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Explore the key steps in building a robust process for plastic injection molding. Learn how to optimize efficiency, minimize downtime, ensure quality, and achieve production longevity.
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The Do’s and Don’ts of Building a Robust Process Garrett Mac Kenzie Owner/ Editor (Plastics Technology Contributor - Articles) Plastic411.com garrett@plastic411.com| 770-298-0821
My background: • 30 years in plastic injection. • Started as an operator, worked my way up into engineering and management in 2001 • Rubbermaid, Stanley Electric, Johnson Controls, Summit Polymers • Process development since 1992— 25 years now • Plastic411.com Owner/ Editor
First, Let’s define what is meant when we refer to a robust process: proc·ess ˈpräˌses,ˈprōˌses/ Noun • A series of actions or steps taken in order to achieve a particular end ro·bust rōˈbəst,ˈrōˌbəst/ adjective • strong and healthy; vigorous.
So when we refer to a robust process, we are referring to: A series of actions that lead to a strong and healthy production event.
But when is a production event strong and healthy in plastic injection? First , we need to look at the 4 primary measurables that define the success or failure of our production operation: • Efficiency The state of achieving maximum productivity with minimum wasted effort or expense. • Down Time Mold changes, color changes, maintenance events, and poor planning • Yield Total parts produced after scrap (which also affects efficiency) • Quality Molded- IN quality, not to be confused with Sorted -in quality
These 4 measurables can be monitored and enforced even further, by requiring that: • Process demands meet or exceed quoted cycle • Minimal down time is repeatable between mold changes • Mold changes are standardized and repeatable • Start up is achieved with minimal scrap (ideally, 2 to 3 parts) • Scrap produced during each production run is preferably zero, • but no more than 1.5% of total production • Maintenance events are planned, not unplanned
Now we must ask, how do we get from: Point A: The development of a part To Point B: A stable and repeatable process that achieves: • Fast and reliable part-to-part change over times • A molding process that quickly starts and offers optimized efficiencies, zero to low scrap and high yields AND • Production longevity, with little to no production hiccups adding to down time
The definition of “Efficient” best describes our approach to developing these type of world class manufacturing systems: ef·fi·cient əˈfiSHənt/ Adjective • (especially of a system or machine) achieving maximum productivity with minimum wasted effort or expense. • Preventing the wasteful use of a particular resource.
Therefore, process efficiency is accomplished through: A series of actions or steps taken in order to achieve maximum productivity with minimum wasted effort or expense.
So the question arises, “What are the steps?” This is where we need to not put the cart before the horse. To outline the steps, we must first establish the engineering phases that occur before a job is turned over to production. Those phases are: • Mold design • Tool building • Press set up • Process Development & Optimization • Process Control
These phases are all critical components of a robust process! Poor handling of any one of these affects our ability to build strength into our end result. • Now let’s break each phase down, and analyze what each will need to accomplish. • My primary focus is on the processing side, so I will just briefly touch on designing and building the tool:
MOLD DESIGN • Mold design is best served as a team participation event. During the design process, include your designer, mold maker and process engineer throughout the development. This valuable approach will help to prevent major headaches, such as a mold that is difficult to process, a design that is difficult to build, etc. MoldFlow™ is an excellent software tool that allows for a process viewing based on likely flow fronts and cooling data prior to building the mold. I highly recommend it to help identify problems before the mold is built and the problem becomes a reality that could have been prevented in the design stage.
MOLD BUILDING • Assure that the mold is cooled/ heated properly • Verify that the movable half has enough support pillars to prevent flexing • Validate that the mold has been sufficiently vented • Assure that draft has been added to areas prone to drags and sticking • Record GPM of individual circuits to achieve base line data for comparison (Historical) • Record Supply/ return PSI of individual circuits for base line data • Record hot runner/ manifold steel temps for base line data
SETTING UP THE PRESS Set up is one of the most important phases of the engineering stage. The goals for developing strong set up procedures are:
Standardize the set up to assure that it is installed exactly the same way every time • Develop methods of quickly centering and leveling mold • Hard plumb tooling to assure cooling is repeatable • Dummy proof connectors (water, hydraulic, electrical) • Standardize knockouts when possible • Standardize bushings when possible • Look for ways to eliminate waste of motion
Maximize change over efficiencies to make changeovers quick and consistent • Plan and prepare for each change prior to the event (NASCAR) • Don’t be afraid to think and communicate outside of the box • Look for tools or approaches that can make change faster and easier(NASCAR) • Take advantage of the strength and skill sets of your entire production crew (NASCAR) • Provide all the tools needed to complete the job (NASCAR) • Use time studies to identify problems and weaknesses
Develop your production operation to be operator friendly • Verify ergonomics • Provide instructions that clearly communicate direction (Lather/ Rinse/ Repeat) • Provide visual aids whenever possible • Let your operators have a voice • Mold in quality • Fast cycle times = less time to inspect • Look for ways to remove human error from the process • Treat your operator like a customer
PROCESSING 101: Developing and Optimizing a Process Earlier we defined what was needed for a stable process as: A molding process that quickly starts and offers optimized efficiencies, zero to low scrap and high yields Now, in the processing stage we add : Dimensional stability Repeatability It all comes down to what steps are taken to identify.. and then clarify, a new process.
Let’s envision this ball pit as a multitude of potential processes. Each of these balls represent a process, both good or bad. Now, let’s identify that only the yellowballs are good processes.. Because -- YES.. it IS possible to have more than one good process.
