Plastic Manufacturing Processes

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Plastic manufacturing processes are many, and numerous parts and products are made of plastics. The plastic manufacturing processes are Injection Molding, Extrusion Blow Molding, Vacuum Forming Rotational Molding, Blown Film Extrusion, Compression Molding, Transfer Molding, Casting, Foaming, and whatever finishing or assembly process is necessary. Let Contech Int’l, in New Orleans, introduce you to top plastic manufacturing companies.

Injection Moulding of Thermoplastics

Resin material is fed to the injection barrel from the hopper. A hopper is a somewhat funnel shaped container for a bulk material; one that tapers downward and is able to discharge its contents. Hoppers may be filled manually or vacuum filled automatically and often have driers attached tot the top to remove moisture from the material.

Drag Flow
As the resin enters the injection barrel, it is driven forward by the rotation of a screw, powered by a motor. The resin plasticizes, or melts, as the turning screw drags it toward the nozzle end. Drag flow causes the polymer molecules to slide over each other creating frictional heat which melts the material.

External heating bands provide additional heat to the injection barrel. The heating bands bring the material to its final temperatureand compensate for the radiation heat loss. The temperature is controlled by thermocouples in the barrel and in the nozzle.

Injection moulding is used for very complex shapes.

Materials Suitable for Injection Molding
ABS
Nylon (PA)
Polycarbonate (PC)
Polypropylene (PP)
Polystyrene (GPPS)

 

Extrusion Blow Moulding of Thermoplastics

Blow molding is utilized for producing hollow products.

Most extruder head and die assemblies are classified as the crosshead type which diverts the melt flow 90 degrees from a horizontal to a vertical orientation. Two kinds of cross head die assemblies are most commonly used in extrusion blow molding, the center feed assembly and the side feed assembly.

Center Feed Assembly
Produces a fairly uniform melt by forcing it to flow vertically downward around the tip of a conical shaped core, called a mandril or pin, resulting in straight flow all around the mandril.

Side Feed Assembly
Melted resin is forced around the perimeter of the mandril, allowing the melt to flow uniformly down around the mandril and then extruded through the die as a parison. A mandril is a rotating spindle, and a parison is a thin tube of plastic. The die consists of a mandril and a bushing which are designed to produced a parison of the desired wall thickness. The parison is then clamped into a mold and air is pumped into it. The air pressure then pushes the plastic out to match the mold.

There are two primary types of die tooling. Converging, which flares inward, and diverging which flares outward.

Materials Suitable for Extrusion Blow Moulding
Polyetheylene Tetrephthalate (PET)
Polyvinyl Chloride (PVC)
Polypropylene (PP)
Polyetheylene (PE)
PET PVC

 

Vacuum Forming Thermoplastics

Thermoforming moulds provide a shape to which a plastic sheet conform to produce a thermoformed part. Typically thermoforming moulds have protruded or convex surfaces, and are referred to as male, or positive moulds; or concave cavity surfaces and are referred to as female, or negative moulds.

Thermoforming moulds are produced from a wide variety of material, depending on the product being produced, and the annual volume of products required. For short runs, prototypes, and one of a kind forms, inexpensive materials such as wood, epoxy, plaster, and urethane are commonly utilized. These moulds are not temperature controlled.

For high volume production runs, temperature controlled moulds produced from aluminum are always used. Aluminum is the ideal metal for thermoforming moulds because of its light weight, machinability, and high rate of heat transfer.

Materials Suitable for Vacuum Forming
Almost all thermoplastics can be formed this way.

 

Rotational Moulding

A thermoplastic powder is tumbled, heated, and cooled in a split hollow mould to produce a hollow moulding of a simple shape. A pre-measured amount of plastic powder is placed inside a two-piece mold. The mold is then rotated about two axis in an oven. The heat in the oven liquefies plastic, enabling it to coat the walls of the mold. During cooling, the rotational motion is maintained. After each cycle, the mold is stopped, opened, and the workpiece is removed.

Materials Suitable for Rotational Moulding
Polyethylene LDPE, LLDPE
Polyvinyl Chloride (PVC)
Polypropylene (PP)
Ethylene Vinyl Acatate (EVA)

 

Blown Film Extrusion

This process uses an extruder to pump moulded plastic vertically up through a die that has a 360 degree angular opening on its top. This produces a tubular film extrusion which is subsequently filled with air. As a result, the tube expands out into a bubble having a diameter larger than the diameter of the anular opening of the die. The tubular bubble cools as it is pulled up. When sufficiently cooled, the bubble is flattened within a collapsing tray. The flattened plastic is then pulled through a series of rolls that help to maintain tension on the flat plastic film as it is guided to a winder, and wound onto a cord for later use.

Materials Suitable for Blown Film Extrusion
Polythenes

 

Compression Moulding

Thermoplastic powder is compressed and heated in a matched die-set to produce a precision moulding.

