Utilizing Advanced Simulation and Melt Management Technologies to Redefine the Industry Standard

Eric D. Bowersox, Tech Molded Plastics, Meadville, PA

Abstract

In the plastics industry products are often grouped into categories, such as; automotive, electrical, medical, etc.  In these categories it seems the trend is to find one material, one process method and one mold layout that will work for all parts.  Grouping products in this manner eliminates creativity and the possibility for advancement in the plastics processing industry.  This paper presents a case study of how Tech Molded Plastics utilizes advanced plastic flow simulation and the latest in melt management technologies in order to create products that re-define the industry standard.

Introduction

In a recent project Tech Molded Plastics was issued with the challenge of building a single cavity time to market mold that would be capable of running Liquid Crystal Polymer (LCP) and Acrylonitrile Butadiene Styrene + Polycarbonate (ABS+PC) materials.  Due to differences in shrinkage values, the mold was to be built specifically for the LCP material with the ability to function with ABS+PC.  The products in this specific plastic industry are typically molded with ABS+PC.  Tech suggested the use of LCP due to the thinner wall conditions of this product and the greater amount of dimensional stability that would be achieved.  The customer was also intrigued by the rigidness of the LCP material compared to the flexibility of the ABS+PC.

An additional challenge was developed by the geometry and assembly requirements of the product.  The necessary gating location and resultant filling pattern develop a weld line in an area of the product where certain force requirements must be met (See Figure 1).  Compounding the weld line challenge is the fact that LCP materials are known for their inherent weakness in weld line strength.  The customer felt certain that the LCP was the material they would prefer for this project; however they were uncertain whether the LCP product would be capable of passing the necessary force requirements.  

 This paper was developed to show how Tech Molded Plastics utilized the latest advances in plastic simulation and mold building technologies to overcome the previously stated challenges.

Theory & Background

Weld lines (or weld planes) are formed during the mold filling process when separate melt fronts traveling in opposite directions meet. The formation of a weld line on a product will regularly be apparent to the naked eye.  This trait of weld lines alone is often grounds for a rejected product.  The deeper and more detrimental aspect of a weld line is that the local mechanical strength in the area of the weld can be significantly lower than the strengths away from the weld [1].

Meld lines occur when two emerging melt fronts are able to create a bond or mixing upon meeting.   Due to this molecular diffusion, the meld line typically exhibits greater quality characteristics than that of the weld line.  Traditionally meld lines are created by altering gating locations so that the material flow fronts combine while flowing parallel to each other [1].

Beaumont Technologies, Inc. performed a case study that shows the strength of a weld line is derived by the temperature of the flow fronts and their ability to entangle their molecules with one another.  When a weld line is formed by “cooler” material, the overall strength in that area of the product is reduced.  However, if the weld line can be formed with hotter material and then allowed to form a meld line, the resulting part properties and strength could be increased.  In Beaumont Technologies Inc’s study the bonding of the two flow fronts form a meld line not because of a meeting angle, but by strategically locating the products hotter, highly sheared material.  The higher flow front temperatures allow the molecules from the flow fronts to entangle to a higher degree than that of the cooler material [2].

Shear is developed in plastic products as a result of the resistance to flow as the material drags against the stationary walls of the mold cavity.  The shear that is developed stretches the polymer in the direction the material is flowing [3]. 

The melt temperature of the flowing plastic within the cavity varies from the mold wall on one side of the flowing plastic to the centerline of the flow and back to the mold wall temperature on the other side of the flow.  Flow rate, heat input from shear heating, mold wall temperature, and rate of heat transfer from the center of the plastic stream to the mold wall will affect the temperature [4].

