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.

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.

Moist nylon resin affects the end product, often producing brittle or dimensionally unstable parts. Cosmetics are also affected; splay being one notable cosmetic defect that can be caused by moist resin. If the processed resin is out of moisture specs, it is essentially degraded. This is called hydrolytic degradation, and the effects and symptoms are akin to thermal degradation. Desired characteristics of many nylon parts include toughness and impact resistance. Parts produced with resin that has been sub-optimally dried can lack these traits.

Moist nylon resin can be hard to process. Nylon has a tendency to drool from the nozzle. Good heat control at the nozzle is important for molding nylons successfully and controlling nozzle drool or freeze-off, but wet resin can make this control almost impossible to achieve.

In addition to drying nylon well, it is important to dry nylon consistently. The same nylon resin dried at different moisture levels will exhibit different melt viscosities, even though the moisture levels may be within the manufacturer’s specifications. Water acts as a plasticizer; therefore wet nylon will fill more easily than dry nylon. This is reflected in peak fill (transfer) pressure and can be reflected in fill times, especially visible in a pressure limited process. For good consistent molding results, especially in a product with demanding dimensional specs, the resin moisture level should be consistent from run to run.

If nylon is allowed to stay in the dryer for too long (over the recommended time), the material can start to degrade as well.  Natural nylon may start to turn yellow.  The finished part may also be very brittle.  This is more common on nylons than most materials.

When inspecting nylon parts it is always good to allow the finished part to absorb the moisture in the air before you do your inspections.  Depending on the environment this can take a few hours or more.  Some nylon jobs require a fixed amount of moisture to be put into the poly bag that holds the parts.  This is common in the processing of nylon straps.

At Matrix Tooling/Matrix Plastic Products, we strictly follow manufacturers’ recommendations for drying temperatures and times to ensure dryness, and we have a moisture analyzer to verify the results. We have found that good, consistent drying gives consistent molding results.

Written By:

Brent Borgerson – Senior Process Engineer (Older Molder)

Patrick Collins – Molding Operations Manager

Our first step was to ask if we could get a sample of the part and the process sheet.  After looking at the part and reviewing the process sheets we noticed the following:   First, key set points like the dryer settings were not included in the process sheets.  We saw this as a potential red flag.  With polycarbonate it is very important that the material be dried correctly with the proper equipment.  Polycarbonate requires a dryer setting around 240 degrees for four hours (following the material recommendations of course, some may vary around 250 degrees for four hours) but doing this requires a high-heat dryer.   It is always good to verify that the moisture is 0.020% or less prior to molding.

Further looking into the setup sheets we noticed that the injection pressures were all on the high side of the recommended range.  This can be a sign that the gate size or nozzle orifice may be a potential suspect.  Running the incorrect gate size or nozzle size can induce molded-in stress.

We also noticed a lack of process monitoring; the set limits would allow the press to continue to run outside the manufacturer’s recommendations.  If uncontrolled, incorrect barrel temps, pressures or screw cushion can all be reasons for in-molded stress.

In looking at the part, the molded stresses were obvious, particularly when looking through a polarized lens under strong lighting.  The stresses create a rainbow effect in the translucent material.  Our next step was to measure the gate size and we found it to be much smaller than what we would recommend for PC.

So we had plenty to consider from the start, and these are just a few possible reasons for PC cracking.  We’ve also been told by the material manufacturer that some mold release sprays can attack polycarbonate.  They even had a story about an operator whose hand lotion was found to be the culprit for cracking parts.  This is one reason we do not allow silicone mold release in the plant and insure our operators use gloves on polycarbonate jobs.

After ruling out all of the above possibilities, it’s possible that some part designs may require annealing for stress relief.  Annealing of the plastic part is the process of heating the post molded part to just below its softening point, then keeping it at the high temperature for a period of time before cooling it slowly back to room temperature.  This can relieve some molded-in stresses but isn’t a desirable solution in most cases.

