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Polymers exhibit some of the best material properties, such as light weight, specific strength, and stiffness. Therefore, they are increasingly used in various parts of the automotive industry to reduce the weight of vehicles, thereby reducing energy consumption. The Journal of Process Mechanical Engineering considers a batch molding process called Single Point Incremental Forming (SPIF). 

Polymer parts are usually mass produced by molding, extrusion, thermoforming, etc., which involve the use of heat and pressure. These processes involve expensive equipment and tools, and do not adapt to changes in product design. The batch molding process called single point incremental molding (SPIF) is economical, uses common tools, and can adapt to changes in product design.

The SPIF process is carried out using CNC, so the tool can advance through a specific path with the help of numerical programming. Generally speaking, this process is performed on a CNC machine, where CNC programming can be used to effectively move the tool on a specific path.

Many studies have analyzed the moldability of polymers under different conditions. However, there are few studies on the influence of parameter changes on the response within the forming limit. This study analyzed the effects of changes in feed rate, wall angle and step length on springback, surface roughness and thinning when SPIF was performed on polypropylene.

For progressive molding, samples were prepared from polypropylene sheets by cutting sheets with a side length of 265 mm. Make holes of appropriate size to fix the sheet to the fixing device using bolts. Experiment with computer numerical control (CNC) machines.

SPIF is performed at room temperature using spherical tools on CNC machine tools. After SPIF, when the sheet was released from the clamp, the size of the sample changed. This change is due to rebound. The amount of springback can be determined by measuring the final size.

A roughness tester was used to measure the surface roughness at the inner surface of the deformed sample at the micrometer level. To measure the thickness after deformation, cut a quarter of the deformed sample. Then use the cursor to measure the wall thickness at both ends. The average wall thickness is taken as the final thickness after deformation.

The main effect presented is that the springback percentage increases as the step length and tool diameter increase. However, as the wall angle increases, the rebound percentage decreases. The increase of the wall angle causes the plastic deformation caused by stretching to be greater than the elastic deformation, so the elastic recovery rate is lower. This reduces the rebound percentage.

It is found that the surface roughness increases with the increase of wall angle, tool diameter and step length. The increase in the wall angle leads to an increase in the thinning. Thinning will decrease as the tool diameter, step length and feed rate increase. However, the change in thinning relative to the feed rate is very small.

When the polypropylene sheet is deformed by SPIF, the tool diameter is considered the key parameter for all three responses. An increase in tool diameter will lead to an increase in springback and surface roughness, but it will lead to thinning. Therefore, if the thickness of the sheet is selected suitably, the smaller tool diameter may be used for performing SPIF.

This means that for parts with smooth surfaces and minimal springback, the smallest tool diameter and step length should be the first choice. The molding time can be controlled by selecting a higher feed rate.

The experiment on SPIF was performed using the L27 orthogonal array. The measured response is: springback, surface roughness and thinning. It was found that three parameters—tool diameter, wall angle, and step length—are important for surface roughness and thinning. The wall angle has the greatest influence on the surface roughness and thinning. It is observed that the average value of the surface roughness increases as the wall angle increases.

As the diameter of the tool increases, a decrease in thinning is also observed. However, as the wall angle increases, the thinning increases. It is found that the refinement decreases as the step size increases. Although the feed rate is critical, the dilution change associated with the feed rate is negligible.

It was found that the most important factor for surface roughness and thinning is the corner. In contrast, it was found that the tool diameter has a significant effect on the rebound of the acrylic plate during the SPIF process.

Higher wall angles will reduce springback, but will increase thinning and roughness. The reduction of the step size will reduce the roughness and springback, but will increase the thinning. An increase in the feed rate will result in a decrease in dilution.

This shows that the molding time and springback can be reduced by a higher feed rate. In order to obtain a better surface finish under the condition of thinning and reduced springback, the tool diameter, step length and wall angle should be set to the minimum.

Jainism, PS, etc. (2021) Analyze the influence of various parameters on surface roughness, springback and thinning of polypropylene sheet during single-point incremental molding. Proceedings of the Society of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering. https://journals.sagepub.com/doi/abs/10.1177/09544089211047203

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