Injection molding lab report PDF

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ABSTRACT Injection moulded components are consistently designed to minimize the design and manufacturing information content of the enterprise system. The resulting designs, however, are extremely complex and frequently exhibit coupling between multiple qualities attributes. Axiomatic design princip...

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ABSTRACT Injection moulded components are consistently designed to minimize the design and manufacturing information content of the enterprise system. The resulting designs, however, are extremely complex and frequently exhibit coupling between multiple qualities attributes. Axiomatic design principles were applied to the injection moulding process to add control parameters that enable the spatial and dynamic decoupling of multiple quality attributes in the moulded part. There are three major benefits of the process redesign effort. First, closed loop pressure control has enabled tight coupling between the mass and momentum equations. This tight coupling allows the direct input and controllability of the melt pressure. Second, the use of multiple melt actuators provides for the decoupling of melt pressures between different locations in the mould cavity. Such decoupling can then be used to maintain functional independence of multiple qualities attributes. Third, the heat equation has been decoupled from the mass and momentum equations. This allows the mould to be filled under isothermal conditions. Once the cavities are completely full and attain the desired packing pressure, then the cooling is allowed to progress.

INTRODUCTION Injection molding is the most commonly used manufacturing process for the fabrication of plastic parts. A wide variety of products are manufactured using injection molding, which vary greatly in their size, complexity, and application. The injection molding process requires the use of an injection molding machine, raw plastic material, and a mold. The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part. The steps in this process are described in greater detail in the next section.

Injection molding is used to produce thin-walled plastic parts for a wide variety of applications, one of the most common being plastic housings. Plastic housing is a thin-walled enclosure, often requiring many ribs and bosses on the interior. These housings are used in a variety of products including household appliances, consumer electronics, power tools, and as automotive dashboards. Other common thin-walled products include different types of open containers, such as buckets. Injection molding is also used to produce several everyday items such as toothbrushes or small plastic toys. Many medical devices, including valves and syringes, are manufactured using injection molding as well.

INJECTION MOLDING OVERVIEW Injection molding 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 mold maker (or toolmaker) from metal, usually either steel or aluminum, 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.

PROCESS CHARACTERISTICS 

Utilizes a ram or screw-type plunger to force molten plastic material into a mold cavity.

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Produces a solid or open-ended shape which has conformed to the contour of the mold.

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Uses thermoplastic or thermoset materials.

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Produces a parting line, sprue, and gate marks.

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Ejector pin marks are usually present.

HISTORY AND DEVELOPMENT 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.

PROCESS CYCLE The process cycle for injection molding is very short, typically between 2 seconds and 2 minutes, and consists of the following four stages: 1. Clamping - Prior to the injection of the material into the mold, the two halves of the mold must first be securely closed by the clamping unit. Each half of the mold is attached to the injection molding machine and one half is allowed to slide. The hydraulically powered clamping unit pushes the mold halves together and exerts sufficient force to keep the mold securely closed while the material is injected. The time required to close and clamp the mold is dependent upon the machine - larger machines (those with greater clamping forces) will require more time. This time can be estimated from the dry cycle time of the machine.

2. Injection - The raw plastic material, usually in the form of pellets, is fed into the injection molding machine, and advanced towards the mold by the injection unit. During this process, the material is melted by heat and pressure. The molten plastic is then injected into the mold very quickly and the buildup of pressure packs and holds the material. The amount of material that is injected is referred to as the shot. The injection time is difficult to calculate accurately due to the complex and changing flow of the molten plastic into the mold. However, the injection time can be estimated by the shot volume, injection pressure, and injection power.

3. Cooling - The molten plastic that is inside the mold begins to cool as soon as it makes contact with the interior mold surfaces. As the plastic cools, it will solidify into the shape of

the desired part. However, during cooling some shrinkage of the part may occur. The packing of material in the injection stage allows additional material to flow into the mold and reduce the amount of visible shrinkage. The mold cannot be opened until the required cooling time has elapsed. The cooling time can be estimated from several thermodynamic properties of the plastic and the maximum wall thickness of the part.

