How to Fix Short Shot Injection Molding?

What is Short Shot Injection Molding?

Incomplete filling of mold cavities during the injection process, known as short shot injection molding, is a frequent problem in plastic injection molding. This occurs when the molten plastic material doesn't entirely occupy the mold, resulting in substandard products. The repercussions include substantial quality issues, rendering the incomplete parts unfit for use. The origins of this problem are diverse, spanning from material viscosity to incorrect machine configurations. Grasping these causes serves as the initial stride in averting and resolving short shot concerns.

Causes of Short Shot Injection Molding

High Viscosity of Material

Plastic viscosity refers to the resistance a melted plastic faces when flowing. If a material has low viscosity, it flows quickly and fills mold spaces before cooling. Conversely, high-viscosity materials may flow too slowly to fill intricate molds. The viscosity rating, like Styrene with a 0 rating, indicates how easily a raw resin material flows. Materials with low viscosity get negative ratings, while those with higher viscosity than Styrene get positive ratings. If your manufacturer finds the material too viscous, they'll suggest solutions.

Venting Issues

Proper mold design should be nearly airtight to prevent injection molding flash. Insufficient venting or improper venting can lead to short shots and other problems. Short shots happen when air in the mold cavity can't escape while plastic is injected, restricting resin flow. Vents in the mold, like standard, ejector, peripheral, or runner venting, should allow trapped air and gases to escape without causing flashing. To fix short shots due to venting, manufacturers may add more venting where needed.

Insufficient Mold Temperature

If material can't flow into the cavity, it may be due to low mold temperature. The current temperature doesn't allow the material to flow sufficiently into all parts of the cavity. In such cases, the plastic injection molding manufacturer will increase the mold temperature, enabling plastic to reach hard-to-reach mold areas.

Frozen Flow Channels

Restricted material flow can be due to frozen flow channels in a hot runner system, requiring clearing for desired flow throughout the mold cavity. Cold runner flow restrictions in molds/simulations, caused by inadequate runner size, may lead to short shots. To correct this, small runners can be opened up to a larger size.

Fill Rate Issues

Short shots may be linked to the fill rate during injection. In such cases, the manufacturer needs to increase injection pressure and/or speed to ensure complete filling of the mold cavity.

Methods to Fix Short Shot Injection Molding

Identify the Culprit

First things first, what causes short shot molding? It's like baking a cake – miss an ingredient, and it won't rise. In molding, inadequate material, low temperature, or improper pressure can lead to incomplete fills. Identifying the culprit is the key to solving the puzzle.

Material Matters

Check your material mix. Sometimes, it's not about the quantity, but the quality. Ensure your materials are up to the mark. Inferior materials can play spoilsport, resulting in a shot that leaves you wanting more.

Change Temperature 

Just like Goldilocks and her porridge, your temperatures need to be just right. Too hot or too cold can throw everything off balance. Adjust the temperature settings, find that sweet spot, and witness the magic of a full shot.

Pressure Matters

Pressure is a silent player in the molding game. Too much or too little, both can be problematic. Check your pressure settings to ensure they align with the requirements of your mold. It's like finding the perfect hug – not too tight, not too loose.

Mold Inspection

Your mold is the stage where the magic happens. Inspect it for any signs of wear and tear. A worn-out mold can lead to uneven fills. If your mold looks tired, it's time for a makeover.

Run It Slow

Sometimes, slowing down is the solution. Adjust the injection speed – a slower pace can give the material more time to spread and fill every nook and cranny of the mold. It's like savoring a good meal; rushing through might leave you hungry for more.

Tips to Avoid Short Shots in Injection Molding

1. Verify Your Heater Settings

Ensure that the heaters on your equipment, including the screw and barrel, are properly configured as incorrect settings can result in short shots. Potential reasons for this issue include a malfunctioning thermocouple or inaccurate temperature readings. Prior to making any adjustments to the processing, it's crucial to double-check the heater settings.

2. Examine Your Tool and Ventilation System

Inspect the ventilation system to ensure there is no accumulation of debris, which can lead to increased pressure within the tool. Additionally, examine the tool for any signs of damage such as burns or marks, particularly if the ventilation system is not easily visible.

Evaluate how the material is filling the tool, particularly if an imbalanced tool is resulting in uneven short shots. Ensure that drops and tips are free from contamination from previous runs by utilizing a purge compound as a preemptive measure before removing the tool.

3. Assess Your Machinery

Take a moment to evaluate the overall performance of your injection molding machine. Confirm that there is an adequate supply of material in the hopper and check for any obstructions in the feed throat, such as elevated temperatures or a closed slide gate. Another consideration is whether the machine is appropriately sized for the part being produced. In cases where the machine is undersized, attempting to compensate can lead to additional processing complications.

Conclusion

In the world of injection molding, a short shot doesn't have to be a showstopper. By understanding the nuances of materials, temperatures, pressures, and giving your mold the care it deserves, you can bid farewell to incomplete shots. Remember, it's a delicate dance of elements, and finding the right balance is the key to flawless injection molding.