FORMING AND SHAPING OF PLASTICS:

Plastics:

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids that are moldable. Plastics are typically organic polymers of high molecular mass, but they often contain other substances. They are usually synthetic, most commonly derived from petrochemicals, but many are partially natural.

Plastics are usually classified by their chemical structure of the polymer’s backbone and side chains. Some important groups in these classifications are the acrylics, polyesters,silicones, polyurethanes, and halogenated plastics. Plastics can also be classified by the chemical process used in their synthesis, such as condensation, polyaddition, and cross-linking.

Thermoplastics and thermosetting polymers

There are two types of plastics: thermoplastics and thermosetting polymers. Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again. Examples include polyethylene, polypropylene, polystyrene and polyvinyl chloride.[6] Common thermoplastics range from 20,000 to 500,000 amu, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as repeat units, derived from monomers; each polymer chain will have several thousand repeating units.

Thermosets can melt and take shape once; after they have solidified, they stay solid. In the thermosetting process, a chemical reaction occurs that is irreversible. The vulcanization of rubber is a thermosetting process. Before heating with sulfur, the polyisoprene is a tacky, slightly runny material, but after vulcanization the product is rigid and non-tacky.

 

clip_image002

Injection Molding (Thermoplastics):

clip_image002[4]

Molding: Injection (thermoplastics)

Process:

INJECTION MOLDING of thermoplastics is the equivalent of pressure die casting of metals. Molten polymer is injected under high pressure into a cold steel mold. The polymer solidifies under pressure and the molding is then ejected.

Various types of injection molding machines exist, but the most common in use today is the reciprocating screw machine (shown schematically). Capital and tooling costs are very high.

Production rate can be high particularly for small moldings. Multi cavity molds are often used. The process is used almost exclusively for large volume production.

Prototype moldings can be made using cheaper single cavity molds of cheaper materials. Quality can be high but may be traded off against production rate. Process may also be used with thermosets and rubbers.

Some modifications are required – this is dealt with separately (see Injection Molding – thermosets). Complex shapes are possible, though some features (e.g. undercuts, screw threads, inserts) may result in increased tooling costs.

STRETCH BLOW MOLDING (SBM) is an important variant of the extrusion and injection blow molding processes. It is most commonly used as injection stretch blow molding for the production of oriented PET drinks bottles. In injection SBM a perform is injection molded ( as for injection blow molding). This is

then transferred hot to the blow mold where it is stretched longitudinally by plunger before being blow radially.

The biaxial stretching significantly improves the mechanical properties (strength and toughness) of the finished part. In extrusion SBM the cut parison is mechanically stretched longitudinally before being blown.

Capital and tooling costs are very high as is production rate. Hence process is used exclusively for high volume production.

Applications:

Injection molding is used to create many things such as wire spools, packaging, bottle caps, automotive dashboards, pocket combs, some musical instruments (and parts of them), one-piece chairs and small tables, storage containers, mechanical parts (including gears), and most other plastic products available today. Injection molding is the most common modern method of part manufacturing; it is ideal for producing high volumes of the same object.

Plunger Type Machine:

clip_image001

These machines are offered with Direct Lock and Toggle lock facilities shot capacity ranges from 30g-500g and mound area available are 6″ , 8″ and 10″. Semi auto and one cycle auto models are available in Direct lock and Semi auto and fully auto models are available in Toggle lock.

Horizontal Screw Type Plastic Injection Moulding Machine:

clip_image001[4]

These machines are available with Horizontal screw and Vertical Screw option .Both Timer based and microprocessor options are available .
Vertical Screw Machines:

These are best suited for smaller components in engineering as well as commercial applications.

Horizontal Screw Machines:

These machines are available in 50 tons and 75 tons, extended Tie bar distance, Precise moulding with operator friendly features are some of the advantages due to which customers prefer these machines. Back pressure and Core puller options are available along with Timer and Microprocessor mode option.

Imported Horizontal Screw machines come from 90 ton onwards up to 1000 ton .The Shine well brand machines which are being imported by us are Trust worthy and the service back up is provided by Texair for better reliability. Variable displacement pump and servo model are also available in these machines.

