Manufacturing plastic parts

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History of Plastic Injection Moulding:

Injection moulding represents the most important process for manufacturing plastic parts. It is suitable for mass producing articles, since raw material can be converted into a molding by a single procedure. In most cases finishing operations are not necessary. The injection moulding has seen steady growth since its beginnings in the late 1800's. The technique has evolved from the production of combs and buttons to major consumer, industrial, medical, and aerospace products.

In 1868, perhaps in response to a request by billiard ball maker Phelan and Collander, John Wesley Hyatt invented a way to make billiard balls by injecting celluloid into a mould. By 1872, John and his brother Isaiah Hyatt patented the injection moulding machine. The machine was primitive yet it was quite suitable for their purposes. It contained a basic plunger to inject the plastic into a mould through a heated cylinder.

Revolutionizing the plastics industry in 1946, James Hendry built the first screw injection moulding machine with an auger design to replace Hyatt's plunger. The auger is placed inside the cylinder and mixes the injection material before pushing forward and injecting the material into the mould. Today, almost all injection moulding machines use this same technique.

PROCESS:

Injection moulding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. The process is used to produce large quantities of identical plastic items. One of the most common types of thermoplastics used in injection moulding is high impact polystyrene (HIPS). It consists of two main elements, the injection molding machine and the injection mold. Injection molding machine can be break down into different parts, plasticating or injection unit, clamping unit, control unit and tempering devices for the mould. Injection moulding machines are socalled as universal machines, onto which various moulds for making parts with different geometries can be moulded, with in certain limits.

Components of injection moulding process

Injection moulding machine

Plasticating unit:

The plasticating units are to melt the plastic material and inject the molten material into the cavity of the mould. To produce the consistent moldings, identical quantities of plasticated materials, of constant quantity. In early days of plastics technology, piston-type injection moulding machines were used. the plastic material was melted only by heat conduction from the cylinder walls. Now a days screw plasticating unit is used. It works with a screw that also serves as an injection piston. The screw rotates and simultaneously takes in material from the Hooper. The rotating action of the screw causes the material to advance towards the nozzle, shearing the material, producing friction, and heating the material. The plasticated melt conveyed forward is stored in the screw chamber in front of the screw tip. It slides back until the screw chamber is filled with the melt volume necessary to fill the cavity. During screw rotation the hydraulic piston behind the screw maintains a certain pressure, to reduce the screw velocity backwards and to obtain better homogenization of the mold.

After the plastication is completed, the screw works as a piston and applying high hydraulic pressure at the hydraulic cylinder because it moves axially forward, pushing the melt from the screw chamber through the nozzle into the mould. The complete plasticating unit is mounted on top of the machine bed in such a way that it can move axially. This motion is necessary because the machine nozzle and the feed brushing of the mould are in contact with each other only during the injection and holding pressure phases. To keep the heated nozzle from heating brushing too much and to keep the cooled mold from cooling the muzzle, these parts should be kept separate as long as possible. If the nozzle cools down too much, the material will solidify upon entering and block it.

CLAMPING UNIT:

Clamping unit of a injection moulding machine has to:

  • Close the mould
  • Keep it closed tightly against the injection pressure, and
  • Open the mould for ejection of the part.

The clamping unit of an injection moulding machine is comparable to a horizontal press. It comprises:

  • A fixed support plate,
  • A movable mold carrier plate
  • A fixed mold carrier plate, and
  • A drive for moving the mold plate

one half of a two-sectioned mold is mounted to the mold carrier plate on the injection side, and the second half is attached to the mold plate on the clamping side, which can be move axially. The support plate is connected to the machine bed and is moved axially only to adjust the machine to molds of different sixes that are to be fixed to the machine. During injection the pressure inside the cavity is more than the ambient pressure and therefore tries to open the mold. To prevent such opening, the clamping unit must keep the mold closed with an adequate force. This clamping force of an injection molding machine is characteristic value used to describe the size of a machine. The clamping force of injection molding machines may range from 25 to 5000 tons.

