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Injection Molding and Moldmaking
with Surgical Precision

Injection Molding and Moldmaking
with Surgical Precision

My favorite part about working on R&D projects is that they tend to challenge you to think outside the box, try new things, and learn about the latest technologies.  One of our recent development projects involved injection molding a long, thin-walled tube (picture a miniature drinking straw) with a wall thickness that shrinks down to .0035” over its nearly half-inch length.  By comparison, that’s roughly the same thickness as a human hair.  Even after running dozens of Moldflow® studies for gating locations and flow analysis, the only thing we were confident of was that it was going to be a challenge to fill the parts out completely.

After struggling on our first sampling, the instinct was to look for higher flowing materials to help make the distance more manageable.  We started with a PE material with a Melt Flow Rating (MFR) around 50 g/10min and then moved on to a similar material with a MFR of 110.  We were expecting to see a noticeable improvement in the 110, but what we found was no appreciable difference on the fill.  It was determined that this was primarily due to leakage at the check ring / non-return valve, common to all traditional, reciprocating screw injection machines.

This brought us to one of the more interesting suggestions on the project.  We decided to sample the tool in one of Sodick-Plustech’s (SPT) micro injection machines.  This machine piqued our interests initially because of its two-stage (plunger-style) injection approach, but as we found is well-suited for this type of application for several reasons.

Like a traditional, reciprocating screw machine, Sodick’s two-stage injection technology (shown here) utilizes a small screw to melt and convey material.  But unlike traditional machines the screw is not responsible for injecting plastic into the mold (or any high speed lateral movements).  It feeds a second chamber, which is metered precisely, and then injected into the mold via a high-speed piston.  The feed screw shuts off after material is loaded into the chamber, which eliminates back-flow without the use of a check ring / non-return valve.

Photo courtesy of SPT

On this particular machine, the piston is capable of reaching injection speeds of 450 mm/s, which isn’t particularly impressive by today’s standards.  Many other press manufacturers tout injection speeds well past 1,000 mm/s.  Sodick, too, offers a high-speed/high-pressure line that boasts an impressive 1,500 mm/s injection speed, but their selling point is based more on acceleration than on speed alone.  The Sodick machine utilizes an accumulator that works with the main piston to reach maximum speed almost immediately upon injection.

The next selling point is the consistent shot sizes due to the tightly metered second chamber.  For our application, this is critical because an inconsistent fill could cause a short shot, which would be nearly impossible to detect with the human eye or a vision system during production.  On a project that could expand to a 16 or 32 cavity tool, this becomes increasingly critical to maintain good production parts.

Another positive about the machine is a more consistent melt and material residence time.  Again, the lack of a check ring helps by allowing for a more reliable first-in/first-out material path.  And since the feed screw isn’t creating excess heat via shear, the material is subject to more uniform heat profiles as it moves through the processing stages.

One last positive about the machine is the capability to swap out injection units (smaller or larger) and match them with differently-sized platen and tie bar configurations.  Matrix is running quite a few bioresorbable/bioabsorbable polymers lately which require minimal shot sizes due to the extremely high material costs.  However the molds associated with these projects are often complicated and require multiple side actions, slides, and/or lifters, so running them in a traditional micro-molding machine with a 4-inch max opening and similar small distances between the tie bars doesn’t always lend itself to the mold design.

Written By:
Andy Ziegenhorn

 

Robots have come a long way…

I remember when I first started working with robotics on molding presses. Back then, they had to be adjusted by climbing all over the robot, and the programs were only capable of the basic “L” and “U” movements. In many cases the drop zone location on the up and down movement had to be set the same as the pick location on the up and down movement. The presses would have a mechanical stop to hold the mold in the open position and an alignment pin above the locating ring on the front half to verify that the mold was perfectly aligned each and every time it was set in the press. Even with this it was a challenge to keep everything aligned. It was also very important for the oil temp to be correct at startup. Old hydraulic presses would not open to the same distance or eject the parts correctly at the wrong oil temperature. You would need to re-adjust after running for a few hours or even days. It was an ongoing battle keeping everything lined up. The robots had pneumatic up and down movement with a servo drive existing only on the traversing and kick movements. For something designed to make a processor’s life easier they certainly brought their fair share of pain!

