COMMONLY USED MANUFACTURING METHODS OF PLASTICS
COMMONLY USED MANUFACTURING METHODS OF PLASTICS
Some applications of extrusion are pipes, tubing,
rods, fibers, coatings of other products (such as wire, cable and paper ans
various profile parts that have crossectional shapes that dont have very
simple geometries.
Products of extrusion [11]
Extrusion molding is a plastic shaping process in which a continuous work-piece is produced by forcing molten thermoplastic material through a shaped die orifice. As the hot plastic work-piece is carried along a conveyor, it is cooled and cut to the desired length.
As seen in the figure below, thermoplastic granules are fed from a hopper into the heated barrel of an extruder. A rotating helical screw inside the barrel toward the die located at the end of the machine as the plastic progresses throughout the barrel. The heating jacket controls the temperature of the plastic. At the end, the plastic takes the shape of the die. If there is mandrel at the die opening where the molten plastic is forced through, the work-piece will be hollow.
Figure 2. Extrusion Process [5]
General Characteristics of Extrusion
Design Details
The figure
below shows various designs. Hollow interior sections are generally difficult
to produce, as seen on the leftmost column. The middle column point out
difficulties in extrusion with varying part thickness. The last column shows
designs that are easiest to fabricate. As can be seen, more uniform material
thickness with open sections are preferred.
Figure 3. Various product shapes [5]
Typical materials that used to produce parts using this process are acetal, acrylic, nylon, and PS. Whereas ABS and polycarbonate can also be used.
With injection molding, parts with very complex shapes
including ribs, pins, bosses, etc. can be molded very easily. Therefore its
product range is extremely wide.
An example of injection molding [13] A
reaction injection molded part [12]
This
process is similar to extrusion in the way of melting and transporting polymer
resins. However this time the molten polymer is forced into a mold cavity where
it cools and hardens to take the shape of the final product. Sometimes
automated rams are used to push the molten material into the cavity.
Figure 4. Injection Molding Process [5]
In injection molding, granular plastics are fed by gravity from a hopper into a heated barrel. As the granules move along the barrel by a screw plunger like in the figure above, the plastic is forced to a heating chamber, where it is melted. After melting, the plastic is forced through a nozzle, allowing it to enter the mold cavity through a gate and runner system. The mold is cold, therefore the most of the plastic solidifies as soon as the mold is filled.
General Characteristics of Injection
Molding
Design Details
To improve quality, some recommended considerations are given below:
· Uniform wall thickness
· Thin walls require less material and curing time
· Streamlining helps prevent gas pockets
· Gates and runners should be round, and sharp corners at runners should be avoided to prevent plastic solidify at the runner
· Contoured parts warp less than flat parts
· Venting of molds removes trapped air
· Filets
should be used at the base of ribs or bosses
Figure 5. Recommended part shapes [5]
Commonly used materials in injection molding are epoxy, nylon, polyethylene, and PS. Thermosets are also possible to shape as well as thermoplastics.
Applications
of blow molding include almost all kinds of bottles, toys, cases, containers,
drums, and housing.
A blow molded
reservoir [13]
Blow molding is utilized
for producing hollow products such as plastic milk bottles
Figure 6. Blow molding process [5]
A heated thermoplastic tube (called parison) is placed in the mold cavity, and then pressure is applied to the inside of the parison, forcing the plastic to blow and eventually take the form of the mold cavity. Parts usually have quite uniform wall thicknesses and are formed within a relatively short cycle time.
General Characteristics of Blow Molding
Design Details
The materials to be used in this process are required to have good melt strength. If the polymer melt strength is too low, the parison will rip when it is blown. The most common plastics are HDPE, LDPE, PP, PVC, PET, polycarbonate, nylon, and FEP
Applications of thermoforming are spas,
bathtubs, sinks, swimming pools, furniture and housewares, point-of-purchase
displays, amusement park rides, automotive components, signs, aircraft
components, molds, lighting fixtures, snowmobiles and ATV's, recreational
vehicle components, burial vaults, vault liners and casket components.
Various thermoformed parts [10]
In this process, a thermoplastic sheet or film is heated to become softer, and is pressed against the contours of a mold and allowed to cool until it retains the shape of the mold.
Figure 7. Thermoforming Process [5]
As the figure explains, a sheet of plastic is heated to its softening point to allow it sag to a specific contour. Then the sheet is pressed into the mold by the plug assist. A vacuum is applies under the sheet to stick the plastic tightly on the mold. The sheet is held on the mold until it cools down and then removed.
