Plastic Blow Molding
The advantages of plastic blow molding are prominent, especially the cost of molds and equipment, which is significantly lower than that of injection molding tools. At the same time, the residual stress of the product is relatively small, and it has high resistance to various strains such as tension, impact, bending, and environment, and has good usability.
Common Problems in Blow Molding Manufacturing
Blow molding is mainly through hollow blow molding (also known as blow molding). It is a method that uses gas pressure to inflate the hot melt parison closed in the mold to form a hollow product. It is the most commonly used plastic processing in mold manufacturing. One of the methods is also a rapidly developing plastic molding method. The mold used for blow molding only has a female mold (concave mold). Compared with injection molding, the equipment cost is lower, the adaptability is stronger, the molding performance is good (such as low stress), and products with complex undulating curves (shapes) can be formed.
Key Points in the Design of Blow-molded Parts
- The connection between the cavity surfaces should be smooth and smooth
- The perimeter of the left and right sides of the product should be as even as possible, otherwise, the wall thickness on both sides will be uneven. In the most serious case, one side may even be too thick and the other side may be damaged.
- During the plastic blow molding process, the raw material polymer is first subjected to high shear force when passing through the die, and then the material exhibits extrusion expansion and shrinkage. When a sagging parison is formed, its expansion rate is close to zero. The parison is then blown up against the mold and exhibits a low expansion rate. Excessive die expansion can produce scrap. Excessive shrinkage leads to uneven wall thickness from the top to the bottom of the part, and in severe cases, it may even fail to form.
Common Defects of Blow Molded Parts and Their Improvement Methods
- The Longitudinal Wall Thickness of Blow-molded Products is Uneven
- Cause:
① The parison sags seriously due to its weight;
②The diameter of the two longitudinal cross-sections of the blow-molded product is too different.
- Solution:
① Reduce the parison melt temperature, increase the parison extrusion speed, replace the resin with a lower melt flow rate, and adjust the parison control device;
②Appropriately change the product design and adopt bottom blow molding.
- The Horizontal Wall Thickness of Blow-molded Products is Uneven
- Cause:
① The parison is skewed during extrusion;
②The temperature difference between the inside and outside of the mold sleeve and the mold core is large;
③The appearance of the product is asymmetrical;
④The parison inflation ratio is too large.
- Solution:
①Adjust the die gap width deviation to make the parison wall thickness uniform; straighten the parison before closing the mold;
② Increase or reduce the heating temperature of the mold sleeve to improve the temperature deviation inside and outside the die;
③ Before closing the mold, pre-clamp and pre-expand the parison to properly deflect the parison in the thin-wall direction;
④ Reduce the parison blow-up ratio.
- Orange Peel-like Patterns or Pitting Appear on the Surface of the Blow-molded Product.
- Cause:
① Poor mold exhaust;
② The mold is leaking or condensation occurs in the mold cavity;
③ The parison is poorly plasticized, causing melt rupture in the parison;
④Insufficient inflation pressure;
⑤Slow inflation speed;
⑥The inflation ratio is too small.
- Solution:
① The mold parison is sandblasted and vent holes are added;
② Repair the mold and adjust the cooling temperature of the mold to above the “dew point”;
③Reduce the screw speed and increase the extruder heating temperature;
④Increase the inflation pressure;
⑤Clean the compressed air channel and check whether the blowing rod is leaking;
⑥Replace the mold sleeve and mold core to improve the parison inflation ratio.
- Reduced Volume of Blow-molded Products
- Cause:
①The wall thickness of the parison increases, resulting in the thickening of the wall of the product;
②The shrinkage rate of the product increases, causing the size of the product to shrink;
③The inflation pressure is small and the product is not inflated to the designed cavity size.
- Solution:
①Adjust the program control device to reduce the parison wall thickness; increase the parison melt temperature and reduce the parison mold-leaving expansion ratio;
②Replace the resin with small shrinkage, extend the blowing time, and reduce the mold cooling temperature;
③ Increase the pressure of compressed air appropriately.
