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Plastic filling analysis cuts waste, lowers costs.

Plastic Filling Analysis Cuts Waste, Lowers Costs

The whole team involved in launching a plastic product - part and mold designers, moldmakers, molders, and suppliers - can improve its efficiency by using plastic filling analysis. This type of computerized analysis provides accurate guidance in every area from design to production. It determines minimum wall thickness; predicts weld line locations, sink marks, ad warpage; and determines minimum runner size, the number of gates required, and the best gate locations. Plastic filling analysis also predicts the last place to fill, balances family molds and naturally unbalanced runner systems, and evaluates different materials and processing conditions. With this multitude of benefits, filling analysis not only aids each member of the product team, but also assists communication between them. It enables the design department to frequently consult the production department during analyses to determine the feasibility of proposed changes.

What Is Plastic Filling


Plastic filling analysis is a finite element program that calculates how plastic will flow into a mold or part. Using viscosity data specific to the material, specified processing conditions (melt and mold temperatures and fill time or fill rate), and a model of the part or mold, the program calculates the required pressure, flow fronts, shear stress (which is an indication of molded-in stress), changes in temperature of the plastic as it fills, and the clamp force that is required to keep the mold closed. Plastic filling analysis software graphically depicts the flow path from the gate to the last point to be filled. A 3-D view of the cavity clearly depicts weld lines and areas of possible gas entrapment. Filling patterns can be modified, and weld lines eliminated, by repositioning the gates and varying the thicknesses of the wall.

How the Part Designer


The part designer is responsible for selecting the proper material, specifying part thicknesses, meeting the functional and structural requirements of the product, and, with the concurrence of the molder, ensuring its manufacturability. Performing a plastic filling analysis can help the designer determine the manufacturability of the part even at the design stage, because it enables the designer to determine whether the specified material can fill the part at the specified thickness. The analysis may show that the material will flow so easily that the thickness recommended in the design handbook is not required and can be reduced. Reduced thickness results in lower weight, less material cost, and possibly even shorter molding cycles.

The designer can also help optimize manufacturability by determining the grade of the specified material that will work best for the part. For example, a more rigid of material will usually have strong mechanical properties, but its flow properties may be poor. Computer analysis would quickly show which is best for a given design.

Simulation can also aid the design by predicting weld lines, which are usually the weakest areas of a molded part. Given the determination of gate position and processing conditions, the simulation will show the locations of the weld lines. If the weld lines are in structurally or aesthetically poor locations, the gates can be relocated. But if the gates cannot be relocated, the designer can at least sense the strength of the weld by reading the temperature of the melt front when the weld is formed. If the melt is sufficiently hot when the weld is formed, it can be assumed that the weld will be reasonably strong unless it is a highly filled material. Also, the shear stress and temperature gradients in the parts give designers a preliminary indication of long-term dimensional stability.

Although part designers probably have the most to gain from performing a filling analysis on a new part design, they are usually under considerable pressure to complete the design and are not allowed the time necessary to perform the analysis correctly. As a form of insurance, companies build a number of prototypes. This is a far more expensive route, considering the time and cost of building prototypes and the hidden costs of a prolonged cycle of product development. On the other hand, plastic filling analysis can often minimize, if not eliminate, the need to prototype.

Such was the case at Black & Decker Household Products Group (Shelton, Conn). The firm used filling analysis software early in the concept stage on its new Spacemaker Plus coffee maker. The polypropylene main housing of the appliance was analyzed for moldability and strength. According to Scott Bojko, senior engineer in Black & Decker's Computer-Aided Engineering (CAE) Department, the very first moldings met dimensional, surface finish, and strength requirements - thus eliminating the need for experimentation on the factory floor. Not only is such experimentation expensive, it can also delay the introduction of the product. Further, computerized molding analysis enabled the firm to study nine different product and mold designs. Because each iteration took into account the lesson learned from the previous one, Black & Decker was confident that the final designs were the best that could be achieved. Performing the analysis simultaneously with design detailing resulted in minimal delay and, ultimately, the saving of thousands of dollars and months of development time.