General Characteristics of Thermoforming
Design Details
Although almost every type pf thermoplastic can be used in thermoforming, the most commonly thermoformed plastics are ABS, PMMA, polyefins, HIPS, and PVC. Common plastics that are not usually thermoformed include acetals and nylons, which melt sharply and therefore are difficult to control in the thermoforming operation.
|
Lawn Mower Grass Chute |
Flow Gauge Mold |
Playground Equipment |
Children's Chaise Chair |
Industrial Lift |
Golf Caddy |
|
||||||
|
Fuel Tank |
Spill Containment |
Agri-grain Distributor |
Commercial Pool Chlorinator |
Retail Display Cooler |
Insulated Hot Water Tank |
|
||||||
|
Tilt Truck |
Tradeshow/Exhibit Display Case |
Riding Mower |
Septic Pump |
Construction Light |
Spill Containment |
|
||||||
|
Spill Containment |
Coffee Display |
Lid |
Childs Rocking Chair |
Retail Cooler |
Wall Bracket |
|
||||||
|
Kayak |
Retail Computer Display |
Baby Changing Table |
Retail Tire Display |
Industrial Tilt Truck |
Bio-filter |
|
||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Product examples for rotational molding [14]
This is a forming process in which melted plastic disperses over the inner surface of a rotating split mold, resulting in a hollow enclosed or open-ended part.
Figure 8. Rotational Molding Process [5]
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.
General Characteristics of Rotational
Molding
Design Details
A good resin for rotomolding should have the following properties:
- Thermal stability
- Excessively high fusion temperature
- Grindability to a fine powder
- Particle distribution (particle sizes should be narrow)
- Mesh size (is a measure of the size of the screen mesh through which 95% of the particles will pass)
- Pourability
- Bulk density (is a measure of the density of the powder before it is heted or compacted. Higher bulk density is better)
- Fusability (the particles must fıse together easily during the heating cycle)
The most suitable material for rotomolding is PE. However, mixed resins may also be employed.
Small objects of rather simple shape such as small radio cabinets, jewelry, and ornamental objects are commonly made by casting.
An example to plastics casting [16]
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)
Figure 9. Casting Process [5]
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.
General
Characteristics of Casting
Design Details
The design of cast parts has fewer restrictions than for most other plastic manufacturing operations. Features that would be very difficult in other processes can be done more easily with casting. These features include severe undercuts, complicated surfaces and detail, and widely varying thicknesses in the part.
Almost all resins can be used for casting, either as liquid resins, hot-melt resins, plastisols (used in slush molding), organisols, solvent based resins, or powders. Some of the commonly used resins are acrylic syrup, polyester (thermoset) resins, phenolics, epoxy resins, polyurethanes, silicones, vinyls, and hot-melt casting resins (these are fully polymerized themoplastic materials that have been liquefied by heating them above their melting point). (Plastisols and organisols are both plastic particles that are suspended in a plasticizing solvent.)
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.
Typical
products are furniture components, TV cabinets, Styrofoam cups and food
containers, insulating blocks, and shaped packaging materials (such as for
electronic appliances).
Packaging using foams [17]
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)
Figure 10. Low-pressure Foam Molding
Process [1]
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. The process of molding will be explained here. Detailed information about these processes is given in [1].
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.
General Characteristics of Foaming
Design Details
Most thermoplastic and thermoset materials can be foamed by one of the foaming processes. The most common resins that are used to make foams include PU, PVC, PS, PET, PP, epoxy, phenolic, ABS, ureaformaldehydes, silicones, ionomers, and cellulose acetates.
Typical, everyday products (primarily from the auto industry) manufactured via compression molding techniques include front and rear end automobile panels, hoods, roofs, scoops, fenders, spoilers, air deflectors, and lift gates.
Compression molded products [19]
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.
Figure 11. Compression Molding Process [5]
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.
General Characteristics of Compression
Molding
Design Details
Typical parts made by transfer molding are electrical and electric components and rubber and silicone parts.
Rubber parts [22]
Figure 12. Transfer Molding Process [5]
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.
General Characteristics of Transfer
Molding
Design Details
Practically all thermoset resins can be compression molded. The starting materials can be liquids, pastes, doughs, granules or performs. Pastes are usually made by mixing a liquid thermoset resin with a filler. Doughs are pastes or, occasionally, pastes with reinforcement added.
After the plastics are molded 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.)
