Shot inventory, or, if this is a 24-oz machine, why can't I run a 24-oz shot?

By: 
December 31, 1996



Old, tired pelletsWhy is shot inventory/residence time so important? The rule of thumb in shot capacity is to use no less than 20 percent and no more than 80 percent of the rated capacity of your barrel. Another way to say this: you should have no more than five shots and no less than one and a quarter shots in inventory in the barrel. Take the total part weight from all the cavities, add the sprue and runner weight, and divide this into the machine's rated shot capacity.

A shot inventory of greater than 5 means you are at the low end of the barrel's capacity. The inability of the machine to accurately control moving a few hundred pounds of steel (the screw) with any degree of precision over a short distance will explain why your parts are remarkably inconsistent.

Molders generally compensate for this by turning down the heats. This puts unmelted material in the front of the barrel, which then is melted through shear heating. This process degrades the physical strength of the part in two ways. One is the mechanics of ripping the material apart; the other is the injection of cold material into the mold, where it cannot properly weld. If you're in this fix, one solution (although I'm generally opposed to it) is to make the runner system larger to take up the volume. To offset the cycle time increase, be sure you have a cooling line under the runner or directly in the runner plate.

A shot inventory of less than one and a quarter means you are over the top of the machine's ability to adequately melt the plastic and hold a cushion. Here is where a hot sprue or a hot runner can really help. Because the sprue/runner is eliminated, there will be less volume in the usable shot, and you might be able to reduce it to result in a higher inventory.

Many people are in the microprecision part molding business—their parts weigh in fractions of a gram. Routinely this means 80 to 90 percent of the shot is the runner system. The shot still uses only 1/10 of the machine's shot capacity. Ideally, this seems like an application for a hot runner. However, there are other considerations.

Many companies sell hot runner systems and only a few molders really know when and how to use them. While virtually any mold can be converted or originally built as a hot runner, when does it make sense and when is it more trouble than it is worth?

The first consideration is figuring out the volume lifetime of the part. Products with a low volume generally don't justify hot runners unless the design demands it because they're expensive to install. The auxiliary equipment can also cost a lot.

The next consideration is the press. I don't know why, but this is usually the biggest failure of communication between the molder, the mold builder, and the salesman for the hot runner system. When we install a hot runner system we are, by necessity, eliminating the sprue and runner. This means no regrind, which is usually considered good. It also means a much longer mold is needed to accommodate the runner system, and the press must be able to open and close.

At the same time, you have a much smaller shot size. In many cases these concepts are mutually contradictory. Since we have to get the mold open and closed, most molders will live with an extensive shot inventory in the system. What if the shot inventory is above the shot inventory level of 5? You need to consider some options:

  • Since the thickest portion of the molded shot is often the intersection of the sprue and the runner, eliminating it will usually improve the cycle. You have two choices: make a smaller sprue so the intersection is smaller, or use a heated sprue that will eliminate the volume of the sprue, leaving you with only a runner. However, eliminating the sprue has now increased your shot inventory. You'll need to offset this by making "poker chips" (round flat dummy parts that take volume but no cycle time).
  • If you're molding in small machines, you might consider a still smaller machine or a smaller screw/barrel combination that will put you back into a good shot inventory.
  • Some parts lend themselves to a semihot runner system. This is a full hot runner system that fills into a foreshortened conventional runner system—ideal for parts that need multiple gates. It saves dramatically on the material usage of a full runner and sprue conventional system.
  • Sometimes fate smiles on us. Look at your cavity layout. It might be possible to put in more cavities with either a full hot runner system or a semihot runner system. You are no longer bound by the "power of two rule" in conventional sprue-and-runner mold layouts that dictates two, four, eight, or sixteen cavities. With a properly balanced runner system, you can make a five, seven, or 13-cavity hot runner tool!
  • For those on an extremely limited budget, there is still some compromise available: What if you used an extended machine nozzle and a short sprue? What if you used a really extended machine nozzle and didn't use a sprue at all? Either of these solutions costs only a few hundred dollars and has a remarkable payoff.

Now, what if the shot inventory is less than one and a quarter? Using almost the entire shot capacity of a machine to fill a mold results in a high amount of sinks, short shots, and warp. This is because you usually cannot melt enough material to pack the part in a single shot. Also, it's hard to hold the cushion to pack the shot. A hot sprue can help to eliminate the amount of plastic required.

The use of a hot runner, hot sprue, or extended machine nozzle is an obvious solution for two reasons. First, by saving material consumed, the shot inventory will go up, hopefully higher than one and a quarter. Second, having the inventory up will save on scrap and cycle time.

When do you not want to use a hot runner? Remember that hot runners and heated sprues are merely an extension of the machine. Every overheating problem you get with a machine you'll get with a hot runner system. Thus, parts with extreme cosmetic requirements are not good candidates for heated runner systems. This is equally true for materials that are not particularly heat stable (medical grade vinyl) or those with thermally sensitive colorants, such as pastels or fluorescents. This is not to say you cannot run any of these materials with a hot runner system. However, your expertise and understanding of the system must be good and your people highly professional.

With multicavity tooling, it is a common practice to block off a cavity. This is bad for several reasons. The major damage is to productivity and profit. But blocking off a hot runner tool also leaves in the mold idle material that is constantly cooking. This burned material will never be truly cleaned out with purging compound until the mold is sent to the manufacturer, disassembled, and properly cleaned.

Many people still think that with a hot runner/hot sprue there is no such thing as a cold slug or the need for a slug well. Do this mind experiment. If you actually had the machine nozzle filling the part directly, each time the mold opened the part would exhibit a string. If it didn't, this would mean the plastic has slightly frozen and the part broke away. This frozen material is your cold slug. If your mold is designed properly, you have a cooling circuit next to each tip. This stops the heated tip from trying to heat the mold and forms the cold slug in the heated nozzle. You therefore need to design a slight depression in the part to accept this slug as part of the molded part. If not, you'll get it anyway as a cosmetic defect commonly known as blush or smearing.

Running quality parts is like carpentry. While you can put up a house with only a tack hammer, there are better tools. Think about your application, then make the proper choice. Everything has its place, which, when done properly, maximizes profit.

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