Hydraulic clamps

Hydraulic clampingsare used on higher-tonnage injection-molding machines, typically in the range 1300 to 8900 kN (150 to 1000 tons). These units are also more flexible than toggle clamps in terms of setting the tonnage at given positions during the stroke.Hydromechanical clampsare designed for large tonnages, usually above 8900 kN (1000 tons); they operate by (1) using hydraulic cylinders to rapidly move the mold toward closing position, (2) locking the position by mechanical means, and (3) using high pressure hydraulic cylinders to finally close the mold and build tonnage.

http://www.oshore.com/products/archived/injectionMolded.html

Advantages of injection molding

  • High production rates
  • Design flexibility
  • Repeatability within tolerances
  • Can process a wide range of materials
  • Relatively low labor
  • Little to no finishing of parts
  • Minimum scrap losses

Disadvantages of injection molding

  • High initial equipment investment
  • High startup and running costs possible
  • Part must be designed for effective molding
  • Accurate cost prediction for molding job is difficult

http://www.jobwerx.com/resources/injection_molding.html

Types of molding includes

  • Injection molding
  • Compression molding
  • Transfer molding
  • Extrusion molding
  • Blow molding
  • Rotational molding
  • Thermoforming
  • Laminating
  • Rotomolding

Compression molding:

Compression moding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured.

Rotational moulding process

Rotational moulding, also known as rotocasting or rotomoulding. Rotational mouldings is a high temperature, low pressure, open moulding, plastic forming process that use heat and biaxial rotation to produce hollow, one piece parts.

http://www.globalspec.com/reference/50649/203279/Chapter-1-Introduction-to-the-Rotational-Moulding-Process

Rotational moulding is a material dependent process. This manufacturing technique could not exist without suitable plastic materials. The attributes of the process impose certain limitations on the materials that can be moulded. In this regard, the process is no different than reaction injection moulding, which is at its best with polyurethane, lay-up, spray up, and resin transfer molding cannot accommodate thermoplastic materials. The machine is capable of biaxially rotating and moving the mold through the four phases of the process. A predetermined amount of plastic material, in the form of a liquid or a powder, is then placed in the mold's cavity. The machine then simultaneously rotates the mold in two directions and moves the mold into the heating chamber. The mold becomes hot and all the plastic material adheres to and sinters onto the inside surface of the cavity.

Rotational moulding design, materials, tooling, and processing by GlennL.beall

More often than not the powdered resin is polyethylene (PE) although other compounds such as polyvinyl chloride (PVC) and nylons can also be used. The oven is preheated by convection, conduction, (or in some cases radiation) to temperature ranges around 500 °F - 700 °F (260 °C - 370 °C), depending on the polymer used. When the powder is loaded into the mould it is closed, locked, and loaded into the oven http://www.rotationalmoulding.ca/process.htm

Materials for Rotational Moulding

Heating and Fusion

Inside the oven the mould is bi-axially rotated (i.e., rotated around two axes) as the polymer melts and coats the inside of the mould. The rotation speed is slow, less than 20 rotations/minute; the process is not centrifugal. During this phase of the rotational moulding process timing is critical. If the mould spends too much time inside the oven the polymer will degrade - this will reduce its impact strength. If it spends too little time inside the oven melting of the polymer will be incomplete and it will not fully coalesce on the mould wall, creating large bubbles in the item.

Cooling the Mould

After the melting has been consolidated to the desired level and the timing is right, the mould is removed from the oven and cooled. Cooling of the mould is typically done with air (by fan), water or sometimes a combination of both. Cooling allows the polymer to solidify to the desired shape and shrink slightly so that it can then be handled by the operator and removed from the mould. The cooling time can typically be measured in tens of minutes. It is important that the cooling rate be carefully measured because rapid cooling causes the polymer to shrink too fast and warps the part.

