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February 1, 2000

5 Min Read
Understanding molds that form threads

Editor’s note: Consultant Bill Tobin of WJT Assoc. spends his time helping molders diagnose molding problems, and offers his comments on how to get molds for threaded parts to work best.

This article is not about screw thread design. There are some fine design manuals put out by resin companies to help you design threads. The important rules are to radius everything, don’t forget draft, and put a generous lead in.

The purpose here is to look at the molds and figure out how to get them to work the best. What kind of molds make threaded parts? The three basic ones: two plate, three plate, and hot runner, depending on your budget. All of them can work equally well with the proper design features.

Ways to Design the Mold
Discussions of thread-forming tools make people immediately think of unscrewing tools. But this type of tool is only one way to mold threads. When the requirement is for deep, small-diameter female threads, an unscrewing core driven by a motor, chain drive, or rack and pinion will do the job. But ejection can become the problem. The most common way is to put "dogs" (these look like miniature saw teeth) on the core plate to keep the part from twisting with the core, advance the stripper plate at the same rate the core is unscrewing, and then let the parts fall free. Another way is to keep parts in the cavity and unscrew with the slow open movement of the clamp, and then use front ejection (usually with a poppet valve).

But do we really need to unscrew a female thread anyway? If we are making reasonably large items, such as bottle caps, why not just strip them off the core? If the material is flexible and the thread is not complex, this is easy to do.

A second option to investigate is a collapsing core. During ejection, core parts move, collapsing the diameter and freeing the threads. Conventional ejection can now remove the parts. While this will leave a witness line, it is a lot cheaper and demands less maintenance than a rotating mechanism. No matter what type of thread forming you use, keep in mind there must be some kind of cooling in the cores. Without it the cycle will slow, and it will cost you.

Male threads, for some reason, are often considered the same as female ones. In my experience many molders go to the expense of building an unscrewing/collapsing-core tool because they are uncomfortable with the alternatives. In reality, male threads are best made with split cores that are opened and closed in one of two ways: in gangs with a large slide block and horn pins, or individually with a split, spring-loaded core that goes into a tapered pocket.

While making male threads with these mechanisms will leave witness lines, there are advantages.

Cycle time. Since a male thread is really a boss with a contoured outside, it presents a thick section. Thick sections take a long time to cool. When a soft, thick section is subjected to the torque of an unscrewing device, there is a high probability that it will break off. Cooling a thick section is most easily done if the cooling is direct and intimate. A cooled slide is easier to make and more effective than attempting to cool a rotating core.While there is some disagreement on the issue, using slides usually means faster cycle times. This is not an argument based on the mechanism; you can take the part out of the mold faster because it doesn’t need to be fully cooled. There is no torque obstacle when using slides.Maintenance. Unscrewing molds are precision tools requiring high maintenance. Everything must be timed, and that timing must be maintained to get the best productivity. Tools with slides, on the other hand, are easy to maintain.Cost. Unscrewing tools are expensive when compared to tools with slides.Venting. The problem with an unscrewing male thread tool is venting. Usually the thread is formed in a blind pocket. While cooling is the main issue, trapped gas (dieseling) will be an ongoing problem.Molding Issues
With all threaded parts, the size of the mold compared to the requirements of molding is often overlooked until it is too late. It is all too common that a 6-oz shot is put into a 50-oz machine. This is because the parts are small, but the mechanisms for unscrewing take up so much space that the distance between the tierods mandates the larger machine. We now have a residence time problem for the plastic. If it lives in the barrel too long (more than 15 minutes) most resin will begin to degrade; at the least the colors will shift. Along with buying the mold, you may have to pay for a smaller-diameter barrel simply to keep the residence time within a controllable limit.When all is said and done, there will still be a lot of argument over which tooling philosophy is best. To aid in your decision, think like a banker. Tooling is an investment. It is the means to an end to produce parts. The real cost of the parts is the tooling, maintenance, setup, and all other costs factored into the lifetime production of the product. The usual rule of thumb is to have the tooling pay for itself in no more than one year’s full production. You’ll need to build robust tools for the life of the product, but you don’t have to spend a king’s ransom and get a tool that never pays for itself.This decision is best made doing a side-by-side budget comparison, one example for each tooling approach. Look at delivery, tool cost, mold productivity, and cost of the parts. Whichever provides the best balance is probably the proper decision.Bill Tobin has a number of books available from the IMM Book Club that address both business and technical issues in the injection molding industry. For more information, call Renee Leatherman at (303) 321-2322, or use the Internet and go to www.immbookclub.com.Contact information
WJT Associates
Louisville, CO
Bill Tobin
Phone: (303) 499-3350

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