Before the parison cools down, a hollow ramrod is injected into its center and pushed to the top of the mold, stretching out the warm plastic preform as it goes. Compressed air is then forced out in controlled low-pressure stages through the hollow ramrod. The plastic form is forced out to the sides of the mold. Because the stretching is performed evenly, the plastic remains uniformly thin and strong. The soda bottle assumes the shape of the mold and is dropped out of the blow molding machine as the two mold halves separate. A new parison is extruded and the entire blow molding process begins again. The actual manufacture of a soda bottle takes only a few seconds.
Since the introduction of bottled water into the market in the 90′s, it has become a household norm. Blow molding a two-liter soda bottle requires a preformed piece called a parison. The parison is mechanically loaded onto a stand and two sides of a bottle-shaped metal mold come together around it and is usually extruded from a plastic injection mouldplaced very close to the blow molding machine. The warm parison looks like an upside-down plastic test tube, with a preformed collar and threads for the cap at the bottom.
Stretch blow molding is a two-stage process similar to injection blow molding. Firstly, a test-tube like preform is made using plastic injection moulding or a similar process. The neck of the preforms is fully finished but the diameter and length of the body portion are much smaller than the final product. The preform then undergos a stretch-and-blow process.
A small piece of thermoplastic material is preformed to a test-tube shape and installed near the tip of the blow stem. The setup is placed in the closed chamber of a divided mold.
Compressed air is injected into the preform via the blow stem while the blow stem is pushing forward to stretch the preform further into the mold.
The blowing and stretching process continues. The blow stem stops near the far end of the mold.
Compressed air continues to blow in until the preform reaches the wall of the mold.
The plastic piece is removed from the mold after it cools and hardens.
The single most important product made by stretch blow molding is the 2-liter PET bottle for carbonated soft drinks introduced in 1978 and updated to one-piece bottle in early nineties. The PET bottles are virtually unbreakable, lightweight, transparent. They and have various barrier properties. The traditional glass containers cannot compete with PET bottles and are almost extinct from the field of carbonated soft drinks.
PET is a common seen resin material in daily life. The melting point of PET is around 245 degrees Celsius. Existing methods for achieving high temperature applications with PET involve the complicated insertion of premoulded neck into the preform mould and the body is then moulded in PET. This in turn is blown into an oversize bottle which is transferred to an oven at 220 to 230 degrees Celsius for 30 seconds producing up to 40 percent crystallinity and causing the bottle to shrink to smaller than the final size. A second blowing step then gives a container of required dimentsions with high crystallinity and capable of withstanding 90 degrees temperatures.
PET bottles are with features of high strength, good transparency, non-toxic, anti-penetration, light weight, high production efficiency, so it has been widely used.
The design purpose of injection process regulation is to make these negative influences ineffective, and thus to attain an even higher reproducibility of the parts.
The dicisive factor for all quality features that are concerned with dimension and weight is the internal pressure of the mould. Constant maintenance of this pressure curve in every cycle guarantees uniformity of the quality of injection moulded parts. If the mould internal pressure curve is maintained at a constant, all of the negative factors mentioned above are compensated.
During plastic injection moulding without injection process regulation, a specified pressure curve is established for injection and holding pressure, which can also be maintained with assurance with a regulated machine. However, the mould internal pressure curve that arises can only be assumed. Pressure losses through the runner manifold as well as the mould specific filling behavior cannot be identified.
With the application of injection process regulation, the mould internal pressure is first measured and compared with a nominal value. If there is a deviation, a hydraulic valve that applies pressure to the injection cylinder is actuated. It is thus possible to follow the nominal value precisely and independently of negative factors. The switch over from injection to holding pressure also occurs as a function of internal pressure. Thus, no pressure spikes can occur since the switch over takes place when a specified threshold value is reached.