Unloading / Demoulding

When it has cooled sufficiently to be handled and the polymer can retain its shape, the mould is opened and the part is removed. The moulding process can then be repeated by adding the polymer powder to the mould.

http://www.rotationalmoulding.ca/process.htm

Rotational molding cycle performed on a three-station indexing machine: (1) unload-load station; (2) heat and rotate mold; (3) cool the mold

IMG: http://www.oshore.com/products/archived/blowMolding.html

Advantages of rotational molding:

Unlike other molding plastic molding processes, roto-molding produces seamless parts with consistent wall thickness and more material in corners. This is in order to absorb shocks and stresses where they are most likely to occur. Roto-molding offers outstanding flexibility and precision. Complex contours such as metal inserts and flanges can be made right into the walls requiring fewer steps to produce your finished product. In addition, roto-molded plastic eliminates sharp corners and burrs and result in a smooth and seamless product. This reduces the risk, a wide spectrum of colours are available with roto-molding.

There is also no chipping because the colour is solid throughout the whole part. http://www.premierplasticsmn.com/molding.php

The ability to quickly change from one mold to another allows the production of the small quantities required for just in time.

The ability to produce two-colour or two material parts with multiple moulds. Two and three-layer hollow parts can be solid, foamed, or combinations of both.

Rotational molding is an excellent process for producing parts with moulded in inserts. Metals, plastic, rubber, and wooden inserts in lengths of one meter have been successfully moulded into one-piece composite parts.

The hollow nature of these parts, with their lack of internal cores, allows the moulded part to shrink away from the cavity and flex inward to accommodate undercuts without complicating the tooling.

The parts produced by this process are free of weld-lines, gate vestiges, and scars left by ejection mechanisms.

The moulds cavity is only thing that comes into contact with the plastic material. There is no time lost in purging from one kind of material or colour to the next. This is a distinct advantage over the melt-flow processes, which can require several hours of purging.

Rotational moulding produces a minimal amount of scrap material. There are no sprues or runners as there are in injection moulding. The finished parts are not cut out of a larger sheet of material, as is required with the thermoforming process. The pinch-off scrap that is an inherent part of extrusion blow moulding is not present.

Rotational moulding in design, materials, tooling and processing by Glenn L.Beall

Limitations

There are no cores inside the hollow parts. Surface details and dimensions can only be provided and controlled on the side of the part that is formed in contact with the cavity. The process requires the most other thermoplastic processing techniques heat and cool only the material. Heating and cooling the mold as well as the plastic results in higher energy costs and longer molding cycles.

The long heating cycles and high oven temperatures increase the possibility of thermal degradation of the plastic material being molded. Pulverizing plastic pellets into a powder adds to the materials cost. The plastic material used for the rotational molding process normally costs more than the same material processed by a technique that can use the material in a pallet form.

BLOW MOULDING

Plastic blow moulding is a process used to produce hollow component parts. It is confined to thermoplastic type resins.

Blow moulding design guide by NORMAN C. LEE

There are basically four types of blow moulding used in the production of plastic bottles, jugs and jars. These four types are

  • Extrusion blow moulding
  • Injection blow moulding
  • Stretch blow moulding
  • Reheat and blow moulding

http://www.engineershandbook.com/MfgMethods/blowmolding.htm

EXTRUSION BLOW MOULDING:

Extrusion Blow Molding is the simplest type of blow molding. A hot tube of plastic material is dropped from an extruder and captured in a water cooled mold. Once the molds are closed, air is injected through the top or the neck of the container; just as if one were blowing up a balloon. When the hot plastic material is blown up and touches the walls of the mold the material "freezes" and the container now maintains its rigid shape.

Extrusion Blow molding allows for a wide variety of container shapes, sizes and neck openings, as well as the production of handle-ware. Some extrusion machines can produce 300 to 350 bottles per hour. Extrusion blown containers can also have their gram weights adjusted through an extremely wide range, Extrusion blow molds are generally much less expensive than injection blow molds and can be produced in a much shorter period of time.

