The quality control of plastic pipes

Quality has become the single most important force leading to organizational success and company growth in national and international markets.
strong and effective quality programs result in substantial increase in market penetration, improvements in total productivity,much lower costs of quality,and stronger competitive leadership.
Successful businesses inevitably place great emphasis on managing quality control carefully planned steps taken to ensure that the products and services offered to their customers are consistent and reliable and truly meet their customer’s needs.
Product quality is becoming the most important factor in customer decision. It is very important for the customer and for organization.
Quality control (QC) is the responsibility of all workers and all divisions, people at all levels are the essence of an organization and their full involvement enables their abilities to be used for organization’s benefit.
This project uses some statistical methods, specifically Statistical Process Control (SPC), to construct control charts for some polypropylene (PPR) pipes produced at plastic factory located in 10th Ramadan City.
For more information about the history and the products of this factory, refer to about Alamal plastic pipes ( Elsharif ) .
Essentially, quality control requires that materials and products be tested at all stages to ensure that:
1-The purchased scrap are fit for specifications
2-Non conforming materials at each stages of manufacturing need a corrective action to achieve the required quality level
3-The final product should be made and inspected according to the international specification.

In any business organization, profit is the ultimate goal. To achieve this, there are several approaches. Profit may be maximized by cutting costs for the same selling price per unit.
If it is a monopolistic business, without giving much of importance to the cost reduction programs, the price may be fixed suitably to earn sufficient profit.
But, to survive in a competitive business environment, goods and services produced by a firm should have the minimum required quality. Extra quality means extra cost.
So, the level of quality should be decided in relation to other factors such that the product is well absorbed in the market.
In all these cases, to have repeated sales and thereby increased sales revenue, basic quality is considered to be one of the supportive factors.
Quality is a measure of how closely a good or service conforms to specified standard.
Quality standards may be any one or a combination of attributes and variables of the product being manufactured.
The attributes will include performance, reliability, appearance, commitment to delivery time, etc., variables may be some measurement variables like, length, width, height, diameter, surface finish, etc.
Most of the above characteristics are related to products. Similarly, some of the quality characteristics of services are meeting promised due dates, safety, comfort, security, less waiting time and so forth.
So, the various dimensions of quality are performance, features, reliability, conformance, durability, serviceability, aesthetics, perceived quality, safety, comfort, security, commitment to due dates, less waiting time, etc.

types and steps of quality control

Types of Quality Control

QC is not a function of any single department or a person.
It is the primary responsibility of any supervisor to turn out work of acceptable quality.
Quality control can be divided into three main sub-areas, those are:
1.Off-line quality control
2.Statistical process control
3.Acceptance sampling plans

1.Off-line quality control:Its procedure deal with measures to select and choose controllable product and process parameters in such a way that the deviation between the product or process output and the standard will be minimized.
Much of this task is accomplished through product and process design.
Example:Taguchi method, principles of experimental design etc.

2.Statistical process control:SPC involves comparing the output of a process or a service with a standard and taking remedial actions in case of a discrepancy between the two.
It also involves determining whether a process can produce a product that meets desired specification or requirements.
On-line SPC means that information is gathered about the product, process, or service while it is functional.
The corrective action is taken in that operational phase.
This is real-time basis.
3.Acceptance sampling plans:A plan that determines the number of items to sample and the acceptance criteria of the lot, based on meeting certain stipulated conditions (such as the risk of rejecting a good lot or accepting a bad lot) is known as an acceptance sampling plan.

Steps in Quality Control

Following are the steps in quality control process:
1.Formulate quality policy.
2.Set the standards or specifications on the basis of customer’s preference,    cost and profit.
3.Select inspection plan and set up procedure for checking.
4.Detect deviations from set standards of specifications.
5.Take corrective actions or necessary changes to achieve standards.
6.Decide on salvage method i.e., to decide how the defective parts are disposed of, entire scrap or rework.
7.Coordination of quality problems.
8.Developing quality consciousness both within and outside the organization.
9.Developing procedures for good vendor-vendee relations.

