VENTING THROUGH POROUS MATERIALS
As an alternative to machining, mold components such as vent plugs can be produced from sintered metals (Fig. 4). The conventional approach is to compact and sinter metal powder around an array of copper wires. After sintering, the wires are pulled out, leaving tiny tubular “pores” straight through the sintered plugs. Sintered vents can be up to 0.59-in. diam. and have anywhere from 70 to 10,000 holes for venting. The venting area can be made larger than with machined vents. Sintered vents also minimize flow and knit lines while reducing clamping pressure and cycle time by reducing backpressure.
Perhaps one of the biggest benefits of sintered vents is that they’re easy to remove from the mold for cleaning. When the vents get clogged, they can be cleaned mechanically or chemically by baking at low temperatures, using ultrasonic cleaning, or MEK (methyl ethyl ketone) solvent (also called butanone). Remember to dry out the solvent afterwards. And keep in mind that after repeated cleanings, the pores may start to clog—time to replace the plugs.
Another advantage of sintered vents is that they can be used with slides and lifters. However, their performance will vary with resin grade and viscosity, and they cannot be ground or milled, or the vent pores will become clogged.
Certain materials are considered “friendly” to sintered vents:
ABS
Acrylic
Biodegradable polymers
Flexible PVC
Nylon 6, 66, etc.
Polyethylene
Polypropylene
Polystyrene
PPO Alloy
On the other hand, sticky materials such as liquid polyurethanes should not be used. Rubber, LSR, rigid PVC, polycarbonate, and talc-filled resins can also be problematic with sintered vents.
Another porous venting option is a proprietary sintered metal called Porcerax II (supplied by Sintokogio Ltd. of Japan and International Mold Steel, Florence, Ky. DME supplies pins, blocks, and inserts of this material.). Its internal structure after sintering contains a network of pores or air spaces, almost like a dense sponge. Porcerax II is heat treated to 30-40 HRC with porosity of around 20-30% by volume. The interconnected pores have an average diameter of 7 or 20 microns.
Because the material is composed of 25% air by volume, it vents gas without the need for fabricated holes, or even a separate venting component such as a plug. Entire core pins or core/cavity blocks made of Porcerax II appear to be solid but allow passage of gasses through the mass of metal (Fig. 5). As a result, “self-venting” core pins and inserts of Porcerax II help reduce mold size, cost, and complexity.
Porcerax II provides a location-specific method of venting gas in a targeted area, where one-fourth of the surface area becomes a vent. The larger the surface area of the Porcerax II insert, the greater the venting capacity. The grade with 20-micron pores offers about 25% greater venting capacity than an equal surface area with 7-micron pores.
This method of venting delivers numerous benefits, including reduced injection pressure, as well as substantially reducing scrap and reject rates. Utilizing Porcerax II for core pins and plugs also enhances part production similarly to using sintered vents by preventing burning and shrinkage, preventing flow and knit lines, reducing cycle time, eliminating short shots, enhancing part appearance, reducing gloss, and aiding in part ejection.
What’s more, Porcerax II can be machined by conventional methods. EDM, texturing, or stoning are effective ways to open the pores after machining. Rigid PVC, phenolics, and natural rubber will work with Porcerax II, but only until the corrosive gases clog the pores. Use of rigid PVC, clear polycarbonate, LSR, and foaming urethanes should be avoided with Porcerax II. For low-viscosity or talc-filled resins, it may be necessary to have an automated system to reverse the airflow after each shot.
Keep in mind that the shorter the distance through the Porcerax II to the exhaust line, the better. And water lines should not be run through Porcerax II, since they are difficult to seal and will rust the material.
This type of porous-metal vent material must be cleaned when the tool shop has first prepared the insert or cavity, and also after the insert or cavity has been in operation and a thin film of mold release, resin residue, shop oil, or other contaminant has partially or totally blocked the airflow.
Common Venting Problems and Fixes
Short shots: Increase the number of vents; check vent location or size.
Weak weld lines: Vent at weld-line location; check vent location or size.
Bubble/splay/ discoloration: Increase the number of vents; check vent size.
Warpage & Shrinkage: Increase the number of vents; check vent location or size
Flash on parts: Vent depth too deep; vent location is wrong.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com