Advantages of extrusion blow molding include a high rate of production, low tooling cost, and a vast majority of machine manufactures. Some disadvantages usually include a high scrap rate, a limited control over wall thickness, and some difficulty of trimming away excess plastic.

  • Extrusion blow molding can be used to process many different plastics, including HDPE, PVC, PC, PP, and PETG.
  • Extrusion blow molding requires relatively small capital investment in equipment.
  • Extrusion blow molding is suitable for small production runs.

Extrusion blow molding: (1) extrusion of parison; (2) parison is pinches at the top and sealed at the bottom around a metal blow pin as the two halves of the mold come together; (3) the tube is inflated so that it takes the shape of the mold cavity; and (4) mold is opened to remove the solidified part.

http://www.petmachine.in/type_of_blow_moulding.htm

INJECTION BLOW MOULDING:

Injection blow molding is part injection molding and part blow molding. Injection blow molding is generally suitable for smaller containers and absolutely no handles ware. Injection blow molding is often used for containers that have close tolerance threaded necks, wide mouth openings; solid handles, and highly styled shapes. Injection blown containers usually have a set gram weight which cannot be changed unless a whole new set of blow stems are built. Generally injection blow molded container's material is distributed evenly throughout, and generally do not need any trimming or reaming. The air is injected into the plastic at a rate between 75 to 150 PSI.

Injection molding can be broken down into three stages.

  • The first stage is where the melted plastic is injected into a split steel mold cavity from the screw extruder.
  • The mold produces a preform parison which resembles a test tube with a screw finish on the top.
  • The preform is then transferred on a core rod to the second part of the injection blow molding stage. The preform is then placed inside another cold and usually aluminum blow mold cavity.
  • Air is then injected through the core rod till the preform takes the shape of the cavity.

While still on the core rod, the container is then transferred to a desired location for the third stage, where it is ejected from the machine.

Injection blow molding: (1) parison is injection molded around a blowing rod; (2) injection mold is opened and parison is transferred to a blow mold; (3) soft polymer is inflated to conform to a blow mold; and (4) blow mold is opened and blown product is removed.

http://www.petmachine.in/type_of_blow_moulding.htm

STRETCH BLOW MOULDING:

Stretch blow molding is best known for producing PET bottles commonly used for water, juice and a variety of other products. Stretch blow molding has been used since the early 1970's especially for packaging detergent, and has grown in existence with the primary use for making carbonated beverage bottles.

One of the major advantages of stretch blow molding is the ability to stretch the preform in both the hoop direction and the axial direction. This biaxial stretching of material increases the tensile strength, barrier properties, drop impact, clarity, and top load in the container. With these increases it is usually possible to reduce the overall weight in a container by 10 to 15 percent less then when producing a container in another way.

Stretch blow molding is divided into two different categories single-stage and two-stage.

Single-stage uses the extruder to inject parison into a preform mold where the plastic is rapidly cooled to form the preform. The preform is then reheated and placed in the bottle mold. Then softened parison stretches to about twice its original length. Compressed air is then blown into the stretched parison to expand to the bottles mold. Once the bottle is cooled the mold is opened and the finished bottle is emptied from the mold cavity. This technique is most effective in specialty applications, such as wide mouthed jars, where very high production rates are not a requirement.

Two-stage stretch blow molding is the same as single-stage, except the preforms are already made. The single-stage process is usually done using one machine, where the two-stage process uses preforms that have already been made and cooled. This allows companies to either make or buy their own preforms. Because of the relatively high cost of molding and RHB equipment, this is the best technique for producing high volume items such as carbonated beverage bottles. In this process, the machinery involved injection molds a preform, which is then transferred within the machine to another station where it is blown and then ejected from the machine. This type of machinery is generally called injection stretch blow molding (ISBM) and usually requires large runs to justify the very large expense for the injection molds to create the preform and then the blow molds to finish the blowing of the container. This process is used for extremely high volume runs of items such as wide mouth peanut butter jars, narrow mouth water bottles, liquor bottles etc.

