INTRODUCTION
Many of the things you own and come into contact with every day are made from plastics which have been injection moulded into shape. These things range in size from pens, up to computer and television cases. Many products have become very cheap and widely available because of the injection moulding 'revolution'. Injection moulding can produce very large or small products with great precision, in very large numbers, and at low cost.
Injection moulding is suitable processing method for following materials:
• Thermoplastics
• Elastomers
• Rubbers
• Thermosets
• Composites
• Foamed plastics
The most used materials are thermoplastics. Hence from here onwards injection moulding refers to thermoplastic injection moulding.
BASIC CONCEPT OF INJECTION MOULDING
The basic concept of injection moulding is the ability of a thermoplastic material to be softened by heating, formed under pressure, and hardened by cooling.
PRINCIPLE OF INJECTION MOULDING
Injection moulding, as its name suggests, involves injecting fused (softened) plastic into a mould. In an injection moulding machine, plastic granules are fed into a heated barrel. They are forced through this barrel either by a piston or a screw (similar to one in a mincing machine). The granules fuse together into a mass like soft chewing gum and this is forced from the end of the barrel into a metal mould.
The mould itself is made from two or more metal parts which fit tightly together leaving only a small hole for the fused plastic to enter. This hole is called the sprue entry. If you look at an injection moulded product carefully, you may see the point at which the plastic entered the mould. It will be a small mark where the sprue has been broken or cut off the moulding.
Many of the things you own and come into contact with every day are made from plastics which have been injection moulded into shape. These things range in size from pens, up to computer and television cases. Many products have become very cheap and widely available because of the injection moulding 'revolution'. Injection moulding can produce very large or small products with great precision, in very large numbers, and at low cost.
Injection moulding is suitable processing method for following materials:
• Thermoplastics
• Elastomers
• Rubbers
• Thermosets
• Composites
• Foamed plastics
The most used materials are thermoplastics. Hence from here onwards injection moulding refers to thermoplastic injection moulding.
BASIC CONCEPT OF INJECTION MOULDING
The basic concept of injection moulding is the ability of a thermoplastic material to be softened by heating, formed under pressure, and hardened by cooling.
PRINCIPLE OF INJECTION MOULDING
Injection moulding, as its name suggests, involves injecting fused (softened) plastic into a mould. In an injection moulding machine, plastic granules are fed into a heated barrel. They are forced through this barrel either by a piston or a screw (similar to one in a mincing machine). The granules fuse together into a mass like soft chewing gum and this is forced from the end of the barrel into a metal mould.
The mould itself is made from two or more metal parts which fit tightly together leaving only a small hole for the fused plastic to enter. This hole is called the sprue entry. If you look at an injection moulded product carefully, you may see the point at which the plastic entered the mould. It will be a small mark where the sprue has been broken or cut off the moulding.
Advantages of Injection Moulding Process:- Parts can be produced in large volume at high production rates.
- Process can be automated and hence increases in production
- Relatively low labor cost per unit is obtainable.
- Many different colors and finishes are available, and good decoration is possible.
- Close dimensional tolerances can be maintained.
- Parts can be molded with metallic and non-metallic inserts.
- Parts can be molded in combination of plastics and additives such as fillers.
- Molded parts require little or no finishing.
- Minimal scrap loss result as runner, gate and rejection can regrind and reused.
- Process creates consistent parts.
- High return on investment.
Limitations of Injection Moulding Process:
- High mold cost.
- Moulding machine and auxiliary equipment cost are high.
- Lack of knowledge about the fundamentals of process causes problem.
- Lack of knowledge about the long term properties of the material may result in long term failure.
- Process requires extremely high temperature.
- Process requires extremely high clamping force.
INJECTION MOULDING PROCESS
In its broadest terms, thermoplastic injection moulding forms a part by forcing a liquid resin into a closed mould, under pressure, until the part has cooled or cured and can be ejected from the mould. This process consists of the following five distinct operations.
1. Feeding of raw materials
2. Plasticizing (mixing (mechanical work), external heating)
3. Injection (filling of the mould cavity)
4. Holding pressure
5. Cooling (cooling of the material in the mould cavity)
6. Ejection of the injection moulded part
The main factors in the injection moulding are the temperature and pressure history during the process, the orientation of flowing material and the shrinkage of the material. This means that the structure and the properties of injection moulded parts are inhomogeneous and the products have always internal stresses.
Feeding of Raw Material
First the molder receives plastic resin in the form of small chopped pellets. These are fed into the hopper of an injection-moulding machine, where they fall into an augur-type screw channel, which feeds the pellets forward inside the heated barrel.
