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What is Common Blow Molding Problems and Solutions

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What is Common Blow Molding Problems and Solutions

Are you looking into the purchase of blow molding equipment?

Do you want to make the right decision for your bottling plants needs and assess the main problems before investing a significant amount in machinery?

You’re not alone; as a plant manager, owner or head of the production, making the right decisions for the business comes down to you!

Reputation, responsibility and ultimately production levels alongside revenue generated are all considerations that must be taken on board when conducting your own needs analysis for the production line.

It could be that you have ageing equipment that has become unreliable, you’ve outgrown the current capacities, or you have decided to expand operations into a new line or two.

Problems, defects and common errors can be an inconvenience at the very least; despite modern advances, there will always be an element of this to contend with.

Knowing how to quickly establish what has caused the problem to be addressed swiftly is critical; your buyers won’t accept sub-standard products.

Overlooking this simple quality checking can result in unhappy customers and reject or return orders.

Some will pertain to aesthetics, whereas others can see a failure in the materials or design of the item which has been blown molded.

Whilst much of the processing these days are automated or semi-automated so that human error is minimised, there can still be occurrences that cause problems for you and the team.

Blow molding as a whole is not exempt from its fair share of challenges, which we’ll explore through this article.

Some common problems will be specifically related to a production method or material, so they may not be entirely relevant to your setup or needs.

Rather than leaving you hanging or searching elsewhere, we’ll also provide the solutions, so you know how to deal with them should they arise or avoid them in the first place.

This table can be a helpful reference for this.

Without further ado, let’s jump in!


What Are The Types Of Blow Molding Available?

The main types of blow molding where we’ll focus our attention in this article are:

  • Extrusion Blow Molding (EBM)
  • Injection Blow Molding (IBM)
  • Injection Stretch Blow Molding (ISBM)


What Are The Common Problems With Blow Molding?

Your bottle leaks

As far as failures go, this is one of the significant failures since the containment of liquids is the primary function.

This error in manufacturing can be caused by a few typical scenarios, which are:

  • Poor quality joins or welds, resulting in a partial breakage or tear in the plastic.
  • Contaminated materials are widespread if reliance is on a re-grind blended with virgin polymers. The dirt or impurities can cause defects, bubbles or cohesion not to be as effective as it could be.
  • Poor design, if tolerances haven’t been correctly calculated or structural weaknesses not acknowledged and dealt with, can result in leaks. Prone areas can be the neck, handles or tail.
  • Incorrect gapping between blow pin nozzle and mold where the neck of the bottle results in the item not being molded effectively to the exact sizes needed; therefore, other components such as lids or screw caps are unlikely to create an effective seal.

Solutions for leaking bottles

To solve these troublesome issues, we recommend doing the following:

  • Check over the mold to ensure it is clean and not damaged or contorted in any way.
  • Be extra vigilant with the quality of the re-grind and ensure exact ratios for optimum performance are adhered to, mixing with virgin polymers or other blends you may need.
  • Revisit the design and prototyping stage if the output batches are riddled with defects; better to do this now than a recall once they’re with your customers.
  • Realign the blow pin nozzle or check sizes against other available options if needed to create a decent seal during the blowing phase.

The bottle is prone to pinch off-line tears.

The weld lines can be a weak point; the bottom of your product or where handles may have been included are particularly susceptible to this.

When de-flashing, ideally, an automated operation will run with a high degree of accuracy, but it can also be done manually.

Not having the correct clamping force can lead to the flash being ripped or tugged at instead of gracefully cut away.

There are a few causes for this problem:

  • Speed of mold closing when parison is placed into it, during which there are two stages, a fast and a slow close.

If they are not timed correctly, it can affect the quality and strength of the joints.

Step one contains the plastic parison holding it in the correct position, a faster action, and step two closes slower to help form a strong weld.

  • The cooling of the mold is not optimised, leading to uneven material cool rate. When this happens, the two sections don’t join together as effectively as they could; some of the plastic has cooled quicker when other bits remain hot.
  • Materials aren’t thick enough, so the weld line cannot form a strong join, resulting in splits or leaks.

Solutions for tears on the pinch off-line

  • Sharp and well-maintained tools will do their job effectively if something has started to blunt, then get it sharpened or replace damaged parts.
  • If possible, switch to automation to eliminate or reduce human error; should this not be an option, then ensure workers are adequately trained.
  • Redesign the product, and material thickness used, test again, repeat the process until happy with the prototype.

There's whitening or pearlescence on your bottles.

Hazing, whitening or pearlescence of the bottles is also known as stress-whitening.

