Matrix Tooling, Inc.

Early in the history of injection molding, molders realized the problems inherent in producing high volume, fast cycling parts of commodity resins with cold runners, especially in high-cavitation molds.  Cold runners can stick or hang in the mold and interrupt or extend the cycle; and often the cold runner being the last part of the shot to set up, can dictate the overall cycle.

It soon became obvious that “runnerless” molding was the way to go.  Early hot runners were of the internally heated (torpedo) type or the externally heated manifold hot runner.  Both were prone to leakage and hard to (especially the torpedo type) change colors with.  Predating these systems were a type of runnerless mold called an Insulated Runner .

Insulated Runners had an oversized internal runner cut into both the top clamp plate and the “A” plate.  This runner was very thick and relied on the thickness of this runner-cull to keep the plastic in a molten state as long as the molding machine was cycling.  The walls of the runner were solid with only a molten center core providing melt delivery.  These led to cylindrical drops (also very thick) and generally to top-center-gated parts.

This system needed fast, uninterrupted cycles to keep the gates open and even momentary interruption caused one or more gates to freeze off.

Startup was also tricky with these molds.  Methods included hand injection of multiple shots into the mold before going to auto, making one big shot and going to auto, or boosting the back pressure way up and extrusion filling the runner cull.

Later the gate drops were heated with a probe which made startup easier and also made keeping the gates open easier, even allowing a brief disruption the the cycle.  With very fast cycles (3 to 6 second range) the heated probe insulated runner can have a fairly small thickness and in some cases, be reground and re-used in the product.

Though sometimes a bit tricky to startup and keep running, these systems could offer advantages over not only cold runners, but hot runners as well.  These include:

• Quick cycles
• Less regrind and scrap , though the thick cull wasn’t generally used back in molding
• The tool was less expensive to build and maintain
• Less chance for melt leakage.
• Color changes were very fast compared to hot runners, as the whole colored cull was pulled after the molding machine’s barrel was cleaned. Often color changes can be preformed in 5 minutes with less than 5 pounds of scrap
• Even if heated gate drops were employed, fewer and less sophisticated controllers were needed.

Yes, the insulated runner is an old technology, but if you have a multi-cavity, fast-cycling job using a commodity resin like PP or PE with frequent color changes, and want a more economical tool that is easier to maintain, then consider insulated runner tools.

Drying is an important part of the process for any product made of hygroscopic (meaning affinity for moisture) thermoplastic.   For medical implants made of bioabsorbable polymers, dryness is particularly critical.  Inadequate drying can produce a variety of problematic results.  These include:  lack of tensile properties and impact resistance, as well as varying flow characteristics.

Bioresins, much like other hygroscopic thermoplastic resins, can suffer three types (or a combination of these three types) of degradation:  thermal, mechanical or hydrolytic. In most thermoplastics these types of degradation occur chiefly during the molding process. With bioresins such as the PLA, PLG , and PGA families, hydrolytic degradation also occurs before and after the molding process.

An implantable device must decay or degrade in the body as part of the absorption process. Different materials and part designs have different rates of degradation in the body (where it is in a moist environment).  The rate of degradation and retention of mechanical properties is affected in no small degree by the way the resin was dried and how the dried resin and finished part were handled.

If a bioresin grocery bag degrades quicker than it was designed to, the results can be the bottom falling out and groceries on the ground. If an implantable device degrades quicker (or slower) than designed to, the results can be harmful to the patient. The degradation process of the implant is key to resorption in the body.

Run of the mill dryers are generally not sufficient to control the moisture as well or reach the super-low moisture levels desired for absorbable implants. Many implant molders opt for vacuum dryers or compressed air with membrane dryers.  Since most implants are small, vacuum ovens designed for lab use is another option for resin drying.  In any case, the drying schedule and temperatures provided by the resin manufacturer must be strictly followed.