Project Equipment

Moldflow Plastics Advisers version 8.1(Flow simulation) was used to simulate the filling of the product with both the LCP and the ABS+PC materials.                                                                                                                        

 The processing equipment used for this project consisted of a 60 ton Nissei NEX500 ELJECT injection molding machine and miscellaneous lab equipment.                                                                                                                    

A single cavity prototype mold was designed and built for this project. The mold incorporated Beaumont Technologies, Inc’s. iMARC TM Multi-Axis Rotary Inserts (rheological control system).  The rheological control system manages the shear profile through multiple axes, shear profile splits, and combinations to produce the desired melt rotation. The rheological control systems’ inserts can be rotated plus or minus 60 degrees, which causes the shear profile to rotate 180 degrees (See Figure 2).

The mold was designed to function while processing two materials. The prime material for the design was SUMIKASUPER® LCP E6007AS (LCP) from Sumitomo Chemical Company. The alternate material for the project was WONDERLOY® PC-540 (ABS+PC) from CHI MEI Corporation.

A Chatillon DFGS 100 500 x .5n (force test apparatus) and a Chatillon Model HTC stand were used to perform the product testing for this project.

Application of Equipment

The first portion of this project dealt with using advanced flow simulation in order to determine the most suitable runner and gate sizes for LCP and ABS+PC.  Simulations were run with full round runners and tunnel gates at the following size: See Chart (Right)

A standard fill and pack analysis was run at each feed system design for both materials.  The following analysis results were considered when choosing the most suitable feed system design for both materials.

The second portion of this project consisted of molding products with the use of the rheological control system. As previously mentioned the rheological control system inserts can be rotated plus or minus sixty degrees.  The LCP and ABS+PC products were molded at the following degrees of rotation with no process parameters being altered. See Chart (Right)

The parts were then collected from the previously mentioned rotational settings and tested for weld line strength on the force test apparatus.  Two categories of force testing were performed on the samples gathered.  The initial force test dealt with testing the weld line strength of the specific product for this project as a standalone part.  This test was performed by placing a direct load on the weld line of the product (See Figure 3).  

The product needs to exceed a minimum 111 N pull force.  The second assembly force test was performed by applying a downward force on the back of the fixtured assembly (See figure 5).  The downward force must exceed a minimum 66.7 N in order for the product to pass force test requirements.  These application force test requirements were determined by the customer in order to establish a high factor of safety with a low degree of variation for the final product.

Presentation of Data and Results

The first portion of this project dealt with determining the most suitable feed system design. The flow simulations displayed that all variations in the feed system designs for the LCP and ABS+PC had a 100% confidence of fill.  When considering the % high quality prediction, the feed system designs that contained either the 2.388 mm or 1.981 mm diameter runners showed higher predicted quality than that of the feed system designs which utilized a 3.175mm diameter runner.  The average % high quality prediction for the LCP material that utilized the 2.388 mm diameter runner was 86.3%.  The same runner dimensions for the ABS+PC materials gave an average measurement of 77.7%.  The average percentage of the % high quality readings for the runners with a 1.981 mm diameter runner were 86.2% for LCP and 77.5% for the ABS+PC.  Decreasing the gate diameters for the LCP material had a negative effect on the quality prediction, while the same decrease in gate diameter for the ABS+PC material had a positive effect on the quality prediction (See Figures 6 and 7). Both the LCP and ABS+PC materials showed an increase in injection pressure as the runner and gate diameters decreased (See Figures 8 and 9).

Additional Findings

While processing the ABS+PC product an unexpected plastic flow related phenomenon was discovered.  Parts that were created at the 0 degree; 0 degree rotational setting had a filling pattern that resulted in a gas trap at the end of fill.  The gas trap developed a highly visible burn mark in an area of the part where it constituted a failed product (See Figure 12).  Adjusting the degrees of rotation of the rheological control system from 0 degree; 0 degree to 35 degree ccw; 35degree cw drastically altered the filling pattern of the product.  Altering the filling pattern resulted in a new end of fill location with no gas trap or resultant burn mark (See Figure 13).  No additional process parameters were altered in order to achieve the visually acceptable product for the ABS+PC.

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