Molding

Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern.

A mold or mould is a hollowed-out block that is filled with a liquid like plastic, glass, metal, or ceramic raw materials. The liquid hardens or sets inside the mold, adopting its shape. A mold is the opposite of a cast. The manufacturer who makes the molds is called the moldmaker. A release agent is typically used to make removal of the hardened/set substance from the mold easier.

The terminology can depend on the application. Sand casting involves both a "pattern" (which is the positive-image model of the desired part) and a "mold" (which is the negative-image hole made by packing sand around the pattern).

Types of molding include:

1. Compaction plus sintering
2. Injection molding
3. Reaction injection molding
4. Compression molding
5. Transfer molding
6. Extrusion molding
7. Blow molding
8. Rotational molding
9. Thermoforming
10. Vacuum forming, a simplified version of thermoforming
11. Laminating
12. Expandable bead molding
13. Foam molding
14. Rotomolding
15. Vacuum plug assist molding
16. Pressure plug assist molding
17. Matched mold

INJECTION MOLDING TERMS

Air Hole : A hole in a molding by air or gas trapped in the melt during solidification.

Air Traps : Converging flow fronts surround and trap a bubble of air or gas which will cause a surface blemish in the part .

Bottom Plate: The plate fixed to the moving half of die and facilitates attachment to the injection molding machine .

Cavity : The mold or die impression that gives a molding its external shape.

Core: Male potion of the mould, which forms internal shape of the component.

Cooling: Channels located within the body of a mold through which a cooling Channels medium is circulated to control the mold surface temperature.

Cycle Time : The time required by an injection molding system to mold a part and return to its original position/state.

Draft : An angle or taper on the surfaces of a pattern or insert that facilitates removal of parts from a mold or die.

Edge gate:Entrance to the part from the runner located on the parting line.

Ejector: A mechanism that pushes the solidified molding out of the die.

Feeding: In a molding, providing plastic melt to a region undergoing solidification, usually at a rate sufficient to fill the mold cavity ahead of the solidification front and to compensate for any shrinkage accompanying solidification.

Flash: A thin section of plastic formed at the parting surface.

Temperature :The material temperature at each point as that point was filled. The result shows the changes in the temperature of the flow front during filling.

Gate: Is a channel or orifice connecting the runner with the impression.

Guide Pillars : Cylindrical members meant to align the mold halves and are & Bushes made of hardened steel. Bushes are meant for wear resistance between housing and pillars.

Hesitation : Hesitation is a surface defect that results from the stagnation of polymer melt flow over a thin-sectioned area, or an area of abrupt thickness variation.

Housing : A metal block which houses inserts (core and cavity), pillars, return pins etc.

Injection: The process of forcing molten melt under pressure into molds.

Injection Location :The injection location is the place where the molten plastic is forced into the mold cavity.

Insert : A part formed from a second material, usually a metal that is placed in the molds and appears as an integral structural part of the final molding.

Locating ring: Is a circular member fitted to top-plate of the mould. Its purpose is to register the mould in its correct position on the injection machine.

Molding: Plastic part molded to the required shape by pouring or injecting melt into a mold, as distinct from one shaped by cutting or a mechanical working process.

Mold Temperature: The temperature at which the mold is maintained. Often the most important benefit of raising mold temperature is that it allows a slower injection rate without the plastic getting too cold.

Nozzle: Hollow metal hose screwed into the extrusion end of the heating cylinder of an injection machine designed to form a seal under pressure between the cylinder and the mold.

Parting Plane : In molding, the dividing plane between mold halves.

Runner: A Channel through which melt flows from one receptacle to another.

Sprue Bush: A bush with a tapered hole, which connects nozzle with runner.

Tie Bar: A bar-shaped connection added to a casting to prevent distortion caused by uneven contraction between two separate members of a casting.

Vent : A small opening or passage that facilitates the escape of gases when the melt is fills the die cavity.

MATERIALS USED IN MOULD MANUFACTURING

1) Mild Steel (C-45)

This steel is relatively cheap. The steel is soft and cannot be fully hardened. It is extensively used to manufacture plates like top plate, bottom plate, ejector plate, core and cavity housings, spacers etc. It has high tensile strength. This steel has good shock resisting and machining properties.