But like all newer technologies, issues were addressed one by one and improvements came out consistently. We now have servo movements on all three axises, with options for rotation and flip servos as well. We are able to tie the robotics directly into the process monitor on the press and automatically divert parts at startup and any time the process parameters move out of tolerance.

We continue to install alignment pins on the front half of the molds but the newer presses hold the open position / ejection forward position much better (especially newer electric presses). Now robots have evolved from the painful era of trial-and-error setup to a nearly scientific setup and operation.

I have worked with many different models and brands over the years and have been lucky to have worked with some of the best built and best supported robots on the market. Recently I attended a Flex Teach class for Yushin robots. The Flex Teach system allows the user to create motion programs for the robot using a personal computer. The same programs can also be modified using the touch panel controller. What I like best about the Flex system is that it utilizes the PC as a training tool for the robot when it is offline. This can save countless hours of down-time and allow operators that would not feel comfortable practicing on a live press to start learning the Flex Teach system. Just knowing that they won’t have to worry about damaging expensive molds or end of arm tools (or more importantly, themselves and others) opens the doors for every operator to catch up to speed.

Even robotic systems from just a few years ago were no comparison.They, too, were fully programmable and also had servos with CNC type controllers, but these models required hundreds of command lines and an extensive knowledge of the programming language to run. The program itself consisted of several parts:a run program, reference program, and home program for every job.Making adjustments to a program became a trial-and-error nightmare.More importantly, valuable press time was lost in the mix.Considering today’s shortened deliveries and 24/7 production jobs, fiddling with the programs is something most molders can live without.I wish I could have done some of the work offline with a program tool like the Flex Teach system.We now have the ability to take our time (with minimal pressure) and do most of the programming offline while the press is still running.

With the old system, programming mistakes would have to be caught during the standard process of verification referred to as “stepping through the program” and tweaked accordingly. The Flex system allows us to run the program or changes through a simulator and verify that it looks good on the computer screen before being transferred to the press via a SD memory card and loaded onto the robot.For good measure we continue to step through the program to verify a second time, but there is no doubt this saves time in the process.

All of these new features have made robots perform more consistently and adds to their versatility, performing tasks like sorting, de-gating, counting, boxing, and stacking.Robots can even place small inserts and verify their placement these days.

So molders, learn to love your robots. They work tirelessly, exactly, and without a complaint or absent day. They can be a molder’s best friend (though you can still keep the dog). Yes, robots surely have come a long way!

Written By:

Pat Collins
Molding Operations Mgr.

Injection molding frequently involves problem solving.  Many of the problems regularly faced by a molder are quality related problems including: short shots, flash, dimensional issues, warp, and cosmetic concerns.  The molder is also frequently confronted with the need to reduce cycle times and/or increase yields.

Most problem solving in injection molding falls into two broad categories: structured (formal) and unstructured (informal).  In a series of blog entries, we will discuss various problem solving tools used in injection molding.

First, let’s examine unstructured/informal problem solving.

Experience

The majority of everyday problem solving in molding relies heavily on experience as the problem solving tool. A good portion of this is the experience of the individual molder. There is an old molding tale that tells of “The World’s Greatest Molder” being interviewed for a trade journal.  He was asked what the key to his success as a molder was.   Being a man of few words, he replied, “Not making mistakes.” He was then asked how he avoided making mistakes.  He answered, “Experience.”  He was further interrogated as to how he obtained experience.  “Making mistakes,” he replied.

Yes, one of the most often used problem solving tools in injection molding is based upon making mistakes… but learning from those mistakes!

The cumulative experience of a truly cross-functional team is powerful as both an informal and formal problem solving tool.  When tackling a complex molding problem, it is a good practice to involve personnel from molding, tooling, design, and quality.  This can be done in an informal shop floor setting or in a conference room.  A group of technical people getting together to share their experiences in solving a problem is a technique called “brainstorming” which leads into our next blog discussion of structured or formal problem solving tools.

Written by:

Brent Borgerson
Senior Process Engineer (Older Molder) 
Matrix Tooling Inc. /Matrix Plastic Products

As an ISO 13485 certified manufacturer, managing risk is always a priority. And the most basic concept of that management process is first understanding what those risks are.

