Ten common problems in blow molding process

In the process of extrusion blow molding products, there are always many technical problems. Now let's share with you the causes of these problems and the corresponding solutions, hoping to provide you with relevant suggestions to solve the problems.
1、 The longitudinal wall thickness of blow molded products is uneven:
Cause:
① The dead weight of the parison is sagging seriously ② The diameter difference between the two longitudinal cross sections of the blow molded product is too large
Solution:
① Reduce the melt temperature of the parison, improve the extrusion speed of the parison, replace the resin with low melt flow rate, adjust the parison control device ② appropriately change the product design, and adopt the bottom blowing method for molding
2、 The transverse wall thickness of blow molded products is uneven:
Cause:
① The parison extrusion is skewed ② The temperature difference between the inner and outer parts of the mold sleeve and the mold core is large ③ The product shape is asymmetrical ④ The parison blowing ratio is too large
Solution:
① Adjust the gap width deviation of the die to make the parison wall thickness uniform; Straighten the mold before closing the mold ② Raise or lower the heating temperature of the mold sleeve to improve the internal and external temperature deviation of the die ③ Before closing the mold, pre-clamp and pre-expand the parison to make the parison shift to the thin-wall direction appropriately ④ Reduce the parison blowing expansion ratio
3、 Orange peel pattern or pitting on the surface of blow-molded products:
Cause:
① Poor mold exhaust ② mold water leakage or condensation in mold cavity ③ poor parison plasticization, melt fracture in parison ④ insufficient inflation air pressure ⑤ slow inflation rate ⑥ low inflation ratio
Solution:
① Sand blasting treatment shall be carried out for the mold parison, and exhaust holes shall be added. ② Repair the mold, adjust the cooling temperature of the mold to above the "dew point". ③ Reduce the screw speed and increase the heating temperature of the extruder. ④ Increase the inflation pressure. ⑤ Clean the compressed air channel. Check whether the air blowing rod leaks. ⑥ Replace the mold sleeve and core, and increase the inflation ratio of the mold parison
4、 Volume reduction of blow molding products:
Cause:
① The increase of parison wall thickness leads to the thickening of product wall ② The increase of product shrinkage leads to the shrinkage of product size ③ The inflation pressure is low, and the product is not inflated to the design size of mold cavity
Solution:
① Adjust the program control device to reduce the parison wall thickness; Increase the temperature of parison melt, reduce the expansion ratio of parison leaving the mold ② replace the resin with small shrinkage, extend the blowing time, reduce the mold cooling temperature ③ properly increase the pressure of compressed air
5、 The outline or graphics of blow molded products are not clear:
Cause:
① Poor cavity exhaust ② low inflation pressure ③ low billet melt temperature, poor material plasticization ④ low mold cooling temperature, mold "condensation" phenomenon
Solution:
① Repair the mold, sandblast the cavity or add an exhaust slot ② Increase the inflation pressure ③ properly increase the heating temperature of the extruder and the head, and add an appropriate amount of filler masterbatch if necessary ④ Increase the mold temperature above the dew point temperature
6、 Too many and thick flash of blow molding products:
Cause:
① Mold expansion, insufficient locking pressure ② mold knife edge wear, guide pillar offset ③ parison deviation during blowing expansion ④ material escape groove at blank clamping knife edge is too shallow or knife edge depth is too shallow ⑤ parison inflation starts too early
Solution:
① Raise the mold locking pressure, suitable for reducing the inflation air pressure ② Repair the mold blade, correct or replace the mold guide post ③ Correct the center position of the parison and the blow bar ③ Trim the mold, deepen the depth of the escape chute or knife ④ Adjust the parison inflation time
7、 Too deep longitudinal stripes appear:
Cause:
① The mold mouth is dirty ② The mold sleeve, the core edge has burr or gap ③ The color masterbatch or resin decomposes to produce dark stripes ④ The filter screen is perforated, and the material is mixed with impurities and deposited in the mold mouth
Solution:
① Clean the die mouth with a copper knife ② Trim the die ③ Reduce the temperature properly, replace the color masterbatch with good dispersion ④ Replace the filter screen and use the stained leftovers
8、 When forming, the embryo is blown out:
Cause:
① The mold knife edge is too sharp ② The parison has impurities or bubbles ③ The blow expansion ratio is too large ④ The parison melt strength is low ⑤ The parison length is insufficient ⑤ The parison wall is too thin or the parison wall thickness is uneven ⑥ The container expands when opening the mold (insufficient bleeding time) ⑦ The mold locking force is insufficient
Solution:
① Properly increase the width and angle of the knife edge ② Use dry raw materials, use wet raw materials after drying, and use clean raw materials; Clean the die opening ③ Replace the die sleeve and core, reduce the blow expansion ratio of the mold failure ④ Replace the appropriate raw material, properly reduce the melt temperature ⑤ Check the control device of the extruder or storage cylinder head, reduce the change of process parameters, increase the length of the parison ⑥ Replace the die sleeve or core, and thicken the parison wall; Check the parison control device; Adjust the die gap ⑦ Adjust the bleeding time or delay the mold start time ⑧ Increase the mold locking pressure or reduce the inflation pressure
9、 Blow-molded products are difficult to demould:
Cause:
① The cooling time of product expansion is too long, the cooling temperature of the mold is too low ② The mold design is poor, and there are burrs on the surface of the mold cavity ③ When the mold is opened, the front and rear templates move unevenly ④ The mold is installed incorrectly
Solution:
① Properly shorten the blow expansion time of parison and raise the mold temperature ② Trim the mold; Reduce the groove depth, and the convex rib slope is 1:50 or 1:100; Use release agent ③ Repair the mold locking device to make the front and rear templates move at the same speed ④ Reinstall the mold and correct the installation position of the two halves of the mold
10、 The quality of blow molded products fluctuates greatly:
Cause:
① Sudden change of parison wall thickness ② Uneven mixing of mixed edge and corner materials ③ Blockage of feeding section, resulting in fluctuation of extruder discharge ④ Uneven heating temperature
Solution:
① Repair parison control device ② Use good mixing device to prolong mixing time; If necessary, reduce the amount of return material at the edges and corners ③ remove the lumps at the material mouth ④ material or foreign matter. Reduce the temperature at the material mouth ⑤ replace the electric heating cabinet