4. Ejection - After sufficient time has passed, the cooled part may be ejected from the mold by the ejection system, which is attached to the rear half of the mold. When the mold is opened, a mechanism is used to push the part out of the mold. Force must be applied to eject the part because during cooling the part shrinks and adheres to the mold. In order to facilitate the ejection of the part, a mold release agent can be sprayed onto the surfaces of the mold cavity prior to injection of the material. The time that is required to open the mold and eject the part can be estimated from the dry cycle time of the machine and should include time for the part to fall free of the mold. Once the part is ejected, the mold can be clamped shut for the next shot to be injected.

After the injection molding cycle, some post processing is typically required. During cooling, the material in the channels of the mold will solidify attached to the part. This excess material, along with any flash that has occurred, must be trimmed from the part, typically by using cutters. For some types of material, such as thermoplastics, the scrap material that

results from this trimming can be recycled by being placed into a plastic grinder, also called regrind machines or granulators, which regrinds the scrap material into pellets. Due to some degradation of the material properties, the regrind must be mixed with raw material in the proper regrind ratio to be reused in the injection molding process.

MACHINERY AND EQUIPMENTS ARBURG ALLROUNDER 520C 2000-800 (2007) Model : 520C2000-800 Year : 2007 Screw diameter : 45mm Shot Weight Max. (PS) : 318g Clamping force : 200-Ton Platen size : 728x728mm Distance between tie bars : 520x520mm Control : SELOGICA Working hours (approx.) : 2,100hrs

MOLD DESIGN The mold or die refers to the tooling used to produce plastic parts in molding. Traditionally injection molds have been expensive to manufacture and were only used in high-volume production applications where thousands of parts were produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy. The choice of material to build a mold from is primarily one of economics. Steel molds generally cost more to construct but offer a longer lifespan that will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear resistant and are primarilly used for lower volume requirements or larger components. The hardness of the pre-hardened steel measures typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining, making them superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminum molds cost substantially less than steel molds, and when higher grade aluminum such as QC-7 and QC-10 aircraft aluminum is used and machined with modern computerized equipment, they can be economical for molding hundreds of thousands of parts. Aluminum molds also offer quick turnaround and faster cycles because of better heat dissipation. They can also be coated for wear resistance to fiberglass reinforced materials. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated.

The mold consists of two primary components, the injection mold (A plate) and the ejector mold (B plate). Plastic resin enters the mold through a sprue in the injection mold, the sprue bushing is to seal tightly against the nozzle of the injection barrel of the molding machine and to allow molten plastic to flow from the barrel into the mold, also known as cavity. The sprue bushing directs the molten plastic to the cavity images through channels that are machined into the faces of the A and B plates. These channels allow plastic to run along them, so they are referred to as runners. The molten plastic flows through the runner and enters one or more specialized gates and into the cavity geometry to form the desired part. The amount of resin required to fill the sprue, runner and cavities of a mold is a shot. Trapped air in the mold can escape through air vents that are ground into the parting line of the mold. If the trapped air is not allowed to escape, it is compressed by the pressure of the incoming material and is squeezed into the corners of the cavity, where it prevents filling and causes other defects as well. The air can become so compressed that it ignites and burns the surrounding plastic material. To allow for removal of the molded part from the mold, the mold feature must not overhang one another in the direction that the mold opens, unless parts of the mold are designed to move from between such overhangs when the mold opens (utilizing components called Lifters).

MATERIALS There are many types of materials that may be used in the injection molding process. Most polymers may be used, including all thermoplastics, some thermosets, and some elastomers. When these materials are used in the injection molding process, their raw form is usually small pellets or a fine powder. Also, colorants may be added in the process to control the color of the final part. The selection of a material for creating injection molded parts is not solely based upon the desired characteristics of the final part. While each material has different properties that will affect the strength and function of the final part, these properties also dictate the parameters used in processing these materials. Each material requires a different set of processing parameters in the injection molding process, including the injection temperature, injection pressure, mold temperature, ejection temperature, and cycle time.

1) Polypropylene (PP) Polypropylene is a very useful plastic for injection molding and is typically available for this purpose in the form of pellets. Polypropylene is easy to mold despite its semi-crystalline nature, and it flows very well because of its low melt viscosity. This property significantly enhances the rate at which you can fill up a mold with the material. Shrinkage in polypropylene is about 1-2% but can vary based on a number of factors, including holding pressure, holding time, melt temperature, mold wall thickness, mold temperature, and the percentage and type of additives. Polypropylene is available in many grades including: glass and /or mineral filled resins; homopolymer and random copolymer visions flame retardant grades and specialty grades. The material in its natural state is translucent and can be molded

in an unlimited number of colors. Polypropylene is also available in many melt flow rates ranging from less than 1 to over 60 g/10min. As the melt flow increases, the ability to mold thin wall parts increases while the mechanical properties of polypropylene exhibit some reduction. The copolymer versions of polypropylene offer enhanced mechanical properties at low temperatures, particularly impact strength.

