CHAPTER - 01

(BASIC CONCEPT OF POLYMER)

Can you live without Plastics......Think It.......🤔🤔

(BASICS OF POLYMER )

Polymer:

Poly(Many)+Mer(Unit)
Polymers are large molecules also known as macromolecule.
Those are made up of long sequences of smaller units
They occur widely in natural and synthetic polymers are a very popular manufacturing material.

Monomer: Polymers are built up by the linking together of a large number of small molecules known as monomer.

Dimer: Formed from the combination of two monomer units.

Oligomer: It is a complex molecule that is made out of a few monomer units.

Polymerization: The process by which, monomer combine to form polymers is known as polymerization

Degree of polymerization: The numbers of repeating unit known as degree of polymerization. It represent by Dp.

THERMOPLASTIC V/S THERMOSET PLASTICS

MELT PROCESSING

Melt processing is a technique used in polymer where polymers are heated until they melt and then shaped or formed into a desired shape while in their molten state. This process is commonly used with thermoplastics,

In melt processing various steps are involved-

Heating
Melting
Forming
Cooling
Solidification
Final Product.

STEPS	DESCRIPTION
Melting the Polymer	Heat the polymer above its melting point. Polymer transitions from solid to molten state.
Flow and Viscosity Control	Control viscosity for optimal flow during processing. Utilize shear thinning properties for easier flow through dies or molds.
Forming the Polymer	Extrusion, Injection Molding, Blow Molding
Cooling and Solidification	Cool the shaped polymer to solidify it. Cooling rate affects properties like strength, clarity, and crystallinity.
Post-Processing and Finishing	Trim, cut, and finish the product as needed. Apply surface treatments, Perform quality control inspections.
Recycling and Reuse	Recycle excess or scrap material by regrinding and remelting.

IMPORTANT KEY POINTS

KEYPOINTS	DESCRIPTIONS
Material Selection	Choose the right polymer based on its properties and the product's needs.
Process Control	Adjust temperature, pressure, and cooling rate for the best results.
Additives	Use additives like colorants and stabilizers to improve performance and appearance.
Mold and Die Design	Design molds and dies to achieve the desired shape and features of the product.
Quality Assurance	Ensure the final product meets the required standards and specifications.

EFFECT OF POLYMER PROPERTIES ON POLYMERS

These are various polymer properties affect processing-

Melt Flow Index (MFI): Indicates ease of flow. High MFI is good for injection molding; low MFI suits extrusion.
Viscosity: Affects flow behavior. High viscosity requires higher temperatures or pressures; low viscosity flows easily but needs faster cooling.
Melting Temperature: Determines processing temperature range. Lower melting temperatures are easier to process but may reduce heat resistance.
Thermal Stability: Impacts ability to withstand heat without degrading. Poor stability can cause discoloration and defects.
Crystallinity: Influences cooling rates and shrinkage. High crystallinity may cause more shrinkage but offers higher stiffness. formed into a desired shape.
Glass Transition Temperature (Tg): Must process above Tg for flexibility; below Tg, the polymer is rigid.
Molecular Weight: Higher weight improves strength and toughness but increases processing difficulty due to higher viscosity.
Additives and Fillers: Modify properties like stiffness and UV resistance but can alter flow and processing conditions.
Moisture Sensitivity: Moisture absorption can cause defects; drying is needed before processing.
Thermal Conductivity: Affects heating and cooling rates. Low conductivity requires longer cooling times, impacting production speed.
Compatibility with Other Polymers: Important for blending; incompatibility can lead to defects and weak products.

VARIOUS PLASTIC PROCESSING METHODS

PROCESS	CONCEPT	LIMITATIONS
Injection Molding*	Molten plastic is injected into a mold, then cooled to form the part. e.g. Bucket, Mugs	High initial mold cost Limited to smaller parts Long lead times for complex designs
Blow Molding*	Used to produce hollow parts by inflating molten plastic inside a mold e.g. Bottles,Container	Limited to hollow shapes Difficult wall thickness control Lower precision compared to injection molding
Extrusion Molding*	Continuous process where molten plastic is pushed through a die to form long shapes e.g. Pipes,Tubes	limited to continuous shapes Some materials difficult to extrude Surface finish may need post-processing
Rotational Molding* -	Powdered plastic is heated and rotated inside a mold to create large hollow objects. Hollow articles	Slow process Limited material options Thickness variation may occur
Thermoforming*	Heated plastic sheets are formed over a mold by vacuum or pressure Storage boxes, trays,	Limited to thin parts Lower precision than injection molding Material waste due to trimming
Compression & Transfer Molding*	Plastic is placed in a mold and shaped under heat and pressure.	Slow cycle time Suited for simpler shapes Less precise,may require additional finishing

* Checkout My Youtube Channel forAnimation of these processes.

