Process Parameters Affecting Plastic Molding

The proportion of plastic materials in life is getting higher and higher, as well as the requirements for its quality. Injection molding is an important means for the production of plastic products, and the requirements for technological improvement are becoming more and more urgent.

There are many factors affecting injection molding products, but the processing condition is one of the important factors. Next, we will introduce the various process parameters affecting plastic molding from three aspects: temperature, pressure, speed and time.

01 Temperature

1.1 Barrel temperature requirements

It should be greater than the flow temperature of the plastic (melting point), and less than the decomposition temperature of the plastic.

1.1.1 The result of high barrel temperature:

Plastics are easily decomposed to generate low molecular compounds and decompose into gases. And that will make the surface of the plasticdiscolored, resulting in bubbles, silver threads and streaks, which further causes the performance degradation.
High material temperature – plastic in the cavity is inconsistent inside and outside, easy to generate internal stress and dents.
When the temperature of the molten material is high, the fluidity is good, and it is easy to overflow.

1.1.2 The result of the barrel temperature being too low:

Low material temperature – poor fluidity, prone to defects such as weld lines, insufficient molding, and ripples.
Uneven plasticization – easy to produce cold blocks or stiff blocks, etc.
Low material temperature – When the plastic is cooled, it is easy to generate internal stress, and the plastic parts are easy to deform or crack.

1.2 Nozzle temperature

1.2.1 The temperature is too high – the plastic is prone to decomposition reaction, etc.

1.2.2 The temperature is too low – the nozzle is easy to be blocked, and it is easy to produce cold blocks or rigid blocks, etc.

1.3 Mold temperature

The actual flow of the melt in the mold cavity is non-isothermal flow. That is, the temperature of each part of the mold cavity is uneven.

1.3.1 High mold temperature – slow cooling, easy to produce mold sticking, plastic parts are easily deformed when demoulding, etc.

1.3.2 Results of low mold temperature:

Reduce the fluidity of the melt, and it is easy to produce insufficient molding and weld lines.
When the molten material is cooled, the cooling of the inner and outer layers is inconsistent, which is easy to generate internal stress.
The temperature of the melt is too high:

It is easy to decompose; it is easy to generate internal stress; the surface viscosity of the melt decreases with good the fluidity. This is especially true for temperature-sensitive plastics. It is easy to fill the mold, and it is easy to overflow and overflow.

Uneven temperature of the melt:

It is easy to generate internal stress. For example, in the actual mold cavity, the temperature of each point is uneven, and the flow of the melt is non-warm flow.

Low melt temperature:

It is not easy to decompose; the surface viscosity is high, the fluidity is poor, the mold filling is difficult, and it is easy to produce insufficient molding, weld lines, cold blocks or rigid blocks, etc.

02 Pressure

2.1 Clamping force

It must be enough, otherwise there will be overflow, burr, etc.

2.2 Injection pressure

2.2.1 Results of overly high injection pressure:

When it is too high, the plastic is forced to condense under high pressure. And it is easy to generate internal stress, which is conducive to improving the fluidity of the plastic. Besides, it is easy to produce overflow. And the residual pressure on the mold cavity is large, the plastic is easy to stick to the mold. It is also difficult to demold, causing the plastic parts deform but with no bubbles generated.

2.2.2 Results of overly low injection pressure:

When it is too low, the fluidity of the plastic decreases, resulting in insufficient molding and weld lines. It is not conducive to the overflow of gas from it, making it easy to generate air bubbles. Then, the plastic fills shrinkage gaps during cooling, resulting in dents and ripples, etc.

2.3 Holding pressure

2.3.1 Too high – easy to produce overflow, increase internal stress, etc.

2.3.2 Too low – insufficient molding, etc.

2.4 Back pressure

2.4.1 Too high – The plasticizing time becomes longer. The melt is easily decomposed. And bubbles, streaks, black spots and so on are generated.

2.4.2 Too low – the temperature of the gas in the melt at the front end of the barrel increases under pressure. And the local heat of the melt is too high, which decomposes to produce black spots and bubbles.

03 Speed & Time

3.1 Mold closing time

3.1.1 Too long – the mold temperature is too low, and the molten material stays in the barrel for too long.

3.1.2 Too short – the mold temperature will increase relatively.

3.2 Injection time, filling speed, shear rate

The injection time is shortened, the filling speed is increased, and the orientation is decreased.

As the shear rate increases, the surface viscosity of most plastics decreases, especially for shear rate sensitive plastics.

When the shear rate is too large, melt fracture occurs. The shear rate increases and the orientation increases.

3.3 Holding time

3.3.1 Short – the plastic parts are not close together, easily generating dents and making the dimensions of the plastic parts unstable.

3.3.2 Long – increase the internal stress of the plastic parts, resulting in deformation, cracking, and difficulty in demoulding.

3.4 Cooling time

3.4.1 Long – difficult to demold, easy to deform, high crystallinity.

3.4.2 Short – easy to produce deformation, insufficient cooling, etc.

3.5 Screw speed

3.5.1 Fast – increased shearing heat, short plasticizing time, etc.

3.5.2 Slow – the shear heat decreases, the plasticization time increases, etc.

3.6 Mould opening speed

If the mold opening speed is fast, the molding cycle will be short. But if it is too fast, it is easy to increase the friction between the surface of the plastic part and the cavity, resulting in scratches.

3.7 Ejection speed

If the ejection speed is too large, the plastic parts are easily deformed.

Conclusion

In order to maintain the production level and quality of plastic products, manufacturers must bear these injection molding process parameters in mind. So that you can avoid the setting mistakes in your production and produce the high quality plastic products.

ONEX MACHINERY