2) Case hardening Steel (17MnCr95)

Initially these steels do not have sufficient carbon content essential for heat treatment process. Carbon is induced into the outer layer of the steel to convert into high carbon steels with carbon content ranging from 0.9 to 1.2%. When heat-treated achieve a hardness up to 50 – 55 HRC on the surface and retains a soft tough core. This is used for finger cam, guide pillars, guide bushes, ejector guide pillars and bushes and push back pins, as they require adequate strength as well as wear and resistance properties.

Composition:
Carbon-0.17%, Silicon-0.25%, Manganese- 1.25%, Chromium-1.15%, Sulphur-0.035%


3) Hot Die Steel (T35Cr5MoV1)

This steel has alloying element like carbon, which increases strength, elasticity and hardness. Chromium improves corrosion resistance, toughness and hardenability. Molybdenum improves creep strength and vanadium, which oxidizes and promotes fine-grained structures which is very helpful in manufacturing core and cavity of the mould. This material is hardened up to 48 – 50 HRC. This type of steel is used for core and cavity inserts, core and cavity sub inserts and side core pins.

Composition:
Carbon-0.37%, Silicon-0.25%, Molybdenum-1.2%, Vanadium- 1.0%, Chromium-5.0%

4. Oil Hardened Non Shrinkable Steel (T110W2CR5)

These steels tend to have good hardening properties and have less dimensional changes during heat treatment. They are relatively inexpensive, readily available, have good machinability, good resistance to decarburization and have a high carbon content to provide good wear resistance. The depth of hardness is homogeneous. This material is hardened up to 48 – 50 HRC. This steel is used for guide rails, wear plates, wedges, side core holder, sprue and sprue puller bushes.

Composition:
Carbon-0.95%, Silicon-0.35%, Tungsten-0.5%, Manganese- 1.20%, Chromium-0.5%

EFFECT OF ALLOYING ELEMENTS

a) Effect of Carbon:

Carbon makes the steel hard and more wear resistance. Steels having less than 0.3% carbon cannot be hardened. Beyond 3% carbon steels can be through hardened. Hardness of steel increases upon 0.8% carbon and beyond that, hardness does not increase but wear resistance increase.

b) Effect of Chromium:

Chromium increases hardenability, toughness and wear resistance. Chromium causes greater hardness penetration.

c) Effect of Nickel:

It improves toughness and wear resistance. When used with increasing hardness element like chromium it lowers hardening temperature and tends the steel towards oil hardening. Higher the percentage of nickel along with the chromium results in rust free steel – stainless steel.

d) Effect of manganese:

It helps to make the steel sound while casting as an ingot. It lowers the critical point and hence the hardenability by 1.5%. Addition of more manganese increases hardenability. The depth of penetration of hardness increases.

e) Effect of Tungsten:

It increases wear resistance when added in fairly large quantity (1.5%). 12 – 20% tungsten with chromium gives a new property names as “Red Hardness”. It unites with the carbon to form tungsten – carbide, which attains very high hardness and wear resistance.

f) Effect vanadium:

It helps in obtaining fine-grained steel. It increases red hardness property. Small percentage increases tensile strength, yield strength, hardness and wear resistance.

g) Effect of Molybdenum:

It has both properties of chromium and tungsten. Like chromium it increases hardness penetration and inclines the steel towards oil or oil hardening. Like tungsten it increase wear resistance and red hardness. 5 – 12% with chromium, tungsten and vanadium forms a molybdenum based HSS material.

SELECTION OF TOOL STEEL

The selection of tool steel is very important for any tool upon which the development and economical operation of the process depends. Care should be taken especially while injection moulding plastics, which may affect the factors like mould life, quality, cost of mould and machine etc.

The steel used in the manufacture of mould varies depending on the applications. Proper material selection and proper combination of alloys in varying percentages are required for finished moulds.