Causes of Variation in Quality

Perhaps the major barrier to perfecting quality in a manufacturing environment is variability. Variability is inherent in every product—no two products are ever identical.
For example, the dimensions of two thin films used for interconnect will vary according to the precise conditions and equipment used to deposit and pattern the films. Small variations might have negligible impact on the final product, but large variations can lead to final products that are unacceptable. Quality improvement may be defined as the reduction of such variability in processes and products.
The variation in the quality of product in any manufacturing process is broadly classified as:
(a)Chance causes
(b)Assignable causes.

a-CHANCE CAUSES

The chance causes are those causes which are inherit in manufacturing process by virtue of operational and constructional features of the equipments involved in a manufacturing process.
This is because of:
1.Machine vibrations
2.Voltage variations
3.Composition variation of material, etc.
They are difficult to trace and difficult to control, even under best condition of production. Even though, it is possible to trace out, it is not economical to eliminate.
The chance causes results in only a minute amount of variation in process.
Variation in chance causes is due to internal factors only the general pattern of variation under chance causes will follow a stable statistical distribution (normal distribution).
Variation within the control limits means only random causes are present.

b-ASSIGNABLE CAUSES

These are the causes which creates ordinary variation in the production quality.
Assignable cause’s variation can always be traced to a specific quality.
They occur due to:
1.Lack of skill in operation
2.Wrong maintenance practice
3.New vendors
4.Error in setting jigs and fixtures
5.Raw material defects
Variation due to these causes can be controlled before the defective items are produced.
Any one assignable cause can result in a large amount of variation in process.
If the assignable causes are present, the system will not follow a stable statistical distribution. When the actual variation exceeds the control limits, it is a signal that assignable causes extend the process and process should be investigated.

Chemical and physical properties

Most commercial polypropylene is isotactic and has an intermediate level of crystallinity between that of low-density polyethylene (LDPE) and high-density polyethylene (HDPE).
Polypropylene is normally tough and flexible, especially when copolymerized with ethylene.
This allows polypropylene to be used as an engineering plastic, competing with materials such as ABS.  Polypropylene is reasonably economical, and can be made translucent when  uncolored but is not as readily made transparent as polystyrene , acrylic, or certain other plastics.
It is often opaque or colored using pigments.
Polypropylene has good resistance to fatigue.
The melting point of polypropylene occurs at a range, so a melting point is determined by finding the highest temperature of a differential scanning calorimetry chart.
Perfectly isotactic PP has a melting point of 171°C (340°F).
Commercial isotactic PP has a melting point that ranges from 160 to 166°C (320 to 331°F), depending on atactic material and crystallinity.
Syndiotactic PP with a crystallinity of 30% has a melting point of 130 °C(266°F).
The melt flow rate (MFR) or melt flow index (MFI) is a measure of molecular weight of polypropylene. The measure helps to determine how easily the molten raw material will flow during processing. Polypropylene with higher MFR will fill the plastic mold more easily during the injection or blow-molding production process. As the melt flow increases, however, some physical properties, like impact strength, will decrease.
There are three general types of polypropylene: homopolymer, random copolymer, and block copolymer. The comonomer is typically used with ethylene. Ethylene-propylene rubber or EPDMadded to polypropylene homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to polypropylene homopolymer decreases the polymer crystallinity and makes the polymer more transparent.

Degradation

Polypropylene is liable to chain degradation from exposure to heat and UV radiation such as that present in sunlight. Oxidation usually occurs at the tertiary carbon atom present in every repeat unit. A free radical is formed here, and then reacts further with oxygen, followed by chain scission to yield aldehydes and carboxylic acids.
In external applications, it shows up as a network of fine cracks and crazes that become deeper and more severe with time of exposure.
For external applications, UV-absorbing additives must be used. Carbon black also provides some protection from UV attack.
The polymer can also be oxidized at high temperatures, a common problem during molding operations.
Anti-oxidants are normally added to prevent polymer degradation.

History

Propylene was first polymerized to a crystalline isotactic polymer by Giulio Natta as well as by the German chemist Karl Rehn in March 1954.
This pioneering discovery led to large-scale commercial production of isotactic polypropylene by the Italian firm Montecatini from 1957 onwards.
Syndiotactic polypropylene was also first synthesized by Natta and his coworkers.
Polypropylene is the second most important plastic with revenues expected to exceed US$145 billion by 2019.
The demand for this material was growing at a rate of 4.4% per year between 2004 and 2012.
Synthesis


An important concept in understanding the link between the structure of polypropylene and its properties istacticity.
The relative orientation of each methyl group (CH 3 in the figure) relative to the methyl groups in neighboring monomer units has a strong effect on the polymer's ability to form crystals.
A Ziegler-Natta catalyst is able to restrict linking of monomer molecules to a specific regular orientation, either isotactic, when all methyl groups are positioned at the same side with respect to the backbone of the polymer chain, or syndiotactic, when the positions of the methyl groups alternate. Commercially available isotactic polypropylene is made with two types of Ziegler-Natta catalysts.
The first group of the catalysts encompases solid (mostly supported) catalysts and certain types of soluble metallocene catalysts.
Such isotactic macromolecules coil into a helical shape;these helices then line up next to one another to form the crystals that give commercial isotactic polypropylene many of its desirable properties.

