Blown Film Extrusion Introduction

Blown Film Extrusion Introduction
Blown film extrusion is a technology that is the most common method to make plastic films, especially for the packaging industry. The process involves extruding a tube of molten polymer through a die and inflating to several times its initial diameter to form a thin film bubble. This bubble is then collapsed and used as a lay-flat film or can be made into bags. Usually polyethylene is used with this process, and other materials can be used as blends with these polymers. A diagram of a polyethylene chain is shown in Figure 1 to the right.
Background Theory on Polymers

In the cooling step of blown film extrusion, the amorphous, transparent melt crystallizes to form a translucent, hazy, or opaque film. The point where opacity begins in the bubble is known as the frost line. 
Fig 1: Model of polytheylene chain frompolyethylene Wikipedia article.The frost line height is controlled by several parameters: the air flow, film speed, and temperature difference between the film and the surroundings. Properties of the film, such as tensile strength, flexural strength, toughness, and optical properties, drastically change depending on the orientation of the molecules. As the transverse or hoop direction properties increase, the machine or longitudinal direction properties decrease. For instance, if all the molecules were aligned in the machine direction, it would be easy to tear the film in that direction, and very difficult in the transverse direction.

The Film Blowing machine Process

Fig 2: Schematic of set-up from User:J.Chiang.

Typically, blown film extrusion is carried out vertically upwards, however horizontal and downward extrusion processes are now becoming more common. Figure 2 shows a schematic of the set-up for blown film extrusion. This procedure consists of four main steps: The polymer material starts in a pellet form, which are successively compacted and melted to form a continuous, viscous liquid. This molten plastic is then forced, or extruded, through an annular die. Air is injected through a hole in the center of the die, and the pressure causes the extruded melt to expand into a bubble. The air entering the bubble replaces air leaving it, so that even and constant pressure is maintained to ensure uniform thickness of the film. The bubble is pulled continually upwards from the die and a cooling ring blows air onto the film. The film can also be cooled from the inside using internal bubble cooling. This reduces the temperature inside the bubble, while maintaining the bubble diameter. After solidification at the frost line, the film moves into a set of nip rollers which collapse the bubble and flatten it into two flat film layers. The puller rolls pull the film onto windup rollers. The film passes through idler rolls during this process to ensure that there is uniform tension in the film. Between the nip rollers and the windup rollers, the film may pass through a treatment centre, depending on the application. During this stage, the film may be slit to form one or two films, or surface treated. Advantages Blown film generally has a better balance of mechanical properties than cast or extruded films because it is drawn in both the transverse and machine directions. Mechanical properties of the thin film include tensile and flexural strength, and toughness. The nearly uniform properties in both directions allow for maximum toughness in the film.
Blown film extrusion can be used to make one large film, two smaller ones, or tubes that can be made into bags. Also, one die can make many different widths and sizes without significant trimming. This high level of flexibility in the process leads to less scrap material and higher productivity. Blown films also require lower melting temperatures than cast extrusion. Measured at the die opening, the temperature of cast film is about 220 C, where as the temperature of blown film is about 135 C. Furthermore, the cost of the equipment is approximately 50% of a cast line.

Blown Film Extrusion Disadvantages Blown film has a less effective cooling process than flat film. Flat film cooling is done by means of chill rolls or water, which have significantly higher specific heat capacities than the air that is used in the blown film cooling process. The higher specific heat capacity allows the substance to absorb more heat with less change in the substance temperature. Compared to cast film, blown film has a more complicated and less accurate method to control film thickness; cast film has a thickness variation of 1 to 2% versus the 3 to 4% for blown film. The resins used for casting typically have a lower melt flow index, which is the amount of polymer that can be forced through a standard die in 10 minutes according to a standard procedure. The melt flow index for cast film is about 5.0 g/10 min where as for blown film it is about 1.0 g/10 min. Consequently, the production rates for cast film are higher: cast film lines can reach production rates of up to 300m/min where as blown film lines are usually less than half this value. And finally, cast film has better optical properties, including transparency, haze, and gloss.

