Cause analysis of short shots of injection molded products

Posted on : Oct 11 , 2022 By  GREFEE

Short shot indicates underfeeding and the products is insufficiently filled, during which the end of the material flows is incomplete or Part of the multi cavity is not fully filled, especially in the thin-walled area or the end area of the flow path. It is presented that the melt condenses before the mold cavity is fully filled, and the cavity is not completely filled after the melt entering in it, resulting in underfeeding.

The major cause for short shot is a large flow resistance, so the melt cannot continue to flow. Factors affecting the flow length of melts include wall thickness of workpiece, mold temperature, injection pressure, melt temperature, and material composition. If these factors are not managed properly, it will lead to short shot.

 Improper equipment selection

When selecting the equipment, the largest injection volume of the injection machine should be higher than the overall weight of the injection part and water inlet. However, the overall injection weight should not exceed the 85% of the plasticizing capacity of injection molding machine.

 Underfeeding

So far, the commonly used method to control feeding is the constant volume feeding, which is about whether the roll amount is uniform with the fruit grain diameter of the raw material, and whether there is a “bridging” phenomenon at the bottom of the feeding port. High temperature at the feeding port will cause poor blanking. In this regard, the feeding port should be dredged and cooled.

 Poor flowability of material

When the flyability of the material is poor, the structure coefficient of mold is the main reason for short shot. Therefore, properly set the runner position, expand the size of gate, runner and injection gate, and use larger nozzles are effective in improving the defects of the mold pouring system. Meanwhile, adding a proper amount of the auxiliary in the material to improve the flowability of the resin. In addition, check whether the recycled material is overweight and reduce the use amount properly can also improve the flowability.

 Excess lubricant

Too much lubricant in the raw material, and when the wear gap between the non-return ring of the injection screw and the barrel is large, the serious reflux of molten material in the barrel will cause insufficient feeding, resulting in short shots. In terms of this, reduce the amount of lubricant, adjust the barrel and the gap between the barrel and the injection screw and the check ring, as well as repair the equipment.

 Cold material impurities block the material path

When the impurities in the melts block the nozzles or the cold slugs block the gate and runners, the nozzles should be removed and being cleaned to expand the cold slug well and runner cross section of the mold.

Improper design of gate pouring system

When it comes to the multiple cavities, improper gate and sprue balance design will lead to appearance defects of the plastic injection molding products. When designing gate pouring system, plastic parts companies should pay attention to the balance of the gate. The weight of the plastic part in each cavity should be proportional to the gate size, making each cavity is filled simultaneously. The selection of the gate position should be in the tick walled area. The design of balanced layout of diversion channel can also be adopted.

If the gate or runner is small, thin and long, the pressure loss of molten material is too much along the way in the flow process, and the flow is blocked, which could lead to short shots. Therefore, the runner section and gate area should be expanded, and the multi-spot feeding can be adopted when necessary.

Poor venting of mold

When a large amount of gas in the mold due to poor venting and is squeezed by the flow material, resulting in a high pressure higher than the injection pressure. This pressure will prevent the molten material from filling the cavity and cause short shots. In this regard, it is necessary to check whether there is a cold slug well or whether it is in the correct position. For the mold with deep cavity, an exhaust groove or vent are required under the injected part. On the mold closing surface, an exhaust groove with a depth of 0.02~0.04mm and a width of 5~10mm should be installed, and the exhaust hole should be set at the final mold filling position of the cavity.

When using raw materials with excessive moisture and volatile content will cause the generation of a large amount of gases, resulting in poor venting of mold. In addition, the raw material should be fired to remove the volatile matter.

Besides, in terms of the processing operation aspect of the mold system, increasing the mold temperature and lower the injection seed and flow assistance of gating system and reduction of clamping force, expanding the gap between molds will improve the venting condition.