Another type of metallocene catalysts produce syndiotactic polypropylene. These macromolecules also coil into helices (of a different type) and form crystalline materials.
When the methyl groups in a polypropylene chain exhibit no preferred orientation, the polymers are called atactic.
Atactic polypropylene is an amorphous rubbery material.
It can be produced commercially either with a special type of supported Ziegler-Natta catalyst or with some metallocene catalysts.
Modern supported Ziegler-Natta catalysts developed for the polymerization of propylene
The catalysts also contain organic modifiers, either aromatic acid esters and diesters or ethers.
These catalysts are activated with special cocatalysts containing an organoaluminum compound such as Al(C2H5)3 and the second type of a modifier.
The catalysts are differentiated depending on the procedure used for fashioning catalyst particles from MgCl2 and depending on the type of organic modifiers employed during catalyst preparation and use in polymerization reactions.
Two most important technological characteristics of all the supported catalysts are high productivity and a high fraction of the crystalline isotactic polymer they produce at 70–80°C under standard polymerization conditions.
Commercial synthesis of isotactic polypropylene is usually carried out either in the medium of liquid propylene or in gas-phase reactors.
Commercial synthesis of syndiotactic polypropylene is carried out with the use of a special class of metallocene catalysts.
They employ bridged bis-metallocene complexes of the type bridge-(Cp1)(Cp2)ZrCl2 where the first Cp ligand is the cyclopentadienyl group, the second Cp ligand is the fluorenyl group, and the bridge between the two Cp ligands is -CH2-CH2-, >SiMe2, or >SiPh2.
These complexes are converted to polymerization catalysts by activating them with a special organoaluminum cocatalyst, methylaluminoxane (MAO).

Extrusion of plastic pipes in PVC and PPR products

Extrusion is a manufacturing process used to create long objects of a fixed cross-sectional profile.
A material is pushed and/or drawn through a die of the desired profile shape.
Hollow sections are usually extruded by placing a pin or piercing mandrel inside of the die, and in some cases positive pressure is applied to the internal cavities through the pin. Extrusion may be continuous (producing indefinitely long material) or semi-continuous (producing many short pieces). Some materials arehot drawn while others may be cold drawn.
The feedstock may be forced through the die by various methods.
A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure), oil pressure, or in other specialized processes such as rollers inside a perforated drum for the production of many simultaneous streams of material.
Extrusion simulation tools help to understand the extrusion process and to optimize development of tools and product.
Commonly extruded materials include metals, polymers, ceramics, and foodstuffs. We will show extrusion of plastic material.
Plastic Extrusion:
Plastics extrusion is a high volume manufacturing process in which raw plastic material is melted and formed into a continuous profile.
Extrusion produces items such as pipe/tubing, weather stripping, window frames, plastic sheeting and wire insulation.
Plastic extrusion commonly uses plastic chips or pellets, which are usually dried in a hopper before going to the feed screw.
The polymer resin is heated to molten state by a combination of heating elements and shear heating from the extrusion screw.
The screw forces the resin through a die, forming the resin into the desired shape. The extrudate is cooled and solidified as it is pulled through the die or water tank.
A multitude of polymers are used in the production of plastic tubing pipes, rods, rails, seals, and sheets or films.

Overview about the extrusion of plastics

In the extrusion of plastics, raw thermoplastic material in the form of small beads (often called resin in the industry) is gravity fed from a top mounted hopper into the barrel of the extruder.
Additives such as colorants and UV inhibitors (in either liquid or pellet form) are often used and can be mixed into the resin prior to arriving at the hopper.
Polymer resin pellets used in plastics extrusion.The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw.
The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic (usually around 200 °C/400 °F). In most processes, a heating profile is set for the barrel in which three or more independently controlled heaters.
gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front.
This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer.
Extra heat is contributed by the intense pressure and friction taking place inside the barrel.
In fact, if an extrusion line is running a certain material fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel.
In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated.
Plastic extruder cut in half to show the components.
At the front of the barrel,the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the melt. The screens are reinforced by a breaker plate (a thick metal puck with many holes drilled through it) since the pressure at this point can exceed 5000 psi (34 MPa). The screen pack/breaker plate assembly also serves to create back pressure in the barrel.
Back pressure is required for uniform melting and proper mixing of the polymer.
After passing through the breaker plate, the molten plastic enters the die. The die is what gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape. Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile.
These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile.
The product must now be cooled and this is usually achieved by pulling the extrudate through a water bath.
Plastics are very good
thermal insulators and are therefore difficult to cool quickly.
Compared with steel, plastic conducts its heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath is acted upon by a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. For products such as plastic sheeting, the cooling is achieved by pulling through a set of cooling rolls.
Sometimes on the same line a secondary process may occur before the product has finished its run.
In the manufacture of adhesive tape, a second extruder melts adhesive and applies this to the plastic sheet while it’s still hot.
Once the product has cooled, it can be spooled, or cut into lengths for later use.