Common Problems Air entrapment between film layers and rollers – this may cause film scratching or wrinkling, or processing problems when winding up the film due to reduced friction. Possible solutions to this is using a vacuum to remove entrapped air or by using winding rolls with a diamond shaped groove in the rubber cover to increase surface area and decrease amount of entrapped air in the film. Large output fluctuations from the die – this causes thickness variations, and can be prevented by keeping the extruder clean and by using more consistently shaped pellets in the extruder. Melt fractures – these appear as roughness or wavy lines on the film surface, and can be eliminated by lowering the viscosity of the polymer melt. This can be done by increasing the melting temperature or by adding an internal lubricant to the material composition. Thickness variations in the film – this can be avoided by centering the die in the extrusion line before every run, adjusting the air speed of the cooling system, or by using heated die lips. Die lines on the surface of the film – this defect reduces the aesthetic appeal of the film, reduces optical properties, and weakens mechanical properties such as tear strength. This can usually be avoided by routinely cleaning the inner surfaces of the die and by refinishing scratched or roughened flow surfaces. Gels – these defects are small, hard globules encapsulated in the film or stuck on the film surface and reduce the aesthetic appeal of the film and cause stress concentration points which may result in premature failure. These are caused by overheating to the point of polymer degradation in the die, and can therefore be avoided by cleaning the inner surfaces of the die on a regular basis. Optimization of the Process Coextrusion One way to improve the line efficiency of blown film extrusion is to implement coextrusion. This is the process of extruding two or more materials simultaneously through a single die. The orifices in the die are arranged such that the layers merge together before cooling. This process saves time because it extrudes two or more layers at the same time, and it provides a method with fewer steps to produce multilayer films. The production rate for a coextruded multilayer film of three layers is about 65m/min, and the production rate for a single layer of blown film is about 130m/min. Thus, in order to produce 10 000m of a three layer multilayer film, it would take almost 4 hours using a single layer blown film process, and only 2 and a half hours using the coextrusion process. Furthermore, the film produced from the single layer process would require an extra step to glue the layers together using some sort of adhesive. Coextrusion is the least expensive means of producing layered films and the coextrusion system is capable of quick changeovers to minimize production line down time.

Minimizing the Melt Temperature
The efficiency of blown film extrusion can be improved by minimizing the temperature of the polymer melt. Reduction of the melt temperature causes the melt to require less heating in the extruder. Normal extrusion conditions have a melting temperature at about 190 C despite the fact that the temperature of the melt only needs to be about 135 C. However, it is not always practical to decrease the melting temperature by that much. By decreasing the melt temperature 2 to 20 C, the motor load can be decreased by about 1 to 10%. Furthermore, reduction of the melt temperature causes less need for cooling, so there is a reduced use of the cooling system. Moreover, removing heat from the bubble is usually the rate-limiting factor in this extrusion process, so by having less heat in the polymer to remove, the rate of the process can be increased, thus yielding higher productivity. A way to maintain the melt temperature at a minimum is to choose an extruder that is matched to the specific processing conditions, such as the material of the melt, pressure, and throughput.

Heated Extrusion Die Lips
Typically, solutions to melt fractures involve decreasing the output or increasing the melt temperature to decrease the shear stress in the extruder. Both of these methods are not ideal because they both reduce the efficiency of the blown film line. Heated extrusion die lips can solve this problem. This targeted heating method allows for film extruders to be run at higher production rates with narrower die gaps while eliminating melt fractures. Direct heat is applied to the surface of the polymer melt as it exits the die so that viscosity is reduced. Therefore, melt fractures, which are caused when trying to extrude too much of the polymer at one time, will no longer act as a limiting factor to increasing the production rate. Furthermore, heated die lips use less energy than increasing the melting temperature because only the surface of the melt is heated and not the bulk of the liquid. Another benefit of using heated die lips is that thickness variations can be controlled by adding heat to certain areas along the die circumference to make the film at that position thinner. This would ensure that no excess material is used.
Determine how much energy each of these processes can save per given volume of material. 

Above article from www.appropedia.org

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Why Houses Aren’t As Bad As You Think

How to Hire the Best Real Estate Agent

You should be able to get a good real estate agent to help you with the process of buying a house. When you hire a real estate agent, you will have a professional helping you search for the best house for you. When you are looking to get the best home possible, you should try and find the best real estate agent to assist you. It is important that you gather information about the real estate agents that are available for you in order to choose the best one for you.The information that you may need for your research will be available online in the websites of the agents. Here are some tips of consideration when you are looking to hire the best real estate agent.

Your budget for hiring the real estate agent should be consider when choosing an agent. You should be able to hire a real estate agent that is within your budget. However it is important to set aside a good amount of money for the agent. Spending enough on the real estate agent will ensure that you do not regret the home you get.Good real estate agents will also be good at bargain and that will ensure that you get a good deal on the home that you are buying.

You should consider the amount of time that the real estate agent has spent in the job. An experienced real estate agent will be able to know all the right places that you can get a good home. They will also have a good customer relationship that will have been gained from the time that they spend in the field. Experience also means that the real estate agent will be able to know what you are looking for when you tell them. They will also be able to know which the best neighborhoods are for you through their experience.

You should also consider the reputation of the real estate agent. It is good that you hire a reputable real estate agent.This way you will be assured that you will get a good home. The reputation is gained from providing great services to the clients. You can find out the reputation of the real estate agent from the previous clients or you can see the ratings from the internet.

The real estate agent should be able to present all required certificates before you can hire them. The certificates are issued after one has undergone all the right trainings in the area. License on the other hand will show that the real estate agent has al he legal requirements to perform the duties.

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Vital Tips for Picking The Suitable Termite And Pest Control Services Specialist

You need to seek the professional services of a termite and pest control firm when you come across termite infestation in your home. You will come across a large number of companies offering these services which can complicate the process of picking the suitable. Carrying out proper research is vital if you are to choose the most suites service provider.

