Car Bumper Design: Materials and Specifications

Bumpers are safety features fixed to the front and back of vehicles. They are design to take a certain amount of impact from a collision by acting as dampers. To improve efficiency and performance modern bumpers are design to be aerodynamic and cheap. By developing wind channels in the bumper air can be used to provide more grip and stability for a safer drive. Modern bumpers are also designed to be interchangeable after impacts for low 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 specification

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Since 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 tests

To 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 isn't enough impact absorption in the bumper resulting in a broken leg test failure will occur. To combat this application material's with high absorption and tensile strength is look at first.

Environmental factor

Environmental factor involves a range of different topics for this project the main ones will be focus on

1. Recycling or disposal of the material safely and cheaply. A polymer that is commonly recyclable with low disposal costs.
2. 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 Process

During the manufacturing process the polymer will have to undergo heat treatment, compression and shearing processes. Preferably a polymer with a high melt flow index and a low melting temperature. This would allow for cheaper heating cost and quicker cycle times.

Mechanical Properties

To 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 D256

Material Selection

After researching different polymers and comparing mechanical properties it was found that Polypropylene polymer was mainly 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, making it suitable for the production of bumpers.

Manufacturing methods

PP is suitable for extrusion, injection moulding 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 leaving as a component. Its low melting point allows for cheaper heating costs.

Mechanical properties

The 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 original shape.

• Average
• Melt flow index
• 26.8 g/10min
• Tensile strength
• 33.0 MPa
• Flexural strength
• 49.0 MPa
• Notched Izod impact test
• 2.00 kJ/m�
• Melting point
• 159 �C
• Table 1

Environmental factor

PP experience small amount of UV degradation but this is relatively small compared to other polymer to solve this additives are added. Though this adds further costs the benefits of the other properties compensates for loses.

PP is considered a type 5 plastic for recycling. It is commonly recycled fulfilling the environmental factor.

Disadvantages

When 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.

Alternative

ABS (Acrylonitrile butadiene styrene) exhibits a lot of PP properties. But due to high UV degradation additives are added increasing costs.

Process selection

Manufacturing of polymer components involve a verity of different ways. Through research injection-compression moulding was the most commonly used form of manufacturing a bumper.

Injection-compression moulding

1. Support wall for injection and apposing compression clamp of mould
2. Mould with coolant attached
3. Main clamping forcing that applies pressure on the mould to counter react the force of injection
4. Second clamping forcing applied after injection mould process is complete. This further compresses the polymer in the mould. Due to a larger surface area more pressure is exerted completely filling the mould.
5. Secondary hydraulic ram
6. Locking levers
7. Tie Bar for supporting movement of clamp
8. Lock of point
9. Primary Hydraulic ram
10. Back support wall

Injection Moulding

Injection 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 sprue into mould.

Compression molding

On 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 partial opened mould the existing polymer in the mould can then be compress completely filling the mould. [2]

By combing these two process techniques compression-injection moulding is performed.

Properties

Moulding machines with clamping range from 8000KN to 13000KN and a shot capacity up to 4000g m/s.

Alternative

Thermoforming forming of the product by place a heated blank sheet of the polymer over the mould and through suction creating the component

Bumper drawings

The 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 create and fairly uniform surface

Mould design

Designing and constructing the mould requires highly precise work in both physical and theatrical ends of the process. To break even on cost a mould has to withstand a prolong cycle time. To achieve this length of time attention product quality most be reliable.

Creating 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 system 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 finish

Mould drawings

To gain a full understanding 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 prediction

Bumper Mould specification

Steel Type

Steel 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.

Injection system

Direct gate system. Reduces risk of blockage and allows for easy maintenance.

Ejection System

Robotic machine to extract the bumper from the mould with the use of suction pads. This allow for constant machine operation.

Surface Finishing

High gloss finish to ensure a polished surface on bumper and reduction of the bumper sticking in the mould.

Mould Cycle Time

It is estimate from the polypropylene entering the mould, till extraction a time cycle of 45s exists.

Cycle life

If run at constant rate it is estimate that a cycle life time for the mould of a 100,000 components produced exists.

Treatment

Coolant channels create to maintain steady temperature. Mould coated with oil to prevent rust.

Assumptions

It 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-up

Before 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.

Procure material

Granulated PP is source from a reliant and efficient supplier

Treatment

Polypropylene is dehumidified. Excess water is extracted before production occurs. This involves a pre-heating process.

Feeding

Granulated Polypropylene is brought to feed hopper where it is either manual feed in or through a conveyer belt machine system.

Mould

Top 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 can't afford to get damaged. Appropriate tonnage is applied compressing the mould.

Clean

Earlier material is purge out from the machine. Final check of machine operation is covered

Production

Polypropylene 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.

Customer

Bumper is deflash of excess polypropylene where injection occurs. This is either done robotically or by human intervention. The bumper is then sent off for packaging and storage where it will then go out to the customer.

Material usage per annum

It 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)
• Mass of bumper 15kg
• Material usage per annum 3000 (metric) tons

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