Blow molding:

Blow molding is a manufacturing process by which hollow plastic parts are formed. In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and injection stretch blow molding. The blow molding process begins with melting down the plastic and forming it into a parison or in the case of injection and injection stretch blow moulding a preform. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.

The parison is then clamped into a mold and air is blown into it. The air pressure then pushes the plastic out to match the mold. Once the plastic has cooled and hardened the mold opens up and the part is ejected.

Working principles of blow moulding:

The principle of blow molding is inputting the compressed air into the preform, to swell the preform up to the inner surface of the mold, then to shape the finish bottles. The blow mold will decide the bottle’s shape and size.
Plastic 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 blowing. 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 product.
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.

clip_image002[6]

Rotational molding

clip_image003

Fig: A three motor powered (tri-power) rotational molding or spin casting machine.

Rotational Molding involves a heated hollow mold which is filled with a charge or shot weight of material. It is then slowly rotated (usually around two perpendicular axes) causing the softened material to disperse and stick to the walls of the mold. In order to maintain even thickness throughout the part, the mold continues to rotate at all times during the heating phase and to avoid sagging or deformation also during the cooling phase. The process was applied to plastics in the 1940s but in the early years was little used because it was a slow process restricted to a small number of plastics. Over the past two decades, improvements in process control and developments with plastic powders have resulted in a significant increase in usage.

Rotocasting, by comparison, uses self-curing resins in an unheated mould, but shares slow rotational speeds in common with rotational molding. Spin casting should not be confused with either, utilizing self-curing resins or white metal in a high speed centrifugal casting machine.

Working principle of rotational moulding:

Rotational molding is a process by which powdered or liquid plastics are converted into hollow articles. This paper is devoted to the theoretical understanding of the process of rotational molding. There are seven sections:

The Art, wherein we describe the process, discuss previous attempts at understanding the process, and mention processes that are similar in principle to rotational molding.

Transient Heating of Mold Surface, wherein we show that the criterion for selection of mold materials is the ratio of the thermal diffusivity to the thermal conductivity, and present the heating curve for a mold in a rotational mold machine.

Melting of Plastic Powder in a Rotating System, wherein we discuss in detail those physical powder characteristics that are necessary for good flow within the mold cavity.

Fluid Flow During Rotational Molding, wherein we discuss the velocity profiles within the melt film, point out that there is very little bulk polymer flow possible within the mold cavity under normal processing conditions, and consider capillary flow forces and surface wetting.

Sinter-Melting, wherein we compare the Kuczynski-Neuville empirical sintering model with the Lontz viscoelastic model, conclude that the latter is correct for the sintering of materials such as ABS, and apply the Frenkel glass densification theory to the prediction of void disappearance in sinter-melt polymers.

Degradation, wherein we compare our experimental tensile strengths of polystyrene, obtained at varying oven cycle times and oven set point temperatures, with values obtained from degradation models given in the literature.

Laboratory Simulation of Rotational Molding, wherein we propose two series of experiments, the first series being carried out without using rotational molding equipment and the second using rotational molding equipment with molds having relatively simple geometries.

Application of rotational moulding:

As mentioned in other areas of our site, rotomolding lends itself very nicely to large, hollow, single wall parts and components. However, not all parts are hollow as they can be very large covers, drainage pipe for run off applications and more. Below is a list of typical applications and markets for rotationally molded plastic parts.

Tanks and containers: fuel, water, industrial, recovery

Portable Barriers

Corner Protection

Fuel Cells

Carts and carriers

Recreational Playground Equipment

Toys

Sporting Goods

Water treatment and containment

OEM components

Enclosures and housings

Consumer products

Medical Appliances

Material Handling

Industrial

Agriculture

Automotive

Defense

Cases and Lockers

clip_image002[8]

Cases and lockers are ideal products for the rotomolding process. Generally large cases, lockers, munitions cases, footlockers, industrial equipment carriers and similar items can be rotationally molded to varying thicknesses, wall densities and exact dimensions in fairly small quantities. Designing in ribs can provide structural support for larger pieces.