Applications and Characteristics Of Plastic Tubing

Applications Of Plastic Tubing:-
Plastic tubing manufacturers provide supply for almost every area of life.
Applications vary widely from beverage tubing to aerospace, from birdfeeder tubing to medical or pharmaceutical, also, filtration, refrigerant, high viscosity, irrigation, oil or fuel, pneumatic or compressed air and even golf tubes.
Available compounds vary almost as much as the applications. Common materials are as follows:
nylon, polyethylene, polyurethane, urethane, latex, silicone, polyolefin, polypropylene (PPr), PVC, CPVC, vinyl and many more
Characteristics Of Plastic Tubing:-

Plastic tubing manufacturers can make their products with various characteristics, including coiled, corrugated (or convoluted), anti-static, reinforced, spark resistant, explosion proof, multi-element and multi-layered Plastic tubes and PVC tubing used for industrial purposes are generally smaller and more flexible than hoses or pipes.
Materials for flexible tubing include polyurethane, santoprene, flexible vinyl (FPVC), low density and linear density polyethylene (LDPE and LLDPE), flexible nylon 11 and nylon 12.
Some applications require rigid tubing.
In these situations, the following materials are commonly used: polycarbonate, rigid vinyl (RPVC), ABS, high impact polystyrene HIPS, butyrate, propionate and PETG.
Semi-rigid materials include polypropylene and high density polyethylene (HDPE).
Although each type of plastic tubing has too many characteristics to provide exhaustive comparisons in this small space, some differences are as follows. Latex tubing has better elasticity, flexibility and gripping power than silicone, though silicone has a better deterioration rate.
Nylon tubing is preferred for pneumatic lines and low-pressure lines and has good impact and abrasions resistance, but some types of nylon can be damaged by moisture.
Polyethylene tubing can be used for liquids and gas lines at temperatures ranging from -60° to 160°F (-51° to 71°C).
Polyurethane tubing provides resistance to chemicals but has low temperature flexibility, while polyolefin is highly flame retardant and has a wide temperature range. PVC tubing can also be used for pneumatic lines at pressures reaching 125 psi with continuous temperatures reaching 100°F/38°C.

Statistical Process Control Methods

The ideal manufacturing process is one that produces products that are identical,However,in reality, this is not possible.
Products will always vary from one to another.
As a result, manufacturers have learned to accept variation as part of the manufacturing process [Chase et a1.(1995)],When products are designed, their specifications include tolerances.
This is the amount or the range of variation that can be tolerated. When the product: variation exceeds the tolerance range, problems can result (e.g., parts will be rejected, or parts that are supposed to fit together with another part may not fit, thereby resulting in product defects, etc.)

The aforementioned sources of product variation can be grouped into two categories, assignable and chance causes [Grenier et al. (1997)].
The former refers to causes that can be avoided, such as human error or broken tools, while the latter refers to causes,often random in nature, that are beyond human control, such as the variation in the hardness of steel. When only chance causes are in action, the process is said to be in-control
 A process that is in-control is expected to have some variation.
The variation resulting from the presence of chance causes is random and follows no discernible pattern [Delmar and Sheldon (1988)].
While on the other hand, the variation due to assignable causes introduces non-random variation, which follows a pattern.

Much of the effort in quality control is dedicated to isolating assignable causes by detecting the existence of patterns in the data.
The presence of assignable causes leads to an increase in product variation, resulting in the production of defects. Since much of the variation in manufacturing follows the normal probability distribution [Besterfield (1998)], it is used in quality control as a means to detect the existence of assignable causes of non-random variability.

statstical process control`s principles

SPC is a quality control tool used to isolate or identify assignable causes so that a process can be restored to its normal state. It involves taking a sample from a process to detect abnormalities (assignable causes) within the process. When the sample indicates that the abnormalities exist,the process is investigated to find the nature of the cause. 

Once established,the cause is corrected and the system is restored to its normal state.

