How to control the stability of injection molding process

Table of Contents

Injection molding is an engineering technique that involves transforming plastics into products that are useful and retain their original properties.

The important process conditions of injection molding are temperature, pressure and corresponding action time which affect plasticizing flow and cooling.

Temperature control

Barrel temperature

The injection molding process needs to control the temperature of the barrel, nozzle and mold, etc. The first two temperatures mainly affect plastic plasticization and flow, while the second temperature mainly affects plastic flow and cooling.

Each kind of plastic flow with different temperature, the same kind of plastic, due to different sources or brand, the flow temperature and decomposition temperature is little different, this is due to the different average molecular weight and molecular weight distribution, plastic in different type of injection molding machine of plasticizing process is also different, so choose the cylinder temperature is not the same.

Nozzle temperature

The nozzle temperature is usually slightly lower than the maximum temperature of the barrel, in order to prevent the “salivation phenomenon” that may occur in the molten material in the straight-through nozzle. The nozzle temperature should not be too low, otherwise the nozzle will be blocked due to the early melting, or the performance of the product will be affected due to the early setting of the material into the injection mold cavity.

Mold temperature

Mould temperature has great influence on the intrinsic properties and apparent quality of products. The mold temperature is determined by the availability of plastic crystallization, product size and structure, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, injection molding cycle, etc.).

Pressure control

Pressure in injection molding process includes plasticizing pressure and injection pressure, which directly affects plasticizing and product quality.

Plasticizing pressure
Back pressure: using screw injection machine, screw top melt in the screw rotation back pressure is known as plasticizing pressure, also known as back pressure.

This pressure can be adjusted by the relief valve in the hydraulic system. In injection, the plasticizing pressure is unchanged with the rotating speed of the screw, so increasing the plasticizing pressure will increase the melt temperature, but will reduce the plasticizing speed.

In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, with uniform color mixing, and discharge of the gas in the melt. In general operation, the plasticizing pressure should be determined as low as possible under the premise of good product quality. The specific value varies with the type of plastic used but rarely exceeds 20 kg/cm2.

Injection pressure

In current production, the injection pressure of almost all injection machines is based on the pressure applied to the plastic by the plunger or screw top (converted from the oil line pressure).

The injection pressure in injection molding plays a role in overcoming the flow resistance of plastic from the cylinder to the cavity, giving the rate of melt filling, and compacting the melt.

Forming cycle

The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle.

Molding cycle directly affects labor productivity and equipment utilization, so in the production process, should be in the premise of ensuring quality, as far as possible to shorten the injection molding cycle in each relevant time.

In the whole molding cycle, injection time and cooling time are the most important, they have a decisive impact on the quality of products.

The mold filling time in the injection time is directly inversely proportional to the mold filling rate, and the mold filling time in the production is generally about 3~5 seconds.

The pressure holding time in the injection time is the pressure time of the plastic in the mold cavity, which accounts for a large proportion in the whole injection molding time, generally about 2~120 seconds (extra thick parts can be as high as 5~10 minutes).

Before the melting at the gate is frozen, the amount of holding time has an impact on the product size accuracy. The holding time also has an optimal value, which is known to depend on the feed temperature, mold temperature, and the size of the main channel and gate.

If the dimensions of the main channel and gate and the process conditions are normal, the pressure value with the minimum shrinkage fluctuation range is usually taken as the standard.

The cooling time is mainly determined by the thickness of the product, the thermal and crystallization properties of the plastic, and the mold temperature.

The END OF THE COOLING time, which is generally BETWEEN 5 and 120 SECONDS, should BE based on the principle that the product does not change during demoulding.

Too long cooling time is not necessary, which will not only reduce the production efficiency, but also cause demoulding difficulties for complex parts, and even produce demoulding stress when forced demoulding.

The other time in the forming cycle is related to whether the production process is continuous and automatic as well as the degree of the two.

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