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Mold design-from Micon Mould

2016/11/2 11:06:11

Mold design and construction requires special attention for optimal product  quality and reliable molding. A detailed specification is required in advance:

 -product shape and tolerances
 -mold in relation to molding equipment
 -parting lines; venting
 -number of cavities
 -runner lay-out and gating system
 -ejection system
 -cooling system lay-out
 -type of tool steel
 -surface finish

Polystyrene Injection Process

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 colored 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 Color

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  minimize 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.

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