REHEAT AND BLOW MOLDING PROCESS (RHB)

Thereheat and blow molding process (RHB)is a type of stretch blow process. In this process, a preform is injection molded by an outside vendor. There are a number of companies who produce these "stock" preforms on a commercial basis. Factories buy the preforms and put them into a relatively simple machine which reheats it so that it can be blown. The value of this process is primarily that the blowing company does not have to purchase the injection molding equipment to blow a particular container, so long as a preform is available from a stock preform manufacturer. This process also allows access to a large catalog of existing preforms. Therefore, the major expense is now for the blow molds, which are much less expensive than the injection molds required for preforms.

There are, however, some drawbacks to this process. If you are unable to find a stock preform which will blow the container you want, you must either purchase injection molds and have your own private mold preforms injection molded, or you will have to forego this process. For either type of stretch blow molding, handleware is not a possibility at this stage of development. The stretch blow molding process does offer the ability to produce fairly lightweight containers with very high impact resistance and, in some cases, superior chemical resistance.

http://www.engineershandbook.com/MfgMethods/blowmolding.htm

MATERIALS USED FOR INJECTION MOLDING:

Most thermoplastics can be processes by injection molding. Some of the commonly used materials are:

  • Acrylonitrile-butadiene-styrene
  • Nylon
  • Polycarbonate
  • Polypropylene
  • Polystyrene

Characteristics of injection molding:

Injection molding is a fast process and it is used to produce large numbers of identical items from high precision engineering components to disposable consumer goods. It is suitable method for thermoplastics, thermo sets, short fiber and particulate filled polymers. It can also be used to manufacture parts from aluminum or brass. The melting points of these metals are much higher than those of plastics. It is a production method for large series and it can be characterized as follows:

  • Part shape can be very complex and it has no specific limitations.
  • Part size is not limited (it vary from small part to big parts)
  • Short cycle time
  • Very good dimensional stability
  • Very good visual quality
  • Integration of different post processing methods (coating, inserts) possible.

INJECTION MOLDING IN THERMOPLASTICS:

Injection molding is the most important molding method for thermoplastics. It is based on the ability of thermoplastic materials to be softened by heat and to harden when cooled. The process thus consists essentially of softening the material in a heated cylinder and injecting it under pressure into the mold cavity, where it hardens by cooling. Each step is carried out in a separate zone of the same apparatus in the cyclic operation.

A diagram of a typical injection-molding machine is shown in Figure PP.6. Granular material (the plastic resin) falls from the hopper into the barrel when the plunger is withdrawn. The plunger then pushes the material into the heating zone, where it is heated and softened (plasticized or plasticated). Rapid heating takes place due to spreading of the polymer into a thin film around a torpedo. The already molten polymer displaced by this new material is pushed forward through the nozzle, which is in intimate contact with the mold. The molten polymer flows through the sprue opening in the die, down the runner, past the gate, and into the mold cavity. The mold is held tightly closed by the clamping action of the press platen. The molten polymer is thus forced into all parts of the mold cavities, giving a perfect reproduction of the mold.

The material in the mold must be cooled under pressure below Tm or Tg before the mold is opened and the molded part is ejected. The plunger is then withdrawn, a fresh charge of material drops down, the mold is closed under a locking force, and the entire cycle is repeated. Mold pressures of 8,000-40,000 psi (562-2,812 kg/cm2) and cycle times as low as 15 sec are achieved on some machines.

Note that the feed mechanism of the injection molding machine is activated by the plunger stroke. The function of the torpedo in the heating zone is to spread the polymer melt into thin film in close contact with the heated cylinder walls. The fins, which keep the torpedo centered, also conduct heat from the cylinder walls to the torpedo, although in some machines the torpedo is heated separately.