INJECTION MOULDING-

Very widely used. High automation of manufacturing is standard practice.
Thermoplastic or thermoset is heated to plasticate in cylinder at controlled temperature, then forced under pressure through a nozzle into sprue, runners, gates, and cavities of mould. The resin undergoes solidification rapidly. The mould is opened, and the part ejected,
Injection Moulding is growing in the making of glass-reinforced parts.
High production runs, low labour costs, high reproducibility of complex details, and excellent surface finish are the merits.

EXTRUSION PROCESS-

Widely used for continuous production of film, sheet, tube, and other profiles; also used in conjunction with blow moulding.
Thermoplastic moulding compound is fed from a hopper to a screw pump where it is heated to plasticate then pumped out through the shaping orifice (die) to achieve desired cross section.
Production lines require input and takeoff equipment that can be complex.
Low tool cost, numerous complex profile shapes possible, very rapid production rates,
Can apply coatings or jacketing to core materials (Such as wire).

ROTATIONAL MOULDING-

A predetermined amount of powdered thermoplastic material is poured into mould; mould is closed, heated, and rotated in the axis of two planes until contents have fused to the inner walls of mould; mould is then opened and part is removed.
Low mould cost, large hollow parts in one piece can be produced, and moulded parts are essentially isotropic in nature.

CHAPTER - 02

(VARIOUS PROCESS & LIMITATIONS)

PROCESSING FUNDAMENTAL

COMPETITIVE PROCESS

PROCESS SELECTION CRITERIA

The following parameters play a key role in selecting the best suited process to produce a specific product for a specific application.

Material Processibility Limitations
Volume of Production
Size & Shape of the Product (Configuration)
Cost to performance balance
Quality

Material Processibility Limitations-

If a specific material is suitable for a particular application, then material processibility will decide the specific method of Processing / Production.

CASE-01

Product- PET bottle
Process- Blow Moulding
Specific Process- Injection stretch Blow moulding
Process Not Possible- Extrusion Blow moulding or Extrusion Stretch blow moulding
Reason- PET material used today does not have the required hot melt strength to hold/self support a parison.

CASE-02

Product- PTFE sheets
Process- Compression Moulding following by Sintering Process
Process Not Possible- Extrusion
Reason- Due to very high melt viscosity above its melting temperature

Configuration of Product-

Size & shape governs the process selection.

CASE-01

Product- Narrow neck containers (Shampoo Bottles, Pharmaceutical Containers, Soft Drink Bottles)
Process- Blow Moulding

CASE-02

Product- Hollow containers such as Tanks (20,000ltrs)
Process- Roto Moulding

CASE-03

Product- FRP Boat
Raw Material- Fiber Glass, Polyester resin and other
Process- Hand Lay Up, Spray Lay Up
Suitable Process- Hand Lay Up for Less Production & Spray Lay Up for Higher Production.

Cost / Performance Balance-

If more than one process is suitable for producing a product based on performance, then cost plays a crucial role in process selection.

CASE-01

Product- 200 litres chemical container
Raw Material- HDPE
Process- 1. Blow Moulding 2. Roto Moulding

S.No.	Parameters	Blow Molding	Roto Molding
1	Specific Process	Accumulator Blow Molding	Three Arm Roto Molding
2	Volume of Production	High	Low
3	Initial Investment on Plant & Machine	High	Moderate
4	Performance	Good	Good
5	Cost / Performance	Best	Good

CASE-02

Product- FRP pipe
Raw Material- Glass fiber, Polyester resin & other additives
Process- 1. Limited lengths & high strengths - Filament winding 2. Continuous lengths & high strength - Pultrusion 3. Centrifugal casting - Batch process

*If medium strength and relatively stress free pipes are required with better optical properties, then centrifugal casting is the best method.

