Car Bumper Design – materials and specifications

Bumpers argon safety features fixed to the front and back of vehicles. They ar fancy to take a certain amount of impact from a collision by acting as dampers. To improve expertness and performance modern bumpers atomic number 18 human body to be aerodynamic and cheap. By developing wind channels in the bumper way can be used to provide more deal and stability for a safer drive. Modern bumpers are also design to be interchangeable after impacts for utter cost repair. Car manufactures tend to design their bumpers from thermoplastics as it is cheaply made and adsorbs impacts more effectively.To design an efficient bumper all possible environmental and unforeseen circumstances most be taken into account.Material conditionSince metal components were more used in the design of larger vehicles a polymer based material was selected. This allowed for cheaper and easier manufacture components.To select the appropriate polymer for the bumper the fallowing applications had to be look at to obtain an overall purpose of the bumper.* Vehicle car crash Frontal-impact showsTo protect pedestrians from low speed frontal-impacts standard test are performed on the front of cars. Bumpers are subject to impacting a dummy at knee height. If there isnt enough impact absorption in the bumper resulting in a broken leg test failure will occur. To combat this application materials with high absorption and tensile strength is look at first. 1* Environmental factorEnvironmental factor involves a range of different topics for this project the main ones will be focus ono Recycling or disposal of the material safely and cheaply. A polymer that is commonly recyclable with low disposal costs.o Protection of the material for weather conditions. Nearly all plastic experience UV degrading from the sun or micro fractures occurring from frost over time. Finding the highest resilient polymer and adding additives should solve this problem for a suitable polymer.* Manufacturing ProcessDuring t he manufacturing process the polymer will have to gift awake treatment, compression and shearing processes. Preferably a polymer with a high thaw flow index and a low melting temperature. This would allow for cheaper heat up cost and quicker cycle times.Mechanical PropertiesTo obtain the required specification of the polymer required the fallowing standard tests are performed.Property ASTM* Melt flow index D1238* tensile strength D638* Tensile modulus D638* Flexural strength D790* Flexural Modulus D790* Notched Izod impact test D256Material SelectionAfter researching different polymers and comparing mechanical properties it was lay out that Polypropylene polymer was in the first place used in manufacturing of bumpers.PP (Polypropylene)Polypropylene is highly common material in manufacturing from furniture to carpets. The reason for its popularity is its low cost, do it suitable for the production of bumpers.Manufacturing methodsPP is suitable for extrusion, guesswork mete and vacuum forming. It has a high melt flow index which allows for quicker flow through the mould. This optimises time between granulated polymer entering and release as a component. Its low melting point allows for cheaper heating costs.Mechanical propertiesThe mechanical properties for PP show high impact resistance even in low temperatures. The material is stiff, and the properties are kept over a wide temperature range. These properties allow for the polymer to experience enough flexibility to absorb small impacts while returning to its accredited shape. cleanMelt flow index26.8 g/10minTensile strength33.0 MPaFlexural strength49.0 MPaNotched Izod impact test2.00 kJ/mMelting point159 CTable 1Environmental factorPP experience small amount of UV degradation but this is relatively small compared to other polymer to solve this additives are added. Though this adds advertise costs the benefits of the other properties compensates for loses. 3PP is considered a type 5 plastic for re cycling. It is commonly recycled fulfilling the environmental factor.DisadvantagesWhen coating and painting the finished component it is found that polypropylene is hard to work with. Colours and dies have to be added at the right time and temperature for quality assured product.AlternativeABS (Acrylonitrile butadiene styrene) exhibits a lot of PP properties. only due to high UV degradation additives are added increasing costs.Process selectionManufacturing of polymer components involve a verity of different ways. Through research injection-compression casting was the most commonly used form of manufacturing a bumper. shaft-compression mouldingFig 11. Support wall for injection and apposing compression clamp of mould2. redact with coolant implement-accessible3. Main clamping forcing that applies pressure on the mould to counter react the force of injection4. Second clamping forcing applied after injection mould process is complete. This further compresses the polymer in the mou ld. Due to a larger surface area more pressure is exerted completely filling the mould.5. Secondary hydraulic ram6. Locking levers7. haul Bar for supporting movement of clamp8. Lock of point9. Primary Hydraulic ram10. Back support wallInjection MouldingInjection moulding involves the feeding in of granulated ABS into the feed hopper. As it is funnelled down the polymer enters the cork screw were it is slowly force up the shaft. During this process the polymer is slowly heat by the heaters surrounding the shaft. At the end the polymer is heated enough to melting temperature allowing for flow to occur. By the cork screw constantly compressing the polymer all air bubbles are force out resulting in a uniform flow of polymer been extruded through the tropical tropical sprue into mould.Compression mouldingOn entering the mould the polymer is forced to spread out to fill the mould. In conventional injection moulding the polymer would only fill about 96% of the mould. By having a range ial capable mould the existing polymer in the mould can then be compress completely filling the mould. 