Recently, the question was raised: "Can any trace mold corrosion be transferred to the final molded product?" This opened a spirited discussion because, while we've been building injection molds for over thirty years, nobody here had a definitive answer.

After consulting with others in the industry, as well as a metallurgist at our steel supplier, we came to the conclusion that while it's theoretically possible to transfer any contaminant from one surface to another, having a problem with bio-burden testing or introducing a contaminant into a product during plastic injection molding production is highly unlikely.

We are involved in a program that overmolds stainless steel with plastic. A stamped steel piece and a metal injected piece are both loaded into a tool steel mold, then overmolded with a high temperature nylon. The stamped piece is passivated; the MIM piece is as-molded and sintered. In speaking with our metallurgist, we learned that the main sources of corrosion on the mold cavities would be degraded resin coupled with high mold temperatures. Fortunately, as we are the molder on this program, we have control over both of these issues. Our processing and mold temperatures are both within manufacturers' specifications, so that helps control a major source of any corrosion. The metallurgist also explained that any corrosion sufficient enough to create a transfer problem would be visible to the naked eye.

The material we chose for this program was a high hardness, general purpose tool steel. We did this due to the fact that two metal inserts are being inserted into the mold cavity for overmolding. There are other grades with similar hardness and stainless properties available, but based on what we learned we do not feel it is necessary to change steel grades at this time. However, we have decided to add a visual check to our regular preventive maintenance plan for this job, a solution we now feel is sufficient to catch a problem before it occurs.

Structured or Formal Problem Solving


There are times when informal or experienced-based problem solving alone may prove inadequate, a “band-aid” approach to a more complicated issue. Informal solutions that ignore the root cause of the problem are not likely to prevent it from rearing its ugly head again later.

Structured or formal problem solving methods and tools allow us to get to the root cause of the problem and solve it permanently. The myriad of analytical tools and structured methods available can befuddle even a trained statistician. A structured problem solving method can enable a molder to choose which problem solving tools best apply to a particular project. Many structured problem solving tools are also process improving tools. (I have always considered process improvement to be problem solving on a grander scale, but I will examine process improvement tools more specifically in a future blog entry.)

Almost all structured problem solving methods hark back to Drs. Deming’s and Shewhart’s PDCA (Plan – Do – Check – Act) Cycle, which can be traced back further to early 1600’s England and Francis Bacon’s Scientific Method. Today’s DMAIC (Define, Measure, Analyze, Improve, Control) from Six Sigma, and the various numbered (5-, 6- 7-) step methods of problem solving, all stem from Deming’s and Shewhart’s methods.

Before a problem can be solved, we must define and get to the root cause of the problem. A technique called The 5 Whys can be used. This technique was gleaned from the 1970’s Toyota Production System (TPS), and can be useful for quickly getting to the root of a problem.

For example, assume that a late delivery on a project has resulted in an unhappy customer.

Apply the 5 Why method:

1. Why is the customer unhappy? = Because the project didn’t get delivered as promised.

2. Why didn’t project get delivered as promised? = Because the job took longer than anticipated.

3. Why did it take longer than anticipated? = Because the complexity of project was underestimated.

4. Why was the complexity of the project underestimated? = Because we made a rough estimate, ignoring the complexity of separate stages of the project.

5. Why did we do this? = Because we were running late on other projects.

It is now apparent that we need to revise our time estimation methods.

We can also apply The 5 Whys to a more typical molding problem; say a reject from a good customer:

1. Why did the customer reject the shipment? = Because of short shots and small dimensions.

2. Why did we get small parts and shorts? = Because not enough plastic got packed into the mold cavities.

3. Why didn’t enough plastic get packed into the cavities? = Because the molding machine wasn’t capable of doing this.

4. Why wasn’t the machine capable of doing this? = Because the melt index of the resin lot in question was too low and the machine became pressure limited.

5. Why didn’t we know this? = Because we didn’t do incoming resin inspection and/or set process alarms on the molding machine.

Conclusion: We clearly need to revamp our incoming resin inspection procedures and assure that our process/machine is robust enough to avoid this mistake in the future.

(To be continued next month)

Brent Borgerson
Senior Process Engineer (Older Molder)
Matrix Tooling Inc. /Matrix Plastic Products

 

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