Understanding Rotationally Moulded Sandwich Foam Parts and Accelerated Creep Testing. BITS Pilani. Dr. Sachin D. Waigaonkar. K K Birla Goa Campus

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1 Understanding Rotationally Moulded Sandwich Foam Parts and Accelerated Creep Testing BITS Pilani K K Birla Goa Campus Dr. Sachin D. Waigaonkar

2 Prof. Roy Crawford ( )

3 Background- Foamed Materials in Rotational Moulding The use of foamed materials is being preferred in rotationally moulded parts. In some cases, foam is used to improve the thermal insulation properties of the part. More Importantly, it is also used to improve the stiffness and stiffness to weight ratio of the part. This also results in good flexural (bending) characteristics of the product. Foamed inner layer is formed by adding polyethylene foam during moulding or by injecting polyurethane foam as a secondary operation. Since the stiffness is very important for RM products, a strong bond between the solid outer skin and the foamed inner layer is very important. Hence polyethylene foam is usually preferred to polyurethane foam because the latter does not bond easily to polyethylene. Applications: Pallets, Road barriers, Boats, Insulating Boxes, etc.

4 Application: Concept City Car by Total Ref: E. Maziers, Total Research & Technology, Feluy, Belgium Skin-Foam-Skin TP-Seal Rotomolded Structures: A New Concept for the Production of Car Bodies for Urban Mobility

5 Understanding Bending Stiffness: Effect of E and I Inference: Doubling thickness increases the stiffness 8 times but not a good solution considering the cost and cycle time

6 Foam effectively acts as a rib

7 Effect on Stiffness, weight : Solid- Foam configuration Solid ( Skin) Thickness Foam Thickness Weight (g/m) Second Moment of Area (I) Stiffness /weight % Increase in weight % Increase in Stiffness % Increase Stiffness /weight

8 Variation of Stiffness as a function of the depths of the solid flange and the foam thickness

9 Variation of Weight as a function of the depths of the solid flange and the foam thickness

10 Solid-Foam-Solid (Sandwich)

11 Comparison with Solid-Foam-Solid Solid ( Skin) Thickness Foam Thickness Solid ( Skin) Thickness Weight (g/m) Second Moment of Area (I) Stiffness /weight % Increase in weight % Increase in Stiffness % Increase in Stiffness /weight Solid-Foam Configuration from previous analysis

12 Observations If we add 5 mm of foam between 2.5 mm solid skins then for the same weight as before the stiffness and stiffness/weight increases quite dramatically to 277%. This is 4 times more than Solid-Foam Configuration and 7 times more than just Solid ( 5 mm thick). As compared with only 5 mm solid having for the same weight, we can optimise the thickness of solid and foam. Thus for almost same weight, a very high stiffness of around 879 % more ( in terms of absolute terms) can be achieved. The solution can be optimised for finding optimal solid and foam thicknesses.

13 Variation of Stiffness as a function of the depths of the Solid-Foam-Solid

14 Conclusions Foam sandwich crosssections is an extremely efficient way of increasing flexural stiffness without any increase in weight. Rotomouded products which are much lighter and still have a greater stiffness can be produced. Use of a foam layer between two solid skins is much better than using a single solid layer with a single foam layer.

15 Creep in Rotational Moulding Resins used in RM are viscoelastic materials whose properties are dependent on time, temperature and rate of loading. Thus instantaneous test results may not be sufficient to predict the long term behavior of the material. Therefore, values of modulus and strength should be expressed under conditions of stress, strain and time. Creep properties become very important for RM products. Creep is the progressive deformation of a material at constant load (stress). Creep test measures the dimensional changes that occur over time under a constant static load.

16 Necessity The long term properties like creep are often ignored in design of RM products. This is true even for RM products are used in engineering applications. It might be due to the fact that there is absence of data related to creep of RM products. It might be also due to lack of recognition of creep as an important factor for such products. As RM products are expected to perform satisfactory for several decades, the lack of creep properties in design of such products may lead to serious premature failures. Long Term creep test like ASTM D2990 requires considerable time and resources. Accelerated Creep Testing can be useful in such cases.

17 Accelerated Creep Testing Using TTS Rotomouldable resins show a time dependence that is proportional to the temperature dependence. Changes in temperature can be used to re-scale time, and predict behavior over time scales, not easily measured. Creep TTS or Stress Relaxation TTS can predict behavior over longer times than can be practically measured. Masters Curve Can be generated using WLF or Arrhenius Equation as follows:

18 Creep TTS Studies : DMA Rotationally Moulded Samples Sample on DMA

19 Creep TTS Studies : LLDPE LLDPE Density g/cm 3 MFI: 4.2 g/10min Applied Stress (MPa) Instantaneous Creep Strain (%) Onset of Secondary Creep Strain (%) Final Creep Strain (%)

20 Creep TTS Studies : PP PP Density- 0.9 g/cm 3 MFI: g/10min Applied Onset of Final Creep Strain Stress Instantaneous Creep Strain (%) Secondary Creep (%) (MPa) Strain(%)

21 Comparative Assessment wrt Creep Strain Ref Temp: 30 0 C

22 Other Importance of the study: Determination of Creep Compliance Material Creep Compliance (µm 2 /N) PP 1783 LLDPE 4100

23 Conclusions Creep TTS Studies can reduce the Time of Creep Testing Significantly. Can be performed in Tensile and Flexural Mode. Effective in comparing Different materials for long term properties Creep TTS studies have shown that Rotomouldable PP has higher Creep resistance than LLDPE. Creep Compliance can be one of the important indices (like Flexural or Tensile Modulus) to compare the long term properties of the materials. Creep Compliance of PP ( lower than LLDPE) indicates that its higher long term strength. (Better Creep Properties)

24 Thanks