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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: 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.

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