A short shot, also known as insufficient filling or short shot, refers to inability of polymer to completely fill every corner of mold cavity.
Causes and solutions for short shots are as follows:
1. Improper Injection Molding Equipment Selection
When selecting an injection molding machine, maximum shot volume should be greater than weight of part (including part, runners, flash, etc.). For optimal results, the total shot volume should be less than 85% of maximum shot volume.
2. Poor Polymer Flow Properties
In this case, an appropriate amount of additives should be added to raw materials to improve resin's flow properties. Also, check proportion of recycled material in raw materials and reduce it appropriately.
Alternatively, consider improving mold's gating system by adjusting runner position, increasing gate, runner and gate dimensions, and using a larger nozzle to correct stagnant flow in mold's gating system.
3. An Improper Gating System
A reasonable gating system takes into account balance of gate and runners. Cavity volume within each cavity should be proportional to gate size, allowing polymer to fill all cavities simultaneously. Gate should be located in a thick wall area. Alternatively, a balanced split runner layout can be employed.
For small, thin, or long gates or runners, melt will experience significant pressure loss during flow, hindering flow and easily causing short shots. To address this, runner cross-section and gate area should be increased, and multi-point feeding can be employed if necessary.

 

4. Low Material and Mold Temperatures
Typically, material temperature and filling length are nearly directly proportional. Lower material temperatures can reduce melt flow properties, shortening filling length. If material temperature is confirmed to be low, check barrel heater for proper function and attempt to increase barrel temperature.
If low-temperature injection is unavoidable to prevent melt decomposition, temperature of front section of barrel can be appropriately increased, or injection speed can be increased to reduce polymer cooling time. However, be aware that excessively fast injection speeds can cause melt fracture, resulting in wrinkle defects.
Low mold temperature can cause molten polymer to cool prematurely, preventing it from filling the entire cavity. In this case, mold should be heated to a temperature that meets process requirements, and cooling medium flow rate should be reduced during initial injection phase. If mold temperature remains consistently low, cooling system design should be adjusted.
5. Low injection nozzle temperature
During injection process, nozzle and mold are in direct contact. Since mold temperature is generally lower than nozzle temperature, and temperature difference is large, frequent contact between the two causes nozzle temperature to drop, leading to condensation of melt at nozzle (also known as dry tip).
To prevent cold slug from solidifying immediately after entering cavity, blocking hot slug from filling cavity, consider creating a cold slug well in runner. Also, ensure that nozzle is separated from mold during mold opening to minimize impact of mold temperature on nozzle temperature.
6. Insufficient Injection Pressure and Holding Pressure
Injection pressure and filling length are nearly directly proportional. Low injection pressure can result in a short filling length, leading to short shots.
In this case, injection pressure can be increased by slowing ram speed and appropriately extending injection time. If injection pressure cannot be increased further, remedy can be achieved by raising material temperature (to a temperature that does not cause melt to decompose) to reduce melt viscosity and thus improve melt flow properties.
Furthermore, short shots can also occur if holding time is too short. Therefore, selecting an appropriate holding time (generally between 30 and 120 seconds, and increasing it for thick-walled parts) can prevent short shots. However, it should be noted that excessively long holding times can also make it difficult for part to be automatically released.
7. Improper Product Structural Design
When overall dimensions of a part are disproportionate (for example, thickness and length are disproportionate), or when part has a complex shape and a large molding area, melt flow can easily be restricted in thin walls of part, resulting in short shots. Therefore, when designing shape and structure of a plastic part, it's important to consider relationship between part thickness and critical flow length of melt during mold filling.
Ratio of the critical flow length of melt during mold filling to part thickness:

In injection molding, part thickness is generally 1-3mm, with large parts ranging from 3-6mm. Part thicknesses exceeding 8mm or less than 0.5mm are generally detrimental to molding.
In addition, when molding complex plastic parts, necessary process measures are also required, such as appropriately adjusting runner layout, properly determining gate location, increasing injection speed, raising mold and material temperatures, and selecting resins with better flow properties.
8. Poor Venting
Poor venting can cause a large amount of gas to remain in mold cavity, which is squeezed by flowing material and generates high pressure. When this pressure exceeds injection pressure, it hinders melt from filling mold, resulting in a short shot.
To address this problem, check whether cold well is properly positioned. For deep-cavity molds, vent grooves or holes should be installed at location where short shot occurs. Vent grooves can be created on mold parting surface, with a depth of generally 0.02-0.04mm and a width of 5-10mm. Vent holes should be located at final filling point of cavity. For volatile polymers or those with a high water content, volatile components should be removed or dried before injection.
Poor venting can also be improved by adjusting mold system's process parameters. For example, measures such as increasing mold temperature, reducing injection speed, reducing mold clamping force and increasing mold gap.