thermoforming

Slide 1: 

بسم الله الرحمن الرحيم Sudan University of Science and Technology College of Engineering Plastic engineering Thermoforming By : Arman Mohammed

Corse contents : 

Corse contents 1. Thermoforming2. Rotation molding 3. Calendaring 4. Compression molding

Thermoforming : 

Thermoforming When a thermoplastic sheet is heated it becomes soft and the techniques for shaping this sheet are known as thermoforming. The term "thermoforming" incorporates a wide range of possibilities for sheet forming but basically there are two sub-divisions : vacuum forming pressure forming.

(a) Vacuum Forming : 

(a) Vacuum Forming In this processing method a sheet of thermoplastic material is heated and then shaped by reducing the air pressure between it and a mould. A heater panel is then placed above the sheet and when sufficient softening has occurred the heater is removed and the vacuum is applied. For the thicker sheets it is essential to have heating from both sides. A sheet of plastic, which may range in thickness from 0.025 mm to 6.5 mm, is clamped over the open mould.

(a) Vacuum Forming : 

(a) Vacuum Forming 1. Negative Forming The simplest type of vacuum forming is illustrated in Fig(1). Advantages The principle is very simple. Dis-Advantages In some cases Negative Forming would not be suitable because, for example, the shape formed in Fig(1) would have a wall thickness in the comers which is considerably less than that close to the clamp. If this was not acceptable then the same basic shape could be produced by Positive Forming.

(a) Vacuum Forming : 

(a) Vacuum Forming 1. Positive Forming. In this case a male (positive) mould is pushed into the heated sheet before the vacuum is applied. Fig(2) Advantages This gives a better distribution of material and deeper shapes can be formed - depth to width ratios of 1:1 is possible.

(b) Pressure Forming : 

(b) Pressure Forming This is generally similar to vacuum forming except that pressure is applied above the sheet rather than vacuum below it. This advantage of this is that higher pressures can be used to form the sheet. A typical system is illustrated in Fig(3) and in recent times this has become attractive as an alternative to injection molding for molding large area articles such as machine housings.

(b) Pressure Forming : 

(b) Pressure Forming

(c) Matched Die Forming : 

(c) Matched Die Forming A variation of thermoforming which does not involve gas pressure or vacuum is matched die forming. The concept is very simple and is illustrated in Fig. 4. The plastic sheet is heated as described previously and is then sandwiched between two halves of a mould. Very precise detail can be reproduced using this thermoforming method but the moulds need to be more robust than for the more conventional process involving gas pressure or vacuum.

(d) Dual-Sheet Thermoforming : 

(d) Dual-Sheet Thermoforming This technique, also known as Twin-Sheet Forming, is a recent development. It is essentially a hybrid of blow molding and thermoforming. Two heated sheets are placed between two mould halves and clamped as shown in Fig(5). An inflation tube at the parting line then injects gas under pressure so that the sheets are forced out against the mould. Alternatively, a vacuum can be drawn between the plastic sheet and the mould in each half of the system. This technique has interesting possibilities for further development and will compete with blow molding, injection molding and rotational molding in a number of market sectors. It can be noted that the two mould halves can be of different shapes and the two plastic sheets could be of different materials, provided a good weld can be obtained at the parting line.

(d) Dual-Sheet Thermoforming : 

(d) Dual-Sheet Thermoforming

(d) Dual-Sheet Thermoforming : 

(d) Dual-Sheet Thermoforming Heaters: The heaters used in thermoforming are usually of the infra red type with typical loadings of between 10 and 30 kW/m2. Normally extra heat is concentrated at the clamped edges of the sheet to compensate for the additional heat losses in this region. Materials: polystyrene ,ABS, PVC, acrylic, polycarbonate, polypropylene and high and low density polyethylene. Co-extruded sheets of different plastics and multi-colour laminates are also widely used nowadays. One of the most recent developments is the thermoforming of crystallisable PET for high temperature applications such as oven trays. The PET sheet is manufactured in the amorphous form. and then during thermoforming it is permitted to crystallise.

Analysis of Thermoforming : 

Analysis of Thermoforming If a thermoplastic sheet is softened by heat and then pressure is applied to one of the sides so as to generate a freely blown surface, it will be found that the shape so formed has a uniform thickness. If this was the case during thermoforming, then a simple volume balance between the original sheet and the final shape could provide the wall thickness of the end product. Where A = surface area , and h = wall thickness ('i' and 'f ' refer to initial and final conditions).

Analysis of Thermoforming : 

Analysis of Thermoforming

Analysis of Thermoforming : 

Analysis of Thermoforming

Analysis of Thermoforming : 

Analysis of Thermoforming

Analysis of Thermoforming : 

Analysis of Thermoforming

Analysis of Thermoforming : 

Analysis of Thermoforming

Analysis of Thermoforming : 

Analysis of Thermoforming

Analysis of Thermoforming : 

Analysis of Thermoforming

Analysis of Thermoforming : 

Analysis of Thermoforming

Analysis of Thermoforming : 

Analysis of Thermoforming

Analysis of Thermoforming : 

Analysis of Thermoforming