Venting molds is extremely challenging because you must create a gap in the molding surface large enough for the gas to escape, but small enough so plastic does not leak out. A very fine line exists between gas traps and flash, and not crossing this line is a constant balance.
As molds get more complex, effectively venting a mold becomes more challenging. Mold flow simulation is a good indication of where gas traps will occur, but the locations identified are not always ideal for venting. Another approach is to insert the area with a custom insert, but this can be costly and add complexity to mold maintenance procedures.
3D printing technology makes it possible to precisely change the density of specific areas of steel using microscopic pores that create an abundance of venting where it is needed the most. Plus, it enables fully dense solid areas for wear resistance. In short, using variable density sintering, a mold builder can get the best of both worlds when eliminating the most challenging gas trap defects in high-wear molds.
Time to Vent
Several types of venting exist, but the most common form is where two pieces of steel come together (fixed or moving) like a parting line or ejector pin with a tenth of thousandths of an inch deep gap letting the gas escape. Another option is metal with a resin binder sintered twice to burn off the binder and fuse the metal, leaving small pores to vent through the thickness of the insert to the atmosphere.
Venting solutions involving direct metal laser sintering (DMLS) are not prone to clogging between preventative maintenance (PM) cycles and can be easily cleaned with ultrasonic cleaning. In most cases, the venting is self-cleaning, which means that with each shot the pressure is pushing gas buildup through the vents into relief and avoiding clogging.
These approaches work when the mold is new and during first trials, but what happens when the mold goes into production and starts to accumulate shots that wear the parting line? Are the vents accessible to a technician to clean every shift by simply opening the press and wiping down the mold?
Suppose the vents are buried in vent inserts or mold areas that are not accessible without mold disassembly. In that case, they will clog, or in some cases, result in burning that eats away the mold steel and causes significant damage and part defects. The mold may also require more preventative maintenance (PM) when in production, which increases maintenance costs and leaves molders struggling to produce defect-free parts.
Developments in direct metal laser sintering (DMLS) allow mold builders to make porous inserts with powdered steel using a laser welding process that creates porous steel regions without the secondary debinding and sintering process. With this approach, a mold builder can create vented inserts with precision venting regions and fully dense steel in the same insert, and in particular, for wear areas such as shutoffs and bearing surfaces that require solid steel like ejector pin and core pin holes.
Even though this process offers an extreme amount of venting in a small area and durability within 48-52 HRC, it is sometimes still not enough for the demands of highly engineered resins that produce gases and burning that can erode steel if not properly vented.
With this approach, a mold builder can create vented inserts with precision venting regions and fully dense steel in the same insert.
Time to Coat
We must push the limits of printed steel to develop a high-wear solution for mold venting. For example, after you design, build and finish machine 3D-printed vented inserts, use a PVD process to coat the inserts for wear resistance or to lower the coefficient of friction, easing part ejection.
For many years our industry has used thin-film PVD on injection mold cavities and cores to protect the steel surfaces without adding a significant amount of material to the surface. This method avoids knocking the mold out of spec and having to consider a large amount of added material (electroplating) during mold design.
PVD coatings result in uniformity across the mold with no buildup on the edges and extremely low surface roughness. Therefore, they do not change the surface texture of the mold. Typically a titanium nitride or zirconium nitride is used with PVD coatings, which provides a hardness of 80-85 Rc. A chromium nitride can also be used, with a lower coefficient of friction but a lower hardness (about 75 Rc).
3D printing injection molds presents some unique challenges for coaters, like more intricate molds that demand a coating process that can reach all areas of a mold. Then there are vented inserts with vent diameters of 25 microns (one thousandth) or less that present a higher probability of damage from corrosive gases that flow through the vents. To protect the inner diameter of the vent inserts, you must apply a coating throughout the length of the vent without clogging or significantly restricting the airflow.
Coatings applied by thin-film PVD technology are typically around five microns in thickness on the surface of injection molds, but for vented inserts, we recommend a coating of two to three microns to allow the vented insert to perform as designed. In addition, a two-to-three-micron hard ceramic coating will also provide a protective barrier that decreases the ability of the corrosive gas to destroy the vented insert steel and increases the time between maintenance cycles.
We must push the limits of printed steel to develop a high-wear solution for mold venting. For example, after you design, build and finish machine 3D-printed vented inserts, use a PVD process to coat the inserts for wear resistance or to lower the coefficient of friction easing part ejection.
PVD coating is known as a “line of sight” coating process, which means you only apply the coating to areas facing the ionized metal cathode. Coating areas within tubes—or narrow vents—requires you to direct the ions down into the holes to apply the coating throughout the inner diameter. It would be best to create a bias on the opposite side (the regress of the vent hole) of the entrance of the vented insert, which attracts the charged particles into the vent and embeds them within the inner diameter along the way. Pay close attention to the area around the exit for the vented gas, as these edges will wear more quickly than the inner part of the vent itself.
Mold builders can provide efficiencies for injection molders by using DMLS to create vented inserts that eliminate gas trap defects. However, combining direct metal laser sintering and thin-film PVD will lengthen the intervals between PM and extend the life of the 3D-printed vented inserts.
ESSENTIAL VENTING REQUIREMENTS IN A MOLD:
A properly designed and built mold is nearly air tight. This is required so that flash does not occur, which is unwanted plastic beyond the boundaries of the part. But without venting, the mold would not fill properly, because the air that’s in the mold cavity has nowhere to go while the plastic is entering. The pressurized air produces heat, and if too excessive can produce burn marks on the molded part. Gasses given off by the molten plastic also increase the air pressure in the mold cavity, further complicating the problem.
