GLOSSARY OF TECHNICAL TERMS - Polymer Process

Amorphous Polymer: Amorphous means irregular, having no discernible order or shape. In the context of solids, the molecules are randomly arranged, as in glass, rather than periodically arranged as in a crystalline material. Amorphous polymer has a glass like structure with tangled chain and no long range order.

Cavity Pressure: Pressure on the melt in side the space between core and cavity as the melt moves to fill the mould.

Change over- Fill/Pack: Switch over point fill to pack: The point on injection stroke at which filling phase with speed profile ends and pack or pressure phase with pressure profile starts.

Clamp Force: It is force applied by the machines clamping unit to the mould during filling, packing and holding phases of moulding cycle. Unit is Tons or kN.

Cooling Channels: It is the channels through which coolant is flown to remove heat from the mould. The channels are to be located thoughtfully in the core and cavity so that the temperature distribution over the mould surface is constant with little acceptable variation.

Co-polymer: Polymers that are derived from more than one species of monomers.

Cross linking: A process in which bonds are formed joining adjacent molecules. At low density, these bonds add to the elasticity of the polymer and at high density eventualy produce rigidity in the polymer.

Crystalline: It is the structure of polymer where the molecules are arranged in a very regular repeating lattice structure.

Cure: The process of changing property of polymer into a more stable and usable condition. This is accompolished by the use of heat, radiation or reaction with chemical addition.

Cushion: It is the a small amount of melt that is left in the barrel at the end of injection, follow up pressure. The cushion prevents the screw tip from making contact to the head of

barrel.

Cycle Time; A sequence of operation that is repeated regularly, the time it takes for one such operation. It is the time required to complete one moulding cycle.

Dosing Stroke: Metering stroke: It is stroke of screw that determines the quantity of melt to be injected to the mould.

EUROMAP: Euromap is the non profit organisation of the national association of machinery manufacturers for plastics and rubber industries in Austria, France, Germany, Italy, Luxembourg, Netherlands, Spain, Switzerland, and U.K. It represents 600 companies. It's Technical Commission deals with mechanical and electrical sandardisation, the communication protocol, and interface for various types of processing machinery and safety standards. It's technical work started in early seventies describe recommendations for functional and specification, testing of machines etc.

FAQ: Frequently Asked Questions.

Feed System: It is the main connecting channel between the machine nozzle and the part cavities of the mould.

Feedback: Feedback in a closed loop system represents the return signal or response of the system to input instruction.

Fill Pattern: It is a visual history of how a mould fills under a specific set of moulding conditions.

Fill Time: Fill time is the time in seconds that it takes to just fill the mould with melt during the filling phase (up to switchover point) of the moulding cycle.

Flow Balancing: Flow balancing is the process of choosing a mould design strategy, which promotes even filling of mould. First, a gating strategy is selected which will promote ease of filling. Second, a runner layout is developed to feed material to the gates. Finally runner dimensions are identified which will cause all of the flow paths to fill at the same time. Even wall thickness can be manipulated by flow leader or flow deflectors.

Flow path: A flow path describes the route that is traveled by a melt front as melt fills a section of a mould.

Flow Rate: Flow rate is a way of describing how much material goes through or past a specific point in a fixed period of time. If nozzle tip is used as reference, flow rate can be described how much melt is flowing out of the machine per second during injection.

Flow Length: Flow length is the total distance that melt must travel from the machine nozzle along a particular flow path in order to fill a section of the mould.

Follow up Pressure: It is pressure on melt after the switch over point in the moulding process.

Fountain Effect: The fountain effect describes how the melt front behaves during the filling of the mould. The leading melt front swells into the shape of a bubble; it behaves in much the same way as water flowing from a fountain.

Frozen Layer / skin: The frozen layer is a skin of solid melt that forms next to the mould surface during the filling phase of the moulding cycle.

Gating Strategy: It is the approach you use to choose the number, dimension and location of gates in a mould.