Following properties are desired from the steels used in mold making

• Characteristics permitting economical machining
• Capacity for heat treatment without problems
• Ease of polishing
• High Wear resistance
• High Thermal conductivity
• Good Corrosion resistance
• Good Toughness
  

GENERAL CONSIDERATIONS OF MOULD DESIGN

In the design of injection mold following machine specifications are to be considered. As frequently an existing machine or a certain machine size poses an important limitation, to which the design engineer has to submit.

1. Injection pressure:

This is the pressure by which thermo-plastic material is injected into the cavity.

2. Shot capacity:

The amount of melt in grams, which can be conveyed into the mold with one stroke of the screw.

3. Plasticizing rate:

This is the maximum amount of material that may be plasticized to an acceptable melt quality per unit time. It is expressed in terms of weight of polystyrene plasticized per hour.

4. Clamping force:

Clamping force is the force required to keep the mold halves together when injection takes place. Clamping force machine should be more than clamping tonnage required of the mold to prevent escape of the melt at parting surface. If the clamping force is insufficient, “flash” occurs at the mold joint or interface of the mold.

5. Distance between tie the bars:

The tool length and width should be within the distance between the tie bars to mount the tool easily without constraints. This decides whether the tool can be accommodated in the machine or not.

6. Max & Min daylight:

The minimum ‘day light’ is the distance between the mold platens when tool is in closed condition. The maximum daylight is the distance between the die platens when the tool is in open condition. The mold height is required to be larger than the minimum daylight and less than the maximum day light.

7. Ejection stroke:

The depth of the component decides the ejection stroke. Ejection stroke is indicative of the maximum ejection movement available.

BASIC TYPES OF INJECTION MOULDS

There are two main types of injection molds:

1. Cold runner.
2. Hot runner.
A runner is the channel in the mold that conveys the plastic from the barrel of the injection molding machine to the part.

1. Cold runner

In a cold runner mold, the runner is cooled and ejected with the part. Every cycle, a part and a runner are produced. The obvious disadvantage of this system is the waste plastic generated. The runners are either disposed of, or reground and reprocessed with the original material. This adds a step in the manufacturing process. Also, regrind will increase variation in the injection molding process, and could decrease the plastic's mechanical properties.
Despite these disadvantages, there are many significant advantages to using a cold runner mold. The mold design is very simple, and much cheaper than a hot runner system. The mold requires less maintenance and less skill to set up and operate. Color changes are also very easy, since all of the plastic in the mold is ejected with each cycle.

2. Hot runner

In a hot runner mold , the runner is situated internally in the mold and kept a temperature above the melting point of the plastic. Runner scrap is reduced or eliminated. The major disadvantages of a hot runner are that it is much more expensive than a cold runner, it requires costly maintenance, and requires more skill to operate. Color changes with hot runner molds can be difficult, since it is virtually impossible to remove all of the plastic from an internal runner system.
Hot runners have many advantages. They can completely eliminate runner scrap, so there are no runners to sort from the parts, and no runners to throw away or regrind and remix into the original material. Hot runners are popular in high production parts, especially with a lot of cavities.

2.8.1 INJECTION MOULDING MACHINE PARAMETERS

Injection Moulding Machines have become highly sophisticated, complex, computerized machines with many features that need to be considered when deciding which machine will be able to make a particular product.

In the design of injection mould, the following machine specifications are to be considered.

1. Injection pressure

This is the pressure in the barrel at the point of injection, expressed in kg/cm2 or kbar. A higher L/D ratio produces greater Injection Pressure. The greater the Injection Pressure the better the quality of product produced.

2. Shot weight

The shot weight is the weight of the component including the feed system. Calculated shot weight should be less than the machine shot weight.

The shot weight of a machine does not tell us the maximum volume of part that that machine can produce, because the pressures for moulding are higher than for extruding. Typically, the maximum volume of a low quality part will be 85% of the Shot Weight, or around 75% for a high quality part.

Parts need to weigh between 35% and 85% of the Shot Weight of a machine, any lower and the machine may be damaged during production, any higher and the mould cavity or cavities will not be filled.

