Faster Is Not Always Better—Optimize Your Molding Cycle

1. Quality is key to the integrity of every molding operation. Running so fast that you can’t provide adequate time for operator inspection defeats the purpose of effective production. Rework and scrap reduce productivity, so steps must be taken to assure that quality is not affected by optimization.

2. Keep optimization tactics real, and consider potential failures that could occur due to optimization. For instance, knocking 10 sec off a job molding nylon parts sounds fantastic, but if the lower cooling time increases shrinkage, it could potentially throw parts out of spec dimensionally.

3. During the optimization process, be vigilant to make adjustments slowly. When changes are made to the process in clusters, first make the adjustments; then allow ample time for changes to take effect; take sample parts for part layout; and then put the settings back until the parts and process have been validated. Sample runs should always be run separately from production as a means to prevent suspect parts from getting to the customer.

Following those three steps helps prevent costly errors through poorly planned optimiza- tion. Optimization is a fantastic tool when properly applied to your molding operation, but only with the understanding that the proper approach is crucial to the success of continuous improvement.

Here are the parameters to be considered when reducing the cycle time of a molding operation:

Cooling Time: This is one of the easiest avenues to optimizing your cycle. In most molding scenarios, cooling time is set 1.5 to 2 sec longer than screw rotate time. It is important to point out that there are situations that may require a longer cooling cycle (such as dimensional requirements or parts sticking); but as a general rule, screw rotate time establishes cooling time. 

Melt Temperature: When setting up a process, using minimal temperature helps reduce cooling time, which in turn helps improve cycle time. It is important to note that every processing approach is different, so the higher viscosity of a lower melt temperature could lead to defects. Start your process at the lower end of the melt-temperature window, and as you make adjustments, raise temperatures in modest increments until you achieve process stability.

Mold Temperature: This temperature also affects cooling time. When establishing mold temperature, start at the low end of normal processing recommendations from the material manufacturer. Higher temperatures might be required to improve aesthetics and even part removal. Don’t forget that mold temperature also affects dimensional properties.

Backpressure: Higher backpressure increases screw-rotate time, which can affect minimum cooling time. Use enough backpressure to achieve melt consistency, but keep it as low as possible to reduce screw rotate time. It is also important to note that higher backpressure might be required for certain material and colorant combinations.