Injection-molding machines are rated by their capacity to mold polystyrene in a single shot. Thus a 2- oz machine can melt and push 2 oz of general-purpose polystyrene into a mold in one shot. This capacity is determined by a number of factors such as plunger diameter, plunger travel, and heating capacity.

http://www.aipma.net/info/plasticprocess.htm#5

INJECTION MOLDING OF THERMOSETTING RESINS:

The machines used earlier were basically plunger-type machines [6,10-12]. But in the late 1960s' shortly after the development of screw-transfer machines, the concept of screw-injection molding of thermo sets, also known as direct screw transfer (or DST), was introduced. The potential of this techniques for low cost, high-volume production of molded thermo set parts was quickly recognized, and today screw injection machines are available in all clamp tonnages up to 1,200 tons and shot sizes up to 10 lb. Coupled with this, there has been a new series of thermosetting molding materials developed specifically for injection molding. These materials have long life at moderate temperature (approximately 2008F), which permits plastication in screw barrel, and react (cure) very rapidly when the temperature is raised to 3508F-4008F (1778-2048C), resulting in reduced cycle time.

A typical arrangement for a direct screw-transfer injection-molding machine for thermo sets is shown in Figure PP.16. The machine has two sections mounted on a common base. One section constitutes the plasticizing and injection unit, which includes the feed hopper, the heated barrel that en closes the screw, the hydraulic cylinder which pushes the screw forward to inject the plasticized material into the mold, and a motor to rotate the screw. The other section clamps and holds the mold halves together under pressure during the injection of the hot plastic melt into the mold.

The thermosetting material (in granular or pellet form) suitable for injection molding is fed from the hopper into the barrel and is then moved forward by the rotation of the screw. During its passage, the material receives conductive heat from the wall of the heated barrel and frictional heat from the rotation of the screw. For thermosetting materials, the screw used is a zero-compression-ratio screw i.e., the depths of flights of the screw at the feed-zone end and at the nozzle end are the same. By comparison, the screws used in thermoplastic molding machines have compression ratios such that the depth of flight at the feed end is one and one-half to five times that at the nozzle end. This difference in screw configuration is a major difference between thermoplastic- and thermosetting-molding machines.

Plastics technology handbook By Manas Chanda, Salil K. Roy

As the material moves forward in the barrel due to rotation of the screw, it changes in consistency from a solid to semi fluid, and as it starts accumulating at the nozzle end, it exerts a backward pressure on the screw. This back pressure is thus used as a processing variable. The screw stops turning when the required amount of material-the charge-has accumulated at the nozzle end of the barrel, as sensed by a limit switch. The screw is then moved forward like a plunger by hydraulic pressure (up to 20,000 psi) to force the hot plastic melt through the sprue of the mold and into the runner system, gates, and mold cavities. The nozzle temperature is controlled to maintain a proper balance between a hot mold (3508F-4008F), and a relatively cool barrel (1508F-2008F).

Molded-in inserts are commonly used with thermosetting materials. However, since the screw injection process is automatic, it is desirable to use post assembled inserts rather than molded-in inserts because molded-in inserts require that the mold be held open each cycle to place the inserts. A delay in the manual placement disrupts an automatic cyclic operation, affecting both the production rate and the product quality.

Tolerances achieved are as low as G0.001 in./in., although ordinarily tolerance of G0.003-0.005 in./in. are economically practical. Thermosetting materials used in screw-injection molding are modified from conventional thermosetting compounds. These modifications are necessary to provide the working time-temperature relationship required for screw plasticating. The most commonly used injection-molding thermosetting materials are the phenolics. Other thermosetting materials often molded by the screw-injection process include melamine, urea, polyester, alkyd, and dially phthalate (DAP).

Since the mid-1970s the injection molding of glass-fiber-reinforced thermosetting polyesters gained increasing importance as better materials (e.g., low shrinkage resins, palletized forms of polyester/glass, etc.), requirement, and tooling became available. Injection-molded reinforced thermoset plastics have thus made inroads in such markets as switch housings, fuse blocks, distributor caps, power-tool housings, office machines, etc. Bulk molding compounds (BMC), which are puttylike FRP (fibrous glass-reinforced plastic) materials, are injection molded to make substitutes of various metal die castings. For injection molding, FRP should have some specific characteristics. A traditional FRP material shrinks about 0.003 in./in. during molding, but low-shrink FRP materials used for injection molding shrink as little as 0.000-0.0005 in./in. Combined with proper tooling, these materials thus permit production of pieces with dimensional tolerances of G0.0005 in./in.