RECORDING YOUR PROCESS • In a nutshell, a robust process has been validated as profitable and repeatable. • Following validation, a "snap shot" of sorts is taken (through process recording) of all settings and monitoring actuals. • This helps to assure that the next time a press is set up, the process can be repeated. • It also develops a standard of recordable data that can be compared historically, which helps to identify changes within the core molding system. • The more care we put into documenting every recordable condition from a previously successful run.. the easier it becomes to repeat that process.
It is absolutely crucial that shot size be established using decoupled molding procedures to assure that parts are filled to 95-98% with no pack or hold pressure. • Once this has been established, hold pressure is added into the process until part aesthetics and dimensions meet established criteria. It is also important to note that there are several important process studies that are part of establishing a robust process. We do not have time to cover the procedures individually, but performing this testing is a critical part of every process foundation. These studies include:
VISCOSITY CURVE STUDIES Graph: Courtesy -FimmTech
Key Injection Molding Fundamentals The key to any successful molding operation is recording all data that is available when the process is producing minimal to no scrap and is running at optimum efficiencies. By replicating these variables at machine start-up, you assure that you are repeating your previous run. Here are factors available that can be used to determine the success of your operation:
OTHER KEY POINTS IN ROBUST PROCESSING SYSTEMS Melt temperature is a key variable that is often overlooked! • Once a process has been validated, melt temp should be recorded • When heat related defects occur, melt temp should be checked prior to making process adjustments • A validated process should not require change. • Process parameters should be able to be repeated each time a mold is set and started. • Before changing your process, it is important to look at your monitoring variables first! What changed? • For instance...if fill time is slower, look at your temperature actuals, pressure limits, etc.. • There may be times you need to change your process to correct a molding condition. • -BUT- • Always check for mechanical changes first! Confirm that your machine, material, mold and auxiliary equipment are correct and functioning properly before making process changes.
Monitor your molding variables for significant fluctuations. • A cushion that varies sporadically can be a sign of a worn check ring, or barrel wear. • Barrel temperature fluctuations can point towards bad heater bands, or thermocouple positioning/ failure • Consider recent mechanical changes made while servicing a press, and whether they might be impacting the validity of your process • Verify the set up has been performed according to standardized procedures • Tools should be cleaned no less than once per shift, and materials that are prone to gassing may require twice per shift. • Slides and guide pins should be lubed, but it is important to remember that over greasing can be detrimental to your process efficiency. • Always clean your mold prior to any process change...defects could be directly related to dirty vents or components • A clean and well serviced mold is imperative to any successful molding operation.
The best approach to introducing a virgin/ regrind mix into your molding equation is to treat it as a different material. • Determine the optimal virgin to regrind ratio. • Once you've established an effective blend and process, record the process separately from a virgin run. • You can gain further process control by re-extruding your regrind with virgin base. This will reduce the potential of inconsistent material drop-down rates. • It is important to understand that adding regrind to your process changes material response. • In some cases where aesthetics and dimensional stability are imperative to process requirements, re-extruding your regrind while adding base might be a viable solution. Establish a separate process for the virgin and the blend to assure that repeatability is achievable for both.
Process Optimization • 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 successfully grow and prosper, the capability of taking on new work suffers because current systems have not been effectively streamlined. One of the first points to address under this topic is that it is possible for machines to run too fast. Every molding job is different, and the following conditions must be satisfied to properly assure that the optimization is successful: • Quality is key to the integrity of every molding operation. Running too fast to provide adequate 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. • Keep optimization tactics real, and consider potential failures that could occur due to optimization. For instance, knocking 10 seconds off of a job molding nylon parts sounds fantastic.. but if the lower cooling time increases shrinkage, it could potentially throw parts out of spec dimensionally. • During the optimization process, be vigilant to make adjustments slowly, and when changes are made to the process in clusters: make the adjustments, allow ample time for changes to take affect, take sample parts for part layout and then PUT SETTINGS BACK until the parts and process have been validated. Sample runs should always be run separate from production as a means to prevent suspect parts from getting to the customer.
Process Optimization The following list outlines many of the parameters that can be considered when reducing the cycle time of a molding operation: COOLING TIME • Cooling time is one of the easiest avenues of optimizing your cycle. In most molding scenarios, cooling time is set 1 1/2 to 2 seconds 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) HOLD TIME • Hold Time is another major contributor to maximizing cycle time. The best method of accomplishing this is through a gate seal study. • Gate seal is the amount of required hold time needed to cool the runner tip into stasis, preventing material from flowing back into the bushing. MELT & MOLD TEMPERATURE • Start your process at the lower end of the melt window • Raise temperatures in modest increments until you achieve process stability • Higher viscosity of a lower melt temperature could lead to defects • Part dimensions or aesthetics may require higher or lower mold temperature • Key is to be a minimalist • Using minimal temperature helps reduce cooling time which helps improve cycle time
Process Optimization The following list outlines many of the parameters that can be considered when reducing the cycle time of a molding operation: MOLD OPEN & CLOSE • mold breakaway and mold close- slow speeds are affected by the complexity of slides, horn pins, etc.. . so keep safety • and care for your mold first priority! • Keep Low Pressure Close settings as low as possible for mold protection • Safety and mold protection take first priority over mold speed optimization. EJECTION • Improper ejection set up adversely affects cycle time • Use only the amount of stroke you need to: • -Remove the part safely • -Prevent sticking • Maximize mold open and close speedsto reduce mold open time
In closing, Lean Manufacturing requires continuous improvement and maximized efficiencies. When cycle optimization is complete, the resulting process will produce: • the highest yield • minimum to zero scrap • and minimum down time events It is important to remember that a true processing regimen continuously evolves, improving upon itself. By applying the steps outlined, we are able to achieve: Full efficiency while maintaining world-class quality parts.
