5 Common Blow Molding Manufacturing Flaws

• 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.

• 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.

• 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.

• 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.