2. thermoforming

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Topic•THERMO FORMING Presented by

Group – DThe members are NAVID RAHMAN 2011332001MD. HASAN ZULFIKER 2011332004MD. LUSHANUR RAHMAN 2011332030NAFISH INTASHER NAFI 2011332033SAINTI CHAKMA 2011332052A.B.M. GOLAM RABBABY 2011332053SADIA BEGUM 2011332058 MUJTOBA RAFID RAHMAN 2011332066

Definition

Thermoforming is the process where a thermoplastic polymer sheet is heated & deformed into a desired shape.

•Process: Step 1: Heating a plastic sheet to the temperature where it softens. Step 2: Stretching the softened polymer against a cold mold surface. step 3: Cooling the finished part and trimming any excess plastic.

Steps of thermoforming1. Heating: Heating is accomplished by radiant electric

heater which is located at a distance of 125mm (5 in.) either on one or both sides of the sheet.

2. Forming: After heating, the polymer sheet is converted or formed into various shapes by either air pressure, vacuum pressure or by mechanical pressure.

Figure : Stage thermo forming machine

So according to the forming techniques thermoforming are 3 types

Vacuum forming

Pressure forming

Mechanical forming

Vacuum forming

It is the earliest method of thermoforming. In vacuum forming, vacuum is created below the preheated

plastic sheet to draw sheet into the cold mold cavity. Basic steps of vacuum forming1. A flat plastic sheet is heated by radiant heater, which is

placed on one or both sides of the plastic.2. The softened sheet is placed over a concave cavity.3. Vacuum draws the sheet to the sheet to the cold cavity .4. The product is cooled and any extra plastic parts are

trimmed.

Figure: Vaccum forming

Advantage:I. Operated at a comparatively low vacuum pressure.II. Relatively cheap.

Disadvantage:III. Region of least thickness occurs near the clamp.IV. Bad finishing or non uniform plastic formation.V. Uneven wall thickness at the corner of the product.

Thin corner

Figure: thin area at the corner

Pressure forming

Alternative to vacuum forming. Here the air pressure is forced to the preheated sheet into a

cold mold. Here the air pressure is much higher than the vacuum forming.

The basic difference from vacuum forming is that the heated sheet is pressurized from above the mold cavity .

Due to high pressure, the heated sheet can be deformed in a fraction of a second. (High production rate)

Figure : Pressure forming

Advantage:• High production rate.• Efficient for large parts.• Low tooling cost. Disadvantage:• Limited shape complexity.

Mechanical thermoformingHere, positive (male) and negative (female) molds are

brought against heated plastic sheet forcing it to the assumed shape.

In mechanical forming, air pressure or vacuum pressure are not used to deform the plastic sheet.

Air between the die and sheet is evacuated by using vacuum pump and the sheet conforms to the mold shape.

Formed part is cooled and ejected.

Figure : Mechanical thermoforming: (1) Heated sheet is placed above a negative mold (2) mold is closed to shape the sheet.

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Advantage 1. Better dimensional control.2. Opportunity for surface detailing of both sides

of the parts. Disadvantages1. Two mold halves are required 2. Relatively costly.

Materials Used:• ABS (Acrylonitrile butadiene styrene)• Cellulose acetate• LDPE (Low density polyethylene)• HDPE (High density polyethylene)• PVC etc.

Applications of Thermoforming I. Food packagingII. Automotive partsIII. Aircraft windscreens IV. Vehicle doors etc.

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Advantages of thermo forming:• Rapid prototype development.• High production rate.• Flexible design.• Low set up cost.• Less thermal stress.

Disadvantage of thermo forming:• Not eligible for thermosets.• All parts need to be trimmed.• Parts may have non-uniform thickness.

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