CASE-03

Product- PVC Flexible sheet
Raw Material- Compounded PVC with additives such as plasticizer, stabilizer, colourants etc.
Process- 1. Calendering 2. Extrusion
Suitable Process- Calendering
Reason- Wider width, Cost effectiveness

CASE-04

Product- Acrylic sheet
Raw Material- PMMA
Process- 1. Casting 2. Extrusion

S. No.	Parameters	Casting*	Extrusion
1	Mechanical Properties	Lower	Higher
2	Optical Properties	High	Low
3	Initial Investment	Low	High
4	Stress built -up	Almost Stress Free	Stress built up due to Orientation

* Since optical properties are very essential based on cost/performance & quality casting process is best.

Quality-

Mainly due to quality reasons, inspite of higher cost of manufacture, certain products are produced by a specific process only.

CASE-01

Product- Gramaphone Records
Raw Material- PVC
Process- Compression Process
Reason- Stress Free Molding

CASE-02

Product- PP Blown film
Raw Material- PVC
Process- Ownward Extrusion Process
Reason- PP, being a crystalline polymer,in order to get transparency, the film emerging out of die lips needs to be quenched.The best /economical way of quenching is to dip in water which is possible only in downward extrusion. So due to quality reason this process is selected.

CASE-03

Product- Plastic float Valve
Raw Material- PP/HIPS
Process-1. Roto Moulding 2. Injection Moulding - Two halves and then joining by vibration welding.

S. No.	Parameters	Roto Molding	Injection Molding & Welding
1	Initial Investment	Lower	Higher
2	Quality	High	Low
3	Production Rate	Low	High
4	Low Volume Prototype	Best	-
5	High Volume Production	-	Best

CHAPTER-03

(BASICS OF INJECTION MOLDING)

Historical Background-

Early Beginnings (1872):

Injection molding was first patented by John Wesley Hyatt and his brother Isaiah in 1872.
They developed a machine to mold celluloid for making billiard balls.

Introduction of Plastics (1940s):

In the 1940s, the use of thermoplastics like polystyrene and nylon made injection molding more versatile, especially for wartime production needs.

Post-War Boom:

After World War II, injection molding technology expanded into consumer goods, thanks to cheaper plastic materials and growing demand.

Technological Advancements (1950s-1960s):

New materials and improved machinery allowed for mass production of complex plastic parts.

Modern Developments:

Today, injection molding uses computer-controlled machines and a wide range of materials, enabling high-precision and large-scale production across industries.

Overview of Injection Molding-

Injection molding is a process where molten material, usually plastic, is injected into a mold under high pressure. The material fills the mold, taking its shape. It then cools and solidifies. After cooling, the solid part is ejected from the mold. This method is ideal for mass-producing complex parts.