A typical product has subassemblies and thus takes a variety of processes to make a single product. Each process works on a particular quality characteristic or variable 

For a product to come together and work as intended, all processes have to meet the design specifications for the particular quality variable they are working on. Montgomery (2005) explains that SPC philosophy integrates an array of quality tools designed to solve the problems that result in process variation and states that,"SPC can be applied to any process".

The seven major technical tools available to sustain an SPC program 

  

Flow Chart:A pictorial representation of a process to identify where problems are likely to occur.   

Ishikawa Diagram:Identify root causes of problems.   

Control Charts:Determines if process is "in-control".   

Histogram:Analyzes frequency of occurrence of items.   

Scatter Plot:Find the correlations among variables.   

Pareto Diagram:Helps in determining which problems to work on first.   

Check Sheet:Helps in organizing information in an efficient way.

Let us assume processes A and C are suspected to be the culprits,then SPC tools should be used in processes A and C to detect the presence of abnormalities.

There are two widely used methods for detecting abnormalities within the process and estimating the number of defects the process is likely to produce;these are control charts and process capability study,respectively.

Rational Subgroups in statstical process control

Other important concept behind SPC is what Shewhart called the rational subgroup concept.
Subgroups should be rational.
A rational subgroup is a group in which all of the observations are generated under conditions in which only random effects are responsible for the observed variation [Nelson (1988),Wheeler(2004),and Hawkins and Olwell (1998)].
Subgroups must be representative of process performance.
Subgroups or samples should be selected so that if special causes are present,the chance for differences between subgroups will be maximized,while the chance for differences within a subgroup will be minimized.
Usually time order is a good basis for the selection because it allows detecting time related assignable causes.
In general the approaches to construct rational subgroups expressed by Montgomery (1985) are as follows:
1-Samples consisting of units produced at the same time or as closely together as possible.
This approach is used to detect process shifts.
2-Each sample is a random sample of all process output over the sampling interval.
This approach is used to make decisions about the acceptance of all products that have been produced since the last sample.
3-If several machines pool their output into a common stream the control charts should be applied to the individual streams to avoid confusing the origin of assignable causes.
Subgroups should ensure the presence of a normal distribution for the sample means
In general,the larger the sample size, the better the distribution is represented by the normal curve. In practice,sample sizes of four or more ensure a good approximation to normality.
Subgroups should ensure good sensitivity to the detection of assignable causes.
The larger the sample size,the more likely that a shift of a given magnitude will be detected. 

Uses of Control Charts

The control chart is an effective tool to achieve statistical control of a process and to give the person who is closest to the process the ability to evaluate the quality performance for which he or she is directly responsible. The control chart does not actually control anything;
it simply serves as the basis for action.
This chart is valuable only if timely actions are taken when required. In explaining the reasons for the popularity of control charts in industry, Montgomery (2005) included the following items:
1.Control charts are a proven technique for improving productivity.
2.Control charts are effective in defect prevention.
3.Control charts prevent unnecessary process adjustment.
4.Control charts provide diagnostic information.
5.Control charts provide information about process capability.
The control chart is a reliable indicator of the two types of process variations (chance cause and assignable cause),and thus minimizes the amount of unnecessary searching for causes with the process when no assignable cause is present.
The control chart helps to get a process into a state of statistical control,after which the inherent variation can be further reduced by a change in the process itself (training,methods,materials,tooling, etc.)to:
1.Increase the yield reducing non-conforming product.
2.Cut the cost per unit by eliminating scrap and rework.
3.Increase productivity by increasing the throughput of the acceptable product with no increase in capacity.
Finally,control charts speak in the language of statistics; this provides the common ground for consistent communications about quality between the various functions in the plant [Hamilton (1993)]. To use control charts, one begins by following these six key steps:
1-Define the objective.
2-Analyze the production process.
3-Choose the characteristics to control.
4-Determine the proper control chart.
5-Collect the data.
6-Interpret control chart run rules.
Define the objectives

Control charts are used as part of the central plan. It is very important that the early steps of gaining management commitment, putting together a management steering group, and training management, technical, and operations personnel are all followed in the quality improvement program before introducing statistical methods such as control charts. When the appropriate ground has been set and work areas are ready to implement such methods, then the objectives of such a program should be well-known by all of the people who are participating.
Analyze the production process

The process should be analyzed in terms of the important factors operating within it (people, equipment, materials, etc.), taking into account its relationship to processes upstream and the process/user requirements downstream. The analysis should involve the technical, operations, and the quality people who are experienced with the process, with the specifications that must be met, and with the major nonconfermance challenges. This will ensure that the analysis is comprehensive, and it will create the groundwork for the operation necessary to make the control chart program effective.