Residual stress creates weaknesses after a load is removed from a material and can be detrimental to your manufacturing products. Stress free molding, and the mold stress relief test can reduce these harmful weaknesses.
Stress free molding is most popularly known in the form of the Mold Stress Relief Test. Any object containing a plastic element or enclosure can be tested.
While the test itself is simple, the evaluation which follows is complex and a rich source of information about the product and its strengths or limitations.
Polymeric Materials and Stress Free Molding
Plastic parts may be made of any number of polymeric materials, or matter made from a series of simple molecules which are then linked.
Understanding polymeric materials is vital in constructing strong and effective adhesives.
It’s also important in comprehending how plastics may react, or fail to react, in the presence of other materials.
Many products can benefit from undergoing the Mold Stress Relief Test, including:
- Eyeglass lenses
- Electrical equipment
- Appliances
Molded parts should be assessed for stress with care and precision when they are paired with other materials or placed inside another element.
Manufacturers should also keep in mind which nations their products will be marketed or used in, and the legal or environmental expectations which come with including any kind of polymeric material.
The Mold Stress Relief Test
When an object is subjected to the Mold Stress Relief Test, it is placed in an industrial oven and subjected to extremely high temperatures, sometimes as much as 70 degrees Celsius, but usually about ten degrees Celsius over what the product can withstand.
Depending on what engineers are looking for, the object may spend some time subjected to such heat, usually as long as seven hours.
Testers are careful, however, to avoid temperatures above 70 degrees Celsius, as a completely melted product wouldn’t yield much information about possible stress points.
Considerations of the Test
Design, cost, materials, and environmental factors should all come into consideration when they are included in a product.
The molded parts may react differently when integrated with other materials than when it is used alone. Stress free molding can provide feedback to how well a product has been:
- Designed
- Sourced
- Produced
Sometimes, even if a product has been carefully designed and constructed with high-quality materials, inconsistencies can result, either in the plastic itself or the material which surrounds it.
Although these might not be readily apparent when the product is first studied, these “invisible stresses” can contribute to mechanical fatigue and eventual part failure if left unaddressed.
What the Mold Stress Relief Test Studies
Once removed from the oven, the item is set aside in a safe, noncontaminated area to cool to room temperature.
Engineers then carefully study it to ascertain how it has withstood heat it may not necessarily have been designed to endure.
- How has the polymeric material reacted?
- How have the surrounding parts or materials corresponded?
- Are there any failure points? Where—and, most importantly, why?
Potential Findings of a Stress Test
Several potential weaknesses can unveil themselves during stress free molding. Below, we discuss the 3 most common.
1. Warping
When a material is deformed after exposure to heat, it is warped. The damage can take the shape of material that is
- Twisted
- Bent
- Raised at both or either ends
- Distorted at the edges
Sometimes, moisture can cause warping without the presence of heat; other materials become warped by dry heat alone.
If engineers see warping after a Mold Stress Relief Test, they will usually cut through the product to examine its interior.
This allows them to study the ratios of the polymers and possible imbalance of mixtures. Tiny samples of the polymers might be studied via a variety of techniques, including Attenuated Total Reflection, which examines each component of the polymer.
The information gathered in such test can help to identify weaknesses.
When a product becomes warped during stress free molding, engineers turn into detectives to try to determine the source of the problem.
Even if the designers are sure that the polymer mixture is appropriate, perhaps it was not mixed properly. The environmental conditions in which it was produced were imbalanced could also lead to problems.
Any problem in the manufacturing process, even if the components arrived from off-site, can contribute to product failure.
2. Shrinking
Shrinkage usually isn’t apparent until after the product has been left to cool at room temperature for a few hours.
Imbalances of polymers are the usual culprits when it comes to the frustrating issue of shrinkage.
Shrinking can occur at a rate of as much as twenty percent and can present significant problems when it comes to matching the tested part to the rest of the system.
3. Cracking
Cracking takes place when the polymers produce an unacceptably brittle plastic.
While it usually manifests in machine parts which have endured too much stress, abrasive chemicals, or overly harsh environmental conditions, a product which emerges from a Mold Stress Relief Test with cracks means its formulation is likely weak.
The polymers have broken into smaller segments, various parts of the enclosures have cooled faster than others, or the mold has experienced pressure the designers did not anticipate.
Residual Stresses
Residual stresses are any forces which create weaknesses that are left behind after a load is removed from a material. In the case of a Mold Stress Relief Test, the load is extreme heat.
Different layers in a material may react in various ways to the test.
- Some may withstand the heat perfectly well, then fail during the cooling process.
- Other areas might exit the oven already compromised.
Understanding how and why this can happen is essential in stopping weaknesses before the part becomes replicated and placed on the market, or inserted into a system to take on immense workloads.
Sometimes these tests can anticipate real-world conditions for failed parts.
Other Problems
Even if such failures as cracking or shrinkage aren’t present, engineers may find a host of other problems following stress free molding, even if they are microscopic.
Sometimes, even when warping isn’t visible to the naked eye, closer examination can reveal material which has become a few millimeters thinner than that which is around it.
While it might not seem like much, such mismatches can contribute to mechanical stress and potential part failure.