BASIC TECHNICAL TERMS

ADDITIVE- A substance added to a plastic compound to alter its characteristics. Examples are plasticizers, and flame retardants.
ALLOY- A combination of 2 or more plastics which form a new plastic.
AMORPHOUS- A plastic material in which the molecular structure is random and becomes mobile over a wide temperature range. .
ANNEAL-To heat a moulded part up to a temperature just below its melting point and slowly cooling it back; down to room temperature. This relieves moulded stresses.
ANISOTROPIC SHRINKAGE- Shrinkage that occurs more in one direction (usually the direction of flow; reinforced materials shrink more across the direction of flow) than another.
AUTOMATIC OPERATION- The term used to define the mode in which a moulding machine is operating when there is no need for an operator to start each cycle.
BARREL- A metallic cylinder in which the injection screw (or plunger) resides in the moulding machine. Also called CYLINDER.
BLEND- A mixture of 2 or more plastics.
BOSS- A projection of the plastic part, normally round, which is used to: strengthen an area of a part; provide a source of fastening; or to provide an alignment mechanism during assembly.
CARTRIDGE HEATERS- Pencil-shaped electrical heater devices sometimes placed in moulds to raise the temperature level of the mould. Especially beneficial when moulding high-temperature crystalline materials.
CAVITY- A depression or female portion of the mould which creates the external plastic part surface.
CHECK RING- A ring shaped component that slides back and forth over the tip end of the screw. The check ring eliminates the flow of molten material backwards over the screw during the injection process.
CLAMP FORCE- The force, in tons, that the clamp unit of a moulding machine exerts to keep the mould closed during the injection process.
CLAMP UNIT- That section of the moulding machine containing the clamping mechanism. This is used to close the mould and keep it closed against injection pressure created by the injection process. The clamp unit also contains the ejection mechanism.
COLD SLUG WELL- A depression (normally circular) in the ejection half of an injection mould, opposite the sprue, designed to receive the first front, or "cold" portion, of molten plastic during the injection process.
COMPRESSION RATIO- A factor that determines the amount of shear that is imparted to plastic material as it travels through the barrel. It is determined by dividing the depth of the screw flight in the feed section by the depth of the screw flight in the metering section.
CONDITIONING- Exposing a moulded part to a set of conditions (such as hot oil) which impart favorable characteristics to the product.
COOLING CHANNELS- Drilled holes or channels machined into various plates or components of an injection mould providing a flow path for cooling medium (such as water) in order to control the temperature of the mould.
CORE- a) An extended or male portion of the mould which creates the internal plastic part surface. b) A pin or protrusion designed to produce a hole or depression in the plastic part.
COUNTERBORE- A recessed circular area. Commonly used to fit the head of an ejector pin (return pin, sucker pin, etc.) in the ejector plate.
CRYSTALLINE- A plastic material in which the molecular structure becomes mobile only after being heated above its melting point.
CUSHION- A pad of material left in the barrel at the end of the injection stroke. It is excessive to the amount needed to fill the mould and acts as a focus point for holding pressure against the cooling melt.
CYCLE- The total amount of time required for the completion of all operations needed to produce a moulded part. Sometimes referred to as the "gate-to-gate" time, meaning the time from when an operator first closes the gate until the time the operator closes the gate again for starting the next cycle.
DECOMPRESSION- A method of relieving pressure on the melt after preparing it for injection during the upcoming cycle. This minimizes the drooling that occurs when a shutoff nozzle is not utilized.
DEFECT- An imperfection in a moulded part that results in the product not meeting original design specifications. These defects can be visual, physical, and/or hidden.
DRAFT- An angle (or taper) provided on the mould to facilitate ejection of the moulded part.
EJECTOR HALF- That half of the mould which is mounted to the moving platen of the injection machine. Sometimes called the "live" half or the "moveable" half because it moves. This half of the mould usually contains the ejection system:
EJECTOR PIN- A pin, normally circular, placed in either half of the mould (usually the ejector half) which pushes the finished moulded product, or runner system, out of a mould. Also referred to as a "knockout" pin, for obvious reasons.
FEED THROAT- The area at the rear end of the injection unit that allows fresh plastic to fall from the hopper into the heating barrel.
FEED ZONE- That area of the screw that is at the rear and receives fresh material from the feed throat.
FILLER- Specific material added to the basic plastic resin to obtain particular chemical, electrical, physical, or thermal properties.
FLASH- A thin film of plastic that tends to form at parting line areas of a mould. May also be found in vent areas and around ejector pins. Flash is caused by too great a clearance between mating metal surfaces, which allows plastic material to enter.
FLIGHT- The helical metal thread structure of the injection screw.
GATE- An opening found at the entrance of a cavity (end of the runner system) which allows material to enter.
GRANULATOR- A machine designed to grind up rejected pre-moulded plastic (products or runners). The material generated by this process is called REGRIND.
GUIDE PINS- A pin (usually circular) which normally travels in a bushing in order to provide alignment of two unattached components, such as the two halves of an injection mould. Also called LEADER PINS.
HEATING CYLINDER- That section of the injection moulding machine in which the plastic resin is heated to the proper moulding temperature prior to injection into the mould.
HEATING ZONE- An area of the heating barrel that is controlled by a temperature controller attached to a set of heater bands. There are four major zones: rear, center, front, and nozzle.
HEATER BANDS- Bracelet-shaped electrical heaters that are placed around the outside circumference of the heating barrel.
HOPPER- A funnel-shaped container mounted over the feed throat of a moulding machine. It holds fresh material to be gravity fed into the feed zone of the heating barrel. Hoppers are normally designed to hold an average of 2 hours worth of material for a given machine size.
HYDRAULIC CLAMP- A term used to describe the use a large hydraulic cylinder to open and close the clamp unit of a moulding machine.
HYGROSCOPIC- A term applied to those plastics (such as ABS and NYLON) which absorb moisture from the atmosphere.
INJECTION CAPACITY- A rating of the maximum amount of plastic material, in ounces, a machine is capable of injecting in a single stroke of the injection screw or plunger. It is based on the specific gravity of polystyrene as a standard.
INJECTION MOULDING- The process of pushing a molten plastic material into a relatively cooled mould in order to produce a finished product.
INJECTION PRESSURE- That pressure which performs the initial filling of the mould. It is supplied by the injection screw or plunger as it pushes material out of the heating barrel and into the mould.
INJECTION UNIT- That section of the moulding machine which contains the injection components, including the hopper, heating cylinder, screw (or plunger), nozzle, and heater bands.