Proper design of parts for injection molding requires an understanding of the flow characteristics of material within the mold. In this respect, injection-molded parts of thermo sets are more like transfer molded parts than to compression-molded parts. Wall-section uniformity is an important consideration in part design. Cross sections should be as uniform as possible, within the dictates of part requirements, since molding cycles, and therefore costs, depend on the cure time of the thickest section. (For thermoplastics, however, it is the cooling time that is critical). A rule of thumb for estimating cycle times for a 1/4-in. wall section is 30 sec for injection-molded thermo sets (compared to 45 sec for thermoplastics). As a guideline for part design, a good working average for wall thickness is 1/8-3/16 in., with a minimum of 1/16 in.

http://www.aipma.net/info/plasticprocess.htm#5

COMMON POLYMERS ADDITIVES INCLUDES:

  • Impact modifiers
  • Fillers
  • Flame retardents
  • Mold release agents
  • Antioxidents
  • Plasticizers
  • Reinforcements
  • Heat stabilizers
  • Lubricants

PROPERTIES OF PLASTIC INJECTION MOLDING :

Now a days, there are many types of plastic materials available are nylon, derlin, acrylic, polycarbonate and polyprophene. Nylon is very strong and tough than any other plastic material. Derlin is much easier than nylon. Polycarbonate is clear, tough plastic and shatter proof. Acrylic is also clear, but brittle compared with polycarbonate. Both acrylic and polycarbonate are comparatively more expensive raw material than the plastics.

Properties of polypropylene:

Some of the properties are Semi-rigid, translucent, good chemical resistance, tough, good fatigue resistance, integral hinge property, good heat resistance.

CNC (Computerized numerical control):

Computer Numerical Control (CNC) means control by computerized numbers. CNC machining is a computer-assisted process to control general-purpose machines from instructions generated by a processor and stored in a memory system or storage media (tape, disk, chip, etc.), for present use as well as future use. Controlling machines by numerical commands has brought about a revolution in manufacturing.

http://www.thinkengineering.net/cnc-machining-technology/

The Sequence Of Part Programming:

Cnc programming is composed of the generation of a process plan from a part drawing and generation of the part program. The detailed processes are as follows:

  • To analyze the part drawing.
  • To decide on the removal volume and to select the machine
  • To decide on jig and chuck.
  • To decide on the setups, machining sequences, cut start points, cut depths for roughing and finishing allowance.
  • To select tools and tool holders and to decide on the tool position.
  • To decide on the technology data such as spindle speed, federate, and coolant on/off.
  • To generate the part program.
  • To verify the part program.
  • To machine.

Theory and Design of CNC Systems

By Suk-Hwan Suh, Seong-Kyoon Kang, Dae-Hyuk Chung, Ian Stroud

Basics of Computer Numerical Control:

The application for CNC machines vary from one machine type to another, all forms of CNC have common benefits. Now a days these machines are very sophisticated and more popular. The benefits of CNC machine :

  • The operator intervention related to producing workpieces can be reduced or eliminated.
  • It can run unattended during their entire machining cycle, freeing the operator to do others tasks.
  • To reduced operator fatigue, fewer mistakes caused by human error, consistent and predictable machining time for each workpiece.
  • To boast almost unbelievable accuracy and repeatability specifications.
  • It is more flexible and reduce the machine changing time.
  • This is imperative with just-in-time production requirements.

Functions of CNC machine:

It is automatic, precise and consistent motion control. The motion is required on most conventional machine tools, CNC machines allow motion control in a revolutionary manner. All forms of CNC equipment have two or more directions of motion, called axes. These axes can be precisely and automatically positioned along their lengths of travel. The two most common axis types are linear and rotary.

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