Polystyrene is one of the best materials for injection moulding. Cycle times with polystyrene are usually short due to ease of melting, fluidity and reasonable set up times. Complex tooling can be used.
The surface finish of the final product is also good, and the low mould shrinkage of polystyrene is a useful factor.
Moreover, due to good inherent thermal stability, polystyrene can easily be recycled.
Finally, polystyrene can be easily compounded or coloured in the melt phase and formulated for specific performance (gloss, anti-static properties,...).
General facts
Total Petrochemicals’s Polystyrene can be processed by every conventional technique used for thermoplastics. The general properties of polystyrene allow for a wide processing window in terms of both temperatures and pressures.
Drying
Polystyrene is not hygroscope, and is delivered in dry pellet form. Drying is not normally necessary. Care must be taken to avoid conditions which can cause condensation, this can lead to the appearance of splash marks on the finished moulding. If necessary, the product can be dried in a ventilated oven for 2 hours at a temperature of about 80°C.
Change of material or colour
All polystyrenes are "compatible", either GPPS or HIPS. The change from one grade to another is straightforward. Polystyrene is not compatible with other polymers such as polyethylene (HDPE or LDPE), PVC (Polyvinyl Chloride), ABS (Acrylonitrile Butadiene Styrene), PMMA (Polymethylmethacrylate), or PA (Polyamides) and, in general, other thermoplastics. This means that the machine has to be purged thoroughly in order to avoid such phenomenon as delamination during molding.
In order to do this efficiently, we advise to let the machine run while decreasing the temperatures, then to feed in the new material, and to start increasing slowly the temperatures. The new material will be more viscous because of low temperature and should "push out" the old material
The change from one colour to another is achieved quite easily by using the same protocol.
Temperature
Standard grades of polystyrene can be processed with a fairly wide temperature range from 180°C to 280°C. Some caution should be exercised when using certain compounds which are heat sensitive e.g. some fire retardant grades.
The choice of temperature to use depends mainly on the component design, the cycle time, and the geometry of the feed system (hot runners, …). Generally an increasing temperature profile from the feed hopper to nozzle should be adopted. The nozzle temperature should be set to a lower value in order to avoid the formation of strings and material leakage from systems without a shut off valve.
In certain cases, where there may be issues relating to plasticising capacity, an inverse temperature profile, where the hottest zone is the feeding section, with an upper limit of 230°C, can be adopted.
Injection speed
The injection speed depends on the machine capacity and general injection parameters e.g. part thickness, hot runners design…. A high speed gives a high level of shear, generating material self heating, which in turn makes it easier for the material to flow by limiting the thickness of the cold layer in the hot runners. Polystyrene, being quite thermally stable, lends itself to this self heating phenomenon. It is recommend to use high injection speeds in order to minimise potential weld line problems. However, there are limits as too high injection speed can cause faults such as material degradation, air inclusion (bubbles), and burn marks due to inadequate tool venting.
Shrinkage
As with every plastic material, polystyrene shrinks during cooling. This value is generally between 0.4 and 0.7% depending on grade, part thickness and issues due to tool design.
Mould Temperature
Generally between 30 and 50°C. For thin wall objects moulded at short cycle times it could be useful to cool down the mould down to 10°C.
Monday, March 5, 2007
Polystyrene Injection Process
Injection moulding preparation
- Preparation of mold
- Preparation of plastic material
- Installation of mold
- Setting temperature
- Setting molding condition
- Starting molding
Those a step by step to prepare moulding, the explanation each step can read below
1. Preparation of mold
Before installing a mold, check the surface of mold, particularly cavity and core for rush, flaws, bare and other material doesn’t needed and then check the condition of mold refers to maintenances record, make sure the mold still have good condition. For efficiency it is also important to check the cooling water pipe and for clogging and nipples for flaws and leakage.
2. Preparation of plastic material
Carefully check the capacity at the hopper, check the color, warming up system in injection machine near hopper make sure it working well, and start drying material three or five our at the specific temperature before starting molding.
3. installation of mold
Molding almost heavy so read and observe carefully the working standard and working safety procedure and direction, position the mold on the platen by using locating ring, then secure it with bolts, clamps etc, after installing the mold, install the cooling water hose according to the piping diagram, confirm that the mold close and open smoothly.
4. setting temperature
See the manual of plastic material, set the plasticizing temperature of the plastic material. Generally, determine the nozzle temperature first, then the lower temperature at the front section, intermediate section and rear section in order, if the temperature of the section under the hopper is too high, the material cannot be fed normally.
