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Injection Molding and Moldmaking
with Surgical Precision

Injection Molding and Moldmaking
with Surgical Precision

Like many thermoplastic resins, nylon has its quirks and accompanying processing considerations. One of nylon’s most notable characteristics is its affinity for water. Nylon is extremely hygroscopic, a veritable sponge, absorbing any humidity in its environment. It is an efficient sponge; quick to suck up water, and slow to give up the moisture.

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

We recently had been asked by a potential customer why a polycarbonate would crack post-molding.  They had been having this issue on a specific part from one of their current suppliers.

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

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

In a recent blog posting we discussed the consequences of molding with wet engineering and commodity resins. The best way of dealing with these consequences is to avoid them entirely. In the posting we discussed our procedures and test equipment for assuring that the dryness of the resin is in the correct range. The most important aspect of resin drying is, of course, the dryer and the maintenance of the dryer.

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)

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.

 

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