The potential of enhanced venting materials

Related Vendors

CUMSA

BORIDE Engineered Abrasives

International Mold Steel Inc.

Venting materials can solve various problems like shrinkage, short shots

Shrinkage. The presence of an air or gas layer between the mould surface and the resin is one cause of material shrinkage. Enhanced venting materials prevent the formation of this layer, allowing for close contact between the mould surface and the resin.

Short shots. Insufficient injection pressure, low resin temperature and trapped gas can cause incomplete filling of a mould cavity, or short shots. Enhanced venting materials can decrease the required injection pressure by reducing back pressure, and can also help to vent blind holes and eliminate gas traps by reducing fill time. Pulling a vacuum during injection will also reduce back pressure and fill time, while reverse blowing during ejection breaks any vacuum and eases the ejection. In some cases, it also is possible to reduce resin temperatures but maintain good fill.

Despite these advantages offered by enhanced venting materials, the revolution I expected 20 years ago never came. Why not? I pulled out a few plastic mould design and engineering handbooks to find references to venting. Each one said basically the same thing: Proper venting is crucial, and lack of proper venting can cause short shots, burns, poor weld lines, splay, internal stress in parts and inefficient cycle times. All of these problems can be solved with enhanced venting materials.

Plastics

Injection moulding sector strong

Consider these real-world examples:

• The mould for an automobile door panel wasn’t filling properly, so a 3-inch-diameter piece of porous steel was installed in the mould, which improved fill and reduced cycle time from 56 seconds to 44 seconds.

• Production of a car’s center console was yielding a 45-percent scrap rate. The majority of the scrap was due to a burn in the cup holder. By retrofitting a piece of porous metal venting steel in the problem area, the burn was eliminated, and the scrap rate (due to other issues) was reduced to 9 percent (see Figures 3, 4 and 5).

• A dishwasher filter screen showed so much stress from being packed out on a 750-ton press (the mould was originally scheduled for a 500-ton press, but it wouldn’t fill in this press) that it sprung out of shape when it came into contact with hot water. After retrofitting with porous metal buttons, the moulds were able to be installed back on the 500-ton press and produced stress-free parts.

The list of examples could go on, from thin-walled medical moulds that would not fill using conventionally designed vents to small automotive wire harness clips with blind holes to high-pressure PVC elbows that had reduced structural integrity due to serious burn problems. Retrofitting enhanced venting materials into the moulds solved each of these problems.

Still, there has been no mould design and moulding revolution. Why? Probably because anything published about proper venting techniques includes a lot of conventional wisdom, which creates a sense of risk associated with trying something new and a sense of safety in doing what has always been done. For example, conventional wisdom dictated 30 percent of the parting line be reserved for vents, vent dimensions of 0.0005- to 0.003-inch-deep by 0.0625- to 0.5000-inch-wide (depending on where the resins flashes), extremities of the cavity be vented, vents be the last area of the mould to fill, grind flats on ejector pins, etc.

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