Processing polycarbonate at the manufacturer’s recommendations is the key to stress-free and crack-resistant parts.  If, for any reason, you are unable to follow the recommendations you should ask yourself why and correct the problem at its roots.

Written By:

Pat Collins
Molding Operations Manager

Bioplastics, aside from being derived from renewable resources, have the advantage of not releasing harmful toxins during their production, processing or degradation. Many conventional plastics can release known or suspected carcinogens such as formaldehyde or benzene during production, processing or destruction.

Growing the sources for bioplastics also reduces carbon dioxide in our atmosphere. Since the production of conventional plastics produces so much CO2 the use of bioplastics in place of a conventional plastic has a cumulative effect, with the substitution of just one ton of bio for conventional plastic having the net effect of reducing multiple tons of CO2 in the atmosphere. This not only takes into account the production methods for each type of plastic, but also the photosynthesis process in growing biomass or raw material for bioplastics. Bioplastics show great promise in reducing both our industry’s carbon footprint and impact on rising global warming.

What can plastics processors do until bioplastics are perfected in properties and reduced enough in costs to truly compete on a large scale with conventional thermoplastics? This is where the 3 R’s apply in injection molding. The 3 R’s in molding don’t stand for “reading, riting and ‘rithmetic,” but rather: reduce, reuse, and recycle. At Matrix Tooling / Matrix Plastic Products, we have been molding with bioresins, including bioabsorbables for a number of years, but as responsible members of the environmental community, we also have been practicing the 3 R’s.

Reduce: Scrap (and resin usage) is reduced through cold runner and sprue size reduction where possible without affecting moldability. In many cases we have reduced sprues and runners to the prescribed percentage of regrind allowed in the product specification. Hot runners and hot sprue bushings also have been used wherever possible. We have also thinned out wall stocks on parts where the product integrity wouldn’t suffer.

Reuse: We reuse what regrind we can and have come up with applications to use up to 100% in-house regrind. We utilize returnable/reusable packaging where possible and where allowed by the customer. We also have a closed circuit water system to reduce consumption and also filter, monitor and analyze hydraulic oil to avoid indiscriminate unneeded oil changes.

Recycle: Where we can’t reuse in-house regrind, we try to find it a good home. We sell the regrind where possible or even give it away for free if it can be used but there isn’t a paying market. Packaging is recycled also. We even collect our soda pop cans!

Matrix is serious about being environmentally responsible, using bioresins, and abiding by the 3 R’s. It not only makes environmental sense, but favorably affects the bottom line.

Written By:

Brent Borgerson

Senior Process Engineer (Older Molder)

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)

At Matrix Plastic Products, we have a dedicated dryer for each molding machine that runs hygroscopic engineering resins. The dryers are of two types:

1. Desiccant hot-air dryers
2. Compressed air dryers

Key to dryer effectiveness is maintenance. If the dryer goes down, the molding machine might as well be down. At Matrix, we take a multifaceted approach to dryer maintenance.

Visual Inspection: Dryers are visually inspected daily for hose condition, clamps, and kinks. Controls are scanned for dew point and temperatures in the proper range. Air flow cones are inspected as are the air flow filters.

Monthly detailed inspection: This includes the moving parts, testing desiccant condition, and confirming dew point meter readings on the dryer with a hand held dew point meter.

All monthly inspections and maintenance are documented on a preventive maintenance spreadsheet, developed here at Matrix Tooling/Matrix Plastic Products. This sheet covers PM for most common injection molding room equipment and is available for free at: http://www.plasticstoday.com on the maintenance forum and also on Bill Tobin’s WJT Associates website: http://wjtassociates.com/site/.

Since the sheet was developed here at Matrix, it will soon be available on our main website, again, for free. The PM sheet has been used all over the world and is a great tool for any molder to have in his or her kit. So avoid the consequences of molding with wet resin and maintain those dryers!
Written by:

Brent Borgerson
Senior Process Engineer (Older Molder)

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

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)

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 1870’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)