Advantages for the Mold


Although often considered as being behind the scenes, the mold designer is second in importance only to the part designer in influencing the manufacturability and cost of a part. The mold designer often determines the locations of the part gate and weld lines, and the pressures required for filling. He can also use filling analysis to determine the location of the gate that will best ensure that the thick and thin sides of the part fill at the same time. If the thickness of the wall varies across the part, the thicker section usually fills first and the thinner side last. As the thinner side fills, pressure increases and the first areas to fill overpack; the result is a part that is likely to warp. But with filling analysis, the mold designer can efficiently and accurately determine the best location of the gate. The results are lower overall pressure to fill the part, and lower molded-in stress.

At the Becton Dickinson Research Center (Research Triangle Park, N.C.), filling analysis determined in a few days the exact change in gate location that was needed to solve an intricate leakage problem in a prototype of a medical product. According to Dr. Towfiq Gangjee, project leader in the Injection Molding CAE Department, a knit line had not sealed properly in the product, a catheter inserter consisting of a flexible polyvinyl chloride tube approximately 0.75 inch long, with wings attached to both sides. When flexed, the wing pinch the tube. The prototype of the part was single-gated on top of one wing. When the plastic was injected into the cavity during molding, it would first fill the gated wing and then flow around the tube into the other wing. Finally, when enough back pressure built up, the central tube would be the last to fill. Because of the length of the flow path, the polymer was too cold to blend, and a knit line had formed by the time the two flow fronts met each other on the far side of the tube

Using filling analysis, the mold designers tried two gating locations. By running a number of iterations in rapid succession, they determined that the optimal gate locations, which minimized the cooling of the polymer, were toward the outer edge of the wings, Filling analysis gave the engineers a new gating scheme and molding parameters; the approach worked perfectly, and the part is now in production. Dr. Gangjee says that Becton Dickinson saved tens of thousands of additional dollars and several months of additional time that would have been required to perform the work through conventional trial-and-error methods.

The mold designer also uses filling analysis as a key to determining the exact size of runner systems for multicavity, family, or even single-cavity molds. Most runner systems are designed larger than necessary; the mold designer can eliminate the wasted cost by determining the absolute size needed. A few mold designers may de-emphasize the benefit of runner size reduction because runners are often reground and reused. But such recycling involves too much cost, and recycled material cannot be use in a considerable number of products (such as those used in medical applications). Finally, large runner systems may limit cycles thus cause higher molding costs. By using filling analysis, the mold designer can determine minimum runner sizes without resorting to overdesign.

Mold designers can also use filling analysis to balance the flow in family molds, so that all cavities fill at the same time and with the same pressure. Family molds, which are often specified when two or more parts are required to fit together in an assembly, can present a challenge to the mold designer because they are usually quite dissimilar in size. Generally, the smaller cavity would fill first. Then, as pressure increased to fill the second cavity, pressure also increases in the filled cavity. As a result, the filled cavity overpacks - it is likely to shrink less and may not fit the mating part that is produced in the same mold. Similar problems are apparent in multicavity, naturally unbalanced runner systems, but filling analysis can prevent these problems by balancing the pressure in the multicavity molds.

Benefits to the Moldmaker

Although moldmakers may seem to be the group least likely to benefit directly from filling analysis, they can provide a valuable service to their customers (molders) by performing the analysis. Reducing the time required to bring a new mold into full production can easily justify the time and cost of the analysis. Considering the frequency with which molders make major changes to the mold during the prototype stage, thereby causing production delays and cost increases, the reduction can be substantial. Further, the changes are sometimes so major that the original mold must be scrapped. But by using filling analysis, moldmakers can drastically reduce recutting time.

An immediate example is a general-purpose electrical connector that was produced by injecting Santoprene (Monsanto) in an insert mold. The mold design was difficult because the runner ejects right into the side of the part. When the molder first tried to make the part, bending and moving insert caused a 50% scrap rate; trying a branch on the opposite side of the part met with little success. But with filling analysis, the problem became clear: The branch failed to work because the flow moved through the inner runner much sooner than through the branch, and it moved the insert. By using the package, the analyst was able to automatically calculate the different runner-size combinations that use a smaller inner runner, and thus equalize flow through the branches. The design that the analyst recommended worked perfectly without further changes.

Another example is the aid that filling analysis provides in determining vent locations in the mold. Generally, the moldmaker utilizes peripheral venting or leaves the mold unvented with the intention of adding vents after the first shots are run. The latter approach often proves unreliable because after the mold is partially vented, flow patterns change and vents must be located in different positions. Filling analysis assumes perfect venting; the melt front accurately show where the vents need to be located, which includes positions inside the perimeter of the part.

