Long fiber materials are usually used for metal replacement opportunities. Retaining long fiber materials in the molded part is essential for final part performance. In order to retain long fiber materials in the final part there are many key ingredients to achieve success. In this tech brief we will give the molder basic rules for achieving success with the final molded part.
Injection Molding Equipment
- Single-stage reciprocation screw injection molding machines are preferred however; plunger type machines have been successful molding Long Fiber materials.
- The use of a mechanical, hydraulic & hydro mechanical clamping unit when used properly, are all suitable for processing Long Fiber materials.
- The injection unit size should allow for the use of 40 – 60% of the barrel.
- General purpose metering type screws are preferred however; Pulsar® type screws from Spirex® are acceptable.
- Compression ratio = 2:1 to 3:1.
- L/D ratio = 18:1 to 22:1.
- Zone distribution = 40% feed, 40% transition, 20% metering.
- Feed zone channel depth = 7.5 mm
- Metering zone channel depth = 3.5 mm
- Pitch = 1D
- Barrier, double wave, and vented barrel mixing screw designs are not suitable for optimum processing of Long Fiber materials and should not be used
Non-return valve and screw tip
- A three piece screw tip assembly with a free flowing non-return check valve provides 100% free flow, is most preferred. All passageways should be sized to provide smooth open melt flow.
Nozzles & nozzle tips
- Both must be general purpose!
- A generous orifice diameter will ensure the restriction-free flow of material.
- Nozzle orifice diameter should be 0.020” smaller than the sprue bushing.
- Do not use internally tapered tips (often called “nylon tips”), or tips without a constant diameter pathway.
Dryers & Drying
- Use a desiccant drying unit with dew points from -20° F to -40° F. This will ensure all moisture is eliminated from hygroscopic materials.
- Non-hygroscopic materials are susceptible to surface moisture that should be removed before processing.
- When drying materials it is important to keep the air returning to the desiccating unit below 130° F. This may require the use of an after cooler. If the air returning from the dryer hopper to the desiccation unit is above 130° F, the desiccant cannot remove the moisture from the air, it, in effect, is regeneration all the time. The hotter the return air is, above 130° F, the less efficient the drying will be. With return air above 150° F there will be no drying at all.
- All material matrixes have their own individual drying conditions. It is recommended to follow all preferred drying conditions outlined in the manufactures molding guide.
- It is recommended to do a moisture analysis on all hygroscopic materials.
Now that we have reviewed the necessary injection molding equipment and drying requirements we must now use the correct processing method to retain the long fibers into the molded part.
Long Fiber Processing Tips
- Set barrel and mold heats accordingly to particular material (PA6, PA66, TPU, etc.) using manufacture recommendations.
- Use low backpressure, i.e. 25 – 50 psi & slow screw speed, i.e. 25 – 50 rpm.
- Use slow to medium fast injection screw speed, i.e. 1 – 3 in/sec.
- Low shear conditions are imperative.
Now that we have the necessary molding equipment, drying and processing requirements we must now use the correct tooling designs to retain the long fibers into the molded part.
General Tooling Information
- Full-round runner systems of ~0.250” are preferred, although trapezoidal equivalents are acceptable.
- Use adequate radii on sharp corners.
- An initial diameter of at least ~0.250” is preferred.
- For smooth flow, gates should be large and rectangular, at least ~0.250” x ~0.125” or 40-90% of wall thickness.
- Width = ~1.5-2.0 x Depth (use higher % for reinforced materials).
- Land Length = 1/2 of gate depth.
- Excessive land length causes jetting; insufficient land length causes blushing and sinks at the gate.
- Vent wherever possible, at parting lines, runners, ejector pins, bosses, ribs, projections, etc.
- Vent Land = ~0.002” Deep
- Vent Channel = ~0.005” – ~0.020” Deep
- Runner = Double preferred vent channel depth
- Ejector Pins (per side) = 0.0065”
Now that we have the necessary information to retain long fibers within the final molded part we can assure success for the adequate parts performance. If by chance the part fails you may want to check the part for fiber length by doing a part burn off in a muffle furnace. If you want more scientific evidence of long fiber retention you can submit the part for an optical microsposy. The best areas of the part to check are the sprue, runner, gate and end of part or material flow.
As you can see there are many key ingredients for retaining long fiber in the molded part.
Following the noted technical suggestions will help.
About the Author
Dallas Cada is a highly trained plastics engineer with over 20 years of sales support experience. Owner of a plastic consulting business (DDC Consulting), his experience includes technical service, application development, market engineering, injection molding, design, tooling, material suggestions and problem solving for plastic manufacturing companies. For more information with troubleshooting plastic problems or helping with new plastic applications, contact Dallas Cada by e-mail at email@example.com. Contact Dallas by phone at (507) 458-5785 or (507) 452-1584 firstname.lastname@example.org
Disclaimer: The editorial content published in this newsletter is the sole responsibility of the authors. The Injection Molding Division publishes this content for the use and benefit of its members, but is not responsible for the accuracy or validity of editorial content contributed by various sources.