- The Outline or Graphics of the Blow-molded Product are not Clear.
- Cause:
① Poor cavity exhaust;
② Low inflation pressure;
③The melt temperature of the parison is low and the material is poorly plasticized;
④The cooling temperature of the mold is low, and the mold has a “condensation” phenomenon.
- Solution:
①Repair the mold, sandblast the cavity, or add an exhaust slot;
②Increase the inflation pressure;
③Appropriately increase the heating temperature of the extruder and machine head, and add an appropriate amount of filling masterbatch if necessary;
④Raise the mold temperature to above the dew point temperature.
- Blow Molding Products Have too much Flash and are too Thick.
- Cause:
① The mold is expanded and the clamping pressure is insufficient;
② The mold blade is worn and the guide pillar is offset;
③When inflated, the parison deflects;
④The escape groove at the edge of the clamping blank is too shallow or the depth of the knife edge is too shallow;
⑤ The parison inflation starts too early.
- Solution:
①Increase the mold clamping pressure and appropriately reduce the inflation air pressure;
②Repair the mold knife edge, correct or replace the mold guide pillar;
③Correct the center position of the parison and the blow rod;
③Trim the mold to deepen the depth of the escape chute or knife;
④Adjust the parison inflation time.
- Excessively Deep Vertical Stripes Appear
- Cause:
①The mold mouth is dirty;
②Mold sleeve, there are burrs or gaps on the core edge;
③The decomposition of color masterbatch or resin produces dark stripes;
④ The filter has holes, and impurities are mixed with the material and deposited at the die opening.
- Solution:
①Clean the die opening with a copper knife;
② Trim the mouth mold;
③ Lower the temperature appropriately and replace the color masterbatch with good dispersion;
④Replace the filter plate and use scraps.
- The Parison was Blown during Molding.
- Cause:
①The mold knife edge is too sharp;
② There are impurities or bubbles in the parison;
③The inflation ratio is too large;
④The melt strength of the parison is low;
⑤The length of the parison is insufficient;
⑤ The parison wall is too thin or the parison wall thickness is uneven;
⑥The container bursts when opening the mold (insufficient deflation time);
⑦The mold clamping force is insufficient.
- Solution:
①Appropriately increase the width and angle of the knife edge;
② Use dry raw materials, use wet raw materials after drying, use clean raw materials, and clean the mold opening;
③Replace the mold sleeve and mold core to reduce the blow-up ratio of the mold;
④Replace appropriate raw materials and appropriately lower the melt temperature;
⑤ Check the control device of the extruder or storage tank head to reduce changes in process parameters and increase the length of the parison;
⑥Replace the mold sleeve or mold core and thicken the parison wall; check the parison control device and adjust the die gap;
⑦Adjust the deflation time or delay the mold opening time;
⑧Increase the clamping pressure or reduce the blowing air pressure.
- Difficulty in Demolding of Blow Molded Products
- Cause:
① The inflation and cooling time of the product is too long and the cooling temperature of the mold is low;
②The mold design is poor and there are burrs on the cavity surface;
③When opening the mold, the moving speed of the front and rear templates is uneven;
④The mold is installed incorrectly.
- Solution:
① Appropriately shorten the parison blowing time and increase the mold temperature;
② Trim the mold; reduce the depth of the groove and the slope of the ribs to 1:50 or 1:100; use a release agent;
③Repair the mold clamping device to make the front and rear templates move at the same speed;
④Reinstall the mold and correct the installation position of the two halves of the mold.
- The Quality of Blow-molded Products Fluctuates Greatly.
- Cause:
① The wall thickness of the parison changes suddenly;
②The mixed scrap materials are unevenly mixed;
③The feed section is blocked, causing fluctuations in the discharge of the extruder;
④The heating temperature is uneven.
- Solution:
①Repair the parison control device;
② Use a good mixing device to extend the mixing time; reduce the amount of recycled material if necessary;
③Remove the agglomerates at the material opening;
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