Glass Transition Temperature: It is the point at which the polymer hardens into an amorphous solid.

Hold on Pressure: Follow up pressure: It is pressure on melt after the switch over point in the moulding process.

Homo polymer: A polymer that is constructed of identical monomers.

Injection Rate: It is the flow rate of melt (cc/sec) coming out of nozzle. Melt comes out in the form of jet. Then it spreads inside the mould in shape of the space between the core and cavity.

Injection Speed / velocity profile: Before the switch over point the set pressure remains constant but set injection speed is varied with stroke position. After switch over point injection speed should remain at lower value and pressure is changed with time till the mould is just filled with out over packing. The variations in the set speed through out the injection stroke is called Injection speed profile.

Injection Pressure Profile: Before the switch over point the set pressure remains constant but set injection speed is varied with stroke position. After switch over point injection speed should remain at lower value and pressure is changed with time till the mould is just filled with out over packing. This set pressure for filling stage and follow up / hold on pressure change with respect to time is called Injection pressure profile.

Maximum Shot Capacity of Machine: In machine specification it is given in terms of weight for PS. For other polymers the density of polymers at moulding temperature should be multiplied to the maximum swept volume of the machine barrel to get the weight for the given polymer.

Metering stroke: It is stroke of screw that determines the quantity of melt to be injected to the mould.

Melt expansion: During the injection stroke melt is compressed by about 5 to 15%. After melt enters the mould it gets the chance to relax and expand.

Melt Front: As the mould is filling, the melt at the leading edge of flow is called the melt front or stream.

Melt Front Velocity: It is not same as injection piston (screw velocity) velocity. As melt enters mould it spreads in all direction depending on the resistance to flow in each direction. Melt front velocity is determined by the area of flow, which does not remain constant with time.

Melt Temperature: Melt temperature is the actual temperature of the melt during processing. The melt temperature is constantly changing. It varies with time and will not be the same at different locations in the mould.

MFI: It stands for Melt Flow Index. It is the weight of polymer melt in grams extruded in 10 minutes through a standard nozzle or die under standard load condition at a certain temperature.

Mould Cooling: Mould cooling describes the process by which the melt temperature is reduced to the point where part can be removed from the mould.

Mould Packing / Follow up pressure / Hold on phase: Mould packing is the process of delivering an additional amount of melt to the mould, to compensate for the shrinkage after filling.

Mould Temperature: It refers to temperature of mould surface in contact with melt. It varies from point to point on surface if cooling design does not provide uniform heat extraction. It also fluctuates if heat is not balanced.

Moulding Cycle: The moulding cycle is the series of steps that result in the machine producing a part. The cycle is usually described by breaking down into four separate phases. The amount of time that it takes to complete one cycle is called cycle time.

No Flow Temperature: No flow temperature is the temperature at which the viscosity of the melt is so high that it effectively can not be made to flow.

Orientation: Orientation is the change in shape that polymer molecules can undergo when they are made to flow.

Parting line: Contour line on the part separating core and cavity.

Polymer: Long chains of covalently bonded atoms.

Pressure Drop: Pressure drop is the loss of pressure that occures when the melt is pushed into a section of the mould during the filling phase.

Pressure Profile: After switch over point injection speed should remain at lower value and pressure is changed with time till the mould is just filled with out over packing. This follow up / hold on pressure change with respect to time is called Injection pressure profile.

pvT diagram: Polymer Melt follows law similar to famous gas law (Boyel's law). It is the relation ship between pressure, volume and temperature for plastic melt.

RAPRA: RAPRA technology Ltd., (UK,) is a leading rubber and plastics consultancy firm with over 75 years of experience. They offer conduct training courses in Rubber and Plastics.

Repeatability: Repeatability is closeness of agreement of a tool movement position from one part to another when cutting several copies of the same part.

Residence Time: Residence time is the length of time that the material is held at melt range temperatures in the barrel.

