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.

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