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What problems can arise from the injection molding process?

1. Why is the injection moulding product underfilled?

1) Improper adjustment of feeding material, lack of material or more material.

A. Improper dosing or improper operation of dosing control system.

B. The injection cycle is abnormal due to the limitation of the injection machine or mould or operation conditions.

C. Pre-molding backpressure is small, or the density of material particles in the barrel is small.

For large particles, more gaps in the granules and crystalline plastics such as polyethene, polypropylene, nylon, etc. and thick plastics such as ABS should be adjusted to a higher volume of material. The material temperature should be adjusted to a higher volume of material.

When the barrel end stock is too much, the injection screw should consume more injection pressure to compress and push the excess stock into the barrel, which significantly reduces the adequate injection pressure of the plastic into the mould cavity and makes it difficult to fill the product.

2) Injection pressure is too low, injection time is short, and the plunger or screw is returned too early.

The molten plastic has a high viscosity and poor fluidity at low working temperatures, so it should be injected with higher pressure and speed.

For example, when making ABS color parts, the colorant’s intolerance of high-temperature limits the barrel heating temperature, which should be compensated by a higher injection pressure and longer injection time than usual.

3) Slow injection speed.

Injection speed for some complex shapes, thickness changes, long process products, and viscosity of plastics such as toughness ABS, etc., has a very prominent significance.

When the use of high pressure can not be filled with products, should consider the use of high-speed injection to overcome the problem of under-injection.

4) low material temperature.

A. The front end of the barrel temperature is low. The melt into the cavity due to the cooling effect of the mould, and the viscosity rises prematurely to the point where it is difficult to flow, preventing the distal end of the mold filling.

B. The low temperature at the back of the barrel makes it difficult for the thick plastic to flow, preventing the screw from moving forward.

The result is that the pressure gauge appears to indicate adequate pressure when in fact, the melt is entering the cavity at low pressure and low velocity.

Low nozzle temperature may be due to heat loss from the nozzle being in contact with the cold mold for a long time during fixed charging, or low material temperature due to insufficient heat supply or poor contact with the nozzle heating ring blocking the inlet channel of the mold.

If the mold is without cold material well, with a self-locking nozzle, using the post-filling procedure, the nozzle can maintain the necessary temperature.

When the nozzle is too cold at the beginning of the machine, sometimes you can use a flame gun to do external heating to accelerate the nozzle warming.

2. Several programs of vertical injection moulding machine adjustment.

1). Adjust the barrel temperature to the middle of the range according to the temperature range provided by the raw material supplier’s information, and adjust the mold temperature.

2). Estimate the required injection volume and adjust the vertical injection molding machine to two-thirds of the estimated injection volume.

3). Initially, adjust the primary injection pressure to half of the vertical injection moulding machine’s limit, and adjust the vertical injection molding machine’s injection speed to the estimated and adjusted required cooling time. Set the back pressure to 3.5 bar. Remove the degraded resin from the barrel. Start the injection process in semi-automatic mode and observe the screw action. 

Adjust the injection speed and pressure as needed, or increase the injection pressure to shorten the mold filling time. Due to the process before complete mould filling, the final pressure can be adjusted to 100% of the primary injection pressure. The pressure should be adjusted high enough so that the set pressure does not limit the speed achieved. If there is an overflow, the speed can be reduced. 

5). Adjust the injection volume and changeover point after each observation cycle. Set the program so that 95-98% of mold filling by injection weight can be achieved at the stage injection. Once the injection volume, changeover point, injection speed and pressure are appropriately adjusted for the first stage, the second stage can be adjusted for the holding pressure.

6). Adjust the holding pressure as necessary, but do not overfill the mould cavity.

Adjust the screw speed to ensure that the melt is completed just before the cycle is completed and that the injection cycle is not limited.

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