PC injection molding

PC injection molding

Polycarbonate(PC) injection moulded product owns excellent performance, high transparency,good  impact resistance and creep resistance, wide temperature window.

Adversely, some other PC injection moulded product features are:

  • poor sensitivity of melt viscosity to shear rate,
  • good sensitivity to temperature change,
  • no obvious melting point, high melt viscosity
  • finished products easily hydrolyzed and easy to crack under high temperature

As a mold manufacturer china,We need keep  in mind these characteristics,:

To increase the melt flow, not by increasing the injection pressure but increasing the injection temperature.

Also mold runner and gate should keep short and thick to reduce the pressure loss of the melt.

Before molding ,the resin need to be sufficiently dried to make moisture content below 0.02%,in addition, in the process ,resin temperature keeping measures should be taken to prevent re-absorption of moisture.

Product design should not only be reasonable, but also consider the correct molding process, such as the mold temperature, after treatment to reduce or eliminate inner stress.

The injection temperature

injection temperature setting should consider shape, size and structure of the injection moulded product.
Various aspects of product performance requirements should be considered .
 
Generally, molding temperature is between 270 ~ 320 ℃, if  temperature is over 340 ℃ , PC will appear decomposition for products  become darker in color, the product surface with silver, dark bar, black spots, bubbles and other defects, at same time physical and mechanical properties also decrease significantly.


The injection pressure

The injection pressure has some influence on the physical and mechanical properties ,internal stress and shrinkage, etc. of PC products, while it has a greater impact on the appearance and  releasing way.
 
Too low or too high  injection pressure will cause the appearance defects. The injection pressure are generally controlled between  80-120MPa.
 
For the thin-walled, long running, complex shape and the smaller gate products,higher injection pressure (120-145MPa) is required to overcome the resistance of the melt flow and fill the mold cavity in time, to obtain a complete and smooth surface of the product.

Packing pressure and time

packing pressure and time have a great impact on the internal stress of PC products.

Too small packing pressure would lead to poor shrinkage ,small vacuum bubbles or surface concave, while too large packing pressure would produce large internal stress around gate.

In the actual processing, we often adopt high melt temperature, low pressure to guarantee good products. Packing time depended on the thickness, gate size, mold temperature, etc..

Generally smaller and wall thinner products don’t need long packing time, on the contrary, as to large and thick products packing time should be longer.

Packing time can be determined by the gate close time.

Injection speed

As to PC products, injection speed seems without very significant impact, except the thin-walled, small gate, deep, long runner products. In general, it should use the medium or slow speed, it preferably adopt multi-stage injection:slow-quick-slow.

The mold temperature

The mold temperature can be controlled at 80-100 ℃.As to complex shape, thin, high demanding injection moulded product ,it can be increased to 100-120 ℃, but not more than the heat distortion temperature.

Screw speed and back pressure

Due to the high melt viscosity of PC, Considering the plasticizing ,venting, maintenance of injection machine and screw load , screw speed can not be too high, generally controlled at 30 -60r/min appropriate, the back pressure should be 10-15% of injection pressure.

Injection molding process

It should strictly control the use of mold release agents for any mold manufacturer china, and the use of recycled materials can not exceed three times, content below 20% about.

Good quality  injection machine is required for the PC products: products weight that require maximum injection volume (including runner, gate, etc.) should not exceed 70-80% of the nominal amount of the injection .