ISOTROPIC SHRINKAGE- Shrinkage that occurs equally in all directions.
LAND- A term used to describe the area in which the gate, or vent, resides. It can also be thought of as the "length" dimension in the "L, W, H" terminology used for describing the dimensions of the gate or vent.
L/D RATIO- The result of a calculation which divides the entire length of flighted area on a screw by its nominal diameter.
MANUAL OPERATION- The term used to define the mode in which a moulding machine is operating when there is a need for an operator to start and finish each phase of the total cycle.
MELT- A term given to describe the condition of molten plastic prior to injection into a mould. A proper melt has the consistency of warm honey.
METERING ZONE- That area of the screw at the front end which contains properly melted plastic that is ready to inject.
MOULD- The term given to the entire tool (cavity, core, ejectors, etc.) needed to produce moulded parts from molten plastic material.
MONOMER- A molecular unit of an organic substance, usually in the form of a liquid or gas. See POLYMER.
MOVING PLATEN- The platen of a moulding machine that travels (opens and closes). It is connected to the clamp unit and is the mounting location for the "B", or traveling, half of the mould.
NON-RETURN VALVE - A mechanism mounted in (or at) the nozzle of the injection machine, which operates to shut off injection flow at the end of the injection cycle. This eliminates material from the upcoming shot from drooling out of the nozzle when the mould opens to eject parts from the previous shot.
NOZZLE- A device mounted at the end of the heating barrel which focuses plastic material to flow from the machine into the mould.
PARTING LINE- A plane at which two halves of a mould meet. Also applies to any other plane where two moving sections come together and form a surface of a moulded part.
PLASTIC- A complex organic compound (usually polymerized) that is capable of being shaped or formed.
PLATENS- The flat surfaces of a moulding machine onto which the two halves of the mould are mounted. One is stationary and the other travels. There is a third platen (stationary) at the clamp end of the machine which serves as an anchoring point for the clamp unit.
PLUNGER- The injecting member of a non-screw design moulding machine. Plungers do not rotate, (auger) to bring material forward in preparation for the next cycle. Nor do they blend the material as a screw does.
POLYMER- A group of long chains of monomers, bonded together in a chemical reaction to form a solid. This term is often used interchangeably with PLASTIC, but there can be a difference.
PURGING- A process of injecting unwanted plastic material from the injection cylinder into the atmosphere for the purpose of changing materials, changing colors, or removing degraded material. Also, the name given to the mass of material that is purged.
RECIPROCATING SCREW- A helical flighted, metal shaft which rotates within the heating cylinder of a moulding machine, shearing, blending, and advancing the plastic material. After rotating, the screw is pushed forward which injects the plastic into the mould.
REGRIND- Plastic material formed by granulating pre-moulded material. Regrind is virgin material that has been exposed to at least one heating cycle.
RUNNER- Grooves or channels cut into either or both halves of the injection mould to provide a path for the molten plastic material to be carried from the sprue to the gate(s) of the cavity.
SCREW SPEED- The rotating speed of the screw as it augers new material towards the metering zone. It is expressed in RPM (revolutions per minute).
SECONDARY OPERATION- Any activity performed after the moulding process required to produce a finished product suitable for its designed purpose.
SEMI-AUTOMATIC OPERATION- The term used to define the mode in which a moulding machine is operating when there is a need for an operator to start each cycle.
SHOT - A term given to the total amount of plastic material that is injected (or shot) into a mould in a single cycle.
SHUTOFF LAND- A raised area of the mould surface surrounding the cavity image. This area is usually between 0.002 and 0.003 inch high, approximately 1/2 inch wide and is used to focus clamping pressure on the mould. The use of a shutoff land reduces the amount of tonnage required to keep a mould closed against injection pressure.
SLIDE- A section of the mould which is made to travel at an angle to the normal movement of the mould. Used for providing undercuts, recesses, etc.
SPRUE- The plastic material that connects the runner system to the nozzle of the heating cylinder of the moulding machine. It is formed by the internal surface of a bushing that joins the mould to the machine's nozzle.
SPRUE BUSHING- A hardened bushing that connects the mould to the moulding machine nozzle and allows molten plastic to enter the runner system.
STATIONARY PLATEN ("A") - The platen at the injection end of the moulding machine that does not travel. It contains the "A" half of the mould and locates the mould to the nozzle of the injection unit. The moving platen travels between this platen and stationary platen "B".
STATIONARY PLATEN ("B")- The platen at the clamp end of the moulding machine that does not travel. The moving platen travels between this platen and stationary platen "A".
STRESS- A resistance to deformation from an applied force. Moulded plastic products tend to contain stresses moulded in as a result of forces applied during the injection process. These stresses may result in fractures, cracks, and breakage if they are released during use of the product.
SUPPORT PILLAR- A circular rod mould component used to support the ejector half of the mould. It is required because of the tremendous amount of pressure exerted against the "B" plate by the injection phase of the moulding process
THERMOCOUPLE- A device made of two dissimilar metals which are used to measure the temperature of a heated area such as a barrel or nozzle. It sends a signal to a controller which then turns off or on to control the temperature of that area.
THERMOPLASTIC- A plastic material which, when heated, undergoes a physical change. It can be reheated, thus reformed, over and over again.
THERMOSET- A plastic material which, when heated, undergoes a chemical change and "cures". It cannot be reformed, and reheating only degrades it.
TIE BARS- Large diameter rods that connect stationary platen "A" to stationary platen "B". The moving platen contains bushings which are used for sliding over the tie bars, allowing the moving platen to travel between the 2 stationary platens.
TOGGLE CLAMP- A term used to describe the use of a mechanical "scissors action" system to open and close the clamp unit of a moulding machine. It is operated by a relatively small hydraulic cylinder.
TRANSITION ZONE- That area in the center of the screw (between the feed zone and metering zone) This section has a tapering flight depth condition which compresses the plastic material in preparation for injection.
UNDERCUT- A recess or extension on the moulded part, located in such a way as to prevent or impede ejection of the part by normal moulding machine operation.
VENT- A shallow groove machined into the parting line surface of a mould in order to allow air and gases to escape from the cavity, or runner, as the molten plastic is filling the mould. Sometimes also located on ejector and core pins.
VENTED BARREL- A heating barrel designed with an automatic venting port which allows moisture and gases to escape from molten plastic prior to being injected into a mould.

