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Abstract
In this work, the bonding strength of overmolded polypropylene is investigated and modeled. A T-joint specimen was designed to replicate the bonding between a base and an overmolded stem made of the same polymer: a previously molded plaque was used for the base, and the stem was directly overmolded. The effect of melt temperature, holding pressure, and localized heating was investigated following the design of experiments approach. Both the melt and base temperature positively affect the welding strength. On the contrary, the holding pressure negatively contributed, as the crystallization temperature significantly increases with pressure. Then, the bonding strength of the specimens was predicted using a non-isothermal healing model. Moreover, the quadratic distance of diffusion (based on the self-diffusion model) was calculated and correlated with the bonding strength prediction. The non-isothermal healing model well predicts the bonding strength when the reptation time is calculated within the first 0.09 s of the interface temperature evolution. The prediction error ranges from 1% to 35% for the specimens overmolded at high and low melt and base temperatures, respectively.
Highlights
In recent years, the integration of various secondary operations in the injection molding process was proposed to satisfy the needs of the industry, primarily when the business is based on high volume production
The joining between parts can be attained in various ways: by mechanical interlocking, where the molten material fills in undercuts on the insert; using adhesives, as in the case of the in-mold-bonding; exploiting an activation process, such as plasma or flame; or using direct welding between plastic parts [4,5,6,7,8]
The negative effect of the packing pressure was unexpected, since it is known that pressure contributes to increasing the degree of contact [31]
Summary
The integration of various secondary operations in the injection molding process was proposed to satisfy the needs of the industry, primarily when the business is based on high volume production. One of the main goals is to join two or more parts during the cycle, and for this purpose, the insert-molding or the overmolding process in its variants, from 2 K up to 4 K, is well known [2,3]. The overall advantage of these processes is to reduce secondary operations, such as post-welding, riveting, and gluing. For this reason, the maximization of the strength of the direct joining is of fundamental importance. The insert molding evolved into the overmolding of metal sheets for the manufacturing of polymer–metal composite parts (PMH) [9]
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