4 Ways to Improve Multi-Cavity Molds

Multi-cavity molds are complex and come with unique challenges. Here are four tips for overcoming these challenges:

1. Adjust the gates.

Gates control the flow of plastic from entry to the point of cooling and ejection. Using tab or edge gates can help improve flexibility which is essential with multi-cavity molds. In addition, tab gates reduce or control the stress to the tabbed area during the ejection. Gate location or placement depends on the specific production expectations. By clearly understanding the expectations early in the process, redesign and issues can be avoided.

Mold imbalances in multi-cavity molds can also be overcome by using hot runner valve gates. The gates shut off flow to cavities that are filling fast, which prevents flashing or overpacking. Simultaneously, slower filling cavities are allowed to continue filling.

A less expensive alternative is using a flow control valve system that is controlled by a worm-gear assembly. In this case, the molder can adjust each nozzle's flow by turning a knob and the setting remains fixed once adjusted.

Another similar solution available in the market is a real-time, closed-loop process control at each gate in the mold to deliver faster startups, easier mold changes and lower reject rates. This allows the molder to establish a pressure and time profile for each drop location which helps control the rate of fill and packing. Through the use of pressure sensors, the valve pins in the mold are controlled to ensure uniform pressure through the filling cycle. This adjusts the pressures at the nozzle to distribute pressure evenly throughout the flow path.

 

2. Design for wall thickness.

It is important to manage wall thickness to control stress marks and to ensure the design meets minimal wall thickness requirements while maintaining consistency in the thicknesses of adjacent features. The gate is an area where there is high injection pressure. If the wall is narrow, this also acts as a restriction increasing injection pressure. Consequently, if the gate and wall thickness are not balanced, shearing, flashing and mold damage can occur. A solution for balancing wall thickness and gate pressure is by either increasing wall thickness near the gate, decreasing injection pressure, or both.

3. Consider side-actions and pickouts.

It is also important to consider side action and pick-outs during design. Side actions may be more appropriate for use in a single-cavity mold. Depending on the part being designed, they will not qualify for multi-cavity tooling in certain situations. The same goes for pick-outs. Manually loaded inserts, or pick out, should be carefully considered.

 

4. Use of sensor-based technologies.

The use of cavity pressure sensing technology is a common solution available to avoid flow variations when filling cavities simultaneously. With the use of sensors, molders can get information regarding the cause of problems in flow pressure and flow imbalance. Once an imbalance is determined, cavity pressure sensing can help with solutions. It can provide information regarding how full the cavity is. Then the molder can use low-velocity injection to ensure the remaining parts are filled with more consistent pressures. Cavity pressure sensing provides a complete picture of how pressure develops as the mold is filling and any imbalance problems. It can also be used to control pressure.

^{Vincent Hua is the Marketing Manager at TSINFA. He is passionate about helping people understand high-end and complex manufacturing processes. Besides writing and contributing his insights, Vincent is very keen on technological innovation that helps build highly precise and stable CNC Machinery. https://www.tsinfa.com/}