Lawn and Garden Products

clip_image002[10]

Lawn and garden products like organic composters, rain barrels, water conservation tanks and simulated stone speakers can all be rotationally molded in plastic to reduce weight and provide leak tight large volume containers. The demand for water conservation solutions is growing and designers are turning to rotomolders to produce colorful, low cost products that can often simulate or blend in with their natural surrounds. Rotational molding intricate designs like this rain water     collection “urn” are achieved as the rotomolds spin in 360 degrees to create seamless, hollow parts. The following are samples of custom rotomolded lawn and garden products:

Composter Tumblers

Rain Barrels

Compost Makers

Rain Collection Urns

Backboards and Spine Boards

clip_image004

Medical spine boards and backboard are excellent applications for the rotational molding process.  Most spine boards are wide and long yet need to be very strong. Designing a mold for the rotational process allows for the molding of a long, hollow part with handles and slots for accessories. Typically, rotationally molded backboards will be injected with poly foam to create an extremely light-weight board with a high level of structural integrity… often rated in excess of 400 pounds. Rotomolded boards also offer customers a wide range of color selections as well molded in graphic opportunities that allow for branding and property identification.

Cover and shields

clip_image002[12]Large-scale covers, shields and lids can be formed rotationally. These can be used to protect snowmobiles and ATVs from road abuse, fastened to trailers, roofing applications and more.

Film Insert Moulding

Film Insert Moulding (FIM) is a versatile and cost effective method of decorating and manufacturing durable plastic parts, it is an advanced form of In Mould Decoration (IMD) or In Mould Labelling (IML). The flat film is firstly reverse decorated (normally screen printed) then optionally formed, cut and finally back injection moulded. With FIM you can easily integrate components such as lens and body into a single unit using just one of our hardcoated films.
MacDermid Autotype has a range of innovative hardcoated PC, PET and PMMA film systems suitable for Film Insert Moulding (FIM), allowing you to decorate moulded components. This hardcoat technology provides enhanced scratch, solvent and chemical resistant surfaces, with systems for altering texture and gloss levels on a part surface. The product systems are suitable for a variety of different depths of form, from flat inserts to complex 3D shapes.
Ideal for consumer and medical devices, appliances and interior automotive applications.

Application of film moulding:

clip_image004[4]

 

Extrusion:

Extrusion is a process used to create objects of a fixed, cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes are its ability to create very complex cross-sections and work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms finished parts with an excellent surface finish.

Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). The extrusion process can be done with the material hot or cold.

Commonly extruded materials include metals, polymers, ceramics, concrete, play dough, and foodstuffs.

Hollow cavities within extruded material cannot be produced using a simple flat extrusion die, because there would be no way to support the center barrier of the die. Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section. The die shape then internally changes along its length into the final shape, with the suspended center pieces supported from the back of the die.

Working principles of extrusion:

For the extrusion process, the approach is typical to that, starting with the classic isothermal melt flow equation. The section on solids conveying follows that of Darnel and Mol with the addition of the energy balance for calculating temperature. The melting model discussed is primarily that of Tadmor, and again the use of flow charts indicates how the model would be used to calculate the melting. The charts again make it easy to follow the strategy of the melting model as compared to the algebraic development found in most books.
Particular attention is given to the overall energy balance of an extruder, as it is fundamental to the overall functioning of the machine. One item not found in most texts is the discussion of heat conduction in the extruder screw as part of the energy balance. Mathematical details of an energy balance are left to an appendix, where an elaborate example is used to demonstrate the phenomena.
A short chapter describing twin screw extruders is included next. This is followed by some operational methods for actual operations. Topics of residence time, control, stability, startup and shutdown, cleaning and other most practical issues are discussed. A short mathematical treatment of flow stability for a single screw is presented in an appendix.
Melt temperature development in extrusion, and the book tries to cover too many large topics, such as dies and twin screw extruders. It would have been better to eliminate twin screws and die, title the book, Single Screw Extrusion Principles and Operation, and add some details of melt temperature development.
The strong point is the overall energy balance in a single screw extruder, which includes heat conduction

in the screw. Consideration of heat conduction in the screw is rarely found in any text (if it exists at all) or in the literature.

clip_image002[14]

Compression molding

Compression molding is a forming process in which a plastic material is placed directly into a heated metal mold, then is softened by the heat, and forced to conform to the shape of the mold as the mold closes.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s