Time filter

Source Type

Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2012

The injection molding process encompasses hundreds of variables. One way to get a handle on them is to organize or categorize the processing variables into groups. Care has to be taken in choosing factors and levels. Keeping both to reasonable numbers can save huge amounts of time and money. Processing factors or variables that may be important should be identified during Operational Qualification (OQ) for medical molders and Production Part Approval Process (PPAP) for automotive processors. Levels should also be established in this initial work-up of the mold, resin, and process. A molder might think all he needs to do is set injection rates of 0.50 in. and 2.0 in/sec in order to achieve those fill times, but the experiment will provide incorrect results. Using the slow velocity first to provide a 3-sec fill time, the molder will find a 0.60 in. transfer position provides a 98% full part by volume. He sets up this transfer to second stage to pack out the part.


Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2013

Black specks in molded parts are one of the most infuriating problems to resolve on the shop floor. They come and go, causing rejects, wasted resin, and downtime; and they are difficult to resolve quickly. Sometimes purging compounds help, but often it is a toss-up whether to purge or just stop and pull the screw so it and the barrel can be cleaned. Another trick to help determine if the problem is emanating from the runner or the barrel is to put a few granules of a different color into the hot sprue bushing, then make a shot. If the black specks occur before the area of where the color comes out, it is the hot runner, if they occur after that it is the screw and barrel. If the problem is the hot runner, you are facing a complete tear-down and a review of the hot-runner design. Check for proper temperature control, burnt-out heating elements, open thermocouples, and incorrect placement of thermocouples. Check the depth of the hot tip relative to gate surface, and account for thermal expansion.


Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2013

Flash is one of the top 10 problems molders face, causing rejects and a significant dent in profits. Flash can appear on the part's edge along the parting line of the mold, or anyplace where the mold has metal meeting metal to form a boundary of the part. To start, one should find out if the flash is occurring in the first or second injection stage. Molders should check out the part after injection under these conditions. It should be visibly short at the fill time they established and at the normal transfer pressure. To check the parting line, one should not use bluing compound. Instead use a special pressure-sensitive paper that develops red color relative to the amount of pressure developed. The parting line may be perfect on a bench press but not tight when clamped by the molding press. If the flash is in the center of the mold, it may be due to inadequate mold support. Molders should consider whether the mold has enough support pillars in the right places for the cavity and core plates.


Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2013

John Bozzelli suggests that a scientific approach needs to be adopted to address problems associated with injection molding. There is a preferred method for troubleshooting, such as taking the hold pressure down to a low level in the range of 5 to 25 psi on a hydraulic press. Another preferred method for troubleshooting involves maintaining the hold pressure at a significantly lower level or in the range of 100 to 250 psi plastic pressure for an electric press set pressure. It is important to leave a minimum of 0.3 sec on the second-stage or pack/hold timer. John Bozzelli informs that the operator has an appropriate Delta P on the first stage in scientific molding, eliminating the need for reducing first-stage pressure.


Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2013

The reasons weld lines are weak has nothing to do with the temperature of the flow fronts bumping into one another. The flow fronts do not have an opportunity to cool; plastic flows like lava-it rolls out, exposing fresh hot melt. As stated, tool design must be considered concurrent with part design. A large part may have multiple gates, resulting in weld lines. If the cosmetics and performance are critical, the weld lines can be eliminated by employing a valve-gated hot-runner system with sequential filling. In multi-gate, cold-runner molds, there will be weld lines. The customer, designer, and moldmaker have to work together to ensure the weld lines will be in areas of least stress or cosmetic importance. Processing can affect the strength and cosmetics of weld lines, but it cannot eliminate the root causes. Processing variations in temperature and pressure usually can provide only marginal results. The root causes are poor polymer chain entanglement across the weld line and air entrapment. Both of these root causes must be resolved in part design or tooling.


Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2014

Developing a process for a new mold involves putting together a multitude of variables. Shot size, injection velocity, backpressure, and temperature settings are often near the top of the list. screw recovery time must be the same. Some argue that reproducing screw circumferential velocity is more important, and in theory this makes sense. Unfortunately my shop-floor experience indicates that better melt-temperature consistency is achieved by duplicating recovery time. There is also the condition where the cycle is minutes long, and it is impossible to turn the screw so slowly to take up all the cooling time for recovery. In this case, take as much time as one can to get the screw back and use screw-start rotation delay in combination with cooling time to provide consistent cycles. Once again, this is a thermal process, so cycle time must be consistent.


Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2014

The article describes how the US molders can minimize mold damage and save related costs every day through mold protection setting. With the machine in mold-safety mode, the molder should find the mold-open position that will allow parts and runner, if any, to drop or be removed by automation. Molder should slowly close the mold and listen and feel the machine for any unexpected noises and bumps. Many times a molder ends up setting a mold only to find the moving side of it bumping the leader pins just as they mate. There is a need to adjust the mold halves if needed. There should be no banging as the mold sides mate. One should start with a low pressure that will not close the mold and increase it until one gets clamp pressure. This should show the minimum pressure needed to trigger clamp-up. If the mold does not close after a period of running, one should recheck one's trigger position and adjust as necessary.


Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2014

Experts provide information on how a machine controls pack velocity. To find the true hold time, the molder must subtract the actual fill time from the set time on the injection timer. A series-logic controller ignores any remaining time on the injection timer after first stage is reached and provides a second-stage time exactly at whatever is set. There are several of these operating characteristics that distinguish between machines, but the emphasis is on second-stage or pack velocity. All machines have a first stage for getting most of the plastic into the part, normally 90% to 99.9% full by volume, and a second stage to pack the part to replicate the steel cavity texture and shape. Second stage or pack-and-hold moves relatively little plastic into the cavity, but how it is done has enormous influence on the process and subsequently the part.


Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2014

Some of the methods on how to mechanically clean a reciprocating screw are discussed. Many molders use one or more of the commercially available purging compounds for screw cleaning. Pulling out and cleaning a screw is a tough job that drains profits because the machine is not producing parts. A new method to clean screws uses high-pressure water. The process is clean, can be automated and will do no damage. Once the screw is out and safely mounted on supports, patiently use tools like copper mesh, brass putty knives, and wire brushes to begin cleaning it. Another method is to burn of the residual resin and degraded material. This works and is easier than hand cleaning, but the temperatures the screw is exposed to in this burn-of process can sometimes cause it to warp, scorch, and/or change its hardness. A newer technique, which works nicely, is to use CO2 pellets to sand blast the baked-on carbon or degraded resin.


Bozzelli J.,Injection Molding Solutions Scientific Molding
Plastics Technology | Year: 2015

Some of steps that need to be followed to perform the gate-seal experiment and learn what is best for a given part and process are discussed. It is better to perform the gate-seal experiment and then test parts made with and without gate seal. The data will tell the user to do what is needed and will allow the processor to make better parts. This will save time and money by eliminating one variable or factor in the design of experiments (DOE). The processor needs to check that timers are functioning correctly and that they span the appropriate time for the part.

Loading Injection Molding Solutions Scientific Molding collaborators
Loading Injection Molding Solutions Scientific Molding collaborators