Residual Stress / Moulded-in Stress: Residual stress is a term that describes the level and pattern of stress, which is left in the part after it is removed from the machine. It can be due to unbalanced flow, non uniform freezing of melt and over packing.

Runner Layout: It is the channels used to get the melt from the machine nozzle to the gates.

Shear Heating: Shear heating is due to friction caused by flow of melt through narrow passages in the mould during filling phase.

Shear Rate: Shear rate is a way to describe how quickly the velocity of the melt changes from the mould surface to the center of flow for a given cross section. The size of the shear rate gives an indication of the shape of the velocity profile for a given situation.

Shear stress: Shear stress is the result of the force that is generated in a melt to overcome its resistance to a particular flow situation. Shear stress is the product of a material and shear rate.

Shear Thinning: Shear thinning is a the description for the physical effects of orientation and affect the flow behavior of the polymer. Shear thinning causes the melts viscosity to drop when it is made to flow within a certain shear rate range.

Shot Composition: The shot size setting and the switch over point give the machine information about how the melt has to be injected into the mould. The stroke positions correspond to volumes of melt that play a role in different portions of the moulding cycle. While the settings relate to the machine barrel, they correspond with the actual events that happen during the cycle.

Shot size Setting: Shot size setting is control setting which limit how far back the screw will travel as it rotates in the cooling phase of the cycle. The shot size setting is measured as a distance from the front of the barrel.

Shrinkage: It is the deviation of dimensions of the moulded part from the dimensions of cavity when measured on moulded part after certain hours. Compressibility, thermal expansion and pvT characteristics influence the dimensions of moulded part.

Sink Mark: A depression on the surface of moulded part caused by differential shrinkage.

Specific Heat: Temperature is measure of heat energy level whereas heat is a measure of total internal energy contained in a body. When the same quantity of heat is given to equal masses of different substances, they do not result in the same rise in temperature. The specific heat is defined as the quantity of heat energy which will rise the temperature of unit mass (1kg) of a substance by 10C. Heat = mass x specific heat x Temperature rise.

Specific Volume: It is inverse of density. It is volume per unit weight. Unit is cc/gr.

Speed profile: Injection speed set up with respect to stroke positions for the filling phase so that melt front speed remains near constant during filling phase.

Split Points: Split points are locations in the a mould cavity where a melt front will split up and advance in more than one direction at the same time. It is also refered as flow junctions, branch points, or nodes.

Stress Cracking: A crack, either external / internal, in a plastic part caused by tensile stress less than its short time mechanical strength.

The moulded-in stress left in the moulded part - due to unbalanced filling, over packing or non uniform freezing - can fail in service condition. The service environment can have objectionable temperature or contact with aggressive chemicals causing the plastic part to fail. Avoiding or minimising moulded-in stress in plastic part can increase the performance of part even under adverse condition.

Switch over point fill to pack: The point on injection stroke at which filling phase with speed profile ends and pack or pressure phase with pressure profile starts.

Thermal Stability: It is polymers thermal characteristic. It is the time interval for which polymer remains stable at certain temperature. Beyond that time if the polymer is exposed to longer duration for a given temperature, it degrades.

Thermoplastic: Linear plastics of finite molecular weight that can be fabricated in a complex shape by melting and injection moulding.

Thermoset: A type of plastics that must be cured, forming network like structures that do not soften at high temperature.

Valcanisation: A process by which a network of cross linking is introduced in to an elastomer to strengthen it.

Viscosity response: The viscosity response is a way of describing how the viscosity of a particular polymer responds to the changes in temperature and shear rate.

Warpage: Uneven bending, Twisting etc on account of differential cooling, differential shrinkage, or non uniform freezing of melt in the mould can cause these conditions in the moulded part.

Weld Lines: Weld lines are locations in the moulded part where two met fronts meet.

Wireframe: It is a geometric model that describes 3D geometry by outlining its edges.

Work Done: Work is said to be done if an unbalanced force moves its point of application through a distance measured in the direction of force. Work = Force x Distance


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