TYPES OF INJECTION MOLDING

There are various type of Injection Moulding Machine available based on their functions and utilizations. The cost of the machines is increased depending on their functions and size.

Hand Injection Moulding
Plunger type Injection Moulding
Reciprocating Screw Type Injection Moulding

HAND INJECTION MOLDING

This type of Injection Moulding Machines are the simplest vertical machine consists of Barrel, Plunger, Band Heaters along with energy regulator, Rack & Pinion system for Injecting the material by the plunger, a torpedo and nozzle.
The clamping is done manually on a working table. The machine is fitted on the working table.
Heating is set manually.
The capacity of the machine is available from 0.5 Oz to 2 Oz.
Once heating is achieved the production starts manually.
The quality of the product is completely depend upon the skill ness of the operator.
The heating set point is achieved by heat & trial method. Although Temperature controller may be fitted on the machine, but the set point is completely depend on the quantity of product produced by the operator.
The cycle time is completely variable and it depends on the competence of the operator.
The function of the torpedo is to help the material for proper melting and create back pressure for help in mixing.

PLUNGER TYPES INJECTION MOLDING

Plunger type injection moulding machine is available horizontal or vertical type and operated pneumatically or hydraulically.
The clamping and Injection may be Semi or automatic.
In a semi Automatic type the clamping cylinder & the Injection cylinders are operated by levers which is connected the pneumatic or hydraulic cylinders.
In Automatic type the cylinders are actuated automatically to a set timers.

Vertical & Horizontal Plunger Type Injection Moulding Machine

SCREW TYPES INJECTION MOLDING

The Screw type Injection Moulding Machine consists of a hopper, a reciprocating screw and barrel assembly, and an injection nozzle.
This system confines and transports the plastic as it progresses through the feeding, compressing, degassing, melting, injection, and packing stages.