3. Clamp force

The melt is injected into the mould under very great pressure, which the mould halves, and any other components or cores, must be able to resist in order to maintain their shape and to avoid any seepage. The larger a component is, the greater the pressure and so, the greater the clamp force.

The clamping force required to keep the mould closed during injection must exceed the force given by the product of the opening pressure in the cavity and the total projected area of all impressions and runners.

4. Distance between Tie bars

It is the minimum distance between the edges of the tie bars that guides the moving platens. This measurement limits the size of moulds that can be placed between the tie-bars and into the moulding machine. Tie bars are used in moulding machine to hold the platens. It also guides the moving platen during closing and opening of the mould.

5. Minimum and Maximum Mould Height

It is the minimum and maximum distance that can be adjusted between the stationary and moving platens. The mould shut height should always lie in between the maximum and minimum mould height.

6. Mould opening stroke

This is the distance that the moving mould half moves from mould closed to mould open. Because the injection moulded part has to clear the mould and have room to be removed from the machine, the Opening Stroke must be greater than:[(2*Mould Height)+Length of Sprue]

7. Ejection force

The force required to eject the component after cooling of the plastic material in the impression. It is about 3-5% of the locking force.

8. Maximum Ejection Stroke

This is the maximum distance through which the component can be ejected during the open condition. The ejection stroke to be adjusted on the machine depends on the component depth.

CLASSIFICATION OF INJECTION MOULDING MACHINE

Injection Moulding machines are broadly classified into two types,
1) Plunger type Injection Moulding Machine.
2) Screw type Injection Moulding Machine


1) Plunger type Injection Moulding Machine

The earliest moulding machines were of plunger type as illustrated in fig and still many of these types of machines are used. In this type of injection moulding machine the resin is fed from hopper into the barrel and heated through the input of thermal energy from heaters around the barrel. The molten resin collects in a pool in the barrel called the injection chamber.
The molten resin is then pushed forward by action of a plunger driven by a hydraulic system at the head of the machine. To facilitate the melting of any residual solid material and to give better mixing of the melt, the molten resin is pushed past a torpedo or spreader that, along with a back pressure plate, imparts shear to the melt. Then the resin flows through a nozzle into the mould.

2) Screw type Injection Moulding Machine

Figure shows a schematic drawing of the injection end of a reciprocating screw machine. The extruder screw, which is contained in the barrel, is turned most often by a hydraulic motor (as contrasted to an electric motor attached to a gear system) point, at which time the screw stops. The rotary shutoff valve is rotated so that when the injection plunger advances, the material is injected into the mold. The main advantages of a two-stage screw are that the material passes over the whole As the screw turns, it picks up material from the hopper. As it progresses down the screw, the resin is compacted, degassed, melted, and pumped past the nonreturn flow valve assembly at the injection side of the screw. This, in essence, is a check valve, which allows material to flow only from the back of the screw forward.

As the material is pumped in front of the screw, it forces back the screw, hydraulic motor, and screw drive system. In so doing, it also moves the piston and rod of the hydraulic cylinder(s) used for injection. Oil from behind the piston(s) goes into a tank through a variable resistance valve, called the back pressure valve. Increasing this resistance requires higher pressures from the pumping section of the screw, and results in better mixing, a slower cycle, and greater energy consumption.

The screw will continue to turn, forcing the carriage back until a predetermined location is reached. Then the rotation is stopped, and an exact amount of melted material is in front of the screw and will be injected into the mold at the appropriate time in the cycle. This is accomplished by using the hydraulic injection cylinder(s).

POLYMERIC MATERIALS FOR MOULDING

Polymers are substances containing a large number of structural units joined by the same type of linkage. These substances often form into a chain-like structure. Polymers in the natural world have been around since the beginning of time. Starch, cellulose, and rubber all possess polymeric properties. Man-made polymers have been studied since 1832. Today, the polymer industry has grown to be larger than the aluminum, copper and steel industries combined.
Polymers already have a range of applications that far exceeds that of any other class of material available to man. Current applications extend from adhesives, coatings, foams, and packaging materials to textile and industrial fibers, composites, electronic devices, biomedical devices, optical devices, and precursors for many newly developed high-tech ceramics.

