Understanding and Preventing Cracking in Acrylic Parts During Overmolding
Time of issue:2024-08-23
Acrylic, also known as PMMA or plexiglass, originates from the English term "acrylic" (acrylic plastic), with the chemical name polymethyl methacrylate. It is an important early-developed plastic high-polymer material that boasts excellent transparency, chemical stability, weather resistance, ease of coloring and processing, as well as an elegant appearance. It finds widespread application in the construction industry. Plexiglass products are typically categorized into cast sheets, extruded sheets, and molding compounds.
Why Does Acrylic Crack? An Analysis of Causes
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Uneven Heating During Overmolding, Leading to Internal Stress
When customers overmold acrylic products using conventional injection molding machines with two sets of molds, the PMMA acrylic parts are often not immediately overmolded with TPE. During the secondary injection molding process with TPE, the acrylic parts are not preheated. Consequently, the acrylic parts encounter the TPE melt, which reaches temperatures of 180-200°C, abruptly within the mold cavity at room temperature. During the overmolding process, the surface of the acrylic part in direct contact with the TPE experiences significantly higher temperatures than the other side. This uneven heating generates internal stress within the acrylic part. If appropriate measures (such as annealing) are not taken to eliminate this internal stress, it can easily lead to cracks in the acrylic part. -
Exacerbated Cracking Due to TPE Shrinkage After Overmolding
The uneven heating during overmolding already poses a risk for cracking in acrylic parts. Furthermore, after overmolding, the elastic TPE undergoes a continuous post-shrinkage process. This shrinkage, combined with the internal stress caused by uneven heating in the PMMA part, accumulates over time and eventually manifests as destructive forces, leading to cracks in the acrylic part at the overmolded interface.While TPE shrinkage is inevitable, the focus lies in preventing the generation of internal stress in acrylic parts. The cause of internal stress is uneven heating of acrylic parts.
Mitigating Strategies to Reduce or Eliminate Internal Stress in Acrylic Parts:
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For conventional injection molding machines used for secondary overmolding, it is recommended to preheat the PMMA acrylic parts before overmolding (at a temperature of 85°C, avoiding discoloration from excessive heat) for 1-2 hours. Alternatively, proceed with TPE overmolding while the acrylic parts are still hot after injection molding. Preheating the acrylic parts helps mitigate uneven heating during overmolding, thereby reducing internal stress.
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If possible, utilize a two-color injection molding machine for overmolding. This machine quickly completes both injection steps, ensuring that the acrylic parts are overmolded with TPE while still hot, eliminating uneven heating and internal stress, and thus preventing cracks.
Injection molding employs granules obtained through suspension polymerization and is performed on standard plunger or screw-type injection molding machines. Table 1 outlines the typical process conditions for injection molding of polymethyl methacrylate.
Post-processing to eliminate internal stress in injection-molded products is conducted in a hot air circulation oven at 70-80°C, with the duration depending on the product thickness, typically requiring approximately 4 hours.
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