets, and boat hulls
  • A mold or cavity is filled with a specific amount of thermoplastic powder or liquid
  • The molds are then placed in an oven and rotated simultaneously about two axes
  • The resin is evenly distributed across the mold walls

Advantages of Rotational Molding

  • Uniform wall thickness
  • No waste
  • Easy color/material changes
  • No pressure
  • Inexpensive molds

Disadvantages of Rotational Molding

  • High energy consumption
  • Long cycle time


Foam Molding

  • A foaming agent is mixed with a plastic resin and releases gas when the material is heated during molding
  • The materials expand to 2 to 50 times their original size
  • Produces low density products
  • Both rigid and flexible foams can be produced
  • Rigid type is used for structural applications such as computer housings, packaging, and shipping containers
  • Flexible foams are used for cushioning


Leave a Reply

Your email address will not be published. Required fields are marked *

Scroll to Top