Short shot is a defect in injection molding, occurring when the molten plastic injected into the mold fails to fill all the spaces within the mold, resulting in partial or incomplete formation of the product.
There are numerous causes of short shots, including issues with mold design, poor material flow, insufficient pressure, improper temperature settings, or other process-related factors.
If during a trial run, we have verified that the injection machine, nozzle, and even the raw materials are functioning properly, yet short shots still occur, there are several key areas we can explore to solve the problem. Let's examine them one by one.
1.Impact of Molding Parameters
Regarding molding parameters, factors contributing to short shots include:
| Reasons | Resulting phenomena |
|---|---|
| Insufficient Plastic Material | Inadequate measurement leads to incomplete filling. |
| Low Mold Temperature | Cold plastic flow, slowing speed, or premature solidification. |
| Low Material Temperature | Cold plastic flow, slowing speed, or premature solidification. |
| Slow Injection Speed | Leads to rapid cooling of flowing plastic. |
| Low Injection Pressure | Inability to overcome resistance, decreasing injection speed, leading to rapid cooling of the plastic. |
During the initial mold trial, a practice known as segmented short shot is employed to approximately adjust the machine to a reasonable injection pressure range. This is done to confirm whether the product can smoothly enter the holding pressure phase. Short shot refers to a scenario where the material is fundamentally insufficient, preventing it from reaching the holding pressure stage. Therefore, our objective is to find a solution ensuring complete filling of the plastic material.
The initial step is to verify if there is an insufficient measurement. If the pressure curve during injection does not show a gradual rise, it may indicate a simple lack of material, which can be rectified by stepwise replenishment. If a noticeable increase in injection pressure is observed, it signifies increased resistance and reduced flowability, making the material prone to cooling.
Moving on to the second step, begin adjusting finer parameters by observing the effects of increasing the injection speed in the final stage. Additionally, consider raising the pressure limit appropriately. If the injection pressure is already too high, reaching the machine's maximum pressure limit of 85%, further increments are not recommended.
Therefore, in the third step, focus on temperature adjustments. Begin by increasing the mold temperature, generally avoiding surpassing the temperature range specified in the material datasheet. Observe changes after reaching the upper limit and adjust accordingly. Adjust the material temperature only after modifying the mold temperature, as material temperature has a more significant impact on plastic properties. It is advisable to start with raising the mold temperature.
2.Mold Venting and Clamping Force
If machine adjustment parameters are already optimized, it's time to review whether the venting is adequate.
At this point, consider taking an initial step by reducing the clamping force in increments of 5 tons. Gradually lowering the clamping force helps reduce resistance for gas escape. Conversely, excessively low clamping force can lead to overflow and flash issues.
If confirming that reducing the clamping force is effective, it indicates insufficient or shallow venting structure. Insufficient venting implies higher vacuum resistance, making short shots more likely at the end of the product. Therefore, initially check if there is a designed venting system or adequate venting dimensions at the end of the plastic flow. Similarly, excessive venting should be avoided as it can lead to overflow and flash issues.
Typically, we design venting based on the plastic's flowability. For highly flowable materials like PA and PP, a venting size of 0.02mm is recommended, while less flowable materials like PC may require 0.03-0.05mm. It's advisable to start with 0.02-0.03mm initially and increase if necessary during subsequent processing.
3.Tooling Design
After mold trial, if the issues persist despite addressing the aforementioned aspects, it's time to examine the mold design. Check and reinforce specific areas as needed.
| Reasons | Resulting phenomena |
|---|---|
| Injection gate location | Choosing a gate location in thicker sections is advisable; injecting into thin sections may result in excessive pressure and incomplete filling in later stages. |
| Injection gate method | Different gate types exhibit varying pressure rise characteristics. Generally, side gates offer smoother pressure rise, while small point gates pose the highest resistance. |
| Injection gate size | Appropriately enlarging the gate cross-section aids in releasing injection pressure. |
| Runner design | More bends and turns, longer flow distances result in greater pressure losses. |
During plastic flow, thinner sections of the mold experience higher cavity resistance. Consequently, during injection, plastic tends to flow initially towards larger spaces and eventually fills the thinner sections.
Therefore, when designing the gate location, injecting into thinner sections leads to significant pressure loss. Hence, the gate position should guide the flow to converge in the thinner sections during the final stages of injection.
4.Product Design
As mentioned earlier, designing with significant thickness variations poses multiple issues. Therefore, when designing a product, it's crucial to avoid excessive differences in thickness, as it leads to significant pressure variations within the mold. Apart from incomplete filling due to excessive thinness, excessive thickness can also result in shrinkage problems.
For plastic parts, the optimal final thickness typically falls within the range of 1.5mm to 3.0mm. Presently, a plastic product is defined as "thin-walled" when the ratio of its flow length to wall thickness (L/T ratio, assuming a 1mm section thickness) exceeds 150.
5.Lastly
If the product design is unalterable and destined to have thin or uneven sections, an initial assessment should focus on high-speed injection machines and vacuum technology.
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