Wednesday, October 7, 2009
COOLING
During the final stage of cooling, mould temperature directly affects product dimensions and stability. Therefore, moulds must have an even distribution of water lines to accurately control the process and part-to-part uniformity.A hot mould will...· Produce parts with less stress and higher gloss.· Usually requires...
Wednesday, October 7, 2009
Wednesday, October 7, 2009
PHASES OF OPERATION
In injection moulding, we can describe the following three phases of operation:· Plastication· Injection· CoolingWe will discuss how to determine the optimum moulding parameters for each phase of operation. As always, it is a good idea to refer to machine manufacturer...
Wednesday, October 7, 2009
MATERIAL PREPARATION
We begin with some initial material preparation before processing the plastics resin.3.2.1 Inspecting MaterialCheck material carefully for consistency and for foreign materials such as dirt, paper, other plastics, etc.SPECIAL NOTE:In all cases, it is extremely important to properly identify and mark...
Wednesday, October 7, 2009
PLASTICS PROCESSING
Injection moulding is a process by which the plastics is melted and injected into a mould cavity. Once the melted material is in the mould, it cools to a shape that reflects the form of the cavity.The resulting form is a finished part requiring no additional work beforeassembly and, in most cases, is the finished...
Wednesday, October 7, 2009
Temperature
The key temperature parameters include:(i) Barrel temperature(ii) Nozzle temperature(iii) Mould temperatureMost temperature controllers will maintain a set temperature to within a few degrees. It is important to accurately control these parameters to develop a consistent process. Reducing part-to-part variation...
Wednesday, October 7, 2009
Wednesday, October 7, 2009
Wednesday, October 7, 2009
Pressure
Typically, the following types of pressure are controlled in the process:(i) Injection pressures· The first stages of injection will fill the mould. Once the mould is filled, an additional amount of packing pressure is used until the gate freezes off.(ii) Screw back pressure· Screw back pressure...
Wednesday, October 7, 2009
MACHINE CONTROL PARAMETERS
Next, we will explain the parameters that an injection moulding machine controls. All injection moulding machines are designed to perform the same basic functions. These include:· Clamping· Injecting· Rotating the screw· Heating the screw barrelThe machine...
Wednesday, October 7, 2009
Shut-Down
To shut down the process:1) Shut the feed throat to prevent any more plastics pellets from entering the screw and barrel assembly. After the machine cycles a few times, put it in semi-automatic mode.2) After the completion of the one cycle, place the machine in the manual mode. Move the injection carriage...
Wednesday, October 7, 2009
Machine Start-Up Procedure
We will begin the start-up procedure with the assumption that the machine has been shut down for a weekend. This type of start-up is referred to as a cold start. Initial preparation is required for the injection moulding machine to warm up and reach its operating temperature and to fully stabilize machine components.(i)...
Wednesday, October 7, 2009
Complete Machine Cycle
The following describes on full machine cycle:1) The injection moulding cycle begins when the mould closes. The screw moves forward, injecting the material through the nozzle to the sprue. The material fills the runners, gates, and finally the mould cavities.2) During the packing phase, additional material...
Wednesday, October 7, 2009
Modes of Operation
Injection moulding machines operate in the following modes:(i) Manual(ii) Semi-automatic(iii) Automatic(iv) Set-up(i) Manual ModeThe manual mode is used for machine set-up. You can perform the following functions in this mode:· Clamp open and close· Ejector stroke· Material and color...
Wednesday, October 7, 2009
Mould Cavity and Core
The mould cavity and core define the shape of the moulded part. The cavity defines the outside of the part, while the core defines the inside shape of the pa...
Wednesday, October 7, 2009
Sprue, Cold Slug Weld, Runner, and Gates
The sprue connects the machine nozzle to the mould. It provides a passageway for the plastics to travel.Opposite the sprue is a circular depression, or a cold slug weld, that captures the first portion of the material injected into the mould. The runners then provide a passageway to deliver the molten plastics...
Wednesday, October 7, 2009
Check Valve
Check valves are used on all machines to prevent material from flowing over the screw as it is moving forward during injection.The most common check valve used is the slip ring check valve. When the slider moves to the right, it seats itself against the screw and blocks the melted plastics from flowing back over...
Wednesday, October 7, 2009
Nozzle
The nozzle connects the machine to the mould.Different types of nozzles exist for moulding materials with special characteristics. The tip screws into the nozzle and can be changed to accommodate moulding different materials. Typically, a heater band is located around the nozzle to control the temperature of...
Wednesday, October 7, 2009
Screw Function
The screw functions in the following way:1) The feed zone conveys the plastics to the transition zone where most of the shearing action takes place.· Sometimes the transition zone is called the compression zone. In this zone, the flights of the screw are not deep so that the material can be compressed...
Wednesday, October 7, 2009
Screw Specification
The general purpose screw is the most common screw found in our industry because it can be used to process many different plastics materials. Other, more specialised screw designs exist to run special materials such as vinyl or thermosetting materials.Figure 6. General Purpose ScrewA term used to specify screws...
