Injection Molder Cuts Scrap From 8% To 0.5% With Closed-Loop Controllers
Nyglass Plastics has cut injection molding scrap rate from 8% to 0.5% on two presses by replacing the original open-loop con
Nyglass Plastics has cut injection molding scrap rate from 8% to 0.5% on two presses by replacing the original open-loop controllers with the latest closed-loop design. The new controller reduces down time by sensing machine faults such as a solenoid failure and stopping the press after only a single bad part is produced. The new controller further reduces down time by allowing use of a profiled injection cycle that slows injection at critical portions of the cycle such as when plastic flows by cores to avoid moving the cores.
Nyglass is a custom manufacturer that produces plastic parts using both structural foam and injection molding methods. The company makes thermoplastic and thermoset components for 1700 customers in a wide range of industries including consumer electronics, computers, and medical diagnostic equipment. Nyglass has 21 presses ranging from a 50 Ton, 2.7 ounce Boy to an 800 Ton Beloit, with a 25 pound shot capacity.
The company uses a mix of new and older machines. Two of its older machines are a 700 ton, 125 ounce shot Van Dorn and a 570 Ton, 78 ounce shot Toshiba. The old controllers on these machines had no way to verify that their commands were actually being carried out as intended. They were also highly dependent upon electro-mechanical devices such as relays and limit switches that would frequently malfunction without any warning. Failures in any of these areas frequently caused large volumes of bad parts to be produced if they were not quickly detected and corrected.
For example, suppose the mold lost clamping pressure. The injection screw rushed forward and the mold flashed, spoiling the part. Even subtle changes in ambient conditions or material properties could have enough of an impact to cause the press to start producing scrap. This made it difficult to handle parts with tight dimensions or other critical features that require a very tight processing window to maintain quality standards. In an effort to overcome these problems, management assigned the shift foreman to make delicate adjustments in the injection pressure and temperature on a day-to-day basis. Foremen had to spend a considerable portion of their time trying to make up for the deficiencies in these controllers.
Nyglass management was considering replacing these presses which would have cost in the neighborhood of $500,000 for each machine. Since the hydraulic and mechanical components of the machine were solid, they decided to investigate upgrading the controllers instead. Management decided to purchase Scoremaster controllers manufactured by Solid Controls, Inc. (SCI), Hopkins, Minnesota. The systems were installed by SCI's integration partner, Hi-Tech Instruments of Pomona, California. Scoremaster controllers provide the important advantage of being able to keep all molding variables under close surveillance. The user can define the conditions that will trigger a fault, normally a pressure or injection velocity outside of the expected range. The controller can be programmed to keep running and sound an alarm or stop the machine. Nyglass typically programs the controller to stop the machine immediately if a fault occurs which means that only one bad part is produced. This was a key factor in reducing scrap by nearly 90%.
Closed loop position control makes it possible to maintain screw travel within hundredths of an inch as compared to the fraction of an inch accuracy possible with the old open loop controls. The improved repeatability virtually eliminates the possibility of overpacking or short shots which can produce significant damage to the mold. Clamping repeatability has also been improved to a few hundredths of an inch. This makes it possible to set low pressure closings more accurately, further reducing the risk of damage to the tool. The Scoremaster control also accurately controls barrel temperature on a multizone basis.
The new controller is capable of adjusting for changes in ambient conditions. For example, hydraulic oil behaves differently at different temperatures. When the machine has just been started and the oil is sluggish, it's very easy to produce short shots or parts with sink marks. The controllers are programmed to automatically compensate for this phenomenon by increasing the injection pressure to maintain a constant velocity profile. It's important to note that, in the past, when this problem occurred on the off shift, the machine might easily go down for the rest of the shift because no one in the plant was capable of making the necessary adjustments.
The ability of the new controllers to profile the injection cycle provides further reductions in scrap. A good example is an appliance housing which is molded from 20% talc-filled polypropylene. In the past, Nyglass had difficulty controlling knit lines on the outside of the housing, which is a cosmetic part. The part has several louvered openings to provide air cooling to the appliance, as well as numerous screw, holes, and bosses. These require a mold with a considerable number of cores. Using the old controllers, material would hit the cores and a subgate and then jam up. The flow would be slowed to the point that by the time the material flowed around the cores and subgate, and formed the knit line, it was too cool to make a good weld. The talc filler would also sometimes rise to the surface, causing a modeling effect that made it necessary to scrap the part.
The new controllers made it possible to profile the injection cycle to eliminate the flow line problem. The flow is started slowly in order to move the material past the gate without any disturbance, then speeded up to a high rate and finally ramped down to avoid flashing. The result is that the material moves much more quickly from the gate to the knit line so that it is still hot enough to make a good weld. Holding was changed to a two-part cycle, first high pressure and then low pressure. The low-pressure part of the cycle forces the resin to the outer surface of the part, eliminating the modeling problem.
Set up time has also been substantially reduced. The old controllers relied on a number of manual settings such as thumbwheels and limit switches and there was no way to be sure they were right except to start the machine. This process typically took about two hours each time the machine had to be set up. The new controllers take only an hour to set up the first time for a new mold because parameters can be entered onto the keyboard directly from the set up sheet. A bit of trial and error is normally required to get it running right the first time. The whole process takes about one hour. After the mold is running right, the configuration is saved into the controller's memory and it can be recalled at any time in the future to produce perfect parts in about a minute.
Besides stopping the machine, the controller also includes diagnostic tools that help maintenance staff identify the problem. When the power is turned on, the controller checks itself as well as key machine functions such as solenoid valves. If a fault occurs, the controller issues a code that provides the location of the problem as well as a graphical depiction of the machine with the fault highlighted. With the problem immediately identified, maintenance can immediately begin work on solving it. In the past, it typically took an hour or two just to locate the source of a machine problem.
After upgrading two machines, Nyglass has drastically reduced its scrap rate. Productivity has also increased because the company has been able to reduce cycle times by profiling injection rates. Foremen have been freed up to spend more of their time on planning production and training press operators. Basically, what the company has done is to eliminate most of the variables involved in injection molding so that parts are produced on a consistent basis with every shot. Current plans are to upgrade additional machines in the plant in the near future and thus achieve these benefits on a larger scale.
Solid Controls, Inc., 820 South 5th Street. Tel: 952-933-9053; Fax: .