Blow Molding Troubleshooting Guide – Top 5 Problems – Extrusion Blow Molding is a popular process for the production of HDPE chemical drums, edible oil jerry cans, beverage, chemical or pharmaceutical bottles, and more. Such popularity comes from being the most economical and highest production output forming process of plastic products that are not only durable, but very high quality. However, there is a chance of encountering some problems during this process, so when it comes to blow molding defects, it is extremely important that you learn to identify their origin and know how to handle each of them.
This blow molding troubleshooting guide will therefore introduce you to the Top 6 Common Defects and Problems of HDPE Extrusion Blow Molding, their possible causes, and solutions and recommendations which can help you obtain the best plastic products possible.
Top 5 Problems and Solutions
Becoming familiarized with blow molding defects should always be a priority. This way, when production problems arise, coming up with an efficient solution will not take too long and overall plant productivity will be kept. Therefore, this leaves no doubt that having a proactive approach to blow molding troubleshooting is necessary to prevent delays and maintain a company’s overall performance.
Blow molding troubleshooting includes a variety of defects including the following:
1. Leakage
Leakage from HDPE bottles can be caused by many reasons, it can be due to tear in the bottle’s wall, poor welding of the molten plastic in areas such as the bottom, side or top handles; it can also be caused by the melted resin being contaminated by dirt or impurities coming from recycled material, which can lead to the product’s walls not being leakproof.
For plastic products with a narrow neck (such as bottles, jerry cans and narrow mouth drums) the source of the leakage can be at the top surface of the neck and might be a little harder to detect. When designing the tooling for your product, the blow molding manufacturer must make sure the blow pin nozzle and the mold are the perfect match, depending on the parison wall thickness, the gap between these two should be narrow enough to let the blow pin push the material inside the mold so the plastic can fill the space between the mold and the blow pin, hence making the top of the neck flat (avoiding leakage when sealing the container). If this gap is too big, the material will not be pushed inwards to fill the space and the top surface of the neck will not be flat, thus, causing leakage.
2. Rocker Bottoms
Why are rocker bottoms caused in bottles? Unstable or rocker bottoms of blow molded bottles is a problem usually caused by insufficient cooling of the blown bottle before removing it from its mold. If this is the case, try increasing the water flow of your cooling system. If there is still no result, you can also check the blow molding machine’s cooling channels for any blockage and clean them. Sometimes, the problem can be caused by excessive thickness of the parison in the flasher area at the bottom, this causes the mold to not fully close at this point and the material not touching the mold completely as it is supposed to; thus, make sure the parison thickness at this area is adequate to allow the mold to fully close.
Additionally, rocker bottoms can also be originated by poor air exhaust from the product’s neck after the blowing cycle has finished and before the mold opens; as when the product is being formed, there is air being blown at a constant pressure pushing the Parison walls against the mold, so when this process finishes, the blow pin nozzle should slightly retract and allow the extra pressure out, otherwise this pressure will most likely expand the product at the bottom, hence resulting in an unstable bottle.
3. Tearing of Welding Line at Pinch-off (or Deflashing)
Tearing of a container when deflashing the flasher material can happen and especially at the welding lines of the product (such as handle areas and the product’s bottom), a very common cause of this issue is the mold close slow speed. What does this mean exactly? To catch the parison inside, the clamping unit must close the blow mold in two steps: 1st mold close (fast speed) and 2nd mold close (slow speed), the purpose of the 2nd mold close with slow speed is to push the material inwards the mold in order to form strong welding lines, and thus, avoid tearing at these areas when removing the scrap material. So, if your bottle, drum or jerry can is having this problem, try reducing the speed of the mold slow close.
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If after reducing the mold slow close speed the problem persists, then it might be that the parison thickness is too thin at the handle area of the jerry can or that the bottom flasher area is not enough to form a strong welding line, therefore, try increasing the thickness profile at these points in the parison so that when the mold closes, there is sufficient material to support the welding.
If the tearing keeps happening after implementing these two solutions, then the problem might come from a poor cooling system design of the mold.
4. Surface wall defects
From almost unnoticeable defects like tiny black spots, to larger ones like horizontal ring-like lines, vertical streaking lines, wall bubbles, and rough, rippled or “orange peel” like surfaces, these can all affect your bottles and appear due to many different reasons.
Black spots
Black spots can be caused by foreign matter or old resin (which has already degraded) inside the extruder die head that contaminated the melt and resulted in different colored spots on your jerry can or bottle. Sometimes, the foreign matter or old resin might be stuck inside the die bushing and causes a distortion or vertical line in the extruded parison, thus marking streaking lines in the product; the solution to this is to find where in the die head the contamination is and clean it thoroughly.
Wall bubbles
As for wall bubbles, these are caused by moisture or water particles condensed in the cold resin due to warm and high humidity climates. To avoid this, you can install a hopper dryer or try to keep the resin warm to evaporate the condensed water before it goes into the extruder hopper; also make sure cooling water flow in the extruder feed opening is not so high.
Horizontal ring-like line
When a horizontal ring-like line is formed around your drum or jerry can, it means the parison thickness difference between two consecutive points is too large, hence, the parison thickness controller will move the die or die mandrel too fast, resulting in a ring-like line showing on your product. To prevent this, gradually increase/decrease the thickness between the profile points, also keep the thickness of the first and last profile points the same or very close.
Rippled surface or orange peel
A rough surface that seems rippled or like orange peel, is caused by the parison wall coming into contact with the cold mold surface two times intermittently instead of just once, therefore, prematurely solidifying the wall before it is blown and then stretching it a second time, which does not allow the parison wall to be smoothed out. This situation can happen especially with oval shaped jerry cans and bottles because the parison is too narrow, so the pre-blow is set high in order to expand it and get an oval shape-like parison before the cycle blow. To solve this, try using a bigger die mouth so you can reduce the pre-blow, the oval shape needed for the bottle will be formed with the cutting and sealing unit of the blow molding machine.
5. Uneven Radial Wall Thickness Distribution
An even wall thickness distribution of your jerry can, bottle or drum´s wall is very important to avoid excessive shrinkage of the product and overall bad quality. How does this happen? A product with an uneven radial thickness will have a thinner side that will be cooled down faster than the one with wider thickness, therefore requiring a longer cooling time otherwise the bottle will shrink too much.
To solve this, try aligning the die mandrel(pin) and the die until the extruded parison looks straight. Because parisons with bigger diameters are easier to control than narrower ones, you can also try to increase the die mouth diameter and reduce the pre-blow of the parison as well.
