The blow-up ratio stands as a key factor in film extrusion. It has a huge impact on things like tensile strength and clarity. In simpler terms, it tells you how much a plastic tube stretches after it comes out of the circular die in the film blowing process. Learning to control this ratio can make a big difference in the final quality of your film.
When we think about blow-up ratio, it seems simple, but it connects to deeper ideas. Picture having a die that’s 100 mm in diameter, and it makes a film with a final bubble of 200 mm. The blow-up ratio here would be 2:1. This idea is the base of the formula the industry uses for blow-up ratios. But its importance goes way beyond just math. Polymers act depending on their type. For example, LDPE often matches well with BUR values from 2 to 3, while HDPE tends to work better with higher ratios like 4 to 6.
Blow-up ratios can range from 1.5:1 often used to make thick and firm films, all the way up to 6:1 for thinner more bendable films.Sure! Please provide the text you’d like me to rephrase, and I’ll follow the outlined instructions . I’m sorry, but I don’t have any sample text to work with in this context. Could you please provide the specific content or passage you’d like me to rewrite? This guide takes a deep dive into calculating blow-up ratio , looks at its effects on film attributes, and shares useful tips to optimize this key parameter for various materials and uses.
Blow-Up Ratio: What It Is and How To Calculate It
The plastics industry relies on knowing key parameters to maintain quality. Instead of sticking to simple terms, I’ll dig into the technical details of an important metric in the extrusion process.
What does blow-up ratio mean in film blowing?
The blow-up ratio, or BUR, works as a key measurement in plastic film production. It shows how much the film expands after the molten polymer leaves the die. BUR calculates the ratio of the final bubble’s diameter to the die’s diameter.
BUR values usually fall between 1.5:1 and 6:1. Thicker stiffer films often require a lower BUR around 1.5:1. Thin more flexible films need higher values closer to 6:1. For stretch films, manufacturers aim for 3:1 to 4:1, while heavier applications stick to lower ratios like 2:1 to 3:1.
Blow up ratio formula and lay-flat width method
Here’s a simple formula used to calculate BUR:
BUR = Final Bubble Diameter / Die Diameter
As an example imagine the die has a 100mm diameter and the final bubble’s diameter is 200mm. In this case, the BUR works out to 2:1. However, measuring the bubble’s diameter during production isn’t always easy.
To simplify this many operators rely on the lay-flat width method instead. The width of the flattened film tube connects to the bubble’s diameter, which gives us another formula to calculate BUR:
BUR = (Lay-flat Width × 0.637) / Die Diameter
Or put another way: BUR = 2 × Lay-flat Width / (π × Die Diameter)
What the extrusion blow-up ratio formula means
Let’s break this formula into parts to make it clear. The diameter of the finished bubble (D₍f₎) is determined by measuring the steady inflated bubble during the production process. The die diameter (D₍d₎) refers to a set specification of the extrusion machinery being used.
Here’s a clear example: if a die has a 50mm diameter and creates film with a lay-flat width of 471mm, you would calculate the BUR to be close to 3:1. Operators carefully manage this ratio by tweaking factors like the air pumped into the bubble, the speed of the take-up, and the melt temperature.
How BUR Changes Film Behavior
Changing the blow-up ratio has a big effect on how the film performs. It plays a key role in shaping the molecular orientation, which decides how the film handles everyday stresses.
Strength Along and Across the Film
The connection between blow-up ratio (BUR) and tensile properties develops in a noticeable way. When the BUR is lower, polymer chains line up along the machine direction. This alignment makes films very strong in the machine direction but much weaker in the transverse direction. Higher BUR shifts the orientation toward the transverse direction boosting TD tensile strength. In some cases, increasing the BUR from 1.7 to 2.8 can improve TD strength by as much as 27 percent.
This trade-off clearly shows up when testing data is examined. For LLDPE films, increasing the BUR from 1.5 to 2.45 allows tensile strengths to reach 50 N/mm² in the MD direction and 48 N/mm² in the TD direction. Real-life cases indicate that higher BUR values tend to cause about a 14% drop in MD tensile strength, but TD strength rises in a similar way.
Impact resistance and tear strength
Impact resistance and tear strength seem to have an interesting relationship with the blow-up ratio. moderate BUR levels, like 3:1 to 4:1, hit the sweet spot for achieving a solid balance of impact strength in all directions. One study found that with proper BUR adjustments, researchers could reach an impact strength of 2,808 J/mm².
For tear strength, MD tear resistance improves when non-crystalline chains get aligned in all directions within the film plane. On the other hand, TD tear resistance works best when crystalline lamellae are straighter and oriented closer to the TD of the film. In practical settings, raising the BUR from 1.7 to 2.8 can increase MD tear strength by as much as 36%, though it lowers TD tear strength by about 16%.
Blow-up ratio to improve shrink film and how it behaves
Shrink films need adjustments to prepare the right blow-up ratio. To achieve better shrinkage ability certain polymer systems may need changes like copolymerization or cross-linking. The way molecules are arranged during the process plays a big role in how the film shrinks when heated.
The percentage of MD shrinkage rises almost as TUR/BUR ratios grow. This happens because higher TUR/BUR pulls on polymer chains more along the machine direction, which boosts MD shrinkage. The way polymer blends are made also plays a role in how they shrink. Blends with more PLA content showed higher MD shrinkage compared to ones with less PLA.