Wednesday, October 7, 2009
Platens/Clamping System/Tie Bars
The fixed or stationary platen never moves, and the stationary side of the mould or “A” side is mounted here.The movable platen moves left to right. This platen supplies the necessary force required to hold the clamp closed during injection and packing. Most of the required tonnage is used during the packing...
Wednesday, October 7, 2009
Screw
The function of the screw is to plasticise, or melt, the plastics material.In addition, the force of the screw turning pushes the melted plastics to the front of the injection unit. The screw then acts like a plunger and forces the melted plastics material into the closed mould.Screw Drive MotorA screw drive...
Wednesday, October 7, 2009
Hydraulic Pull-In Cylinder
A hydraulic cylinder retracts the entire injection assembly from the mould and moves the injection unit into the mould to begin mouldi...
Wednesday, October 7, 2009
Barrel
The barrel of the machine contains the screw and check valve assembly.Heater bands wrapped around the barrel assist in machine start-up and accurately control the moulding temperature during the moulding process.CAUTION:To prevent personal injury, avoid contact with the barrel heaters and do not stand on the...
Wednesday, October 7, 2009
Feed Throat
Plastics pellets enter the barrel through the feed throat. This area usually becomes extremely hot as heat from the barrel rises into the feed throat. This heat can cause the plastics pellets to cling or stick together and will sometimes clog the feed throat.To prevent clogging or bridging, a cool water jacket...
Wednesday, October 7, 2009
Hopper
The hopper holds and funnels the plastics pellets into the screw and barrel assembly.Drying units are often attached to the hopper to reduce the moisture content of the material. Moisture will affect processing conditions, as well as the physical and visual characteristics of the moulded part.Many hoppers are...
Wednesday, October 7, 2009
Reciprocating Screw Injection Moulding Machine.
The reciprocating screw injection moulding machine is the most common machinefound in today’s injection moulding operations.This machine uses a reciprocating screw that rotates counterclockwise while moving backward and forward. This rotating action causes a shearing of the plastics pellets between the barrel...
Wednesday, October 7, 2009
Crystallinity
Some thermoplastics polymers are called semi-crystalline polymers. Examples of semi-crystalline polymers include polyethylene, polypropylene, nylon, acetal, polyester, and most of the specialty resins. Semi-crystalline polymer chains develop rigid crystalline structures during cooling.Semi-crystalline polymers...
Wednesday, October 7, 2009
Orientation
When melted, thermoplastics polymer chains are in a randomly coiled and twisted state. During injection, the polymer is forced through the sprue, runner, and cavity. When this occurs, many of the polymer chains orient, or align, in the direction of polymer flow.Increasing the injection velocity, or injection...
Wednesday, October 7, 2009
Melt Flow Index
Melt flow indexing is the most popular, and yet least accurate way to determine material viscosity. Melt flow index uses a standard testing apparatus with a standard capillary to measure the flow of the material. The melt flow indexer tests the polymeric material at a single shear stress and melt temperature.The...
Wednesday, October 7, 2009
Capillary Rheometry
Capillary rheometry uses a capillary rheometer to measure the viscosity of a polymer. The capillary rheometer melts the polymer inside a small barrel, and then a plunger forces the polymer melt through a small capillary.The rheometer measures the amount of force required to push the polymer through the capillary....
Wednesday, October 7, 2009
Viscosity
The viscosity of the polymer is a measure of the material’s resistance to flow. A material which flows easily has a low viscosity, while a material with a higher viscosity does not flow as easily.Most polymers are available in different grades, each grade has its own flow characteristics. Typically, materials...
Wednesday, October 7, 2009
POLYMER PROCESSING
Most injection moulding processes use a reciprocating screw injection moulding machine.The reciprocating screw injection moulding machine uses a large screw inside a heated barrel to melt the plastics pellets, and convey the polymer melt. The screw typically turns counterclockwise, and the friction created by...
Wednesday, October 7, 2009
Polymer Classification
Most plastics materials used in the industry today fall into one of the three categories. These are the commodity, engineering, and specialty resins.(i) Commodity ResinsCommodity resins are the least expensive and most commonly used polymers. These polymers are easy to produce and process. Commodity resins...
Wednesday, October 7, 2009
Types of Polymers
The two types of polymers commonly used in the plastics industry are thermoplastics and thermosets.(i) ThermoplasticsThermoplastics are the most widely used type of polymer. Thermoplastics polymers are comprised of many long polymeric chains. The entanglement of these polymeric chains is what gives thermoplastics...
Wednesday, October 7, 2009
UNDERSTANDING PLASTICS MATERIALS
DEFINITION Although the terms polymer, plastics, and resin are not technically the same, they are used interchangeably in the plastics processing industry. The word polymer can be broken down into two parts, ‘poly’ meaning many, and ‘mer’ meaning unit.Although there are many different types and classifications...
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