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July 7, 2004

6 Min Read
Tooling Corner: Using prototypes to evaluatethermoforming molds and parts

Editor?s note: This story is adapted with permission from a presentation given at the SPE ANTEC 2004 in Chicago.

Designing a new thermoformed part is a complex process in which market needs, piece function, customers? requirements, use conditions, part geometry, manufacturing process, and production feasibility must be studied. Prototypes can be used to evaluate a new part or optimize an existing design, such as in the following example.

A broadly used method for creating mold prototypes is computer numerical control (CNC) machining. Rapid prototyping (RP) techniques use modeling software to produce a part that is almost identical to the original piece in just a few hours, with an excellent quality/cost ratio.

Prototyping Techniques

To design a new thermoformed thin-wall container and its mold, two prototyping techniques were used. The fused deposition RP technique was used for the piece prototype, and the CNC machining technique was used for the mold. To achieve the process objectives, the study was divided into the following stages:

  • Stage I?Thin-walled part design conceptualization. A marketing study was carried out to determine the new piece characteristics. Then, a 3-D model was developed in commercial CAD software. Originally, two designs were proposed?one with a rectangular base and the other with a pentagonal base.

  • Stage II?Mold design. The new male and female mold designs were drawn in a CAD program. The corresponding female mold was replaced with a plate to carry out the vacuum. A preliminary simulation with the thermoforming simulation software was made to evaluate the part?s thickness distribution and moldability. Based on these results, design modifications were made using the CAD software.

  • Stage III?Export the part?s finite element analysis (FEA) model in stereolithography (STL) format. The part model was produced by a Stratasys (Eden Prairie, MN) 3-D printer, Model Genesis XS (see Figure 2). The male mold was manufactured in wood via CNC machining (see Figure 3). CAM software was used to generate the CNC code.

  • Stage IV?Thermoforming the part with the mold prototype. A Brown (Beaverton, MI) S1150 thermoforming machine was used to evaluate the influence of major process parameters on the part thickness distributions. A high impact polystyrene sheet was used. The thickness of the sheet was 1.26 mm. Standard parameters were chosen after experimental pieces were obtained. For this experimental condition, wall thickness distribution through the cutting line was measured. These experimental values were compared with the theoretical ones.

Modifying the Mold and Part

After analyzing commercial containers, the design considerations were established. The design should:

  • be disposable,

  • be lightweight,

  • have a holding system,

  • provide appropriate material transportation,

  • have rounded corners, and,

  • have a capacity of 250 cm3.

For the design conceptualization, it was necessary to analyze the conditions under which the part would be used. It would usually be used at ambient temperature and be stressed by the weight of the granular, but nonaggressive, material it carried. The part design required dimensional stability and a broad surface to avoid overflow when adding the material to be weighed.

Although the capacity required by the users for the container is 250 cm3, it was necessary to adapt the piece dimensions to the single-cavity mold available for the prototype manufacturing. The core male mold dimensions available were 108 mm by 121 mm.

For the part design, an original proposal was chosen based on the designer?s experience. It is known that this configuration presents smaller restriction for the material slip inside the container; and it presents smaller angles that could cause problems during forming (see Figure 1). Also, by using an additional CAD module, it was possible to determine the smaller material volume necessary to thermoform the chosen geometry.

The holding system designed takes into account that at the end of the manufacturing process a certain quantity of unformed sheet remains at the upper border of the piece, which can be cut on the required form (see Figure 2). To impart stability in the holding system, it was necessary to introduce rigid elements in the part structure to avoid excessive warpage (see Figure 1). Most commercial containers use this type of reinforcement.

These reinforcements were studied to determine the quantity and dimensions necessary to reinforce the walls of the container appropriately. In the proposed design, the borders and the corners have been rounded to avoid having the material ripped during the forming process.

In stages II and III (mold prototype manufacturing), the core or male mold was designed starting from the part pattern. The process simulation runs were evaluated in order to verify problems presented during the forming state. For the simulation runs, standard process conditions were used.

Thin areas were observed among the two reinforcements included in the part structure. These areas of the piece have local deformations that occur when the part is removed from the mold surface.

Consequently, the areas present a smaller resistance than the rest of the piece and could fail under normal use. To correct the weakness, the reinforcement channel depths were decreased and the surface area and the round were increased (see Figure 3).

To verify whether the modifications corrected the appearance of thin areas in the forming stage, it was necessary to carry out a simulation under the same standard process conditions. A more homogeneous thickness was observed in the walls of the container through the cutting line length.

We checked for thin areas that could damage the mold, but did not find any. For this reason, the part design changes were accepted for the male mold prototype manufacturing.

After defining the final geometry of the part, a prototype of the same part was created to verify its functionality. If it fulfilled the requirements settled upon in the first design stage, qualitative experiments would be carried out.

A powdered material was poured into the newly formed part to verify that the material could easily slip out of the container when emptied. The new reinforcements avoided excessive grip system flexibility. Also, it could be established that the proposed dimensions were adapted to fulfill the established requirements.

Starting from the male?s pattern generated in the CAD program, the necessary sequence of CNC was generated by the CAM software to define the toolpaths necessary to reproduce the part?s complex surface. The male prototype mold was produced in plywood to minimize costs. Plywood presents the necessary characteristics for prototype construction where high volume production is not needed.

In stage IV (mold prototype validation), the simulated and experimental thickness distribution results obtained by the wood prototype mold are different due to the frictional coefficient used and the heat capacity of the material employed for simulation runs. The mold materials were not the same?for simulation runs an aluminum mold was used, and for the experimental runs a wood mold was used.

A Pattern for Success

By using CAD/CAE tools for part design, mold design, and thermoforming process simulation, it is possible to identify and correct design and manufacturing failures in products and molds. It is also possible to estimate the optimal process conditions so that a part has a homogeneous wall thickness distribution. This estimation yields money and material savings during the manufacturing process.

The use of rapid prototyping techniques allows the designer to obtain a solid, 3-D CAD pattern in which design concepts, dimensions, and part function can be verified before the part is produced.

Contact Information
Universidad Simón Bolívar
R. A. Morales, M. V. Candal
[email protected]
[email protected]

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