A part in this molding condition is also unlikely to fill completely, since the pressurized air is restricting the flow of the plastic resin. The result is “short shot”, a part which is incomplete. To resolve this issue, vents are machined into the mold tooling. A properly designed vent is deep enough to allow the trapped air and gasses to escape but not deep enough to allow the plastic to flash. There are a few different ways in which mold tooling can be vented.
STANDARD VENTING
In a standard vent, a small channel is machined at the parting line. The “primary” area of this vent is very thin, allowing only the air to escape, and is usually very short, usually .030 to .050, depending on the material being used. The “secondary” area of the vent is a deeper channel that connects the “primary” vent to the outside atmosphere.
EJECTOR PIN VENTING
Ejector pins themselves provide natural venting; since the clearance required for an ejector pin to operate is often enough to effectively vent a plastic part. In areas where more aggressive venting is required, primary and secondary vents can be added to the ejector pin.
PERIPHERAL VENTING
In this venting scenario, the primary vent continues around the part or all of the parts periphery. This is used when maximum venting is required at the parting line.
RUNNER VENTING
If the runner is not properly vented – all of the air that was originally in the runner system will be added to the mold cavity as it fills with plastic. When a large runner is used, this can create more trapped air, especially when the runner is significantly bigger than the size of the part. Runners are usually vented with a standard vent as close to the gate as possible.
Venting has a big impact on part quality so the number of vents and locations need to be designed correctly. Below are best practices for optimizing venting.
Allow as many escape routes as possible for the enclosed air to vent from the mold.
Vents should run from the edge of the mold to the mold exterior.
When sizing the depth of the vents, they should be sufficiently deep to let air escape but not so deep that plastic can seep out.
When designing vents for the mold, ensure you consider all of the tooling such as runners, sprues, actions, or slides.
Machined vents need regular cleaning and inspection to prevent clogging.
If you are experiencing issues associated with poor venting such as short shots or warpage, consider increasing the size or number of vents, or perhaps a different vent location may resolve the issue. Additionally, if you are having issues with flash, the vent depth may be too deep.
Bad venting design will Reduce injection speed will allow the air to escape from the cavity and to limit the problems indicated, but this will lead to other problems, such as short shots. As injection speed slows, the temperature of the plastic decreases. The lower the temperature of the plastic, the higher the melt viscosity: the plastic will begin to harden before the mold is completely full and therefore produce incomplete parts. Packing is possible as long as a center-core flow path of melted material can transfer the packing pressure from the injection point to the entire injected part. Long filling time will cause the material to cool down, which can result in poor packing, causing warpage and less conformity to the cavity design and texture. Efficient filling time is essential for achieving proper welding lines and quality parts.
To avoid all these problems, “standard” ways are used during design and machining, to realize a proper venting of the mold.
Standard mold venting involves machining pathways between the mold’s mechanical elements through which air can escape. Typical vent locations may include, for example, the space between parting lines, or around pins or ejector pins, or at sliding elements such as sliders and cams. When parting lines and sliding elements do not allow space for enough venting, the mold cavity can be divided into several inserts, in order to increase the number of possible venting areas.
Material viscosity limits the recommended size of vents that should be used in a mold. Lower the viscosities require shallower than recommended vent depth. Industry standards for vent depth are determined by the material to be injected into the mold. Viscosity is also a function of melt temperature and injection pressure. Moreover, gases produced during molding also impact venting. Materials such as polyamide (PA), polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polycarbonate (PC) with glass fiber, polyethylene terephthalate (PET), polyoxymethylene (POM) and materials with flame-retardant properties, produce gases during molding. This gas production generates oily deposits that contaminate and with time block venting grooves, channels and the cavity surfaces, causing part defects. This can also lead to an increase of mold cleaning frequency and mold cavity’s contamination.
Although there are well known guidelines for designing and machining vents to optimize venting capacity, without removing excessive parting-line bearing surface, standard venting systems offer a limited venting capacity and performance that most of time is not sufficient to achieve reasonable part quality.
Proper mold venting is essential for producing quality parts. During the molding process, the air contained in the mold needs a way to escape. If this doesn’t happen, the melt will compress and trap the air in the cavity. A combination of melt pressure and high temperature will then ignite the oxygen, in a process known as “diesel effect”, causing burnings, gloss marks and stress cracks in the finished plastic part, compromising part quality and appearance. Poor venting, together with the diesel effect, can also cause the mold to wear on the parting lines, resulting in flash on the injected part.
Building venting system for releasing exhausted air is a very important step in injection mold design. A high-quality venting system can effectively release gas in mold, which will eliminate the defects caused by residual gas and improve the quality of product. However, the surfaces used to create venting system are usually complex, and traditional manual design of venting system is time consuming and difficult to guarantee the design quality on complex surface. To improve the design efficiency and quality of venting system, this paper introduces an automatic approach for designing venting system on complex surfaces of mold. It mainly consists of three steps: (i) original centerline generation, (ii) vent centerline optimization, and (iii) vent channel generation. In step (i), methods of offsetting curves and creating vertical lines on complex surfaces are proposed to generate original vent centerlines. In step (ii), algorithms of connecting curves and curve fairing on complex surfaces are presented to optimize vent centerlines. In step (iii), a hybrid way of extruding and thickening is developed to generate vent features. Based on these three steps, a smooth and continuous venting system can be created on complex surfaces in an automated manner. Finally, a case study with complex surfaces is provided to validate that the design efficiency and quality of venting system can be dramatically improved by using this approach.