What is molded in stress?
Lesson 2: Understanding Flow-Induced Molded-In Stress Injection molding is a complex manufacturing process involving temperature and pressure. During the molding process polymer materials experience complex molecular chain deformation while undergoing temperature and pressure changes.
Does polycarbonate mold?
Also known as PC, polycarbonate is an adaptable thermoplastic that’s durable and pliable, able to be injection molded and thermoformed, depending on the project.
What causes crazing in polycarbonate?
The majority of times you see cracking and crazing in plastics such as Acrylic, Perspex, Plexiglas (PMMA), Polycarbonate (PC) and Ultem (PEI) is due to lack of the correct heat treatment. These such plastics need careful normalising and annealing steps throughout the machining, bonding and polishing process.
What is a major source of internal stresses for injection molded products?
There are three typical causes of residual stresses in the injection molding process: 1) Nonuniform shear action – as can be seen in Figure 1, molecules are highly oriented near the mold walls as compared to the bulk and this shear tendency due to the melt flow behavior results in nonuniform orientation through a part …
What is residual stress in plastic?
Residual stresses are generated when an object (especially a welded component) is stressed beyond its elastic limit, resulting in plastic deformation.
How is polycarbonate processed?
Since polycarbonate is highly viscous, it is usually processed at high temperature to reduce its viscosity. In this process, the hot polymer melt is pressed through into a mold with high pressure. The mold when cools, gives the molten polymer its desired shape and characteristics.
Does polycarbonate crack when drilled?
Using the wrong type of bit can result in the tool grabbing as you are drilling, which can lead to chipping at the edge of the hole. That’s a real problem because chips can propagate into cracks. According to Alison Svoboda of Boda Corporation, “polycarbonate is “notch sensitive” so a sharp drill bit is imperative.
Can you fix crazing?
Although crazing is considered a glaze defect, it can also be corrected by adjusting the clay body. A glaze adjustment might not be possible if it is under so much tension that there is no room in the recipe for correction.
Are residual stresses good or bad?
In general, compressive residual stress at the surface of a component is beneficial: it tends to increase fatigue strength and fatigue life, slow crack propagation, and increase resistance to environmentally assisted cracking, such as stress corrosion cracking and hydrogen-induced cracking.
Is polycarbonate a stress crack victim?
Polycarbonate frequently appears in the role of stress crack victim. One survey performed by Rapra estimated that while polycarbonate makes up 1 percent by weight of the global consumption of plastic resin, it constitutes 10 to 15 percent of the case studies for field failures. This is not an indictment of polycarbonate.
What causes residual stress in injection molded parts?
When these parts are injection molded, far more internal residual stresses are developed. Why? Here are some factors that cause residual stress in plastic parts: 1. Thermoplastics are long-chain molecules. Their resting or stress-free state is a coiled chain something like a slinky.
Can mold release spray damage polycarbonate?
We’ve also been told by the material manufacturer that some mold release sprays can attack polycarbonate. They even had a story about an operator whose hand lotion was found to be the culprit for cracking parts.
What is the tensile stress in plastic molding?
Additionally, the molten plastic continues to flow inside this frozen material and the polymer chains immediately adjacent to this layer are also placed in a state of tension. The thickness and magnitude of this tensile stress zone are often driven by how fast the mold is filled and the mold surface temperature.