2By combing these two process techniques compression-injection moulding is performed.PropertiesMoulding machines with clamping range from 8000KN to 13000KN and a shot capacity up to 4000g m/s.SuppliersEngel, Demag, Klockner, Windsorhttp//www.sumitomo-shi-demag.eu/products/AlternativeThermoforming forming of the product by place a heated blank carpenters plane of the polymer over the mould and through suction creating the componentBumper drawingsThe fallowing drawings show the basic model of the part been produced. The overall thickness of the bumper throughout was taken as 10mm. The part had to be design to suit the mould so the polymer could flow easy. This involved keeping a constant flow of lines so no corners would be frame and fairly uniform surfaceFig. 1 Auxiliary view of bumperFig. 2 BumperMould designDesigning and constructing the mould requires highly precise work in bo th physical and theatrical ends of the process. To break even on cost a mould has to withstand a prolong cycle time. To achieve this distance of time attention product quality most be reliable.Fig. 3 Mould and bumperCreating the mould involve the fallowing design parameters* Shape of bumper and tolerances* Mould had to be design for injection process* Direct gate hot runner injection agreement for funnelling the melted polymer from the injection system to the mould. By insulating the mould from the runner the temperature of the sprue can be maintained allowing the same left over material to be injected into the next product.* A draft angle throughout the mould and the sprue to allow for easy release. 1 per side of the sprue* Cooling system to allow for uniform temperature during cooling* It was desire not to have sharp corners in the mould as it reduces the bumper strength while creating mould flow problems* Type of tool steel* Surface finishMould drawingsTo gain a proficient und erstanding of the environment in the mould finite element modelling would have to be applied. This would give a computer aided design of the melt flow analysis throughout the mould, expected expansion and compression forces exerted during the process and cycle time predictionFig. 4 Top of mould designed so internal dimensions of the bumper was takenFig. 5 Bottom of mould designed so external dimensions of the bumper was takenFig. 6 Assembly of mouldBumper Mould specification trade name TypeSteel H13 was selected as it has long cycle life and shows great resistance to constant temperatures, wear, cracking and thermal shock. All these properties are associated with the manufacturing process. 5Injection systemDirect gate system. Reduces risk of blockage and allows for easy maintenance.Ejection SystemRobotic machine to extract the bumper from the mould with the use of suction pads. This allow for constant machine operation.Surface FinishingHigh gloss finish to ensure a polished surface on bumper and reduction of the bumper sticking in the mould.Mould round TimeIt is estimate from the polypropylene entering the mould, till extraction a time cycle of 45s exists.Cycle lifeIf run at constant step it is estimate that a cycle life time for the mould of a 100,000 components produced exists.TreatmentCoolant channels create to maintain steady temperature. Mould coated with anoint to prevent rust.AssumptionsIt was assumed that due to high compress and prolonged cycle times the mould would have to be highly strong and rigid. This involved making a highly thick walled and solid mould.On cooling of the polymer it was assumed that a shrinkage allowance of 0.5mm would have to be taken into account in the design of the mould.Manufacturing set-upBefore production begins a manufacturing process is set. This takes into account standard working procedures and outside factors. This creates a more safe and quality assured environment. obtain materialGranulated PP is source from a re liant and efficient supplierTreatmentPolypropylene is dehumidified. Excess water is extracted before production occurs. This involves a pre-heating process. nutritionGranulated Polypropylene is brought to feed hopper where it is either manual feed in or through a conveyer belt machine system.MouldTop and bottom of the mould are load onto the clamps were their securely fixed. This involves crane operation and coordinated procedure as the moulds are highly heavy and cant afford to get damaged. Appropriate tonnage is applied compressing the mould.CleanEarlier material is purge out from the machine. Final check of machine operation is coverProductionPolypropylene is slowly released into the cork screw and gradually begins to fill and compress. Fine tuning is performed on injection speeds, heaters, coolant temperatures, etc.. Tuning is performed till maximum efficiency is maintained. After the bumper is complete the mould is opened and the bumper is removed.CustomerBumper is deflash of e xcess polypropylene where injection occurs. This is either done robotically or by human intervention. The bumper is then send off for packaging and storage where it will then go out to the customer.Factory layoutFig. 7 Factory layout from a plan viewMaterial habit per annumIt was assumed a cycle time of 45s between polymer entering and leaving mould. By taking into account other assumptions an overall estimate of components per year was found.Assumptions* Average time of 12 work hours per day* Maintenance of machinery, loading of material, organising and other factors estimate of 1 component every minute. (Estimate of 720 a day)* Average of 200000 bumpers per year. (Estimate 260 work days)* green goddess of bumper 15kgMaterial usage per annum 3000 (metric) tons