Fierce competition within the plastics industry is driving productivity and the need to do more with less. These same forces demand a very fast part-to-production leadtime to win OEM molding programs, and, going beyond these challenges, require mold builders and processors to qualify and test molds within very tight time parameters.

In this competitive environment that demands both speed and accuracy, the mold builder and manufacturer need to go to production as quickly as possible with tools to ensure solid quality control from their molds long before the presses start producing parts by the millions.
Additionally, leadtimes for new mold builds have decreased drastically over the past few years.

  Although molds frequently are manufactured within the quoted leadtime, it is in the mold sampling process that costs can spiral upward and time is lost; both factors negatively impact time-to-market and manufacturing efficiencies. During initial mold commissioning, there is a great deal of additional lost time and money when sampling and debugging the mold and the molding process to produce an acceptable product.

  Thus, the end result is typically a part-to-production leadtime increase of weeks or even months. Many of the delays can be attributed to cavity-to-cavity variations hidden inside of the mold that tie up numerous human capital resources like toolmakers, processors and engineers to diagnose and make corrections. Too often, the proposed solutions do not solve the root cause of these variations, and a band-aid solution is hastily applied under the pressure of ever-shrinking leadtimes. Such a move creates long-term issues throughout the production life of the mold that must be dealt with on a daily basis.

   In 1999, we participated in a two-part article in Moldmaking Technology magazine that discussed the primary causes of filling imbalances-and offered the means to identify, diagnose and solve these challenges with a new technology that analyzed molds to pinpoint the source of filling imbalances.   The solution was a patented melt rotation technology that could quickly and accurately debug the shear-induced imbalances in a mold, getting it into production at record speeds. Today, this technology is in use in thousands of molds worldwide, not only improving production startup and improving part quality (and thus providing a quick ROI), but also enabling many companies to be more competitive in today's global market.
Since then, other solutions also have been developed.

  At NPE 2003, newer, better and faster ways to get from the proverbial art-to-part were introduced, and a new five-step process offered processors new, quicker and more accurate options. Optimally used in combination with proven melt-management advances, the new technologies helped processors correctly diagnose and quantify variations and accelerate part-to-production leadtimes. As an added benefit, the new process ensured a more stable running mold throughout its production life.
The Five-Step Process Explained

  Step 1: Mold Samples
Getting a representative sample from the mold is very important. Each cavity and flow is identified for the particular layout and number of cavities according to their respective flow group. The test is done as follows: Once a reasonable process for the mold has been established, the processor can set the hold pressure and hold time to the minimum value that the process controller permits (zero where possible). The screw feed can be reduced, if necessary, until the best-filling cavity in the mold is about 80 percent full to avoid masking any imbalances due to unvented air, thin regions in the part geometry or other hesitation effects. The original injection rate should remain constant.

    Step 2: Weigh Parts
Collect all the short shots from a single shot and weigh them by their individual cavity location, within the respective flow groups.

  Step 3: Determine Steel Imbalance in Flow 1
Identify short shots molded from Flow 1 (typically two parts in a four cavity mold, four parts in molds with eight or more cavities) and then contrast the weight of these short shots to each other using a graph. These differences are typically variations resulting from dimensional discrepancies in the mold steel.

 Step 4: Determine Steel Imbalance in Other Flows
Distinguish each of the other flows in the mold. This step also requires that processors repeat Step 3, isolating the effect of the dimensional variations in the mold steel on each of the other flow groups.

  Step 5: Determine Shear-Induced Imbalance
To determine shear-induced imbalances, the optimization process requires that the processor identify short shots molded from Flow 1 to determine their average weight. Contrasting this to the average weight of the other flow groups, gives the molder the shear-induced variation created within the runner. The shear-induced variation is independent of dimensional differences in the mold steel and cannot be corrected by machining the cavities, runner or gates in the mold.

  The Fall of Injection Molding Machines
For many injection molders, the injection molding machine's control options are regarded as the key factor to produce identical parts from a multicavity mold. It comes as no surprise that machine manufacturers have, over the years, significantly advanced the controls and options available to the plastics processors in efforts to improve part-to-part quality and consistency. As manufacturers have learned, however, advances in controls and process variables haven't necessarily resulted in better quality parts.

For example, at NPE 2003 it was demonstrated that part-to-part quality within a given shot, contrary to conventional wisdom, is one manufacturing function in which the molding machine plays a minor role. The melt rotation technology was used on a 1944 Van Dorn manual plunger injection molding machine and a top-of-the-line 2002 all-electric molding machine. The same mold was run in each machine to provide a true comparison between the machines.

Half of the runner system of the mold was retrofitted with this advanced technology, while the other half simply used a traditional geometrically balanced runner system. , the filling patterns were identical from both machines. The half with a geometrically balanced runner shows the common filling imbalance problem, while the half with melt rotation technology shows a balanced filling between all the cavities for both machines.

Given fifty-eight years of technological advancements in the molding machines tested, there has been nothing within the machines to deal with and correct the cavity-to-cavity variations seen within a given shot-until the development of the now-proven melt rotation technology. It's a given that the 2002 all-electric IMM provided a better shot-to-shot consistency, which only means that any scrap being molded will be consistent from shot to shot without variation. 

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