At he mold set the temperature of molding by using mold temperature regulator, it takes some time to rise the mold temperature to set the level after circulation of the medium is started.
5. setting the molding condition
- Set condition
Set condition include injection pressure, dwelling pressure, injection speed, injection time, pressure change offer time, cooling time, metering finishing time, screw speed, it all can be prepare as well as we need.
- Resultant condition
It is will related to injection, filling time, filling temperature, gate sealing time, injection finishing point, and other
6. Starting molding
After all prepare well, and the mold temperature and the cylinder temperature reach the respect set levels, start the molding, since the mold temperature is unstable and the plastic material is uneven just after molding is started, the quality of molding is unstable, basically after the operation cycle of the molding machine stabilized, discard the first several moldings, then start collection the moldings.
Saturday, March 3, 2007
Injection Molding Basic Knowledge
To put is basically, injection molding is the process of forcing melted plastic in to a mold cavity. Once the plastic has cooled, the part can be ejected. It is useful when the parts are too complex or cost prohibitive to machine. With this process, many parts can be made at the same time, out of the same mold
Injection Molding is the process of forcing melted plastic in to a mold cavity. Once the plastic has cooled, the part can be ejected. Injection molding is often used in mass-production and prototyping. Injection molding is a relatively new way to manufacture parts. The first injection molding machines were built in the 1930's.
Injection molding is a plastic-forming process used in the production of most (about 70%) of plastic parts. Other plastic-forming processes include blow molding, pressure-forming, and thermo-forming. Injection molding is generally used in the high-speed manufacture of low-cost, high-volume parts, like videocassette cases, plastic cups, printer parts, refrigerator parts, automotive parts, and other electronic parts like casing, gear.
The process of injection molding begins with a barrel full of hot, liquid plastic. The plastic is rammed at high pressure into a mold. Once the plastic fills the mold, it is allowed to cool and solidify. The finished part is then extracted (usually automatically) from the mold. You will learn more about the injection molding process in next week's lab. This week, we will concentrate on mold design and analysis.
The mold defines the shape of the part, as well as the path by which the molten plastic flows from the barrel. A simple mold has several features:
- Fixed and Moving Platens - These are rectangular blocks of aluminum or steel into which the shape of the part is cut.
- Cavity - When the fixed and moving platens are touching, the space formed by the cut-out portions, called the cavity, defines the shape of the part.
- Sprue - The sprue is a hole cut into the center of the fixed platen. Molten plastic flows from the sprue to fill the cavity.
- Runners - Runners are channels cut into the platens that direct molten plastic from the sprue to the gates.
- Gates - Gates are small openings between runners and cavities. These are the points at which plastic enters the cavity. They are generally small so that the finished part may be easily broken away from the useless sprue and runner material
Generally step process in the injection molding process are mould close - injection carriage forward - inject plastic - metering - carriage retract - mould open - eject part
There are six major steps in the injection molding process
1. Clamping
An injection molding machine constists of three basic parts; the mold plus the clamping and injection units. The clamping unit is what holds the mold under pressure during the injection and cooling. Basically, it holds the two halves of the injection mold together.
2. Injection
During the injection phase, plastic material, usually in the form of pellets, are loaded into a hopper on top of the injection unit. The pellets feed into the cylinder where they are heated until they reach molten form (think of how a hot glue gun works here). Within the heating cylinder there is a motorized screw that mixes the molten pellets and forces them to end of the cylinder. Once enough material has accumulated in front of the screw, the injection process begins. The molten plastic is inserted into the mold through a sprue, while the pressure and speed are controled by the screw. Note: some injection molding machines use a ram instead of a screw. · basically the screw extends from the hopper to the injection chamber.
3. Dwelling
The dwelling phase consists of a pause in the injection process. The molten plastic has been injected into the mold and the pressure is applied to make sure all of the mold cavities are filled.
4. Cooling
The plastic is allowed to cool to its solid form within the mold. It usually uses cooling process in mold base, the cooling process was made compact with injection machine so its can do automatically by setting in injection machine
5. Mold Opening
The clamping unit is opened, which separates the two halves of the mold in two plates model, but in three plate model, mold will separates in three halves, include cutting runner process.
6. Ejection
An ejecting rod and plate eject the finished piece from the mold. The un-used sprues and runners can be recycled for use again in future molds. Ejection of parts usually doing by ejector system, ejector system consist of ejector rod, spring, ejector plate and ejector backing plates.