Property

Value

Technical Name

Polypropylene (PP)

Chemical Formula (C3H6)n

Resin Identification Code (Used For Recycling)

Melt Temperature

130°C (266°F)

Typical Injection Mold Temperature

32 - 66 °C (90 - 150 °F) ***

Heat Deflection Temperature (HDT)

100 °C (212 °F) at 0.46 MPa (66 PSI) **

Tensile Strength

32 MPa (4700 PSI) ***

Flexural Strength

41 MPa (6000 PSI) ***

Specific Gravity

0.91

Shrink Rate

1.5 - 2.0 % (.015 - .02 in/in) ***

1) Acrylonitrile Butadiene Styrene (ABS) Acrylonitrile Butadiene Styrene (ABS) is an opaque thermoplastic and amorphous polymer. “Thermoplastic” has to do with the way the material responds to heat. Thermoplastics become liquid (i.e. have a “glass transition”) at a certain temperature (221 degrees Fahrenheit in the case of ABS plastic). They can be heated to their melting point, cooled, and re-heated again without significant degradation. Instead of burning, thermoplastics like ABS liquefy which allows them to be easily injection molded and then subsequently recycled. By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy), resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials poor candidates for recycling. ABS is also an amorphous material meaning that it does not exhibit the ordered characteristics of crystalline solids. ABS is very structurally sturdy, which is why it is used in things like camera housings, protective housings, and packaging. If you need an inexpensive, strong, stiff plastic that holds up well to external impacts, ABS is a good choice.

Properties of ABS Property

Value

Technical Name

Acrylonitrile butadiene styrene (ABS)

Chemical Formula

(C8H8)x· (C4H6)y·(C3H3N)z)

Glass Transition

105 °C (221 °F) *

Typical Injection Molding Temperature 204 - 238 °C (400 - 460 °F) * Heat Deflection Temperature (HDT)

98 °C (208 °F) at 0.46 MPa (66 PSI) **

UL RTI

60 °C (140 °F) ***

Tensile Strength

46 MPa (6600 PSI) ***

Flexural Strength

74 MPa (10800 PSI) ***

Specific Gravity

1.06

Shrink Rate

0.5-0.7 % (.005-.007 in/in) ***

ADVANTAGES 

High production rates.

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Design flexibility.

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Repeatability within tolerance.

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Can process wide range of materials.

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Relatively low labour.

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Minimum scrap losses.

DISADVANTAGES High initial equipment investments. High start up and running cost possible. Parts must be designed for effective molding. Accurate cost prediction for molding job is difficult.

DEFECTS IN INJECTION MOLDING Warping The permanent bending of a part that occurs when certain section of the part shrink faster than others, as result of a non-uniform cooling rate.

Bubbles Balloon shape cavities because injection too high, too much moisture in materials and nonuniform cooling rate.

When molten material is injected into a mold, voids can occur if certain sections solidify first.

DISCUSSION There are several problems faced during the completion of the test. One of the problems faced was the wrong temperature selection for the injection of the plastic for both plastic. Initially, for ABS material the injection speed and pressure are set to half of the screw barrel temperature. However, it is noticed that the mold does not spread evenly on the 6 cavity card holder mold causing the product become incomplete. In order to counteract this problem, we set the variable to 20% of the initial setting and it eventually make the product become complete and the material spread evenly through the mold. For the polypropylene material, the setting for the ABS material is used to make the product and. However, it is noticed that the material is complete and evenly spread through the mold but the problem is that the product is attached to one another because of the excessive of material. So, the solution to solve the problem we set up the variable for the injection pressure to 25% of the ABS material and the product formed perfectly. Not only that, we use the minimum setting so that we can make a product by using the least material that we can consume. Once the test is executed, the settings for both material were then inserted into the machine for the injection process. Both material has different settings as they are not from

the same type. The table below shows the comparison between the ABS material and polypropylene (pp) material.

Table : Parameters setting of 6 cavity card holder mold Material