Plasticizing
The cycle begins with the extruder plasticizing the resin and accumulating it in the forward section of the barrel. The heater bands maintain the melt's temperature as the shot it built up. The mould is closed. The cycle is typically timed so that there is minimal time between the closing of the mould and the next shot. Injection of the Resin into the Closed Mould.
Injection
After the mould closes, the screw (not rotating) pushes forward to inject melt into the cooled mould. The air inside the mould will be pushed out through small vents at the furthest extremities of the melt flow path.
Holding pressure
When the cavity is filled, the screw continues to push forward to apply a holding pressure. This has the effect of squeezing extra melt into the cavity to compensate for the shrinkage of the plastic as it cools. This holding pressure is only effective as long as the gate(s) remain open.
Cooling the Resin inside the Closed Mould
Cooling starts immediately when polymer mass flows into the mould cavity including the injection and holding phases, and continues after these too. Therefore the cooling time is the longest phase in the injection moulding process. Because the injection and holding phases are included in the cooling phase, they also effect on the cooling of the moulded part. Cooling time depends on the raw material, wall thickness of the part and the heat transfer capability of the mould. In order to have a nonporous and unstressed part the polymer melt should solidify uniformly.
Opening the Mould and Ejecting the Moulded part
Once the gate(s) freeze, no more melt can enter the mould and so the screw-back commences. At this stage the screw starts to rotate and draw in new plastic from the hopper. This is conveyed to the front of the screw but as the mould cavity is filled with plastic, the effect is to push the screw backwards. This prepares the next shot by accumulating the desired amount of plastic in front of the screw. At a pre-set point in time, the screw stops rotating and the machine sits waiting for the solidification of the moulding and runner system to be completed. When the moulding has cooled to a temperature where it is solid enough to retain its shape, the mould opens and the moulding is ejected. The mould then closes and the cycle is repeated.
THE PROTOMOLD PROCESS
At Protomold we have automated the process of designing and manufacturing molds based on customer supplied 3-D CAD part models. Due to this automation, we typically cut the lead time for the initial parts to one-third of conventional methods. Cost saving varies with the number of parts being produced, but Rapid Injection Molding may also have a substantial cost advantage in runs of up to thousands of parts.
The Protomold Rapid Injection Molding process gives design engineers a fast and affordable way to get real injection molded parts in prototype or low-volume quantities.
So from an overall market perspective we see Rapid Injection Molding as in between Rapid Prototyping and Conventional Injection Molding as illustrated in the figure.
THE INJECTION MOULDING PROCESS CYCLE
The typical process cycle time varies from several seconds to tens of seconds, depending on the part weight, part thickness, material properties, and the machine settings specific to a given process.
Process control of injection molding has a direct impact on the final part quality and the economics of the process. The various components of process control must be fully understood to maximize profit and part quality.
The various stages are:
Injection phase (filling phase)
Packing phase
Holding phase
Cooling phase
Fill time
At the beginning of the injection molding cycle (point 1), the mold has just closed and the molten polymer, which is maintained at a fairly uniform temperature inside the barrel of the injection machine, is forced to flow through the nozzle, runner, gate, and then into the cavity under controlled flow rate or pressure, depending on the control scheme of the injection unit. The fill time is defined as the time needed for the polymer to fill the entire cavity (duration between points 1 and 4.
Post-fill time
After the mold is completely filled (point 4), more material is packed into the cavity and the polymer continues to cool. The post-filling stage ends when the polymer temperature is sufficiently low and the part is rigid enough to be removed from the cavity without significant deformation (point 7). The post-fill time is defined as the time between the moment when the cavity is completely filled and the instant when the mold opens (duration between points 4 and 7).
Mold open time
The mold-opening stage begins when the mold is opened (point 7) and ends when the mold is closed (point 8) to start the next cycle. The mold-open time includes the time taken for mold opening and closing actions as well as part ejection (duration between points 7 and 8). Because this can be a significant portion of the cycle time in processes with extremely short cycles, each action of the mold clamp and ejection systems should be analyzed for possible time delays and wasted energy. During this stage, additional heat transfer occurs between the mold and ambient air.
To further illustrate the machine motion within the process cycle, the hydraulic (cylinder) and cavity pressure traces, screw position, and mold face separation position are shown below
1. Filling (injection stage) 2.Packing and cooling stage 3.Mold opening 4.Part ejection 5.Mold closing.
In its broadest terms, thermoplastic injection moulding forms a part by forcing a liquid resin into a closed mould, under pressure, until the part has cooled or cured and can be ejected from the mould. This process consists of the following five distinct operations.
1. Feeding of raw materials
2. Plasticizing (mixing (mechanical work), external heating)
3. Injection (filling of the mould cavity)
4. Holding pressure
5. Cooling (cooling of the material in the mould cavity)
6. Ejection of the injection moulded part
The main factors in the injection moulding are the temperature and pressure history during the process, the orientation of flowing material and the shrinkage of the material. This means that the structure and the properties of injection moulded parts are inhomogeneous and the products have always internal stresses.