In compression molding, a thermoset is introduced directly into a heated metal mold, softened by heat, and forced to conform to the shape of the mold cavity as the mold closes.

A calculated amount of thermoset plastic in powder, perform, or granular form is positioned in the heated female female mold cavity. The mold is then closed, and the part is formed by heat and pressure. After the molded part has cured, the mold is opened, and the ejection pin pushes the part out of the mold.

 

Transfer Moulding

In this process a pre-measured quantity of thermoset plastic is softened by heating and then forced into a mold cavity where it conforms to the shape of the cavity.

A measured amount of thermoset on powder, perform or granular form is placed into a heating chamber. After the mold is closed, a plunger forced the plastic through the sprue and runners into the mold cavity. After curing, the mold is opened and the part is lifted out of mold cavity using ejection rods.

 

Casting

Small objects of rather simple shape
Casting processes are characterized by the use of a liquid or power starting material that is shaped without the application of significant pressure. The absence of pressure is such an important characteristic of casting process that all process for forming processes that do not require pressure are called considered as casting processes. Therefore there are numerous different casting processes. These are :

  • Mold casting (exlained below)
  • Slush casting (this processing technique is closely related to rotational molding)
  • Static powder casting (very similar to slush molding, except the mold is filled with powder)
  • Cell casting (uses a mold that is defined by two parallel plates – used to produce sheets)
  • Continuous casting (another method for making plastic sheets. This time the resin is poured between two continuous belts separated by a gasket)
  • Film (or solvent) casting (used for producing plastic films)

Mold casting is the one most commonly associated with casting. The casting material is poured into an open mold until the mold is filled. The resin hardens to form the part. In an advanced form of mold casting (as in Figure 9), gates are used to fill the mold. Risers, which are placed at the far ends of the mold, help determine when the mold is full and ensure that the material flows to the upper portions of the mold. This process, called surface casting or gated casting, is similar to traditional metal casting in sand molds.

Materials Suitable for Casting
The most common materials used in mold casting are liquid resins. These materials do not have any evaporating solvents that would be difficult to remove from thick parts typically made by the mold casting technique.

 

Foaming

Foaming processes are characterized by techniques that cause tiny bubbles to form within the plastic solidifies the bubbles, or at least the holes created by the bubbles, remain. The solidified bubble-containing material can be thought of as a cellular structure. The products made by these processes are referred to as foams or cellular plastics.
There are several techniques used to make foams in resins. Four major methods widely utilized are,

  • Mechanical foaming (a liquid resin or resin solution is mechanically beaten or whipped to disperse air throughout the material)
  • Chemical foaming (results from formation of a gas through the breakdown of special chemicals called foaming or blowing agents)
  • Physical foaming (If a gas is forced into a liquid or molten resin and then the pressure is reduced, the gas is liberated quickly and a foam is created)
  • Hollow glass spheres (these can be mixed into a liquid or molten resin as a filler)

After the foams are created, several processing methods can be used to shape and solidify the foams into useful products. Common methods are molding, extrusion foaming, casting, and expanded foam molding.

In molding, foamed liquid materials are injected or poured directly into molds that define the shape of the product after solidification. There are two methods used to control the manner in which the foam expands within the mold. In one method, called low-pressure foam molding, a metered volume of liquid resin containing the foaming agent is introduced into the mold. The volume of this yet unfoamed or partially foamed material is much less than the volume of the mold, but it is soon allowed to expand to fill the mold.

Materials Suitable for Foaming
PU, PVC, PS, PET, PP, epoxy, phenolic, ABS, ureaformaldehydes, silicones, ionomers, and cellulose acetates

Finishing Plastic Manufacturing

After the plastics are moulded or cast to their desired shape, they require additional finishing before they are in their final use form. Common finishing operations are as follows:

  • Runner system and flash removal
  • Machining
  • Non – traditional machining (with lasers, hot-wire cutting, etc)
  • Shaping (postmold forming) (used if the molded part is at an intermediate shape)
  • Mechanical joining and assembly (by using rivets, metal screws, or designing snap joints and internal hinges)
  • Adhesive bonding (with adherents and adhesives)
  • Nonadhesive bonding (i.e. fusion bonding, ultrasonic welding, Radio frequency welding, friction welding, and induction welding)
  • Coating and decorating (painting, printing, metallizing, etc.)

 

Machining Plastics
Since frictional heat does not disipate easily through a plastic work piece, the parts surface finish may be affected if it’s allowed to reach the softening point. Extensive heat build up can also dull the cutting tool. For these reasons, the use of coolant may be required to reduce frictional heat. typical coolants used for machining plastics include clean compressed air, which aids in chip removal and limits part contamination; and the use of mist sprays, water soluable oils, light cutting oils, and other solutions for use in high speed and automatic operations. Coolant must be selected with care as some may react with certain materials.

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