In many cases, the resins must be dried to less than 0.02% (200 ppm) and the resin and finished product must be maintained dry. This requirement mandates an inert gas such as nitrogen atmosphere in any non-vacuum dryer hopper, humidity controlled atmosphere in the cleanroom, vacuum packing with a desiccant and nitrogen, and refrigerated storage of the resin prior to drying.

It is not enough to strictly follow the drying and handling procedure, the resin dryness must be well tested, documented, and controlled.  The dryness data is so important because it must be correlated with part degradation data to be able to predict implant device performance and absorption in the body.  Lost weight or halogen type moisture analyzers are relatively economical devices but should be equipped with data acquisition and logging technology.

Drying bioabsorbable resins requires specialized knowledge, methods, and equipment, but is key to successful bioresin implant molding.

Drying engineering resins is crucial to obtaining desirable end products with these high–performance and often expensive resins. Thermoplastic resins are being called on to be as strong as metal and to survive in harsh environments. To achieve these end properties, a resin must be processed correctly, and one area of proper processing is to ensure that the resin is molded at or under the manufacturer’s specified maximum moisture content (%).

At Matrix Plastics Products, we are very careful (almost to the point of being neurotic) about our resin drying and dryness assurance procedures. We take a multi-pronged approach to these issues including some of the techniques and procedures as follows:

• •  Drying Time: We follow the manufacturers’ recommendations as a minimum for drying time before beginning molding as well as residence time in the dryer. These steps are carefully documented for accountability.
• •  Drying Temperature: Again, resin makers’ guidelines are strictly followed.
• •  Dew Point Monitoring of the Dryer: Our dryers feature dew point monitors and alarms which are consistently observed. The dew point on a dryer is the best indication of the proper function of the dryer, which allows us to foresee many impending problems.
• •  Moisture Analyzer: Our Quality Inspection lab features an OMNIMARK Mark IV moisture analyzer which can be used to test and verify results. This is the last line of defense and is used whenever there is any doubt about the dryness of a resin. In the case of sensitive jobs, moisture analyzing test are routinely used and documented.

A part molded with wet resin (moisture content above the manufacturer’s suggested max percentage) may not be a cosmetically unappealing part, but it is almost always a structurally weak part. Hydrolysis – the result of heating moist resin – produces an action in the resin that is essentially akin to thermal degradation. The molecular structure and integrity are affected, and a weak and/or brittle part is the result. Some of these problems are not always readily detectable, especially during the early life of the product, but premature and unexpected failures can result from molding with “less-than-dry” resin. We try our best to avoid this situation.

Written By:

Brent Borgerson
Senior Process Engineer (Older Molder)

Pat Collins
Molding Operations Manager

The thought of processing PEEK (polyetheretherketone) or other high-temp resins can send nervous tremors through many a molder’s body. I know, as a molder who learned the craft on a steady diet of PP and PE closures with their low melt temperatures and cold molds, my first PEEK experience made me edgy to say the least. But I’ve since come to realize that PEEK is just another thermoplastic resin and, like the others, can be molded safely and efficiently with just a few precautions.

PEEK is widely believed to be one of the highest performing thermoplastics on the market and its end properties more than justify any trials and tribulations you may encounter processing it. PEEK is a linear aromatic, semi-crystalline thermoplastic having excellent wear, chemical and hydrolysis resistance. It has very low flame/smoke toxicity and excellent electrical properties that preclude the need for additives in many cases.

PEEK processes at a high melt temperature nearing 720°F, and both the press barrel and controls must be capable of this. On many molding machines the high heat software is an option and I recommend ceramic high-temp heat bands whenever possible. A special screw and barrel are generally not needed, but consider hard units if running filled PEEK resins. We typically use sliding ring non-return valves, GP or Eliminator™ tips and don’t recommend ball checks or shutoff nozzles.

