Precise control over material behavior is the primary reason for requiring an adjustable blank holder force on a lab press. By varying this pressure, the equipment can simulate distinct forming conditions—specifically, shifting between clamping the material to test pure stretch and allowing it to slide to test friction. This capability is essential for isolating specific failure modes and accurately determining the forming limits of paperboard trays.
Without the ability to modulate blank holder force, it is impossible to distinguish between material failure caused by stretching and failure caused by friction. Adjustability allows researchers to switch between measuring ultimate elongation limits and analyzing flow characteristics, ensuring a complete evaluation of the material's formability.
The Mechanics of Material Evaluation
To fully evaluate paperboard, one must understand how it behaves under two distinct physical conditions: stretching and flowing. An adjustable blank holder force is the mechanism used to toggle between these states.
Fixed Blank Settings: Testing Elongation
When the lab press is set to a fixed blank setting, it applies high blank holder force to the perimeter of the paperboard.
This high pressure clamps the material firmly in place, effectively preventing any material flow into the mold.
Because the material cannot slide, it is forced to stretch. This allows researchers to determine the ultimate elongation limits of the paperboard, revealing exactly how much strain it can withstand before rupturing.
Sliding Blank Settings: Analyzing Friction
Conversely, a sliding blank setting utilizes significantly lower pressure on the blank holder.
This reduction allows the paperboard to flow physically into the mold cavity as the press descends.
This mode is essential for investigating friction characteristics. It shifts the focus from the material's internal tensile strength to the interaction between the paperboard surface and the mold.
Common Pitfalls in Parameter Selection
While adjustable force provides critical data, it requires precise calibration to yield valid results. Misunderstanding the relationship between force and flow can lead to incorrect conclusions about a material's viability.
Misidentifying Failure Modes
The primary risk in forming analysis is confusing a stretch failure with a friction failure.
If the force is too high during a friction test, the material will be artificially restrained. This causes rupture due to excessive tension, falsely suggesting the material has poor formability.
If the force is too low during an elongation test, the material will slip into the cavity. This invalidates the stretch data, as the material was never tested to its true breaking point.
Making the Right Choice for Your Goal
Adjusting blank holder force allows you to tailor the test to the specific physical property you need to validate.
- If your primary focus is determining material strength: Use a high-pressure fixed setting to prevent flow and measure the ultimate elongation limit.
- If your primary focus is process optimization: Use a lower-pressure sliding setting to permit material flow and evaluate friction characteristics against the mold.
Mastering this variable is the key to optimizing stamping parameters and preventing material rupture during production.
Summary Table:
| Setting Type | Pressure Level | Material Behavior | Primary Test Objective |
|---|---|---|---|
| Fixed Blank | High Force | Material Clamped (Stretch) | Ultimate Elongation Limits |
| Sliding Blank | Low Force | Material Flow (Slide) | Friction & Flow Characteristics |
| Hybrid/Variable | Adjusted | Controlled Draw | Process Optimization & Formability |
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References
- L. Berthold, Jens‐Peter Majschak. Role of blank moisturisation in press forming of paperboard. DOI: 10.15376/biores.19.2.2272-2285
This article is also based on technical information from Kintek Press Knowledge Base .
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