5 Common Blow Molding Manufacturing Flaws

January 27, 2016 - "Industry Insight"

Blow molding is a complicated process, and there are several manufacturing flaws that can occur. The best way to avoid these problems is to work with an experienced supplier, who has the expertise to avoid, or fix, all of these problems.
Here is a rundown of five common blow molding manufacturing flaws to watch out for.
Rocker Bottoms
A rocker bottom is when the bottom of the product is rounded and won’t sit flat. This defect is caused by a warping of the plastic, which can occur if the part is removed from the mold before it’s adequately cooled.
There are several ways to solve this problem:
  • Increase the amount of cooling water to the mold. This will help the plastic cool faster.
  • Lower the resin temperature. The part will need to be monitored closely to make sure there aren’t cold spots in the part wall.
  • Make sure the cooling channels aren’t blocked. If they are, the mold needs to be cleaned.
  • If none of these work, look at the part design. If the variation in the resin distribution is too high, the part may require an abnormally long time to cool completely. Fixing this problem may require redesigning the mold.
Defects Within the Blown Wall
Bubbles and cold spots in the wall of your part ruin its appearance.
Bubbles can be caused by moisture in the resin. Here are two ways to avoid this problem:
  • Let the resin warm up and the moisture evaporate. Bubbles are less likely to occur if the temperature is high enough before production starts.
  • Increase the temperature in the transition section. This will eliminate any residual moisture.

Cold spots occur when the resin is not melted uniformly. This problem is most common with resins that have a lower melt temperature.

Poor Weld and Parting Lines
Problems at the weld and parting lines can weaken the final product and may result in tears or breaks. This threatens the structural integrity of the part and is especially problematic for parts that are meant to hold liquid.
There are three weld defects that you should watch for:
  • Thinning of the weld. This can occur when the resin temperature is too high, the pinch-off land length is too short, or the pre-blow air is happening too early.
  • Tearing of the flash during trimming. This can occur if the pinch-off land length is too long. Tearing can also happen if the mold is damaged, the mold clamp pressure is uneven, or the pinch-off is worn.
  • Cutting at the pinch-off. This is when there is a hole in the part’s weld. It can happen if the resin temperature is too low or if the mold closes too quickly.

Poor Part Surface

Rough or pitted surfaces can be downright ugly. There are several reasons that your part may look rough or pitted.

  • The mold surface is old or worn. If this is the case, the surface should be refinished.
  • The vents are clogged or too small. Vents are necessary for air to escape the mold. If they’re blocked, the air can’t escape and may cause surface problems.
  • The air lines are clogged. Blow pressure needs to be high enough that the air can quickly and fully blow mold the part. The air lines should be checked for clogs and the blow pin for any leakage.
  • There is moisture in the resin or the resin isn’t melted uniformly. The same issues that can cause bubbles and cold spots can also cause rough or pitted surfaces.
A blow-out is essentially a hole in your product. Blow-outs have many possible causes:
  • There are problems with the pinch-off. The pinch-off shouldn’t be too sharp, too wide, or too hot.
  • The clamp pressure is too low. This can cause the mold to separate and the part to blow out.
  • The blow pressure is too high. When this happens, the parison is stretched too quickly.
  • The parison is too short. If the parison is too short to be caught at the bottom pinch-off, there may not be enough resin to adequately fill the mold.
  • The blow-up ratio is too high. If the parison expands too much, the walls can get too thin.
  • There’s moisture in the resin. See above for ways to reduce moisture.
Many other manufacturing issues can occur while producing blow molded products, like parison curl, stringing, die lines, and shrinkage. A qualified supplier with a dedicated quality assurance procedure will be able to identify any potential issues in the process and correct them so your part is manufactured correctly to spec. Click to learn more about blow molding.
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