a) Determine the most common type of polymer used to produce this part and explain why such polymer is used.

The most common type of polymer used to produce this part is polycarbonate, which has high impact resistance, relatively good temperature resistance, and its also extremely flexible. From the research done, polycarbonate can be as much as 300 times stronger than average plastic. Polycarbonate is one of those strange "premium materials" because it's not always apparent by sight and touch that it is higher quality. With glass and metal, we know automatically that we're dealing with premium materials. But, polycarbonate can look and feel just like regular plastic, even though it is far stronger and more durable. Polycarbonates are also a transparent engineering thermoplastics that can maintain rigidity up to 140C and toughness down to -20C or special grades even lower. The material is amorphous (thereby displaying excellent mechanical properties and high dimensional stability), is thermally resistant up to 135C and rated as slow burning.Unlike most thermoplastics, polycarbonate can undergo large plastic deformations without cracking or breaking. As a result, it can be processed and formed at room temperature using sheet metal techniques, such as bending on a brake. Even for sharp angle bends with a tight radius, heating may not be necessary. This makes it valuable in prototyping applications where transparent or electrically non-conductive parts are needed, which cannot be made from sheet metal. As the market becomes squeezed by decreasing profit margins, the low price of polycarbonate relative to glass or metal is also a significant advantage that can't be overlooked. Many polycarbonate phone manufacturers also offer additives or optional coatings to prevent scratching or degradation from UV ray exposure. Despite the durability of polycarbonate, its a light-weight material that wont weigh down your pocket or purse.

b) Explain the sequence of operations in producing this part.

The plastic injection moulding process produces large numbers of parts of high quality with great accuracy, very quickly. Plastic material which is polycarbonate in the form of granules is melted until soft enough to be injected under pressure to fill a mould. The result is that the shape is exactly copied. Once the plastic moulding has cooled sufficiently to harden the mould opens releasing the part. The whole injection moulding process then repeats.

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. Dwelling - after the molten plastic has been injected into the mould pressure is applied to ensure all cavities are filled.

4. 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.

5. Opening - the moving platen moves away from the fixed platen (see next section) separating the mould tool.

6. 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..

c) Discuss the common defects that might be occurred for this part, the cause of the defects and how to prevent the defects from occurring.

Flash

NormalFlash

Charactaristic - Excess molding material that penetrates into mold gaps (i.e., between parting faces, slide push-out faces, and inserts, etc.) in a molten state.

Molding Machine ConditionsDie ConditionsMaterial Conditions

Causes Insufficient mold clamping pressure in the molding machine Aging of the die Insufficient mold clamping pressure Insufficient precision at the die's mating faces The molded product is arranged in a cantilever-type setup as a result of die design Plastic has low viscosity

Counter-measuresIf there is insufficient clamping pressure, estimate the correct pressure using the following equation and choose an appropriate molding machine.

Die clamping pressure = Projected surface area x die-internal effective injection pressure / 1,000 kg

In the case of ABS, a rough value for 400 kg/cm2 can be used in this calculation for the die-internal effective injection pressure Switch to a suitable molding machine. If the problem is related to mating faces, a joint examination must be carried out with the die machining company.If the problem is related to the plastic's viscosity, either lower the temperature of the plastic or switch to a grade with higher flowability.

Flow Marks The term "flow mark" is used to describe the phenomenon where a striped pattern is formed around the gates when plastic has flowed through the die.CharacteristicUneven luster in locations where the material thickness varies.

CauseImmediately after molten plastic has passed through a change in die thickness, factors such as the flow speed, cooling speed, and pressure applied to the die surface will change, and luster differs as a result of this.

Countermeasures Raise the die temperature Increase the speed of the injection flow Raise the runners and gates Review the shaped of the molded component and round off corners on steps between areas of different thickness so that smoother flow may be realized.

Warping or Twisting 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.Cause(1)Molded component shape Distribution of wall thickness: Areas with non-uniform distribution are included. Insufficient structural stiffness: The structural stiffness of ribs and the like is insufficient. Ribs can actually contribute to warping, and therefore, a detailed examination of thickness and height factors must be undertaken.

Cause(2)Die Cooling circuit: Die temperature distribution is non-uniform, the cooling circuit is too long (i.e., large temperature difference between in and out points), control is inadequate, or the cooling method is not suitable. Die material : Low thermal conductivity (i.e., low cooling efficiency) Gate and runner: Non-uniform distribution of pressure in the dwelling process due to an insufficient number of gates or poor positioning. Push-out mechanism: Poor push-out balance or excessive ejection load with respect to the pin surface area. Parting: Insufficient polishing in the core extraction direction, inadequate extraction angle.

Cause(3)Molding machine and accessories Insufficient die clamping force : Inability to setup suitable clamping conditions (i.e., pressure and time) Die temperature regulator: Flow volume of cooling agent is insufficient (i.e., Reynolds number is not large enough for turbulent flow), insufficient performance in terms of die thermal capacity.

Cause(4)Molding conditions Resin temperature: Pressure transmissibility drops when the viscosity is high; consequently, uniformity in the degree of shrinkage is not possible in the dwelling process. Die temperature: When excessively low, the viscosity increases and the pressure transmissibility drops; consequently, uniformity in the degree of shrinkage is not possible in the dwelling process. Crystallization (or solidification) takes place before directional or stress relaxation can take place, and anisotropic residual stress remain. Injection pressure : Either too high or low (i.e., flow length in excess of the plastic's flow characteristic) Dwelling pressure : Either too high or low (Over-packing in the vicinity of the gates, or back-flow as a result of poor gate sealing) Dwelling time : Either too long or short (Over-packing in the vicinity of the gates, or back-flow as a result of poor gate sealing) Cooling time : Too short (dependence of material strength on temperature)

Sink Mark Sink marks or shrink marks are hollows or indentations that occur on the outer surfaces of molded components.FactorCauseCountermeasure

ProductMaterial thicknessThick or uneven (thin)Removal of material, standardizing thickness, thickening

Gate position and methodInadequate (design restrictions)Addition of gates, modification of method

Molding conditions/Molding machine performanceResin temperatureHigh (or low)Reduce (or increase)

Die temperatureHigh (or low)Reduce (or increase)

Injection speedHigh (or low)Reduce (or increase)

VP switchingFastSlower position

Dwelling pressureLowIncrease

Dwelling timeShortLengthen

Back pressureLow (measurement disparity)Increase

Screw speedTemperature increase due to rotationReduce

Amount of cushioningInsufficient, disparityIncrease (excess is unacceptable)

Mechanical back-flowReplace with normal component

MeasurementInsufficientIncrease

Die clamping pressureInsufficient (presence of flash)Increase

DieNozzle holesSmall cross sectionIncrease

SpruesSmall cross sectionIncrease

RunnersSmall cross sectionIncrease

GatesSmall cross sectionIncrease

Cooling circuitInsufficient (temperature disparity)Increase, standardize

Unsuitable method (i.e., efficiency)Change method

Small flow volume / high pressure lossImprove efficiency

Die materialInsufficient cooling abilityUsage of material with good thermal conductivity