A hot mold is the key to achieving crystallinity in PEEK parts. Purging PEEK allows you to see the color change from a translucent to a solid colored crystalline state. If the mold is too “cold” (i.e. not hot enough) the parts will have that discoloration or partial translucency, and the quality of the end product will be compromised. The mold, in most cases, must be between 350°F and 450°F. This is steel temperature and requires oil or cartridge heat to maintain this level. Complex parts may require better temperature control so oil would be the preferred option. We also recommend the use of thermocouples to verify and monitor the steel temperature.

These molds must be specifically designed to run high-temp materials with draft, finish, undercuts and steel types all factored in from the beginning. Insulator plates between press platen and mold clamp plates are a must. The preferred steel type would depend on whether or not the resin uses any abrasive fillers but should have a minimum hardness of 52-54 Rc.

The resin also must be very dry to process well and achieve the desired end properties. This means that the resin must be at 0.02% moisture or below. We typically recommend drying the resin at 300°F for at least 3 hours. We also suggest the use of a moisture analyzer to assure dryness.

PEEK can be quite costly, but you should be able to use 30% dry first-pass regrind with unfilled PEEK and 10% with filled PEEK.

Safety should be a primary consideration when molding PEEK, both for purging and while working with the mold. Wear safety glasses and/or a face shield, Kevlar or Kevlar/stainless steel sleeves, and heavy cotton cloves when purging and reaching into the mold.

When preparing for your PEEK experience, research it well with your resin supplier. The above information is based on our experience, but it should used as a reference only. Also, make sure you don’t neglect recognized scientific principles when working with any thermoplastic material. With a bit of common molding sense, your PEEK experience can and should be a rewarding one.

Matrix Tooling, Inc. & Matrix Plastics Products has a great deal of mold design, building, as well as processing experience with PEEK resin, much of it in the medical field.

Posted by:

Brent G. Borgerson
Senior Process Engineer (Older Molder)

In Part 1 , we discussed the history and sources of biodegradable and compostable plastics – what we call “green resins.” They are here; they are with us; and they come with unique challenges in both the mold building and injection processing of these resins. Most are still in the developmental stages.

Bio-resins are formulated to give end properties mimicking well known thermoplastic resins, and in some cases the processing techniques of a bio- resin emulate the conventional resin for which it is designed to mimic, but in many cases they are processed quite differently.

Most bio-resins thermally degrade quite readily. Many run at a melt temp around 350°F or less, and even if kept at the recommended melt temperature, can degrade in the barrel or a hot runner system over time. Generally, the molder wants between 50-75% of his barrel capacity for each shot, and barrel or hot runner residence time shouldn’t exceed two minutes by much.

A good processing method is to begin and end the molding with an easy flow PE resin. While the PE is going through the barrel, the heats can be lowered to the bio-resin range. The hot runner can be purged with the PE either through air shots or by running parts. When shutting down this procedure is also followed. The mold and machine, after being cleaned with Polyethylene, are shut down with the PE in them. In addition to avoiding thermally degrading the bio-resin, this technique mollifies the tendency of the bio-resins, particularly PLA, to be corrosive, especially when degraded.

A well designed hot runner system with a well placed thermocouple for each drop, as well as manifold and sprue bushing thermocouples is needed to process bio-resins successfully. The hot runner system capacity should not exceed two shots.

A well fit sliding ring-type, non-return valve and a general purpose screw with a 20:1 or 22:1 L/D ratio will nicely plasticize most any bio-resin. Shear will greatly affect most of the bio-resins, so gate size can’t be too small, and fast injection speeds should be reserved only for the thinner walled parts.

Bio-resins should be well dried prior to processing. Most should be dried to under 0.010% moisture. Either desiccant, or compressed air dryers can be used. Temps can be moderate to low when compared to many of the common thermoplastic engineering resins.

Always consult the resin maker’s guideline literature before processing any resin, especially bio-resins.

Mold construction must be tight to successfully mold bio-resins without flashing. Vents are important, and care must be taken when cutting them. The experience gained from building molds for Nylons and LCP can be applied to constructing a mold for bio-resins. Again, the resin’s corrosive nature must be taken into account when selecting steels for the injection mold. Matrix Tooling has experience in designing, constructing, and running precision injection molds for bio-resins.