Low mold temperature

After the melts enter into the low temperature mold cavity, the cavity will not be filled completely due to the fast cooling. Thus, pre-heat the mold to the required temperature before starting. When it just starts, control the passing amount of cooling water in the mold. If there is no increase in mold temperature, check whether the cooling system of mold is proper.After the melts enter into the low temperature mold cavity, the cavity will not be filled completely due to the fast cooling. Thus, pre-heat the mold to the required temperature before starting. When it just starts, control the passing amount of cooling water in the mold. If there is no increase in mold temperature, check whether the cooling system of mold is proper.

 Low melts temperature

Typically, within an allowed molding range, The relationship between material temperature and mold filling length is close to positive proportion. The decreased fluidity of low-temperature melt shortens the filling length. When the material temperature is lower than the temperature required, check whether the barrel feeder is intact and try to increase the barrel temperature. When the machine is just started, the temperature of the material barrel is always lower than the temperature shown by the instrument of the material barrel heater. It should be aware that the machine will take a while to be ready after the material barrel is heated to the temperature of the instrument.

To prevent molten material decomposition, low-temperature injection is adopted when necessary. The injection cycle time can be appropriately extended to overcome under injection. For screw injection molding machines, increase the temperature of the front section of the barrel can improve short shots.

Low nozzle temperature

During the injection process, the nozzles will get contact with molds. Since the mold temperature is lower than nozzle temperature most of time, the temperature of nozzles will decrease after a frequent contact between both due to a big temperature difference, resulting in the material condensed around the nozzles.

If there is no cold slug well in the mold structure, the cold material will condense immediately after entering into the cavities, resulting in The hot melt blocked behind cannot fill the cavity. Thus, separate the nozzle and mold before mold opening is beneficial to reducing the impact to the temperature of nozzles, allowing the temperature of nozzles maintained within the required range.

If the temperature of nozzles is low and does not raise up, check whether the heated of nozzles is broken or not, and manage to increase the temperature of nozzles, otherwise, the pressure loss of flow material will lead to short shot.

 Insufficient injection pressure or pressure holding

The injection pressure is close to the positive proportional relationship with the filling length. Low injection pressure will shorten the filling length and cause the cavity is not fully filled. In terms of these issues, extend the injection time or employ other methods to increase the injection pressure. If the injection pressure cannot be elevated anymore, increase the material temperature and decrease the viscosity of melts or increase the flowability of molten to improve the conditions. One thing should be aware of is that too high material temperature will lead to the decomposition of the melts, affecting the use properties of the plastic parts.

In addition, short pressure holding time will lead to short shots. Thus, the pressure holding time should be controlled within a suitable range. What should be note is that long pressure holding time will also cause other breakdowns. During molding, it should be adjusted according to the specific conditions of plastic parts.

 Slow injection speed

The injection speed is associated with the mold filling speed directly. Slow injection speed will slow down the melt filling out the mold, whereas melts flow in low speed will cool easily, resulting in the short shots due to the decreased flowability.

In terms of this, raise the injection speed, but also should be aware that fast injection speed will cause other breakdowns.

 Inappropriate structural design of plastic parts

When the thickness and length of the plastic part are out of proportion, the shape is very complex, and the forming area is very large, the flow of molten material is easy to be blocked at the entrance of the thin-walled part of the plastic part, making it hard to fill the mold cavity. Hence, one thing that the injection molded parts manufacturer and plastic products suppliers should be aware is that the thickness of plastic parts is related to the limit flow length of molten material during mold filling when designing the shape structure of plastic parts.

During injection molding, the thickness of plastic parts is 1~3mm at most, and that of large plastic parts is 3~6mm. Typically, the recommended minimum thickness is; Polyethylene 0.5mm, cellulose acetate and cellulose acetate butyrate plastic 0.7mm, ethyl cellulose plastic 0.9mm, polymethylmethacrylate 0.7mm, polyamide 0.7mm, polystyrene 0.75mm, PVC 2.3mm. Generally, the thickness of plastic parts more than 8mm or less than 0.5mm will affect the injection molding, and this thickness should be avoided in design.

In addition, when forming structural plastic parts with complex shapes, necessary measures should also be taken in the process, such as the  correctly deciding the gate position, appropriately adjusting the layout of the runner, increasing the injection speed or adopting rapid injection. Increase the mold temperature or select the resin with good flow performance.

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