Optical and Structural Changes Using BUR
More than just mechanical traits how films look can affect their market appeal. Changing the blow-up ratio has a big role in altering the visual and optical qualities of films.
Clarity and amount of haze
The connection between the blow-up ratio and haze involves various factors. Higher BUR levels improve biaxial orientation, which helps films become clearer by forming more even crystalline patterns. Faster stretching minimizes crystal growth resulting in smaller and more consistent crystals that scatter less light.
Haze comes from two key sources:
- Surface haze makes up 63-97 percent of the total haze and happens because of surface roughness.
- Internal haze occurs due to light scattering inside the bulk material of the film.
When you increase the BUR in polyethylene films from 2:1 to 4:1, haze values drop . They go from 15-20 percent to under 10 percent (link_5).
Gloss and smooth surfaces
Measuring gloss at a 45° angle works well to spot differences between various polyethylene films. Larger blow-up ratios often boost gloss by smoothing the surface through steady inflation of the bubble. The smoothness of the surface has an impact on gloss since the strength of reflected light depends on the surface it bounces off.
Issues with Film Thickness and Consistency
Keeping film thickness controlled gets harder when BUR values get higher. When BUR crosses 4:1, bubble stability tends to drop. Small changes in temperature or air pressure can cause wobbling, which leads to uneven film thickness. To calculate thickness, the formula is: final thickness equals die gap thickness divided by BUR.
Deciding on BUR Levels to Use with Different Polymers
Choosing the best blow-up ratio values depends on the type of polymer undergoing processing. Each polymer’s melt behavior and molecular structure create unique challenges during film blowing.
LDPE: Flexible and easy to process
Low-Density Polyethylene works well due to its flexibility and solid melt strength. It can handle a broad BUR range from 2:1 up to 4:1. LDPE stays stable under different processing setups. When used at the right BUR, it creates films with well-balanced strength in both machine and transverse directions while keeping clarity intact. This balance makes LDPE suitable to make shopping bags and food packaging materials.
LLDPE: Strength and durability
Linear Low-Density Polyethylene offers impressive toughness and resists punctures well. It is processed at BUR ratios between 1.8:1 and 3.5:1. Studies show that the best LLDPE samples reached top-notch impact strength measuring 2,808 J/mm² when processed at a 2.0 BUR. These films also stretch achieving elongation of 870% and 825% in MD and TD directions. This makes LLDPE a great choice to create heavy-duty and courier bags.
HDPE: Strong stiffness with great tear resistance
High-Density Polyethylene needs higher BUR ratios falling between 3:1 and 5:1. The crystalline structure of this polymer aligns more during the blowing process, which boosts its crosswise strength. HDPE films provide great stiffness and strong barrier abilities. These qualities make them ideal to create thin yet durable bags like T-shirt bags and garbage bags.
Polypropylene: Lower BUR ratios
Polypropylene has long struggled on regular PE film lines because it lacks good melt strength. However improved PP grades now work with standard blown film equipment. BUR ratios for PP range from 3:1 to 6:1. The newer grades also deliver top-notch optical results, like 70% gloss and 4% haze in films with a 40-micron thickness.
Application-specific BUR recommendations
| Material | Typical BUR Range | Key Characteristics | Common Applications |
|---|---|---|---|
| LDPE | 2:1 – 4:1 | Balanced properties, flexible | Shopping bags, food packaging |
| LLDPE | 1.8:1 – 3.5:1 | Puncture resistance, impact strength | Heavy-duty bags, courier bags |
| HDPE | 3:1 – 5:1+ | Stiffness, barrier properties | T-shirt bags, garbage bags |
| PP | 3:1 – 6:1 | Clarity, stiffness | Hot-fill pouches, technical films |
Conclusion
Knowing the blow-up ratio is a key skill to master in plastic film production. This guide has shown how this simple parameter has a significant influence on properties like mechanical strength optical features, and the overall quality of the film. When manufacturers control BUR , it gives them a strong way to create films that match specific applications.
The connection between BUR and film characteristics often follows clear patterns. Lower ratios tend to boost strength in the machine direction, while higher ratios focus more on transverse qualities. Picking the correct BUR also helps balance important traits such as dimensional stability tear resistance, and impact strength, all of which matter for how the film will perform.
Different types of polymers need varying methods to process them. LDPE performs best in a moderate range between 2:1 and 4:1. On the other hand, HDPE achieves its ideal properties at higher ratios from 3:1 to 5:1. LLDPE stands out by offering great toughness within a range of 1.8:1 to 3.5:1, which makes it an excellent choice to use where puncture resistance is a priority.
Changing BUR values also has an effect on optical properties. Higher ratios tend to enhance both gloss and transparency but may cause issues with how the material’s thickness spreads out. Manufacturers should weigh and adjust these factors based on what their product needs.
Instead of seeing BUR as just another setting to tweak, think of it as a critical factor that can elevate ordinary films into standout products. Even minor changes in the blow-up ratio have the potential to bring noticeable benefits to how a film performs, looks, and holds up .
When you face problems with film quality, start by checking your blow-up ratio. This key parameter could be the solution you need to achieve improved results.