polymer processing

Raising the injection velocity will reduce the time taken to fill the cavity and it is therefore possible to achieve faster cooling of the preform. However, it will also increase shear in the material. Shear is a major factor affecting overheating of the material, A.A. Generation and I.V. reduction, therefore increasing velocity will damage the PET resin. When working with hot preform method , the injection velocity will also make a significant difference to the material distribution in the finished container. Filling faster means that the preform will be colder when the mold opens and its temperature balance will also have changed. Typically, the shoulder area will become relatively cooler than the base area giving less stretch at the top of the preform. When working with warm / cool / cold preform method the temperature related effects of increasing the velocity are either greatly reduced or non-existent Oil flow into the injection cylinder must be increased. For machines fitted w...

Keeping the injection velocity low will reduce the shear that occurs in the material. Shear is a major factor affecting overheating of the material and I.V. reduction, therefore reducing velocity will protect the PET resin from excessive damage. When working with hot preform method, the injection velocity will also make a significant difference to the material distribution in the finished container. Filling slower means that the preform will be hotter when the mold opens and its temperature balance will also have changed. Typically, the shoulder area will become relatively hotter than the base area giving more stretch at the top of the preform. Excessive injection velocity can also disturb the alignment of the injection core, especially if the design is long and thin. Reducing the injection velocity will also have the effect of making the holding time shorter since the V/P time will increase. When working with warm / cool / cold preform method the temperature related effects are either...

Hot PET stretches more easily, cooler PET is more difficult to stretch. Therefore the primary method of adjusting the positioning of material in the finished container is to use relative temperature in various parts of the preform. If the temperature is balanced, the wall thickness of the container can be optimized and the overall strength of the container can be improved. If the balance is incorrect, some areas may become thick leading to a mottled or grainy appearance while the thin, overstretched areas such as the corners may show pearlescence or crystallization. Balance the temperature of the material within the preform to give the most equal strength in the finished product. There are two major methods of doing this. In the first method, injection velocity is used to control the temperature balance of the preform. Since most of the retained heat from the injection process is used in the blowing of the container, this method can have dramatic effects on the finished container. Fil...

The sixth step in validating a injection mold with the overall process shown in injection mold validation flow chart is balance of fill analysis . The steps before: 1. Mold certification 2. Dry cycle mold 3. Process stability test 4. Gage repeatability & reproducibility (R&R) test 5. Mold viscosity test Purpose: The purpose of the balance of fill analysis is to evaluate the thermal and flow balance of the plastic distribution system in the mold. The plastic distribution system encompasses the hot or cold runner system, as well as the core and cavity. Only a naturally or symmetrically balanced manifold should be specified. The cavity-to-cavity weight difference is an indicator of the quality of the hot runner system. It is critical to have the flows balanced to each cavity or the part-to-part variation may be large and process capability may not be achievable. Recall, the purpose of the mold validation procedure is to reduce variation throughout the injection mold. A typical m...

The purpose of the Secondary Blow air is to make the final shape of the container with all details. The Secondary Blow air should NOT be controlling or influencing the inflation of the preform. As with the Primary Blow Delay timer, there is no "correct" setting for Secondary Blow Delay . The optimum setting will depend on many other factors upstream of the stretch blow process and will include the setting of the Primary Blow Delay . Therefore, adjusting the Secondary Blow Delay time should be the last processing adjustment to be made when setting up a machine. As a general rule, the Secondary Blow Delay timer should be set as short as possible to get the best definition in the container, but without upsetting the blow-up of the preform. Having the Secondary Blow Delay set too short is likely to cause off center gates and neck rings. If there is too much delay between the introduction of the primary air and the secondary air, it may cause pearlescence in the corners of the c...

The purpose of the primary blow air is to inflate the preform as much as possible to the final shape of the container. The setting of the primary blow pressure can be extremely critical with containers having flat panels although it is not normally so important for round containers. If there is insufficient inflation in the primary blow, the extreme high pressure of the secondary blow may cause damage to the inflating preform such as pearlescence in the corners or splitting in the base. Too much primary air pressure can cause uncontrolled inflation of the preform leading to splitting preforms or buckling of flat panels. Turn off the Secondary Air and test the inflation of the preform at varying Primary Pressures. Referring to Optimize Primary Blow Time , increase the time delay of Secondary Air to the same as the blow time. This will prevent the secondary air from starting so that the Primary Air inflation can be more easily observed. Reduce the Primary Air Pressure to around 0.3~0.4 ...

The purpose of the Primary Blow Air is to inflate the preform as much as possible to the final shape of the container. The setting of the Primary Blow Delay time is one of the most critical processing adjustments that can be made in the molding process. Very small changes can have a significant effect on the container quality. There is no "correct" setting for Primary Blow Delay . The optimum setting will depend on many other factors upstream of the stretch blow process and it is also important to understand that the optimum setting will change if one of those upstream conditions changes. Therefore, adjusting the Primary Blow Delay time should be one of the last processing adjustment to be made when setting up a machine. If Primary Blow Delay is too early, the container may suffer from neck rings or off center gates. If it is too late, the shoulder may show a grainy appearance, there may be a constriction or the preform may split in the body. If there is insufficient infl...