Why need PET preform mold tryout before production
the improper design of the PET preform mold often cause the final product with some defects,especially for injection molding part for it need more precise size than blowing molding and rotational molding often. So, before the this kind of plastic mold modification, usually it need conduct a tryout or evaluation for optimization of PET preform mold design and process parameters, so as to avoid unnecessary errors, achieve good products, then meet the quality requirements of mass production.
The steps of tryout of PET preform mold
- Step 1. Set the barrel temperature.It should be noted that the initial temperature of the barrel must be set in accordance with the plastic material suppliers’ recommendation. This is because different manufacturers produce different brands of the plastic material which may have quite a big difference, and those material suppliers often have a very indepth study and understanding of their products.Users can have basic settings according to their recommendations, and then have the appropriate modification on the real production.In addition, it also need use the detector to measure the actual temperature of the melt. Because we tend to set the barrel temperature effected by environmental temperature, type and location of the depth of the temperature sensors,which does not guarantee 100% consistent with the real melt temperature. Sometimes, because of the presence of oil or other factors, the temperature difference between the actual temperature of the melt and the barrel is very large.
- Step 2. Set the temperature of the PET preform mold.Similarly, the initial mold temperature settings must also refer to the recommended values from the material supplier.Note that the mold temperature here refers to the temperature of the cavity surface, rather than the displayed temperature of the mold temperature controller .In many cases, due to environmental , improper selection of the mold temperature controllers and other reasons, the temperature and the temperature of the mold cavity surface are not consistent. Therefore, before the formal tryout, cavity surface temperature must be measured and recorded. Meanwhile, the different mold cavity position should also be measured, to verify the balance temperature at various points, and record the corresponding results, provide to optimize the reference data.
- Step 3. Based on experience, the initial plasticizing setting, injection pressure limit, injection speed, cooling time , screw speed and other parameters could be figured, and make its appropriate optimization later.
- Step 4. Have a test to identify the switching point. Which is a stage point from the injection stage to the packing stage, it may be the screw position, the filling time and filling pressure. This is one of the most essential and basic parameters of the injection molding process . under the actual test, the following points should be followed:(1) holding pressure and holding time is usually set to zero for the first trial;(2) products filling depends on the wall thickness of products and structural design of the mold;(3) for the injection speed would effect the position of switching position, so after each change of the injection speed , we must reconfirm switching points, the material flow path in the mold cavity could be found, thereby trapped air area could be easily determined,then decide how to improve to vent the mold.
- Step 5. Find the limit of the injection pressure. In this process, it should be noted the injection pressure and injection speed is related.For hydraulic systems, injection pressure and injection speed are interrelated. Therefore, these two parameters can not be simultaneously set best to meet the required conditions.Injection pressure on the screen is a limit of the actual injection pressure, and therefore, injection pressure limit should be set larger than the actual value of the injection pressure . If the injection pressure is set too low, the actual injection pressure is close to or exceeds the limit value of the injection pressure, which would affect the injection molding cycle time.
- Step 6. Optimizing the injection speed. Injection speed referred to the speed which could meet the shortest filling time demand and the lowest filling pressure demand.the following points should be noted:(1) Most of the product surface defects, especially defects near the gate, are caused due to the injection speed.(2) multistage injection can not be adopted only when the one time shot couldn’t meet the needs of the process , especially in mold test stage.(3) In the good mold condition, correct switching point setting and the enough injection speed , there is no direct relationship between the injection speed and product flash.
- Step 7. Optimization of holding time.holding time is time of the gate solidifying. In general, the holding time may be determined by weighing the products to obtain different time, and optimized holding time is when the products get its max weight.
- Step 8. Optimizing other parameters, such as holding pressure and clamping force and so on.Finally, it should be emphasized that the purpose of PET preform mold trial is to optimize the tooling and injection processes to meet the requirements of mass production, rather than just a good test sample.