The Molder's Job Made


In the process of bringing a new mold efficiently into production, molders can benefit from using, for each mold, a preliminary setup sheet that indicates the specific material, melt and mold temperature, and fill rate. Filling analysis enables an engineer to calculate these processing conditions long before the mold is even completed. In addition, the engineer can evaluate the effects of changing these conditions to achieve the highest quality part.

The software can also be used to determine the size of the press required for a new job. One of the results of the analysis is the calculation of the clamp force required: the sum of the pressures on each finite element, adjusted for its angle relative to the clamp direction. After beginning a new job, molders frequently find that they can run a mold in smaller press - but they often lose the job because they have to quote it for a larger press with a higher machine hour rate. The software also calculates the volume of the mold's finite element mesh. If the model is prepared accurately, the molder can multiply volume by material density to obtain the weight of the part, from which he can accurately calculate material cost. The result is a more accurate quote.

Competitive Edge for


A small number of companies, including consultants, resin suppliers, corporate design or service groups in large companies, and a few top quality molders and moldmakers, have already seen the advantage of offering mold filling analysis services to their customers. Some purchased the software to preserve their as leaders in the industry; others wanted to be able to provide services that were needed by other part or mold designers but not available to smaller shops. Still others recognized the value of these tools and justified the cost solely on the basis of the savings that it could yield.

Today, there is yet another reason why companies are adding plastic filling analysis capabilities to their engineering departments: Their major customers demand as much. This is especially true of primary suppliers to the automobile industry. The major automakers are giving their suppliers the opportunity to do more of the design work. But before they approve mold building and production, they want to see the computer simulation. Primary suppliers risk losing business, whereas smaller companies view this as a means of obtaining new business from the automakers. Regardless of the reason, to display the capability of providing plastic filling analysis enhances a company's image in the same way that CAD/CAM did a decade ago.

A case in point is Monsanto Elastomers Business' Marketing Technical Service Group in Akron, Ohio. The group is responsible for providing technical support to prospective and current customers of their Santoprene thermoplastic rubber and Geolast thermoplastic elastomer. Filling analysis software helps them generate new applications for these materials and solve problems on jobs that are currently in production. The software package allows Monsanto to reduce start-up time for companies that use their products. The group's use of the software prior to production usually enables customers to produce good parts on the first trial run, without needing to adjust operating parameters or recut the die. As a result, the firm's elastomers business has increased. In a study of the forty-five applications in which Monsanto used filling analysis software during the last three years, the firm generated figures showing that a substantial increase in its business can be attributed to its use of the software.

Why Not Now?

Given the many benefits of plastic filling analysis that are available at every stage of the production cycle, it is something of a mystery why the analysis has not been used more widely. Perhaps the fear of new technology, and an overly confident attitude on the part of some experienced plastics engineers and designers, are reasons. The cost factor is another possible reason. Unlike other software, the prices of which have decreased over time, analysis software has held its price - a reflection of the enormous development effort that is required initially, as well as ongoing enhancement costs. On the one hand, a company's justification of the cost is that a single analysis can save the total cost of the software. But on the other hand, the high initial investment often discourages companies from seriously evaluating what the software can do.

The additional step in the design-to-production cycle is another reason why the software is not used more often. Who is to perform the analysis? If the designer does, the design may take longer, especially if he finds a problem that must be corrected. End-users are reluctant to pay mold designer or moldmakers to run an analysis. And because molding is so competitive, molders cannot afford to invest time or money in this venture. As a result, most part designs are not optimal, new molds may require several tryouts and some reworking, and material may be wasted.

Companies that are most likely to use plastic filling analysis have learned from experience that the investment at the beginning of a project yields greater rewards later. They realize that today's competitive edge will be tomorrow's basic requirement. Today, CAD/CAM capabilities are expected from a high quality vendor. In the next decade, mold filling analysis capabilities will be among the offerings required of top vendors.

PHOTO : The time required to fill the mold of an automotive console, and each area of the mold. Red areas are nearest to the gates, blue areas are farthest.

PHOTO : As the plastic fills, the software graphically depicts changes in the temperature of the plastic, allowing for optimal blending conditions.

PHOTO : Pressure readings from the filling analysis of a computer disk.
COPYRIGHT 1990 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Title Annotation:processing
Author:Caren, Stuart
Publication:Plastics Engineering
Date:Nov 1, 1990
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