Bio-resins seem to be the wave of the future, and though new and a bit delicate, can be injection molded successfully with a little knowledge, care, and common sense.

Written By:

Brent Borgerson
Senior Process Engineer (Older Molder)

“Green” is all the rage now, and a major part of the green revolution is biodegradable and bio-compostable plastic resins. Our landfills are quickly filling up and a large portion of the contents are plastic items. These items remain in their state “forever.” Incineration isn’t an option due to air pollution and grinding the plastic only reduces the size but not the volume.

This problem is a product of the disposable or throwaway society that we now live in. I can still remember families having one TV set, telephone, refrigerator, and radio per household. When these malfunctioned, they were repaired. People had one fountain or ball point pen, and they were refilled when the ink supply was depleted. Nuts, bolts and most everything else you used came from a bulk barrel and were wrapped in old newspaper or put into a brown paper bag which was then used for your lunch. There wasn’t such a thing as blister or clamshell packaging. You took your soft drink bottles back for deposit, and your empty milk bottles were picked up by the milkman. We live in a convenience-first throwaway society and plastics, particularly plastics packaging, play a big part in that convenience.

Enter biodegradable and bio-compostable plastics. With these two “green” families of plastics we can have our throwaway convenience without overfilling our waste disposal sites. There also is the advantage of deriving these plastics from renewable plant resources rather than finite resources based on crude oil and natural gas.
The earliest plastics were bio-plastics that were derived from cellulose (wood). Billiard balls were the first use. Later Henry Ford used soybean derived resins in his early automobiles. The Yo -Yo is cellulosic and most of today’s screwdriver handles are of this renewable resource, non-petroleum based plastic. This plastic won’t biodegrade though, and when disposed of becomes a permanent part of the landscape.

During the Arab oil embargo of the 1970’s, corn starch was used to extend polyethylene due to the lack of petroleum feed stocks. The resultant packaging which included milk caps and pails was functional though the resin was problematic to run. The products broke down into smaller pieces but never completely degraded as only about 25% of the product was corn starch.

My first exposure to a truly biodegradable, and in this case, bio-absorbable was at Matrix back in 2002. The resin was PLA (Polylactic acid) . This was a corn based resin for a PLA encapsulated implantable radioactive “seed” for treating prostate cancer. This medical grade resin at the time of development was $1,500 per pound. Medical uses of PLA soon became a commonplace.

PLA continued to be developed, and commodity grades of the resin soon were made available allowing for prices to fall until it became a viable alternative to petrochemical resins.

Development of “green” plastics continued at a frenzied pace between 2003 and the present, and soon soy and milk (casein) based plastics reappeared and new resins came out based on the castor bean. Characteristics and properties of the new resins began to mimic the traditional resins that we are used to. Cutlery, plates, cups and clamshell packages for the fast food industry are becoming common. Writing instruments are being developed that are made from these resins, and even the pesky and environmentally ugly shopping bag as well as beverage bottles are being developed using biodegradable resins.

One developmental problem of these resins is combining the lasting properties of conventional resins with the biodegradable and compostable properties of the “green” resins. You want your pen to decompose in the landfill but not in your pocket. Your shopping bag must survive the trip home from the store; your plate or cup can’t leak all over you.

These bio-plastics, especially PLA have come under criticism though. Many people blame the rise in food costs largely on these bio plastics (and bio fuel). Defenders and makers of PLA say that they only use feed corn not consumer corn for PLA. Detractors say that the shortage of feed corn raises milk, egg, and chicken costs as well as red meat costs, and more land is diverted to feed corn for PLA and less to consumer corn.

New development in castor bean plastics makes use of this product which is poisonous to human beings. Soy based resins also take some of the pressure off of corn, though it puts the beneficial soy plant under pressure. Recent plastic resin and also fuel developed from biomass may prove the most promising development in “green” plastics and fuel. The corn kernels and soy beans can be used to feed the world’s people, directly or indirectly, and the once discarded biomass can be used to make the fuel and plastics we need to save the environment. Exciting new developments in this area are forthcoming.

