What are the hidden risks of tool transfers? Part 3: Noninstrumented tool transfers

The tool arrives with no documentation and, of course, the customer needs parts right away. How do you quickly create your own set of procedures?

“I have to have these tools shot by tomorrow.” Sound familiar? These words, spoken by our customer, ring true for a lot of people. On a recent trip through the Northeast we heard good news: Molding shops are getting busy again. However, with that come a few headaches. Transfer molds are coming in the doors from molders who have shut down and customers are looking to get these molds up and running again. Now. Tool transfers come in many forms, and RJG categorizes them into three groups: high risk, medium risk, and low risk. Let’s address the high-risk transfers.
 
When you have nothing

Figure 1: Will it run? Inspection of the tool is a critical first step.

The high-risk tool transfer is the mold that comes in your door without any information. Unfortunately, that constitutes the majority of tool transfers, and can present a hostile situation if, for instance, the customer pulled a tool from its molder or the molder went out of business. Moving a tool between a company’s facilities could present another sticky situation; many sister facilities have a competitive streak and don’t like giving up work to their colleagues.

Trying to get any pertinent information about the process and mold in those situations is probably next to impossible, but it may be worth a shot. In this situation, you are essentially starting from scratch, so there needs to be a game plan in place to address the situation as proficiently as possible.

Inspection. Where do you begin when a mold shows up at your door? A practical first step is to inspect the tool. Is this mold capable of making parts in its current state? Are there functional defects? Is there any visible damage to the parting line, cavities, cores, slides, hot runner? What is the condition of the coolant lines? Does everything move as it should? How do you start to develop inspection procedures that properly ensure the tool is in good working condition before it’s put in the press.

Figure 2: Example of supplier-recommended material processing guidelines.

Calculating area. While looking at the tool, don’t forget to calculate the projected area of cavities and runners—this number establishes tonnage. Also, try to calculate the volume of the cavities and runners. This information will become helpful when establishing the process, (discussed later).

The part. Now that you have either confirmed the mold as is or returned it to good working condition, it’s time to figure out what you’re making. In our previous article, we addressed how to take risk out of a part design. If you have a sample part, that’s helpful for determining where the traps lie. If not, then you have to take a peek at the mold to figure this out. Remember, when establishing the amount of risk to a part, you want to identify nominal wall issues, sharp corners, draft, additions and subtractions, and gate location, to name a few. If you are seeing a lot of issues, what are the possibilities for fixing them? You may find out quickly that this tool is high risk and may be more of a problem than it’s worth.

Material requirements. Next, what material is being used in this part? Do you know the processing conditions? Do you have all the necessary auxiliaries to run this material such as dryers or a hot oil system, if needed? If you don’t have the material manufacturer’s processing guide, get online and find it. You won’t have the processing parameters from the previous molder in a high-risk situation so you are going to have to set up your process from scratch, starting with the spec sheet. The most important information to look for is recommended melt temperatures, mold temperatures, and drying information (if applicable).

Figure 3: Figure 3: Make sure the mold uses no less than two-thirds the tiebar spacing.

The molding machine. In what machine is this mold going to run? And no, it’s not whichever press will fit it between the tiebars. There are many factors to consider when making sure the mold is placed in the right machine; tiebar spacing is just one of them. The mold should take up a minimum of two-thirds of the tiebar spacing. Take your calculation of cavity and runner volume and you can determine what machines have the appropriate shot capacities to run this job. You want the shot to be no less than 20% and no more than 80% of the machine’s capacity. This comes down to two factors.

