Material Selection for Milk Bottle Manufacturing

Abstract/Executive Summary

An experiment was conducted to determine the most appropriate material for milk bottles by comparing the results of a tensile test on dog-bone test specimens. Materials used included PVC, HDPE, Polycarbonate, and Perspex. The study measured parameters such as Ultimate Tensile Strength (UTS), Young’s Modulus, 0.2% offset yield, and elongation. Based on the results, High-Density Polyethylene (HDPE) was recommended for milk bottle manufacturing due to its favorable properties.

1. Introduction

1.1 Background

Tensile Testing is an effective method for evaluating material properties.

This report aims to assess various materials under tensile stress to determine the most suitable one for designing blow-molded milk bottles.

1.2 Problem Statement

The objective is to identify the most appropriate material among PVC, Perspex, Polycarbonate, and HDPE for producing blow-molded milk bottles.

1.2.1 Tensile Testing

Tensile testing involves subjecting a specimen to a tensile force, measuring load and displacement, and calculating key parameters.

1.2.2 Engineering Stress

The engineering stress is calculated as:

= ^F/_A

Where is stress, F is the tensile load, and A is the initial cross-sectional area of the specimen.

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1.2.3 Young’s Modulus

Young’s Modulus, denoted as E, measures a material's ability to withstand changes in length under tension or compression:

E = /

Where E is Young’s Modulus, is stress, and is strain.

1.2.4 0.2% Offset Yield

The 0.2% offset yield marks the transition from elastic to plastic deformation. It is determined by drawing a line with a gradient equal to the Young's Modulus and cutting the strain axis at 0.002.

1.3 Blow Moulding

Blow molding is a manufacturing process used to create hollow plastic parts, such as milk bottles.

It involves several steps, including mixing, melting, extrusion, molding, blowing, cooling, and part removal.

1.4 Materials

1.4.1 PVC

Polyvinyl Chloride (PVC) is a synthetic polymer known for its flexibility, durability, and insulation properties. It finds applications in construction, packaging, electrical insulation, and healthcare.

1.4.2 Perspex

Perspex is an acrylic glass used in various applications due to its resistance to weathering, abrasion, and chemicals. It has a high Young's Modulus and rigidity.

1.4.3 Polycarbonate

Polycarbonate exhibited an average yield strength of 29.99 MPa, an average ultimate tensile strength of 66.75 MPa, an average % elongation of 183.71%, and an average Young's Modulus of 1077.41 MPa.

1.4.4 HDPE

High-Density Polyethylene (HDPE) is a thermoplastic with excellent chemical stability, wear resistance, and fracture toughness. It is commonly used in pipes, milk bottles, and kitchen cleaning products.

2. Methodology

2.1 Apparatus

The experiment utilized:

• A Tensile Tester
• An Extensometer
• Computer for data processing
• Dogbone-shaped specimens for each material

2.2 Method

1. Measure specimen dimensions using Vernier calipers.
2. Place specimen in the tensile tester, ensuring cleanliness to prevent slippage.
3. Tighten grips securely.
4. Apply tensile stress until fracture or tester limit is reached.
5. Record load vs. displacement data.
6. Measure final specimen length.
7. Convert load vs. displacement into a stress vs. strain curve.
8. Repeat steps for all specimens.

3. Results

3.1 PVC

PVC exhibited an average maximum tensile strength of 49.89 MPa, an average % elongation at failure of 23.02%, an average Young's Modulus of 1232.67 MPa, and an average 0.2% offset yield strength of 29.17 MPa. The average fracture point occurred at a strain of 0.2554.

3.2 Perspex

Perspex demonstrated elastic behavior up to a proportional limit stress of 26.28 MPa. The Young's Modulus was 1206.90 MPa. Beyond the proportional limit, plastic behavior was observed up to a yield stress of 32.24 MPa. The ultimate tensile stress was 81.06 MPa with a strain of 0.1321. The percentage elongation was 15.45%.

3.3 Polycarbonate

Polycarbonate exhibited an average yield strength of 29.99 MPa, an average ultimate tensile strength of 66.75 MPa, an average % elongation of 183.71%, and an average Young's Modulus of 1077.41 MPa.

3.4 HDPE

HDPE did not reach a fracture point due to equipment limitations. However, it showed an ultimate tensile strength of 22.77 MPa, an average Young's Modulus of 403.32 MPa, a yield stress of 12.45 MPa, and an average percentage elongation of 323.45%.

4. Discussion

Comparing the material properties, PVC was found to have the highest Young's Modulus, indicating its stiffness. Perspex had the longest elastic region, while Polycarbonate exhibited the highest ultimate tensile strength. HDPE showed exceptional percentage elongation.

4.1 Conclusion and Recommendations

HDPE is recommended as the best material for milk bottle manufacturing due to its high elongation and resistance to fracture under stress.