This is due to the PET material being overstretched and causing the polymer structure to break apart and create white circles inside your bottles.

It can also be a temperature or timing problem, which can, in turn, lead to the over-stretching of a PET bottle.

Solutions for pearlescence problems

  • Temperature adjustment by heating the area more if thick walls make it more workable and less likely to stress.
  • Should the affected area be on the thinner side, cool it and heat the space underneath it or close to it; in your attempts to rectify it, you’ll need to be precise with how you target the heat.
  • Adjust your ventilation and airflow, affecting your products heating and cooling.
  • Vary the heating or cooling timings and lamp settings to see if this adjustment helps increase or decrease.
  • Your plastic injection speeds can also be altered where applicable to help remedy this.

Bottles break easily when dropped on the floor.

Your products need to live up to expectations, with standard tests being dropped on the floor, with liquid in, from around 4-5 ft.

Naturally, you can expect bottles to break when mistreated, dropped, smashed or subjected to high amounts of force.

This in itself is reasonable; however, bottles that display weakness here are usually not manufactured to the best of their capabilities, or there have been inconsistencies within the processes.

During ISBM, the orienting polymers are compacted and fused closer together, forming a tighter lattice at the molecular level.

Had this step not been carried out to the highest potential, it can present problems for you and your customers.

Weak bottles are likely to fracture or explode, especially with carbonated beverages.

Solutions for brittle or weakened bottles

  • Timings at each stage of the process must be reassessed to deal with this; revisit your steps to see how long the material spends on each one and increase or decrease accordingly.
  • The temperature of the blow molding process may need to be increased, so plastic is fully workable and in the ‘glass state’ when blown.
  • The amount of stretch exerted on the walls of your bottle may need adjusting, so it can achieve maximal structural integrity before cooling.

Top load strength problems

Having weak bottles is not ideal, and you can see your goods being damaged in transit as they stand up to the demands placed on them when handled, stacked, driven around town.

When the pallets of bottles are stacked 3, 4 or 5 high, the pressures exerted downwards can be referred to as a top load, so this strength is a critical aspect of your design.

Flimsy designs are setting you up for problems and can be addressed during the design or manufacturing stages.

Solutions for weak designs

  • Timings at each stage of the process must be reassessed to deal with this.
  • The addition of more material into the design of the preform or parison may be required.
  • Total redesign of the mold or bottle is the final step if the above solutions cannot rectify this issue.

The base doesn't sit flat on a surface.

This relates to the push up not being formed correctly; instead of being nicely contoured upwards into your bottle, it instead protrudes and pushes downwards somewhat, resulting in bottles rocking rather than standing flat and steady.

Caused by benign too hot after the molding process, if it’s released from the mold and the plastic remains very soft, warm and pliable, it is likely to set in an undesirable position.

This insufficient base depth can present problems further down the line if left unnoticed, checked or passed out when carbonated beverages are filled.

Having liquids stored at high pressure means the bottle needs a bit of giving, which comes built into the design in the form of an inwardly curved base.

Once the bottle has been blown, the curved section on the base can also become deformed by air not venting properly from the neck, so when the blow pin nozzle tries to retract and cannot, the base is blown outwards.

Solutions for bottle rock or protruding bases

  • Allow sufficient cooling after the mold is formed; increasing the water flow rate in the cooling system can solve this quickly enough.
  • Introduction of additional cooling, such as fans or a jet of air aimed explicitly at this problematic area to assist faster, more effective cooling can also be helpful here.
  • Ensure machinery is all operating as it should; a sticking blow pin nozzle or one that doesn’t retract at all will be causing you issues, including the possibility of a protruding bottom.
  • Recalculate or adjust the material thickness of the parison base in case the mold is not closing fully around it.

Top load strength problems

Having weak bottles is not ideal, and one can see the goods being damaged in transit as they stand up to the demands placed on them when handled, stacked, driven delivered.

When the pallets of bottles are stacked 3, 4 or 5 high, the pressures exerted downwards can be referred to as a top load, so this strength is a critical aspect of your design.

Flimsy designs are setting you up for problems and will exploit the wall thickness at its weakest point.

Solutions for weak designs

  • To solve this is one of the easier problems theoretically; you need to revisit the design and make the walls thicker; you’ll need to know which part has caused the failure to address this.
  • Don’t stack the products as high when in storage or transit.
  • More careful transportation, from the packaging to the people responsible for moving the items.
  • Extra vigilance and checks for correct oriented stretching for optimum strength on walls; this is quality control in the factory.