Feeding of Raw Material
First the molder receives plastic resin in the form of small chopped pellets. These are fed into the hopper of an injection-moulding machine, where they fall into an augur-type screw channel, which feeds the pellets forward inside the heated barrel.
PlasticizingThe cycle begins with the extruder plasticizing the resin and accumulating it in the forward section of the barrel. The heater bands maintain the melt's temperature as the shot it built up. The mould is closed. The cycle is typically timed so that there is minimal time between the closing of the mould and the next shot. Injection of the Resin into the Closed Mould.
Injection
After the mould closes, the screw (not rotating) pushes forward to inject melt into the cooled mould. The air inside the mould will be pushed out through small vents at the furthest extremities of the melt flow path.
Holding pressureWhen the cavity is filled, the screw continues to push forward to apply a holding pressure. This has the effect of squeezing extra melt into the cavity to compensate for the shrinkage of the plastic as it cools. This holding pressure is only effective as long as the gate(s) remain open.
Cooling the Resin inside the Closed Mould
Cooling starts immediately when polymer mass flows into the mould cavity including the injection and holding phases, and continues after these too. Therefore the cooling time is the longest phase in the injection moulding process. Because the injection and holding phases are included in the cooling phase, they also effect on the cooling of the moulded part. Cooling time depends on the raw material, wall thickness of the part and the heat transfer capability of the mould. In order to have a nonporous and unstressed part the polymer melt should solidify uniformly.
Opening the Mould and Ejecting the Moulded part Once the gate(s) freeze, no more melt can enter the mould and so the screw-back commences. At this stage the screw starts to rotate and draw in new plastic from the hopper. This is conveyed to the front of the screw but as the mould cavity is filled with plastic, the effect is to push the screw backwards. This prepares the next shot by accumulating the desired amount of plastic in front of the screw. At a pre-set point in time, the screw stops rotating and the machine sits waiting for the solidification of the moulding and runner system to be completed. When the moulding has cooled to a temperature where it is solid enough to retain its shape, the mould opens and the moulding is ejected. The mould then closes and the cycle is repeated.
THE PROTOMOLD PROCESSAt Protomold we have automated the process of designing and manufacturing molds based on customer supplied 3-D CAD part models. Due to this automation, we typically cut the lead time for the initial parts to one-third of conventional methods. Cost saving varies with the number of parts being produced, but Rapid Injection Molding may also have a substantial cost advantage in runs of up to thousands of parts.
The Protomold Rapid Injection Molding process gives design engineers a fast and affordable way to get real injection molded parts in prototype or low-volume quantities.So from an overall market perspective we see Rapid Injection Molding as in between Rapid Prototyping and Conventional Injection Molding as illustrated in the figure.
THE INJECTION MOULDING PROCESS CYCLEThe typical process cycle time varies from several seconds to tens of seconds, depending on the part weight, part thickness, material properties, and the machine settings specific to a given process.
Process control of injection molding has a direct impact on the final part quality and the economics of the process. The various components of process control must be fully understood to maximize profit and part quality.
The various stages are:
Injection phase (filling phase)
Packing phase
Holding phase
Cooling phase
Fill timeAt the beginning of the injection molding cycle (point 1), the mold has just closed and the molten polymer, which is maintained at a fairly uniform temperature inside the barrel of the injection machine, is forced to flow through the nozzle, runner, gate, and then into the cavity under controlled flow rate or pressure, depending on the control scheme of the injection unit. The fill time is defined as the time needed for the polymer to fill the entire cavity (duration between points 1 and 4.
Post-fill time
After the mold is completely filled (point 4), more material is packed into the cavity and the polymer continues to cool. The post-filling stage ends when the polymer temperature is sufficiently low and the part is rigid enough to be removed from the cavity without significant deformation (point 7). The post-fill time is defined as the time between the moment when the cavity is completely filled and the instant when the mold opens (duration between points 4 and 7).
Mold open time
The mold-opening stage begins when the mold is opened (point 7) and ends when the mold is closed (point 8) to start the next cycle. The mold-open time includes the time taken for mold opening and closing actions as well as part ejection (duration between points 7 and 8). Because this can be a significant portion of the cycle time in processes with extremely short cycles, each action of the mold clamp and ejection systems should be analyzed for possible time delays and wasted energy. During this stage, additional heat transfer occurs between the mold and ambient air.
To further illustrate the machine motion within the process cycle, the hydraulic (cylinder) and cavity pressure traces, screw position, and mold face separation position are shown below
1. Filling (injection stage) 2.Packing and cooling stage 3.Mold opening 4.Part ejection 5.Mold closing.
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