The termite and pest control services firm that has the capability and capacity to professionally undertake the work is the one that has long establishment in the market. You can consider visiting the web where you will find important information that will aid you in making the suitable choice. Ensure that you look at the experiences of clients that have enlisted the services of the specialist in the past. The specialist that you pick must be highly regarded in the profession.

It is your duty to choose a termite and pest control services company that is properly registered and licensed to operate the business locally. Ensure that you are hiring a firm that has membership to a supervisory agency that is responsible for regulating operations of stakeholders in the industry. How the company is rated at the Better Business Bureau will influence the decision you will make in the end.

You need to enlist the services of a termite and pest control specialist that is aware and adheres to the requisite safety guidelines. They must demonstrate commitment to the preservation of the environment as the chemicals used in their work are highly toxic. See to it that you are contracting a properly and adequately insured pest company.

A thorough evaluation must be undertaken at your property by the termite and pest control services firm that you have gone for. This will allow them to know which method to apply in the fumigation process. Knowing the scope of the work to be performed ensures that the work will be accurately quoted.

You are better off settling for a termite and pest control services specialist that has its business operations in your region. This is not only a cost-efficient measure but it will give you guarantee that you will be served efficiently and promptly due to the close proximity. You will experience greater confidence when you hire a firm that is well known by the local community.

It is important to find out how much will be the cost of the service provided by the termite and pest control specialist. It is good practice to compare the rates available from other service providers. A specialist who has confidence in the quality of their work will be willing to give a guarantee.

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Benefits of Playing Dice Games

Generally gaming is one way of relieving stress and anxiety and this is very healthy for the mind and soul. When people participate in gaming they tend to have fun and feel relieved away from depressions or stressful situations. Gaming is a good stress reliever since it soothes the mind and soul more so gaming is one way of keeping fit. All in all there are various types of gaming that depends with someone’s interest. And that’s why you will find that in all games there must be people who are participating as all games have their fans meaning all games are fun both for participants and non-participants.

Dice game is a cool game that doesn’t need a lot of energy for someone to participate rather it is a game that is played while sited. And dice game has been embraced a lot by aged people due to its calm nature wit dice you won’t have to struggle nor strain you only have to think and focus and you are done. Using the dice cup the roller is then rolled and threw on the table to find the figure and this game can have more than two players. There are large and small cups also medium depending with preferences but again all the cups serve the same purpose in this game and that’s to shake the dice roller. Dice cups vary in sizes that is small, medium and large sizes all these are used for the same purpose the only difference is the size and the prints on the sides of the cups. Looking at the dice rollers these are small items that have a square shape with spots prints on each corner that shows the figures one to six. Dice rollers are small items that get shook by the use of the dice cup and then it is thrown on the board. However the rollers are made of various materials such as the ivory the wood and the bone depending with ones preference they are all in the market.

Dice game manufacturers have come up with new ideas that help in motivating the gamers and grow the passion these ideas help in growing the gamers hobby allowing them to want to play even more. The companies have improved the quality of dice gaming products to players and also gaming stores to keep them motivated in growing their passion about the game. Dice game companies have a unique way of keeping their players motivated by improving gaming products. Experts say too much of gaming can be addictive thus should be played less often all in all dice game is healthy for the mind.

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Things to Note before Buying A Sous Vide Machine

This has been a cooking that has been existing for years but never been there in the home cooking for long until a few years ago when it took roots and is now greatly spreading across homes. The emergence of this cooking has brought relevance in the market on purchases of sous vide machine and its related products. Before you commit yourself to buy home sous vide machines, it is important to look at the following factors.

Understand the capacity of a water bath that is required for your machine. For most models, the capacity ranges from 2 to 3 gallons. This water is enough for cooking and very sufficient for such purposes. Nevertheless, in some cases, you may need one ranging between 3 and 10 gallons depending on the volume of food that you cook at a time. Always ensure you know this and in that case, you will choose sous vide machine whose immersion circulator capacity is compatible with the volume of food you cook. You may go for those with a storing water bath for keeping some when you might be in a crisis for a water bath.

The period that you will be using to cook with the sous vide also matters. There are recipes for cooking that may take you more than a day. In case you may take longer days in cooking it is good to look for what ovens. These eliminate any evaporation. They include the lids whereby they are easily found that fits your machine. They include lids that are designed to fit into some sous vide machines. The water ovens can save you big time because of their energy efficiency. Some particular brands have double insulated water baths, and this does not allow any single loss of heat energy. It is efficient more so because of less energy involved because there is no need for motor circulation.

The amount of food being cooked each time also marks the type and kind to choose. If the cooking involves single steak then you do not require a huge water bath. Size and design of the sous vide machine should also come into place if you want to walk home with the best choice. There are diverse designs and sizes that you will need to choose from if you have to get the right one. take time to look into the needs within your home so that you may select one that perfectly suits them. Having a budget is something else that you should not ignore because it plays a significant role in this. It is always important if you can get the best value for your money without feeling like you have been robbed of your hard-earned money.

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