Family mold flow imbalance is a very common and challenging issue. It refers to a situation in a single mold where, due to significant differences in the shape, size, wall thickness, and runner length of different products (family members), the molten plastic cannot fill all cavities simultaneously.
This leads to a series of quality problems:
Short Shot: Some cavities are not completely filled, while others are full or even over-packed.
Flash: Cavities that are easy to fill develop flash, while difficult-to-fill cavities remain incomplete.
Dimensional Variation: Products from different cavities have inconsistent dimensions and weights.
High Internal Stress: Products from certain cavities, due to excessive packing pressure or fast filling, develop high internal stress, making them prone to warping or cracking.
Longer Cycle Time: The process must wait for the most difficult cavity to fill, reducing production efficiency.
Solving family mold flow imbalance requires a systematic approach across mold design, process adjustment, and material selection.
I. Mold Design Solutions
- Runner System Balancing Design
Natural Balance: This is the ideal scenario. It involves designing runners with different diameters and lengths so that the flow resistance for the melt to reach each cavity is identical. For family molds, this means designing longer, thinner, or more tortuous runners for products with lower flow resistance, and shorter, thicker, or smoother runners for products with higher flow resistance.
Using CAE Software like Moldflow for Analysis: In modern mold design, it is essential to use mold flow analysis software for simulation before manufacturing the mold. This optimizes runner dimensions and layout, predicting and resolving imbalance issues beforehand.
- Artificial Balancing Elements – Flow Control Valves
Hot Runner Systems: Install pin-valve sequential valves at the inlet of each nozzle in the hot runner system. The control system opens and closes each valve pin according to a specific sequence. This can delay the opening of valves for easy-to-fill cavities, allowing the melt to prioritize filling the difficult cavities first. Once the difficult cavities are nearly full, other valves can open for synchronized packing. This is the most advanced and effective method for solving family mold imbalance.
Cold Runner Systems: Install adjustable restrictor pins within the runners. These allow for manual mechanical adjustment of the local cross-sectional area of the runner, thereby altering the local flow resistance.
- Gate Size and Type Optimization
Adjusting Gate Size: Design smaller gates for easy-to-fill cavities to increase flow resistance, and larger gates for difficult-to-fill cavities to reduce flow resistance.
Changing Gate Type: Select different gate types (e.g., pin-point gate, edge gate, submarine gate) based on the characteristics of different products, utilizing their varying resistance properties to aid in balancing.
- Cooling System Balance
Flow imbalance often accompanies cooling imbalance. It is crucial to enhance cooling (e.g., with more dense cooling channels) in areas that are easy to fill and tend to have heat concentration (often thick-walled sections). Conversely, reduce cooling appropriately in difficult-to-fill areas to prevent the melt from freezing too early.
II. Injection Molding Process Adjustment Solutions
When mold imbalance is identified after the mold has been manufactured, process adjustment becomes the primary means of resolution.
- Injection Speed Profiling
Core Method: Utilize multi-stage injection speed control. During the initial filling phase, use a slower injection speed to allow the melt to flow steadily through the main and secondary runners. Once the melt front has reached the gates of all cavities, switch to a high-speed injection. This leverages shear heating to reduce viscosity, aiding in filling all cavities simultaneously. This effectively avoids the “flow racing” phenomenon caused by inertia.
Principle: “Slow first, then fast” – slow through the runners, fast through the gates into the cavities.
- Melt Temperature and Mold Temperature Adjustment
Increase the Overall Melt Temperature: This reduces the melt viscosity, making flow easier and helping to fill the difficult cavities. Attention must be paid to the risk of material degradation.
Differentiated Mold Temperature Control:
Increase the mold temperature in the area of the difficult-to-fill cavity to prevent the melt from cooling too prematurely, thereby reducing its flow resistance.
Decrease the mold temperature in the area of the easy-to-fill cavity to increase its flow resistance and enable it to cool faster.
This typically requires the use of mold temperature controllers for zoned temperature control of different sections of the mold.
- Packing Pressure and Time Adjustment
Due to different fill end times, uniform packing parameters are not equitable for all cavities.
For the difficult cavity: It may require higher packing pressure and longer packing time, as it is likely filled last and its gate may freeze off sooner.
For the easy cavity: It may require lower packing pressure and shorter packing time to prevent over-packing, which can cause flash and high internal stress.