Optimum Processing Parameters for HIPS Thermoforming

Pages: 11 (1326 words) Published: April 16, 2014


This report assesses thermoforming parameters such as forming temperature, drape delay and heater dwell and observes the differences between plug assist and negative forming’s and their resulting characteristics. The results from this report show that plug assist forming can be used to provide a more carefully controlled thickness distribution within thermoformed parts. Furthermore, it has been shown that temperature and drape delay both have significant influence on thermoforming parameters and both need to be carefully controlled to produce a consistent output. Also, it has been shown that the ability of the polymer to regain its original shape (plastic memory) is reduced at higher forming temperatures.


1 Introduction

1.1 Thermoforming process background

Thermoforming is a high yield manufacturing process that involves heating thermoplastic sheet above the glass transition temperature where it behaves as a viscous liquid and forcing it against the contours of a mould using mechanical, air or vacuum forces to create the desired part geometry. Upon cooling, the polymer solidifies and retains its new form and hence this process has many engineering applications which capitalise on the low cost, low cycle time, large geometry attributes of the process which make it very suited to large, thin walled parts such as containers and panels etc. [1] [6] This report will assess the processing parameters within the thermoforming process that result in the optimum outputs and will describe the potential causes of defects.

2 Results & Discussion
2.1 Thickness Distribution Analysis
The main characteristic of thermoforming is that a polymer sheet is stretched and deformed as it is forced around the mould contours. This consequently results in an uneven thickness distribution due to differential cooling rates. Typically, the first area to make contact with the mould wall will be the thickest and the last area to make contact will have stretched the furthest before cooling and hence have the thinnest sectional thickness. [4] This phenomenon is shown below in Figure 1. This shows that in the forming of a cup geometry the thickness extension ratio (thickness/initial thickness) increases and the contour extension ratio (distance/initial distance) decreases with increasing radial sector, shown in the top right of the diagram. This shows that the material that stretches the least, has the thickest section and vice versa.

2.2 Influence of Drape Delay on Plug assist forming’s
To mitigate the thickness variation shown in negative vacuum forming, a plug is used to pre stretch the sheet in a controlled manner to produce a more uniform thickness throughout the part, which is especially important in parts with deep draw ratios such as cups. The comparison between plug assist and negative forming is shown in Figure 2. This shows that there is a wider thickness variance associated with the negative forming and that the plug assist helps to increase the average thickness and create a more even thickness distribution across the part.

Figures 3 & 4, shown below, show that drape delay, on average, has a positive influence on thickness but an inverse relationship on the thickness distribution, with a larger thickness measurement deviation. However, with increasing drape delay, the temperature of the sheet results in uncontrolled sagging and therefore less control over the thickness distribution which explains why the results show a greater spread of thickness at increased delay. 2.3 Influence of forming temperature on Recovery

Thermoforming does not melt the polymer and therefore the polymer chains are still connected and have a memory of their previous position. For example, in thermoforming the polymer molecules are held in a state of stress, which upon heating will try to assume there unstressed state,...

References: [1] Birley, Arthur W, Haworth, Barry and Batchelor, Jim, Physics of plastics: processing, properties and materials engineering, Munich; New York: Hanser Publishers: Oxford university press (distributor), 1991.
[2] Throne, James L. Thermo Forming. Munich: Hanser, 1987. Print.
[5] P. W Klein, Plastics Thermoforming Tool design: Plug VS Cavity Moulds. Available at: http://technologyinterface.nmsu.edu/Fall06/02_Klein/index.cgi
[6] Throne, James, Thermoforming, p.120, 1987.
[9] G.Gruenwald, Thermoforming. A Plastics Processing Guide. (1998)
[10] Peter Klien, Fundamentals of Plastics Thermoforming, Morgan & Claypool Publishers, 8 Jul 2009
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