| In a Blow Molding process a hot tube of molten | | | | part is controlled by the machine while also the |
| plastic is extruded through a die. The hot tube of | | | | mold design does play a part in the wall thickness |
| molten plastic is suspended in mid air before the | | | | of the finished part. When designing a blow |
| mold closes, while the tube is in mid air it is | | | | molded part many shapes are possible but blow |
| stretching and cooling. | | | | ratio's have to be maintained. A hole is required on |
| With Injection molding the plastic is contained in | | | | a blow molded part as the plastic is expanded in |
| the barrel of the machine and the runner of the | | | | the mold with air pressure although some parts |
| mold, the process is controlled precisely until the | | | | can be made using trapped air to blow. |
| part is ejected from the mold. | | | | Apart from a sprue in a typical Injection mold |
| With Blow Molding there is precise control in the | | | | there is no waste in fact on a Hot Runner mold |
| machine but the process has elements that can | | | | there is no waste. A blow molded part does have |
| be difficult to control. | | | | waste because the plastic trapped between the |
| The big difference on the molded part is the fact | | | | mold does have excess outside the molded edge. |
| that Injection molded parts molded in one shot | | | | This molded edge on a blow mold is called pinch |
| have a single wall while Blow molded parts need | | | | edge or nip edge. It has a cutting effect to |
| to be a complete shell with an air gap on the | | | | separate the waste plastic from the molded part. |
| inside. A simple blow molded part would be for | | | | Designing a blow molded part needs many |
| example a shampoo bottle. A more technical blow | | | | considerations and an understanding of the |
| molded part could be a bottom shell lawnmower | | | | process is vital. Generally a shape can not be |
| seat hollow on the inside, this type of lawnmower | | | | blown with reasonably uniform wall thickness |
| seat would have many shapes and cut out's with | | | | unless the width of the cavity is equal to or less |
| some performed in the mold. | | | | than the depth of the cavity. Another |
| When designing an Injection Molded part the | | | | consideration is sharp corners, the only place a |
| general rule is to maintain a uniform wall thickness | | | | sharp corner is allowed on a blow molding is the |
| as much as possible, this is controlled by the mold | | | | parting line. |
| and mold design. Wall thickness on a blow molded | | | | |