Plastics are synthetic materials called polymers, which are long-chain molecules made up of repeating units joined together. These units contain various combinations of oxygen, hydrogen, nitrogen, carbon, silicon, chlorine, fluorine, and sulfur. Although plastics are soft and moldable and approach a liquid condition during manufacture, they are solid in their finished state. As more repeating units are added, the plastic’s molecular weight increases. Addition of more repeating units to the chain makes the molecule heavier.

The mechanical and physical properties of plastics are directly related to the bonds between molecular chains, as well as to the chain length and composition. Plastic properties can also be modified both by alloying and blending with various substances and reinforcements.

CLASSIFICATION

The classification of plastics can be extensive and confusing. However, two major groups can be identified: Thermoplastics and Thermosets. In addition to the broad categories of thermoplastics and thermosets, polymers can be classified in terms of their structure, i.e., crystalline, amorphous, and liquid crystalline. Other classes of plastics commonly referred are copolymers, alloys, and elastomers. Finally, additives, reinforcements, and fillers play a major role in modifying properties.

THERMOPLASTICS

Thermoplastics are resins that repeatedly soften when heated and harden when cooled. Most thermoplastics are soluble in specific solvents and can burn to some degree. Softening temperatures vary with polymer type and grade. Because of thermoplastics’ heat sensitivity, care must be taken to avoid degrading, decomposing, or igniting the material. Nylon, acrylic, acetal, polystyrene, polyvinyl chloride, polyethylene, and cellulose acetate are just a few examples of the many rigid thermoplastic resins currently available. Also within this group are highly elastic, flexible resins known as thermoplastic elastomers (TPEs).

Most thermoplastic molecular chains can be thought of as independent, intertwined strings resembling spaghetti. When heated, the individual chains slip, causing plastic flow. When cooled, the chains of atoms and molecules are once again held firmly. When subsequently heated, the chains slip again. There are practical limitations to the number of heating/cooling cycles to which thermoplastics can be subjected before appearance and mechanical properties are affected.

THERMOSETS

Thermosets are plastics that undergo chemical change during processing to become permanently insoluble and infusible. Phenolic, amino, epoxy, and unsaturated polyester resins are typical thermoset plastics. Natural and synthetic rubbers such as latex, nitrile, millable polyurethane, silicone, butyl, and neoprene, which attain their properties through a process known as vulcanization, are also thermoset polymers. The structure of thermoset plastics is also chainlike and, prior to molding, very similar to thermoplastics.

However, cross-linking is the principal difference between thermoset and thermoplastic systems.

When thermosets are cured or hardened, crosslinks are formed between adjacent molecules, resulting in a complex, interconnected network. These cross bonds prevent the individual chains from slipping, thus preventing plastic flow when heat is added. If excessive heat is added to the thermoset resin after the cross-linking is complete, the polymer is degraded rather than melted. This behavior is somewhat similar to an egg when it is cooked; further heating does not return the egg to its liquid state, it only burns.

INJECTION MOULDING PROCESS

Injection molding is a cyclic process of forming plastic into a desired shape by forcing the material under pressure into a cavity. The shaping is achieved by cooling (thermoplastics) or by a chemical reaction (thermosets). It is one of the most common and versatile operations for mass production of complex plastics parts with excellent dimensional tolerance. It requires minimal or no finishing or assembly operations. In addition to thermoplastics and thermosets, the process is being extended to such materials as fibers, ceramics, and powdered metals, with polymers as binders.

Injection moulding of thermoplastics was patented by John and Isaiah Hyatt in 1872 to mold camphor-plasticized cellulose nitrate (celluloid). The first multicavity mold was introduced by John Hyatt in 1878. Modem technology began to develop in the late 1930s and was accelerated by the demands of World War II. A similar surge in the technology of materials and equipment took place in the late sixties and early seventies.

The injection molding process is used to turn plastic stock into finished products. The process involves many steps:
1.Feeding raw material.
2.Plasticize the raw material.
3.Fill the mould.
4.Pack the mould.
5.Hold pressure.
6.Cooling of mould.
7.Opening of mould and Part ejection.