At Matrix Plastic Products, we are involved in ongoing R&D projects involving “green” plastics. They require specialized mold building techniques and processing methods. It is exciting to be involved in projects that not only lessen our dependency on foreign oil, but are good for our fragile environment. I think the future will accentuate the positives and minimize the negatives of “green” plastics.

Written By:

Brent Borgerson
Senior Process Engineer (Older Molder)

Medical grades of biodegradable/bioabsorbable PLA, PLGA, and PLG resins can sell for upwards of $1,500 USD per pound. This and the delicate nature of biopolymers can send shivers down the spine of any injection molder. Really though, the standard precautions and best practices of any high level molding operation familiar with running engineering-grade resins similarly apply to the processing of these bioabsorbable resins. Every aspect of the manufacturing process must be closely monitored, and proper procedures strictly adhered to.

Mold construction must take into account the rheology or flow characteristics of the resin , shrinkage , and venting requirements. Due to the relative infancy of the bio-materials there is little data available from the resin makers, so all mold construction should be steel safe. Selection of mold material should keep corrosion resistance in mind.

Cleanliness, of course, is paramount. There can be no contamination present in an implantable device. Purging an injection unit after a run with a different resin, no matter how thorough, is not enough. The screw must be pulled and all residue removed from the screw and barrel physically. Don’t use copper gauze (here we differ slightly from the standard molding procedures) for fear of fragmenting and heavy metal contamination. A plastic abrasive cloth would be better, and the unit should be wiped, dusted, and blown or vacuumed free of any minute debris after cleaning. Having a dedicated injection unit is a better option, with a dedicated molding machine, dryer, and loading system being the best case scenario.

The molding machine really is not specialized. Usually a general -purpose screw is fine with about a 3:1 compression ratio and 20:1 length over diameter ratio (again, a general – purpose unit). Clearances should be tight in the injection unit, and the check ring non-return valve should be in good shape. Bioresins are not very heat or shear stable, so sizing the barrel capacity of the machine to the intended shot size is very critical, and high compression screws are not advisable. Since the resins are so costly, any scrap will immediately affect the bottom line. Savvy molders start and stop the run with a “commodity” bioresin with similar flow characteristics.

PLA/PLGA/PLG for implantables, like most bioresins, are slow to give up their heat and also can be slow to obtain optimum crystallization, so cooling times and pack/hold times can be quite long which lead to longer cycle times. Here, corners can’t be cut by running a colder mold in quest of faster cycles because you risk endangering the end characteristics of the implant.

In addition to shear and heat degradation, bioabsorbable resins are susceptible to hydrolytic degradation. Careful drying is important, with drying in the cleanroom next to the molding machine being advisable. Minimum out-of-dryer/in-molding-machine hopper time is also essential. Like many engineering resins, bioabsorbable resins must be dried and maintained at or below 250ppm (0.025%) moisture. A moisture analyzer is mandatory.

It is important to fast-track the process building and optimization portion of any injection project for PLA implants, if only due to the resin costs. One tool for this is DOE, or Design of Experiments. Here again, process building with a commodity PLA resin and final tuning with the PLA implantable resin is advised.

The implantable device must be made, handled, and packaged under the strictest of cleanroom conditions. Packaging must protect the implantable and be hermetic to keep it from picking up ambient moisture. Vacuum packing with laminated foil pouches is an option, and inclusion of a desiccant in the master carton is also popular. Sealing options include vacuum and heat sealing. Finally, sterilization methods and their impact on product end characteristics must be cautiously considered.

Care must be taken with these expensive and fragile resins; but the discipline is good for a molder, and the results for following good methods and procedures can be quite rewarding.

Written By:

Brent G. Borgerson
Senior Process Engineer (Older Molder)