Q. Write short note on blow molding.
OR
Explain the Thermoforming process.
What are the plastic fabrication method write in brief with neat sketch.
OR
What is blow moulding? Explain briefly.
OR
Describe the plastic processing method with the help of neat sketches.
(i) Blow moulding (ii) Thermoforming
Ans. Several techniques have been developed for fabrication of plastics to convert them into usable forms. Some popular techniques of these are the following:
1. Blow moulding
2. Forming methods
1. Blow moulding: It is used for production narrow neck plastic containers, like bottles and similar other articles.
An examples showing the production of a screwed neck plastic bottle is illustrated in given figure.
In this process a heated closed end thermoplastic tube is placed bottle in the mould and air pressure is applied to inflate it to acquire the shape of mould cavity. After the plastic is cooled the mould is opened and the product taken out.
2. Forming methods: The common method of forming or shaping plastic sheets into useful shapes are the following:
(i) Drape Forming: It is the simplest of all method of following and consists of draping the heated plastics sheet over the contours of a male form, followed by applying pressure and cooling.
(ii) Vacuum Forming: Intricate shapes are easily formed by the application of pressure or vacuum on the plastic sheet draped over the mould. The operation consists of stretching the sheet over the mould cavity to form a seal, heating it by suitable means and then drawing the air out of the space between the sheet and the mould. A few typical vacuum forming examples are illustrated in fig.
In (a) only vacuum is used for drawing the heated sheet into the mould.
At (b) the formed by pulling the vacuum through the mould to complete the forming.
At (c) a cored plug is used to push the heated sheet into the mould, followed by applying air pressure through the plug to complete the forming operation. Relief holes in the mould allow the air to escape.
3. Snap Back Forming: It is also a vacuum forming method. In this
method. In this method the heated plastic sheet fitted of the top of the vacuum box is pulled to a certain depth in the cavity by pulling the vacuum. Male plug is then moved down until it reaches a predetermined position. Vacuum is then pulled through the male plug to form the plastic sheet around the contours of the plug.
4.Pressure Forming: In this process the heated plastic sheet is formed into the required shape between a pair of male and female dies as shown in figure. No vacuum is used in this process.
5. Draw Forming: It is similar to the deep drawing process for metals, In this, a heated blank of plastic sheet is placed over a die and held firmly by means of a hold down plate. Flange, if remaining, may be trimmed separately or within the same operation.
6. Plug and Ring Formation: It is a combination of stretch and draw forming method. In this, the heated plastic sheet is clamped over a draw ring which forces the sheet on the form block. Finally the mould mounted of the bottom of s ram is forced into the clamped sheet to complete the forming to final shape.