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CHEN WAY - Top 6 Common Problems in HDPE Blow Molding

Issue 200 2021-08-05 Author: CHEN WAY MACHINERY CO., LTD.

HDPE Blow Molding is the most economical- highest production output forming process for hollow products: beverage, chemical or pharmaceutical bottles, edible oil jerry cans, chemical drums, etc. And even though you can produce an almost perfect, high quality container, you might encounter different problems along the blow molding process of your products, so it is vital that you can identify where the problem comes from and how to handle it.

That being said, we will introduce you the top 6 common defects and problems when making HDPE Blow Molding products, the possible causes of the defects, and solutions and recommendations that can help you solve this problem:

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.

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.

 

6.    Finish Trim Defects 

Trimming the flasher of the jerry cans and bottles should be done without too much effort from the operator (or if done with auto-deflashing unit, smoothly). If your product’s deflashing is not ideal, it might be due to not enough clamping force in that flasher area in the mold. This happens frequently when a small mold is run in a blow molding machine that is for a higher volume range, for example, when a 1L or 5L jerry can mold is run in a machine made specially for 10L – 25L jerry cans, which is not ideal nor recommended, as the clamping force will not be centered, and the mold will not properly cut around the flasher area. Therefore, the ideal solution is to not run a mold that is out of that specific blow molding machine.

 

 
If you are running the molds in the corresponding machine and you have this problem, then it is due to not enough cooling of the flasher, which can be caused by a poor mold cooling system design or flow, or because the parison thickness is too big in the flasher area and the mold does not close completely, hence, not cooling down the flasher enough to trim it. To fix this, try reducing the thickness in the flasher areas of your bottle, drum or jerry can.
 

What to Keep in Mind?
Defects are never good in a product, as a blow molding plastic products manufacturer, it can largely affect your production output rate and thus, the relationship with your valuable clients. And as a purchaser of blow molding plastic products, it is simply unacceptable as it would not meet the quality standards.
So, it is important that you inspect thoroughly your bottles, jerry cans or drums and if you encounter any of the above-mentioned defects, that you can identify where they come from and how to avoid or fix them. Of course, there might be some rejected pieces every here and there, but the key is to keep these numbers below 2% or to the absolute minimum.

Purchasing the Right Blow Molding Machine

Purchasing high-quality machinery is the best prevention step you can take to avoid problems, when purchasing an Extrusion Blow Molding Machine, an assured quality consistency in the produced containers is of top importance, as it is of no benefit to have a high output if you still have a high percentage of rejected jerry cans or bottles in your production line.

Therefore, choose the best blow molding machine manufacturer that can provide you with all the incredible benefits that extrusion blow molding can bring to you! Assured quality consistency, high production output, efficient after-sales service, latest technology, and a competitive price. 

When purchasing a blow molding machine from Chen Way Machinery, you can rest assure that our high-performance machines will run with maximum effectiveness and meet all your requirements. Moreover, as a Chen Way customer, you will benefit of our lifetime after-sales and our rapid online customer service that will respond to you within 24 hours.

You are just one step away from great plant productivity and profit with customized blow molding machines, all these through Chen Way Machinery, your lifetime friend and support!

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