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From responses to the first in the series of WACKER's "Ask the Expert" Technical Articles, there was one question regarding LSR that by far was the most common. This question was about predicting cycle time for costing purposes.

Predicting Cycle Time

After determining the machine movement times associated with mold opening, parts removal and mold closing (all specific to part design and mold construction), the silicone portion of the cure time equation relates to injection, hold and vulcanization times.

While there is work within the industry using various mold flow predictors for effective injection and hold times, absolute minimum times for these parameters are not determined until a first run DOE is completed on the production mold. "Depending on part size and complexity, experienced LSR tool builders can offer some assistance with accurate estimations for the injection/hold values," said Vic Wilcik. "What is predictable is the required cure time for the LSR once injection and hold are completed. Most rubber fabricators are familiar with some type of moving die rheometer."

According to Vic, there are two different temperature profile types used to determine LSR cure: linear and isothermal. "While the linear test has proven a great asset for repeated injection molding performance," Wilcik said, "any 't' numbers or percentage of cure based on the torque curve generated do not directly relate to required cure time for individual molded articles."

When testing LSR via an isothermal temperature, t90, or 90% of maximum torque, is the most common value used to evaluate cure in relationship to removing a molded article from the LSR tool with no signs of under cure. "One misconception that WACKER regularly fields questions on is in regard to the t90 value being used as an 'as-is' or 'absolute' number," Vic explained. "Cure time in an LSR mold is a function of cross-sectional thickness vs. temperature. Individual rheometer test procedures involving degree of arc of the moving dies, open or closed test chamber, with or without Mylar[R]/nylon test sheets, with or without material dams, and sample size based on volume (specific gravity) or a fixed weight can play a small role in determining the torque/cure curve. In finding the highest contributor to the t90 value, it is WACKER's experience that maximum cross-sectional thickness of the LSR material in the test die is by far the largest factor."

To demonstrate differences that can be realized from various rheometers, WACKER's ELASTOSIL [R] LR 3003/50 LSR material was tested in an isothermal temperature environment from 110[degrees]C to 210[degrees]C on two different machines. One has a maximum cross-sectional thickness of 3mm while the second measures at 6mm (see Fig. 1)

The resulting test plot of the t90 values are displayed in Fig. 2.

Neither of these curves by themselves demonstrate the final cure time required for any given molded article. For the prediction of cure time, the t90 value should be broken down into a "required seconds of cure per millimeter of wall thickness" value at the appropriate molding temperature the fabricator will be using. At this point, the thickest cross-section of the intended part can be plugged into the equation to determine theoretical minimum cure time required.

With the same LR 3003/50 material as represented in the cure curve plot displayed (Fig. 2), examples of a wire wrap for ear buds were molded (Fig. 3).

The thickest cross-section of this example measures 7.5mm. Initial moldings were completed at 190[degrees]C. Using the t90 value in the 6mm column (32.0 s) from the isothermal chart generated, seconds of cure required for every millimeter of wall thickness calculates to 5.3. With a wall thickness of 7.5mm, predicted minimum cure time would be 39.75 s. At 40.0 s cure and above, all cross-section checks immediately out of the mold yield a cured part (Fig. 4). However, when the cure time was reduced to 35.0 s or 37.5 s, uncured LSR Is still present at the thickest cross-section (Fig. 5).

Similar results were realized at any temperature comparison.

This model has been used successfully In many applications. For molded articles less than 5mm thick, the 3mm test has provided the most accurate prediction. For parts 5mm-12mm, the 6mm test is most realistic. When cross-sectional thickness on intended moldings becomes greater than 12mm, extra time must be added to the 6mm calculation.

Should you wish to contact Vic for more Information on WACKER's views on additional pump topics, or, more importantly, if you have a specific question or topic on silicone rubber and would like to see it addressed in this series, please forward your inquiry to


Vic Wilcik began his career as a laboratory technician with a chemical company in Toledo, Ohio. His work quickly focused on all formulating and molding disciplines associated with the production of thermoset epoxy, urea, melamine and thermoplastic alkyd products.

In July 1990, Vic joined Wacker Chemical Corporation (Adrian, Michigan) in the position of senior laboratory technician. His first assignment was quality testing of WACKER'S liquid silicone rubber (LSR) products. As the LSR market grew and other WACKER LSR team-specific members were added, his duties were expanded to include development of new LSR products and test procedures. A specific goal was to bring injection molding knowledge to customers starting out in the LSR fabrication industry.

To that end, WACKER Adrian now has at its disposal, in its test lab, three LSR injection presses, one of which is capable of two-shot molding (thermoset LSR over various grades of thermoplastic).

WACKER's further support of the complete LSR molding system includes multiple molds capable of demonstrating the various molding techniques available (cold runner valve gate, open nozzle and molds with runners), LSR only and two-shot overmolding, along with the capability to demonstrate various dosing systems from different suppliers. This allows Vic to work with the customer in choosing the correct material for an application while developing a process capable of producing molded articles that will meet or exceed requirements.

Customers seeking specific information on LSR or the LSR molding process can be accommodated through WACKER ACADEMY training seminars.,

Caption: Fig. 1

Caption: Fig. 3

Caption: Fig. 4

Caption: Fig. 5
Fig. 2

ELASTOSIL[R] LR 3003/50, t90 Value
Isothermal Temperature Profile

[degrees]C   3mm     6mm

110          143.1   223.3
120          59.9    120.0
130          40.4    76.2
140          30.8    56.9
150          20.6    42.4
160          17.2    35.9
170          15.8    33.4
180          15.2    32.6
190          13.2    32.0
200          12.8    29.2
210          12.6    28.8
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Author:Wilcik, Vic
Publication:Rubber World
Date:Dec 1, 2017
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