Wall thickness problems with your bottles

It should come as no surprise here that inconsistencies with thickness are undesirable characteristics, leading to a multitude of potential problems.

Parisons of preforms that are not calculated or designed correctly will likely be contenders and first places to look.

Misshapen bottles can happen if the wall thickness is not even, which can be rejected for use.

Another time you can discover this problem is if you’re attempting to inflate your parison or preform too much.

Solutions for the thickness of bottle walls

Unfortunately, a few variables at play here can cause these problems, so you’ll need to get your detective hat on and work through them one by one.

  • To increase the amount of material in the preform or parison.
  • Adjust temperatures during the forming of the parison, including the die sleeve temperature.
  • Speed of extrusion during parison being created.
  • Check and adjust the parison gap width concerning the deviation of the die.
  • Monitor the outputs more regularly to catch any slight variations from the desired result of the parison.
  • The inflation rate of the parison can need adjusting.

Bottles have imperfect surfaces and appear pitted, bubbly or have a texture like citrus fruit.

Visually this isn’t such a great look and can also result in weakness within the item; this should, therefore, be addressed as a matter of importance as the issue tends to worsen if left unnoticed or ignored.

The main culprits for this are:

  • Moisture or condensation introduced into the process can be at the stage of performing or parison production but is more likely when blow molding occurs.

Condensed water particles in the resin will produce bubbles.

  • Incorrect parison size/ thickness calculations cause low inflation; not enough pressure to inflate or flow rate is insufficient.
  • A leaking mold cavity inhibits the parison from forming its full expression.
  • A dirty mold or die and contamination of the polymers can lead to black spots or foreign bits embedded in the products.
  • This problem can be in multiple places within your blow molding setup but will commonly be within the extruder die head, mold itself or die bushing.

Solutions for surface imperfections on your bottles

To rectify these common issues, you’ll need to work through them systematically, starting with the most accessible first: Clean the whole setup, including the blow molding dies and parison mold, which also gives the chance to inspect for any damages or parts that may need replacing.

Sandblasting of the metal faces may be needed to ensure a smooth and clean surface.

  • Adjust the mold cooling temperature to not introduce moisture into the plastic.
  • Sufficient ventilation of each stage, especially the parison mold.
  • Regular maintenance and cleaning of the blow rod and nozzles; check they’re not partially blocked.
  • Adjust inflation ratios and pressure of the parison.
  • Install a hopper dryer to keep the residual moisture away from the polymers as they are heated and fed into tHe extruder.
  • Pre-heating the plastic resin or aiming to keep it warm before this step can also prove to be a solution for eradicating air bubbles.
  • Thorough cleaning of the die head and extrusion setup.

For some visuals to see common problems check this link

Problems with ring marks on your bottles

These unsightly formations can be seen to reject a batch of the products and are caused by your parison thickness not being uniform, so when heated, some of it is stretched more than the rest.

It is relatively straightforward in the grand scheme of things as they die or the mandrel is moved too quickly.

The solution to deal with ring marks

To sort this issue out, we recommend adjusting the thickness of the profile points and the distances in between in small increments.

Having continuity here is essential, and the lack of it has been the resulting factor in this particular problem.

Test, reassess and adjust again where needed.

Sometimes these more minor refinements can be time-consuming but are the finishing touches that ensure a perfect blow molded product.

Problems with rippling on the bottle walls

Another visual imperfection issue, with this, you’ll discover lines or a rippling in the plastic walls, which can be pretty discreet or very pronounced depending on its severity.

This is usually caused by the warm plastic parison coming into contact with the surface of the mold, which is cold in comparison before it has been entirely blown to size and shaped.

Particularly tricky to solve when dealing with shapes of the bottle that are spherical or oval, as the parison undergoes a ‘pre-blow’ to roughly establish the body before the final form is taken.

A wider parison through the standard diameter opening of the mold can lead to an intermittent touch of the surfaces and premature cooling or solidification in the areas that make contact.

Since the process is designed to take a warm plastic [parsion and blow mold to shape in one step, this cooled down surface area will respond differently from the rest of the finished product.

Solution for your rippled bottles.

To resolve this issue, there are a couple of methods we can suggest, which are:

  • Use a larger die mouth, so the warm parison doesn’t make contact before being blown to shape.
  • Change the dimensions of your parison or output units if it’s an option.



Blow molding is a great process and can be used extensively within your factory for bottles, hollow containers, and various products.

Keeping up to date with regular cleaning and servicing will assist with this; however, quality and reliable machinery will also be a serious consideration to help the production of bottles.

You can find more information about these and other manufacturing-related queries on our website or by emailing [email protected].

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