The main factors in the injection moulding are the temperature and pressure history during the process, the orientation of flowing material and the shrinkage of the material. This means that the structure and the properties of injection moulded parts are inhomogeneous and the products have always internal stresses.

1.Feeding raw material:
This is a primary step of injection moulding process. The moulding machine operator feeds the resin in the form of pellets into the hopper of injection moulding machine either with hands (distributed feeding) or through under pressure pipes (centralized feeding). Drying and mixing of raw material can be done at the same time. where they fall into an augur-type screw channel, which feeds the pellets forward inside the heated barrel.

2.Plasticize the raw material:
The resin entered in the barrel through feed throat is plasticized by screw extruder and accumulating it in the forward section of the barrel. The heater bands surrounded on barrel maintains the melt temperature of the resin. This plasticized resin is then injected into the mould after mould closing.

3.Fill the mould:
After the mould closes, the entire screw moves forward (usually driven by a hydraulic mechanism at the drive end of the machine) and pushes the molten resin out through the end of the barrel and inject melt into the cooled mould. The air inside the mould will be pushed out through small vents at the furthest extremities of the melt flow path. This phase takes time from tenth of seconds to few seconds depending on the plastics grade, wall thickness and the shape of the part. This phase fills about 95% of the mould cavity.

4.Pack the mould:
After filling the mould with resin reduce the pressure to the pack value and maintain it for a specified time to assure the mould is full. Normally the screw will stay in the forward position until the resin begins to harden in the mould. This creates the pressure in the mould, thus ensuring that the mould fills completely.

5.Hold pressure:
The screw is held in the forward position for a set period of time, usually with a molten 'cushion' of thermoplastic material in front of the screw tip such that a 'holding' pressure may be maintained on the solidifying material within the mould, thus allowing compensating material to enter the mould as the moulded part solidifies and shrinks. Reduce and maintain the pressure at the hold value while the plastic cools. This holding pressure is only effective as long as the gate(s) remain open.

6.Cooling:
The cooling process starts immediately upon the injection of the molten polymer, but the cooling time is referred to as the time from the solidification of the gate, when the holding step is finished, to the ejection of the part, when it has reached a temperature low enough to withstand the forces during ejection.
Critical dimensions, surface finish, cycle time, etc., are all affected by mold cooling. Hence mold cooling is a decisive factor if a product will be manufactured with good quality and at a competitive cost.

7.Opening of mould and Part ejection:
When the cooling phase is complete the mould is opened and the moulding is ejected. This is usually carried out with ejector pins in the tool, which are coupled via an ejector plate to a hydraulic actuator, or by an air operated ejector valve on the face of the mould tool. Ejection is the removal of the cooled molded part from the mold cavity and from any cores or inserts. Once the moulding is clear from the mould tool, the complete moulding cycle can be repeated.

Injection moulding

Injection molding (moulding) is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity. After a product is designed, usually by an industrial designer or an engineer, molds are made by a moldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars.

Injection Moulding is the process of heating plastic granules to melting point before injecting them at high pressure through a nozzle into a mould. When the plastic cools the mould is opened and the newly formed plastic part is removed.
The process has been modified and developed in numerous ways and now there are many different types of Injection Moulding, such as:

• Injection Blow Moulding.
• Twin/Triple Injection Moulding.
• Multi-component injection moulding.
• Multi-station injection moulding.
• Reaction injection moulding.
• Gas injection moulding – and many more.
Both thermoplastics (including thermoplastic Elastomers, the Thermoplastic ‘rubber’) and thermosetting plastics are injection moulded to produce an enormous and ever increasing range of products and components.

Injection Molding is the most widely used for the formation of intricate plastic parts with excellent dimensional accuracy. This is the most commonly used method for plastic fabrication. These materials can be moulded into various shapes through the application of heat and pressure. A large number of items associated with our daily life are produced by way of injection moulding.