The first issue is the material’s residence time. To calculate the appropriate volume, you need to know the screw diameters and the maximum shot limit of the machines. Let’s say we calculated the volume of the cavities and runners to be 2.25 in³ and we have a machine with a screw diameter of 1 inch and a maximum shot stroke of 5 inches.
• Calculate the area of the screw: dia x dia x 0.7854 = 0.7854 in².
• Multiply the area of the screw (0.7854 in2) by the maximum shot stroke (5 inches) and you get the maximum volumetric shot stroke (3.927 in³).
• Figure out the percentage by dividing the volume of cavities and runners (2.25 in³) by the maximum volume of the machine (3.927 in³) and multiply that by 100. That computes to 57.29% of the barrel capacity, which is a great choice for a machine.

The second is machine capability. Don’t ask your machine to go fast if you’re molding below 20% of the barrel capacity; it won’t make it, and you will be in a flow-limited situation, not to mention the high residence times. How do you know if you are flow limited? Let’s use the following example.

We have a machine with a maximum shot stroke of 10 inches. The shot size is set to 2 inches and transfer is set at 1 inch. The velocity setpoint is 5 in/sec. From this information we can calculate how close we are to achieving the set velocity.
• First, calculate the expected fill time: distance (1 inch) divided by rate (5 in/sec) equals time (0.2 second).
• Compare this number to the actual number on the machine’s control. Let’s say it’s 0.28 second. Divide the actual fill time (0.28) by the expected fill time (0.2) and you get 0.4.
• Multiply 0.4 by 100 and you get a 40% difference.

Figure 4: Which machine is the right one for the job?

That is a significant number because it tells you the machine is not getting up to speed. If we flip the equation around and divide distance (1 inch) by fill time (0.28 second), you get a fill speed of 3.57 in/sec. In this situation, the machine is incapable of filling any faster than 3.57 in/sec.

The big unknown is fill pressure. Since you have nothing to go by, you’ll have to guess and see what you get. Your experience will go a long way here. You’ll have to take into account the wall thickness, flow length, and material to estimate fill pressure. A machine with available fill pressures in the 30,000-psi range can accommodate most applications.

Trial run
Now that you have the mold in the right machine, it’s time to shoot some plastic. Where do you start? Based on the material processing guide, barrel temps are set in the middle of the range, mold temps are at the middle of the range, and backpressure is set at 1000 psi. Keep in mind these are all starting points and are subject to change.

In addition to the starting points from the spec sheet, you can calculate shot size and transfer positions using the cavity volume of 2.25 in3. This figure represents a completely full part, but you don’t want a completely full part just yet. If you multiply 2.25 in³ by 90%, you get 2.025 in³. This number represents how much plastic is needed to get a 90% full part. If you divide 2.025 by the area of the screw (0.7854 in²), you get 2.57 inches, which represents the shot stroke. In other words, if you set the shot limit to 2.57 and went to zero, you would have a part that was approximately 90% full.

Since we typically mold with a cushion, we need to account for that. Let’s add 10% to make the math easy and be on the safe side. Then add 0.25 inch to the shot, which gives a new shot limit of 2.72 inches and a transfer position of 0.25 inch. At this point you are trying to achieve a fill-only part so you can continue trialing the mold. By doing some calculations up front you will save a lot of guesswork when it goes into the machine.

Now you’re ready to continue putting the mold through its paces. Start with a rheology curve, cavity balance, gate seal, pressure loss, temperature mapping, coolant flow checks, and any additional procedures you may have in place. After you have what you think is an established process, throw in a viscosity shift to see what happens to the parts. You can accomplish this by using two lots of material or going from virgin material to regrind. This will give you a better idea of what the process is capable of during a long production run.

So there you are: You have inspected the tool, found the material information, risked the part design, put the mold in the right machine, and developed a process. All of these exercises should lead you to answer the following question: Can I mold this part and make money at it? Hopefully these fundamental steps will help you develop your recipe for success.

Author Shane Vandekerkhof is a trainer/consultant with Scientific Molding specialists RJG Inc.

More on this topic:
Coordinating an acceptable transfer, Part 3: The receiving company
What are the hidden risks of tool transfers? Part 1
What are the hidden risks of tool transfers? Part 2: Assessing risk in part design