Venting grooves on parting surface are easy to clean, not easily clogged, and have good venting effect, making them main place for gas to escape. Parting surface venting includes venting between cavity and core, as well as between cavity, core, and slide block parting surfaces. There are two ways to create venting grooves on parting surface: one is to create partial venting grooves, and the other is to create venting grooves along the entire circumference of cavity.
It is best to create a full circumference venting structure, with a depth of 0.025~0.030mm. Full circumference venting provides the best effect, with venting throughout the entire PL line. Theoretically, parting surfaces never contact each other. If full circumference venting is not possible, only partial venting can be done. When mold cannot achieve full circumference venting, venting must be considered at last filling position and second-to-last filling position; this type of venting is called partial venting.
Full circumference venting is shown in Figure 8-2. Full circumferential venting is commonly used in flat plastic parts with simple mold structures. In this case, parting surface is a single plane, and both primary and secondary venting grooves are machined with flat-bottomed milling cutters. When plastic part structure is complex and parting surface has significant undulations, full circumferential venting cannot be designed.

 

Figure 8-2 Full circumference venting design
If full circumferential venting is not possible due to irregular parting surfaces, a better approach is to implement localized venting in fixed mold. During mold design, localized venting grooves are designed around cavity, and their dimensions are marked. Based on mold flow analysis results or design experience with similar molds, specific locations prone to air trapping are selected for venting. When machining localized venting grooves, first, secondary venting grooves are machined to size using a flat-bottomed milling cutter with chamfering, reserving space for primary venting grooves. After machining secondary venting grooves, primary venting grooves are ground using a hand-cranked small grinder. Primary venting grooves can also be machined with a flat-bottomed milling cutter.
Parting surface venting also includes runner venting. For two-plate molds, runners are located on parting surface, requiring venting grooves to be designed at the ends and corners of runners. Refer to Figure 8-3 for design method of localized venting. In a three-plate mold, runner is located between A-plate and ejector plate. Venting grooves need to be located at the end of runner on the back of A-plate.

 

Figure 8-3 Partial exhaust design of parting surface
Venting groove depth is relatively small, just enough for air to pass through. Venting depth depends on flowability of plastic used in finished product; the better flowability, the shallower depth, and vice versa. Venting groove depth is generally between 0.015 and 0.05 mm.
Too many venting grooves will reduce contact area of parting surface. Insufficient contact area will cause local collapse of parting surface, which in turn will reduce depth of venting grooves, leading to poor mold venting. The thinner wall of product and the further away from gate, the more important venting grooves become.
A common misconception is that too much venting causes burrs. In fact, opposite is true. Inappropriate venting requires excessive pressure. When gas is allowed to escape, plastic material will escape with gas. The only thing that causes excessive venting and burrs is an excessively deep venting groove. Therefore, with proper design and machining of venting channels, venting channels will not cause burrs on injection molded parts.

In thin-walled cavities, ends of melt flow, bottoms of blind holes, ends of solid pillars in molded part, bottoms of reinforcing ribs and screw pillars, and closed corners of cavity are the most prone to air trapping. Venting in these areas mainly relies on venting channels between insert contact surfaces. Figure 8-4 shows a typical example of insert venting. Following four points should be noted when using insert venting:

 

Figure 8-4 Insert Arrangement
① Design dimensions of insert venting channels are exactly same as those for parting surface venting. See Figure 8-1 for venting channel depths of commonly used plastics.
② Insert venting channels must connect to bottom of insert. Airflow passes through venting channels, and from bottom chamfered three-stage venting channels, gas is vented to outside of mold and connected to atmosphere.
③ Venting channels between inserts are easily clogged by glue powder or dirt and must be cleaned regularly.
④ For deep bone areas, column areas, small pin points, or thin glue areas, special consideration is needed for venting. Venting pins or flat ejector pins can be used.

Ejector system venting typically includes three types: push rod venting, push tube venting, and flat ejector pin venting. Ejector system venting is far more effective than pin venting. Due to its periodic reciprocating motion, ejector system is less prone to clogging and has a self-cleaning function. Venting groove design of push tube is shown in Figure 8-5. Venting groove design of ejector pin and pin is shown in Figure 8-6.

 

Ventilated steel is a material sintered from spherical particle alloy. It has relatively low strength but a porous texture, allowing gas to pass through. Placing a piece of such alloy in the area requiring venting will achieve desired venting. However, diameter of bottom vent hole should not be too large to prevent deformation due to cavity pressure. Following are precautions for using ventilated steel:
① Ventilability of ventilated steel is inversely proportional to its thickness, so a thickness of 30~50mm is recommended.
② During rough machining, permeable steel can be processed using milling machines or other machine tools. During finish machining, except for electrical discharge machining (EDM), no mechanical machining (grinding or milling) should be used.
③ Venting grooves should be made at the bottom of inserts.
④ Permeable steel can be directly connected with screws.
⑤ If water circulation is to be implemented in permeable steel, water circulation holes should be plated with Dichtol.
⑥ Factory hardness of permeable steel is 35~38 HRC, which can be quenched to 55 HRC. Specialized instruments are required for hardness testing.
⑦ To check permeability, apply a small amount of liquid, such as a release agent, to surface of permeable steel workpiece, then blow compressed air through vent. Observing amount of foam that rises indicates permeability performance.
⑧ Method for cleaning blocked vent holes:
Heat workpiece to 500°F (260℃) and maintain for at least 1 hour; After cooling to room temperature, immerse in acetone and maintain for at least 15 minutes; Remove workpiece and use a high-pressure air gun to blow out blockage from bottom of workpiece; Repeat steps a and b until no blockage is blown out.
Figure 8-7 shows design method for vented steel insert.

 

Figure 8-7 Ventilation of ventilated steel

Guide post and guide sleeve venting is very important, especially for large molds. Venting grooves at the ends of guide posts are mostly located at the bottom of guide posts, allowing air to escape from bottom of guide post to outside of mold. Venting grooves are added to guide sleeve position of mold blank as shown in Figure 8-8. Venting grooves are located on plate A, 6mm wide and 3mm deep. Two venting grooves on the bottom side of mold face downwards, while venting groove on the top side should not face upwards to prevent dust from falling in. Two venting grooves on the top side face operating side and non-operating side respectively.

 

Figure 8-8 Guide sleeve venting

For deep cylindrical plastic parts and box-shaped or simplified shell-shaped plastic parts with large height dimensions, if material is fed from bottom through a side gate, air is easily trapped at the top. Therefore, in most cases, a top insert is used for venting. However, because this will leave a crease on the surface of plastic part, customer approval is required.
Light guide columns are common light-transmitting components in modern electronic products, mostly made of PMMA and PC materials, and a few made of GPPS. Top of light guide column is an appearance part and must not have any defects. Due to characteristics of plastic part structure, it is very easy for air to be trapped. Figure 8-9 shows a common insert method for this type of plastic part, where an integral insert is designed at the top of plastic part, a ejector pin is added to bottom of each column, and venting is designed at the end of runner (dimensions in figure are in imperial units).

 

Figure 8-9 Light guide column venting