Published 10/18/2016

Faster Is Not Always Better—Optimize Your Molding Cycle

It is possible for machines to run too fast, so find the sweet spot where maximum output overlaps with good parts.

Garrett MacKenzie, contributor

In today’s fast-paced injection molding business, lean manufacturing is a primary driver of profitability. Without lean, organizations find their operations are sluggish and ineffective. This not only affects a company’s ability to grow and prosper, but also its capacity to take on new work is diminished because current systems have not been effectively streamlined. This column addresses cycle-time optimization, outlining the different variables within the molding process that can be used to maximize profits.

One of the first points that needs to be made when addressing this topic is that it is possible for machines to run too fast—that is, too fast to ensure product quality. Every molding job is different, and the following conditions must be satisfied to properly assure that the optimization is successful:

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.

Hold Time: This is another major contributor to maximizing cycle time. The best method of optimizing hold time is through a gate-seal study. Gate seal is the amount of required hold time needed to cool the hot-runner tip into stasis. This prevents plastic from leaking back out of the runner, which leads to molding inconsistencies.

Performing a gate-seal study is simple. Once a decoupled process has been established, set the hold time well above what is generally common for the material and part size you are working with. While running, make reductions to the hold time and weigh each part as it relates to the change. Watch for a weight reduction, and when the part weight drops, add 1 sec back to the hold time, and the test is complete.

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Fill Time: Here is another parameter that affects cycle time. Injection speed controls how long or short the resulting fill time is. Of course, the type of material and the mold’s complexity also put constraints on fill time. Based on this, the goal of optimizing fill time is to shoot material as quickly as possible without affecting the aesthetics and functionality of the parts being produced. It is also important to note that proper venting is crucial to faster fill time. Poorly vented tooling does not allow for the proper escape of gases, which can cause defects such as burns, splay, etc.

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.

Mold Open/Close Speed: Maximize these speeds to reduce mold- open time. Here it is important to note that mold-breakaway and mold-close speeds are affected by the complexity of the given tool’s slides, horn pins, etc., so make the mold’s safety your first priority as you set these speeds. In addition, watch for low-pressure close. You want to keep the pressure as low as possible for mold protection, but remember that setting the speed and/or pressure too low can add to overall cycle time. Again: safety and mold protection take first priority over minimizing cycle time.

Ejection: Improper ejection setup can adversely affect cycle time. During ejection setup, use only the amount of stroke you need to remove the part safely without it sticking in the mold. Ejection speed and pressure are also important to faster ejection time, but it is important to note that when increasing the speed and/or pressure setpoints, you must watch for pin push or cracking. Minimal pressure and maximum speed will generally produce the optimum result.

Robot: Automation also affects cycle. There are two primary effects that can be optimized. First, the robot needs to get in and out of the mold quickly to minimize mold-open time. Second, the robot must be in position waiting for the mold to open. When possible, establish the robot “wait ” position as low on the “Y” axis as possible to improve the extraction time.

Lean manufacturing requires continuous improvement and maximized efficiencies. When cycle optimization is complete, the resulting process will yield the highest production with little to no scrap and a minimum amount of downtime. It is important to remember that the primary goal of optimization is full efficiency while maintaining world-class quality. Through a careful and meticulous approach, process optimization can be an effective tool in the journey towards lean manufacturing.

ABOUT THE AUTHOR: Garrett MacKenzie is the owner/editor of plastic411.com, as well as a consultant/trainer to the plastic injection molding industry. He has spent more than 31 years in plastics processing, engineering, and development, including experience with U.S./Japanese automotive OEMs and handgun manufacturers. Contact: garrett@plastic411.com