Here are the answers to the question of how to determine the size of the venting groove in the design of the venting system for a PET jar preform mold:
1. Consider the properties of the plastic material
The properties of the PET material have an important influence on the size of the venting groove. The melt viscosity and gas generation of PET during the injection molding process are key factors. Generally speaking, the melt viscosity of PET is relatively high, which means that it is more difficult for gas to escape from the melt. If the material has poor fluidity, the depth and width of the venting groove need to be appropriately increased. For example, for some high-viscosity PET modified materials, the depth of the venting groove can be set between 0.02 - 0.03mm and the width between 0.5 - 1mm, so that the enclosed gas has enough channels to be discharged and prevent defects such as bubbles or gas marks from forming in the preform.
2. Based on the mold structure
The structure of the mold determines the flow path and accumulation area of the gas in the cavity. For PET jar preform molds with complex structures, more internal structures or thicker parts, the size of the venting groove should be adjusted accordingly. In places where gas accumulation is likely to occur, such as corners and ribs in the cavity, the venting groove should be appropriately deepened and widened. For example, in a preform mold with a complex internal support structure, the depth of the exhaust groove can be increased to 0.03-0.04mm and the width can reach 1-1.5mm to ensure that the gas in these local areas can be effectively discharged and the molding quality of the preform can be guaranteed.
3. Combined with injection molding process parameters
Injection molding process parameters such as speed and pressure affect the difficulty of gas generation and discharge. Higher injection molding speeds make it easier for air to be drawn into the melt, and larger exhaust grooves are needed to deal with this. When the injection molding speed increases, the width of the exhaust groove may need to be increased from the conventional 0.5mm to 0.8-1mm, and the depth from 0.015mm to 0.02-0.025mm. Similarly, higher injection molding pressures will also make it more difficult for gas to be discharged, and the size of the exhaust groove should be increased accordingly to ensure that the gas can be discharged in time under high-pressure injection molding.
When determining the size of the exhaust groove, the design experience of similar PET jar preform molds in the past is very valuable. If there is experience data on the successful production of the same type of preforms, it can be used as a reference. At the same time, conducting experiments is also an indispensable link. By making a test mold and using different venting groove sizes for injection molding tests, the molding quality of the preform is observed, such as whether there are bubbles, surface defects and other problems. The venting groove size is optimized and adjusted according to the test results, and finally the venting groove size suitable for the specific PET jar preform mold is determined.
