Short filling in the injection molding process:Reason and Solution

Short filling is one of the most common defects in the plastic injection molding process. The plastic part is not full, and there are voids, bubbles, shrinkage, etc. These defects mostly occur in relatively thick plastic parts or far from the gate.

What’s the reason of short filling?

  1. Plastic injection mold

Short filling due to the irrational mold structure is often been seen, and the handling of this problem is also cumbersome, which is described below.

(1) The design of the pouring system is unreasonable. The pouring system is the channel through which the melt enters the mold cavity, which has a great relationship with the molding quality of the plastic part. If the gate is unbalanced, the quality of the plastic parts in each cavity is not proportional to the size of the gate, and the melt cannot fill the cavity at the same time, which may easily cause insufficient filling; Insufficient filling; if the cross-sectional area of ​​the gate or runner is small and long, the melt pressure loss will increase, the flow resistance will increase, and poor flow will easily occur and the filling will be insufficient. The above reasons will cause the plastic parts to have insufficient filling defects, and the mold can be modified according to specific conditions.

(2) The mold venting structure is not good. When the melt flows into the mold cavity, the gas in the mold cavity should be driven away. If the gas is not discharged smoothly, it still stays in the mold cavity, and it is difficult for the melt to fill the mold cavity. Insufficient filling. To this end, it is necessary to modify the exhaust structure or increase the exhaust clearance. For molds with deep cavities, vent grooves or vent holes should be added to the insufficiently filled parts.

(3) Impurities or cold materials in the melt block the flow path, making the melt flow poorly, making it difficult to fill the cavity and causing insufficient filling. In this regard, we must wake up in time to clear the flow.

(4) If there is no cold material cavity in the mold or the design of the cold material cavity is unreasonable, the cold material will enter the cavity and take the lead to condense. Therefore, the design of the cold material cavity should be paid enough attention.

  1. In terms of raw materials

There are many raw materials used for injection molding process, and all kinds of raw materials have different functions. For example, improper selection of raw materials or improper ratios may cause insufficient filling, which are described below.

(1) The moisture content of the raw materials is too large. This moisture vaporizes into water vapor when heated, and the additional gas adds to the burden of the exhaust structure, and it is difficult to exhaust it quickly after entering the mold cavity, making it difficult for the melt to fill the cavity within a predetermined time. In this regard, those materials which are easy to absorb moisture should be pre-dried.

(2) The volatile matter in the raw materials exceeds the standard. Some raw materials contain more volatile substances, and they quickly volatilize into gas when heated. These gases cannot be eliminated in time and lead to poor exhaust. To this end, the volatile substances in the raw materials need to be removed.

(3) Too many recycled materials are added to the raw materials. They have poor fluidity after melting, making the melt fluidity worse. In this regard, the amount of recycled materials should be strictly controlled.

(4) Improper raw material formula. There are many types of plastic formulas, and their fluidity is greatly affected by the formula. For example, a lubricant is an additive that has a large impact on the fluidity of plastics. However, if there is too much lubricant, the melt has good fluidity, and it will cause backflow in the barrel, causing insufficient supply. In this regard, the amount of additives that can affect the melt fluidity is strictly controlled.

(5) There are many impurities in the raw materials. It is mainly non-meltable solid impurities. Their presence will block the nozzle-type flow path, making the melt flow poor, the supply amount reduced, and the cavity filling not full. To this end, solid impurities must be removed from the raw materials in advance.

3, injection molding machine

(1) The injection volume of the injection molding machine is insufficient. If the quality of the plastic part is greater than the maximum injection volume of the injection molding machine, the cavity cannot be filled and the filling is insufficient. In this regard, when the injection molding machine is selected according to the plastic part, the total injection amount cannot exceed 85% of the plasticization amount of the injection molding machine.

(2) The gap between the injection rod and the barrel is too large, causing the melt to flow back along its gap, causing insufficient supply. The gap between the injection rod and the barrel is mostly caused by wear. In order to solve this problem, relevant parts need to be replaced.

(3) The nozzle of the injection molding machine is blocked by foreign objects, which makes the melt flow poor and causes insufficient filling. For this reason, the nozzle can only be removed for dredging.

(4) The nozzle hole is too small, which makes the melt filling speed not fast and it is difficult to fill the cavity within a predetermined time, resulting in insufficient filling. For this reason, the nozzle with a larger nozzle hole diameter should be replaced.

(5) Insufficient supply of raw materials. The supply of raw materials cannot keep up with the requirements of the injected materials. At this time, check whether the particle size of the raw materials is uniform and whether there is a “tower bridge” phenomenon at the bottom of the feed port. In this regard, materials with uniform particle size should be selected, or the “bridge” should be crushed.

(6) The check valve is faulty. The check valve is to prevent the melt from flowing backwards. If it fails, the melt will be insufficiently supplied. In this regard, check whether the check valve works normally, and then wake up accordingly.

(7) Insufficient injection stroke makes the injection volume insufficient, which makes it difficult to fill and insufficient filling. In this regard, the injection stroke can be appropriately increased to increase the supply amount.

How to improve the process to solve the short filling?

(1) Unstable material feed adjustment, lack of material or multiple materials.
In the injection molding process,Incorrect feeding measurement or abnormal operation of the feeding control system, abnormal injection cycle, injection molding machine or mold or operating conditions, which lead to abnormal injection cycle, low pre-plastic back pressure or small particle density in the barrel may cause material shortage. For large particles, granular materials with many voids and plastics with large changes in specific volume such as polyethylene, polypropylene, nylon, etc., as well as plastics with higher viscosity, such as ABS, should be adjusted to a higher amount. When the temperature of the material is high, the amount should be increased.

When there is too much material at the end of the barrel, the screw consumes extra injection pressure to compact and push the excess material in the barrel during injection, which greatly reduces the effective injection pressure of the plastic entering the mold cavity and makes the Products are difficult to fill.

(2) The injection pressure is too low, the injection time is short, and the plunger or screw is retracted too early.
Molten plastics have high viscosity and poor fluidity at low operating temperatures, and should be injected at a higher pressure and speed. For example, in the manufacture of ABS color parts, the high temperature resistance of the colorant limits the heating temperature of the barrel, which must be compensated by a higher injection pressure and longer injection time than usual.

(3) The injection speed is slow.
The injection speed is very significant for some products with complex shapes, large thickness changes, and long processes, as well as plastics with high viscosity, such as toughened ABS. When the product cannot be filled with high pressure, high-speed injection should be considered to overcome the problem of dissatisfaction.

(4) Material temperature is too low.
The temperature of the front end of the barrel is low, and the melt entering the cavity rises prematurely to the point where it is difficult to flow due to the cooling effect of the mold, which prevents the filling of the distal end. The temperature at the back of the barrel is low and the viscosity is high. Difficult flow, hinders the screw from moving forward. As a result, it appears that the pressure displayed by the pressure gauge is sufficient and the melt actually enters the cavity at low pressure and low speed;

Low nozzle temperature may be caused by the nozzle losing contact with the cold mold for a long time during fixed feeding.  Or the low temperature of the nozzle heating ring due to insufficient heating or poor contact may block the feeding channel of the mold.

If the mold does not have cold material Wells, with self-locking nozzles, using a post-feeding procedure, the nozzles can maintain the necessary temperature. The nozzles are too cold when they are just started, and sometimes a flame gun can be used for external heating to accelerate the nozzles’ temperature.

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