A single screw injection Moulding machine for thermoplastics, showing the plasticizing screw, a barrel, band heaters to heat the barrel, a stationary platen, and a movable platen.

Steps of Injection Process-

Plasticisers the material by reciprocating Screw.
Injects the molten material to a closed mould (via a channel system of gates and runners).
Cools the Mould.
Refills the material for the next cycle.
Ejects the Product.
Closes the Mould for further cycle

Machine Operation Sequence-

The mould closes and the screw begins moving forward for injection.
The cavity fills as the reciprocating screw moves forward, as a plunger.
The cavity is packed as the screw continuously moves forward.
The cavity cools as the gate freezes off and the screw begins to retract to plasticize material for the next shot.
The cavity is packed as the screw continuously moves forward
The cavity cools as the gate freezes off and the screw begins to retract to plasticize material for the next shot.
The mould opens for part ejection
The mould closes and the next cycle begins

Advantages of Injection Moulding-

Large volume production is possible due to high production rate.
low labour cost per unit.
Process is highly susceptible to automation.
Parts require little or no finishing.
Many different surfaces, colours, and finishes are available.
Good decoration is possible.
This process is the most economical way to fabricate many shapes.
Very small parts which are almost impossible to fabricate in quantities by other methods are also manufacture by this process.
Minimal scrap loss result as runners, gates, and rejects can be reground and reused.
Same items can be moulded in different materials, without changing the machine.
Close dimensional tolerances can be maintained.
Parts can be moulded with metallic and non-metallic inserts.
Parts can be moulded in a combination of plastic and such fillers as glass, asbestos, talc and carbon.

Limitations of Injection Moulding-

Due to competition often results in low profit margins.
Mould costs are high.
Moulding machinery and auxiliary equipment costs are high.
Lack of knowledge about the fundamentals of the process causes problems.
Lack of knowledge about the long term properties of the materials may result in long-term failures.

Aplications of Injection Moulding-

Automotive Industry: Produces car parts like dashboards, bumpers, and door panels.
Consumer Goods: Used to make everyday products like toys, kitchenware, and packaging.
Medical Devices: Creates precise components for syringes, surgical tools, and implants.
Electronics: Manufactures casings for phones, computers, and other gadgets.
Aerospace: Produces lightweight parts for aircraft interiors and structural components.
Packaging: Used for producing containers, bottles, and lids.
Construction: Makes tools, fasteners, and pipe fittings.

THE RECIPROCATING SCREW

This reciprocating screw has three zones-

Feeding Zone
Compressing or Transition Zone
Metering Zone

The Feed Zone- where the plastic first enters the screw and is conveyed along a constant root diameter;
Transition Zone- where the plastic is conveyed, compressed and melted along a root diameter that increases with a constant taper
Metering Zone- where the melting of the plastic is completed and the melt is conveyed forward along a constant root diameter reaching a temperature and viscosity to form parts.

L/D Ratio-

L/D ratio is the ratio of the flighted length (Effective Length) of the screw to its outside (major)diameter.

L/D Ratio= flighted length of the screw/ Nominal (Outer) diameter

Several injection moulding machine manufacturers now offer a choice of injection units.
Most injection screws use a 20:1 L/D ratio.
For thermoplastic- 18:1 to 24:1.
For thermoset- 12:1 to 16:1.

Effect of changing the L/D Ratio-

Larger the L/D Ratio (longer flighted length)

More shear heat can be uniformly generated in the plastic without degradation.
Greater the opportunity for mixing, resulting in a better homogeneity of the melt.
Greater the residence time of the plastic in the barrel possibly permitting faster cycles of larger shots.

Compression Ratio (CR)-

The ratio of the first feed zone channel depth to the last meter zone channel depth, or first flight depth of feed zone to last flight depth of metering zone.

CR Ratio= first flight depth of feed zone / last flight depth of metering zone

Ranges from 1.5:1 to 4.5:1 for most thermoplastic materials.
Most injection screws classified as general purpose have a compression ratio of 2.5:1 to 3.0:1.
Thermoset screws have a 1:1

Effect of CR Ratio-

Higher the CR Ratio-

Greater shear heat imparted to the resin
Greater heat uniformity of the melt
High potential for creating stresses in some resins
High energy consumption

Helix Aatio-

Plastics Processing Techniques-I (PGD-PPT-I)