Mold Venting
IMM 03/01 & 02/01

How many toolmakers are putting parting line vents in their mold up-front? We are a custom molder that makes about 80 to 90 percent of our own molds. I'm the process engineer and believe that you can't have too much venting. Our toolroom puts runner vents in all molds right away, but prefers to hold off on parting line vents until after the initial mold run, letting the part specify where vents are needed

We are a custom moldmaker and service several customers. Our position on venting is to always put in runner vents, as mentioned, and to always put in parting line vents. In most cases, since flow analysis has not been conducted, we use experience to determine the end-of-fill areas. We also place vents on multiple gated parts where knit areas form as flow fronts converge. Some part designs allow periphery vents around the entire perimeter or at least a large percentage of it. These alleviate all gas traps at the parting line.

We also install vents away from the partying line before the first test, when it is obvious that a trap will occur. These vents can be on the bottom of slides, on lift cores, out the seal-off side of pass cores, and in special places such as those described in a recent By Design column by Glenn Beall (December 2002 IMM, pp. 60-63). The most common configuration we use is with dynamic or static vent pins. We have installed laminate inserts, vent plugs, or porous metal in special situations.

After the mold has been tested, it can (if necessary) have the venting fine-tuned. We will sometimes deepen specific vents or areas of the periphery vent. However, most of the time no additional vents are needed, and the initial venting scheme yields the desired results.

In today's world of short critical deliveries and lower costs, put vents in during the initial build. - J. Buss, Buss Precision Mold Inc., Clackamas, OR, (503) 652-5804, jbuss@bussmold.com.

I agree with the sentiment that you can't have too much venting. My experience with molds is that mold vents are not added until the first trial run is completed with a high degree of confidence.

Typically there will be some type of partying line venting, but to properly place a vent or vents depends on the flow characteristics of the material through the geometric mold configuration of the part being molded. Once a confident trial run is completed, we inspect the molded parts completely using visual stress testing, sectioning, preliminary first articles, test assemblies and so forth. Once we are confident that the part is complete as to design, then we add vents where we feel it is necessary, and resample again.

Most of our molded components are polycarbonate and are used in the medical industry, where one of our biggest concerns is molded-in stress. By adding vents in high-stress areas we have enhanced the mechanical integrity of our products drastically. With new molds, and with the use of flow analysis, engineers can evaluate the venting of a tool before any steel is cut. Mold manufacturers with these capabilities have already added venting in key points of the mold. But again, additional venting can only be determined during the first trial runs. You can't over vent a mold; however, I suggest you vent as you go. - T. Hahn, Edwards Lifesciences Corp. of Puerto Rico, (787) 826-2200, tim_hahn@edwards.com.

Placing vents in a mold after running parts used to be the standard way of determining where to put vents because the moldmaker looked for burn marks on the part. These marks were created by trapped air compressed to the point of auto-ignition by injection pressure. The compressed air (and / or other gases) would then burn the surrounding plastic.

Today, it is understood that not all trapped air gets compressed to the point of auto-ignition, so burn marks do not appear at all locations of trapped air. I agree that you can't have too much venting. I use a rule-of-thumb that states, at a minimum, 30 percent of the parting line perimeter should be vented. If the part perimeter is 10 inches, for example, I would have at least 3 inches of vents. If my vents are 1/4 inch wide (which I like) I would have 12 vents around the perimeter of the part, strategically located at obvious areas, such as corners. Using another reference, the GE Plastics website (www.geplastics.com) suggests having a vent at every inch of length around the part's perimeter. A final thought: When in doubt, vent it. - D. Bryce, Texas Plastic Technologies, Georgetown, TX, (512) 863-5933, dbryce@texplas.com.

More venting is definitely better and perimeter vents work very well. Although end-of-fill vents are probably the most critical, additional venting only improves the molding by allowing easier filling and packing at a given melt temperature. There should also be a vent at every knitline, if possible. Make sure the vents are dimensioned correctly for the material, the land is polished for self-cleaning, and there is a dump into the atmosphere. Having extra vents is money well spent and will make processing more efficient and forgiving. - B. Tutmark, GM Nameplate, Oregon Div., Beaverton, OR, (503) 646-0444, bradt@gmnameplate.com.

We sample the molds that we build because it's time consuming and costly to pull a mold from a machine to add vents during customer sampling. Customers expect to walk in, sample a mold, and walk out with good parts the same day. We incorporate venting in the initial design and base our venting decisions on part design, resin, tool design, and experience. - R. Cox, Armin Tool & Mfg., South Elgin, IL, (847) 742-1864, rick_cox@armin-ind.com.

I think the article on p. 60 (December 2000 IMM) addresses this question. What we are seeing in the industry is that more people are using materials like Porcerax II for their venting needs. This material is usually inserted in a problem area on the core side. However, if a large enough piece is used, there is no need for parting line vents. This material is 25 percent air by volume, so backpressure is greatly reduced in the cavity. You put the vent where you need it and seal up the parting line, thus reducing flash and wear in the area. - R. Bowen, International Mold Steel Inc., Florence KY, (513) 235-5131, russlbowen@aol.com.

I used to get frustrated during mold sampling when gas burning was the only factor preventing me from getting a good-quality part. Not any more - the vents always go in before first trial. - I. Hoskins, Proen International Pty. Ltd., Hilton, Australia, +61 (8) 8152 0800, ianh@moldsupply.com.

Proper venting of any mold is just as important as having proper cooling. As a plastic engineer for a Tier One automotive supplier, I am responsible for both new and existing molds. Before any tool is set up for initial sampling, all venting must be in place. Any new tool that come in-house without the proper venting will not be set up until the necessary venting has been applied. - D. Napominceno, VDO dnapomiceno@vdo.com.