Most of the injection molding tools contain air inside the cavity when tightly closed. When plastic is injected the air needs to squeeze and go out somewhere. Mold venting enables air to pack and free up space for the plastic.
The Problem With Poor Mold Venting
Failure to vent a mold properly increases mold temperatures and increases the melt’s pressure. The combination of these issues can extremely heat the oxygen in the mold, causing an array of visual defects and part integrity issues, including:
Weak and visible weld lines
Poor surface finish
Poor mechanical properties
Incomplete filling, especially in thin sections
Local corrosion of the mold cavity surface
The Mold Venting Methods
Mold venting methods are split into standard and non-standard processes for cavity venting. Regardless of the specific method used, the result should always be that gasses leave the mold, thus increasing the quality of the end product.
1. Standard Processes
The following are venting methods that are often built into the molds or machinery manufacturers use.
1.1 Parting lines
Which are formed by the points where the two halves of a mold meet. This meeting point naturally leaks gases and can be opened for venting.
1.2 Vent pins
Which are ejector pins that usually have between six and eight grooves along their bodies. They’re often used during the ejection phase to release gas and air.
1.3 Ejector pins
Which apply the force required to eject a part from the mold. Some manufacturers used them to vent deep features in a mold, thus preventing gas traps.
1.4 Tool clearances
Which can be used for mold venting, including that of the parting surface, ejector parts, or the core pulling parts. In all cases, manufacturers must be vigilant to ensure blockages don’t occur in the clearance used.
1.5 Injection mold sliders
Which switch the vertical movement of a mold opening to a horizontal motion. They typically consist of a slider body, forming surface, wedge, wear plate, and guide pin. They can prevent the displacement caused by pressure building in the injection molding process.
1.6 Mold Inserts
Some manufacturers use core inserts to prevent air traps. These inserts are usually placed where melt streams converge to decrease pressure and gas build-up.
2. Non-Standard Processes
Beyond the standard methods, manufacturers can use several other techniques for mold venting.
2.1 Using Porous Sintered Materials
Porous materials, such as breathable steel, allow gases to flow through them freely. Unfortunately, these materials often have low strength, though their loose texture can help with mold venting.
2.2 Vacuum Technology
Vacuuming devices, such as pumps, solve the air trap problem because the cavity is empty prior injection. However, they add cost to the mold and require well-matched parting surfaces to implement.
2.3 Overflow Systems
In an ideal world, a manufacturer can design a mold with gas channels, allowing the mold geometry to expel air. When this isn’t possible, they can use overflow wells to direct air into specific areas of the part or increase gas penetration. The idea is that overflow wells provide a path of least resistance away from the main part that the air should follow.
2.4 Venting Valves
Venting valves come in external and internal varieties. External valves are usually connected to the mold via a cold runner or channel to allow gases to escape. Internal valves are built into the mold cavity to create a venting channel for gases.
2.5 Exhaust From Vent Groove
Molds for medium and large parts require the removal of more gas. Manufacturers place vent grooves on the concave mold, usually at the end of the melt flow. They ensure smooth exhaust while helping prevent overflow.
2.6 Active Air Venting
In their paper Active Air Venting of Mold Cavity to Improve Performance of Injection Molded Direct Joining, Kimura F., et al, proposed the use of a micro or nano-structured metal plate that is joined to an injection mold. The plate was made using a porous metal.
They evaluated an active system based on this concept on several injection molded specimens. They found that the surface micro or nanostructure used can help with venting, depending on the composition of the structure.
2.7 Air Vents
These may be placed on the cavity, near the cavity, or at the furthest end of the melt flow. Venting isn’t limited to the mold. Vents can also be placed on the gate, sprue, and runners used in the process. Though they’re all in different locations, these vents help expel gases as the melt moves from the feed to the mold.
2.8 Mandatory Exhaust
This involves placing a vent pint into the mold at the place where gas collects. Though effective, the method leaves a mark on the finished product.
Factors to Consider in Designed Mold Venting
A wide range of factors affects how manufacturers design mold venting systems. All of the following must be considered before a manufacturer selects the technique they’ll use.
Injection Molding Temperature and Pressure
Manufacturers have the ability to change mold temperatures and pressures prior to filling. The flow rate depends on these temperature and pressure settings. Temperature, pressure, and the material’s rheological properties also affect viscosity. More viscous materials flow slower than less viscous ones, which affects cooling speeds. Viscous materials also often require deeper venting solutions, as discussed below.
Number of Vents
There are differing schools of thought on the number of vents needed. Some recommended placing one vent for every inch of the mold, with others stating that at least 30% of the mold’s perimeter should be vented.
Wall Thickness and Vent Depth
Wall thickness affects the depth of a manufacturer’s venting. As wall thickness increases, the vent depth should do the same. If shear reduces material viscosity, the manufacturer must use a vent depth at the lower end of the range. This often occurs near the gate or at the bottom of a thin rib. Depth should increase if the material’s viscosity increases due to sheer, such as in the thicker areas of the part.
The following is a list of vent depth ranges for various materials:
CELLULOSE ACETATE: .001-.0015
ETHYLENE VINYL ACETATE: .001-.0015
Vent Land Length
Long vent land lengths require more pressure to exhaust gases through them. The shorter the land length, the faster gas expulsion becomes.
Mold Vent Width
Vents are typically 0.25 inches for small parts and 0.5 inches for large parts. These are standard widths. However, nothing stops a manufacturer from using smaller or larger vent widths.
Mold Vent Surface Finish
Polishing a vent with diamond paste on a felt bob prevents the vent from leaving a milled finish on a surface. Alternatively, manufacturers perform stoning and grinding operations that match the direction of airflow to prevent the vent’s surface finish from being applied to the part.
Mold Vent Relief
This is the machined area that connects to the vent. The area feeds into a channel, which may connect with other channels. Ideally, the primary relief channel’s width should match that of the vent it’s connected to, thus improving efficiency.
Mold Vent Shape
Shape impacts the pressure applied within the vent. Think of vents as constrictions. With this thinking, manufacturers can use the coefficient of discharge to compare theoretical discharge to actual discharge.
The gases produced during injection molding aren’t pure. They contain chemical substances that deposit inside the vents, eventually leading to clogging. In the paper Development of the vent clogging monitoring methods for injection molding, Bongju Kim, et.al, proposed using cavity-gas-pressure (CGP) sensors inside vents to detect deposit buildups. These sensors detect changes in internal vent pressure, which can indicate deposits that need to be cleared.
Avoiding and Minimizing Bad Mold Venting Defects
In addition to accounting for the factors that affect venting decisions, manufacturers can also take steps to minimize defects. Processing and flow analysis both help in this regard.
Processing - Injection Molding
Processors have to analyze the end product for defects and make adjustments based on what they see. For example, a processor may slow the melt injection velocity if burning is present because doing so creates more time for gases to escape. However, this isn’t a catch-all solution. Processors must examine everything from barrel temperatures to material viscosity to determine which approach works best.
Flow analysis exists to help manufacturers determine weld line locations, dead zones, and how the mold will fill. The process can also help identify venting issues. DuPont recommends spraying the mold with kerosene or hydrocarbon-based spray prior to injection. These sprays leave black spots on the part in areas where the air is trapped.
Mold venting is a crucial part of the injection molding process. If manufacturers fail to take venting into account, they’ll likely produce deformed parts that have visual and structural defects. Understanding the factors that influence how a mold should be vented, coupled with the standard and non-standard techniques used for venting, can prevent these issues.