Process Characteristics

* Utilizes a ram or screw-type plunger to force molten plastic material into a mold cavity
* Produces a solid or open-ended shape which has conformed to the contour of the mold
* Uses thermoplastic or thermoset materials
* Produces a parting line, sprue, and gate marks
* Ejector pin marks are usually present


History

The first man-made plastic was invented in Britain in 1851 by Alexander Parkes. He publicly demonstrated it at the 1862 International Exhibition in London, calling the material he produced "Parkesine." Derived from cellulose, Parkesine could be heated, molded, and retain its shape when cooled. It was, however, expensive to produce, prone to cracking, and highly flammable.

In 1868, American inventor John Wesley Hyatt developed a plastic material he named Celluloid, improving on Parkes' invention so that it could be processed into finished form. Together with his brother Isaiah, Hyatt patented the first injection molding machine in 1872. This machine was relatively simple compared to machines in use today. It worked like a large hypodermic needle, using a plunger to inject plastic through a heated cylinder into a mold. The industry progressed slowly over the years, producing products such as collar stays, buttons, and hair combs.

The industry expanded rapidly in the 1940s because World War II created a huge demand for inexpensive, mass-produced products. In 1946, American inventor James Watson Hendry built the first screw injection machine, which allowed much more precise control over the speed of injection and the quality of articles produced. This machine also allowed material to be mixed before injection, so that colored or recycled plastic could be added to virgin material and mixed thoroughly before being injected. Today screw injection machines account for the vast majority of all injection machines. In the 1970s, Hendry went on to develop the first gas-assisted injection molding process, which permitted the production of complex, hollow articles that cooled quickly. This greatly improved design flexibility as well as the strength and finish of manufactured parts while reducing production time, cost, weight and waste.

The plastic injection molding industry has evolved over the years from producing combs and buttons to producing a vast array of products for many industries including automotive, medical, aerospace, consumer products, toys, plumbing, packaging, and construction.

Applications

Injection molding is used to create many things such as wire spools, packaging, bottle caps, automotive dashboards, pocket combs, and most other plastic products available today. Injection molding is the most common method of part manufacturing. It is ideal for producing high volumes of the same object. Some advantages of injection molding are high production rates, repeatable high tolerances, the ability to use a wide range of materials, low labour cost, minimal scrap losses, and little need to finish parts after molding. Some disadvantages of this process are expensive equipment investment, potentially high running costs, and the need to design moldable parts.

Machining

Molds are built through two main methods: standard machining and EDM. Standard Machining, in its conventional form, has historically been the method of building injection molds. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditional methods.

The electrical discharge machining (EDM) or spark erosion process has become widely used in mold making. As well as allowing the formation of shapes which are difficult to machine, the process allows pre-hardened molds to be shaped so that no heat treatment is required. Changes to a hardened mold by conventional drilling and milling normally require annealing to soften the mold, followed by heat treatment to harden it again. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered onto the mold surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mold causes spark erosion of the mold surface in the inverse shape of the electrode.

Cost

The cost of manufacturing molds depends on a very large set of factors ranging from number of cavities, size of the parts (and therefore the mold), complexity of the pieces, expected tool longevity, surface finishes and many others. The initial cost is great, however the piece part cost is low, so with greater quantities the overall price decreases.

Different types of injection molding processes:

1. Co-injection (sandwich) molding
2. Fusible (lost, soluble) core injection molding
3. Gas-assisted injection molding
4. In-mold decoration and in mold lamination
5. Injection-compression molding
6. Insert and outsert molding
7. Lamellar (microlayer) injection molding
8. Low-pressure injection molding
9. Metal injection molding
10. Microinjection molding
11. Microcellular molding
12. Multicomponent injection molding (overmolding)
13. Multiple live-feed injection molding
14. Powder injection molding
15. Push-Pull injection molding
16. Reaction injection molding
17. Resin transfer molding
18. Rheomolding
19. Structural foam injection molding
20. Structural reaction injection molding
21. Thin-wall molding
22. Vibration gas injection molding
23. Water assisted injection molding
24. Rubber injection
25. Injection molding of liquid silicone rubber