Screw Boss Insert Prevents Sink Defects in Injection Molded Parts
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Solution Overview
Problem
Molded polymer components with screw bosses often experience defects like 'sink' due to differences in cooling rates, which existing approaches such as material modifications and 'dog houses' have not adequately addressed.
Innovation Solution
A method involving a polymer screw boss insert with a pin cavity and guide surfaces, where molten polymer material flows into side passageways around the insert, bonding it to the primary cavity and preventing rotation, and a self-tapping threaded screw forms threads for secure interconnection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a molded screw boss is formed in the injection molded polymer part, then the part can accommodate threaded fasteners, but sink defects occur due to differences in cooling rates between the main geometry and the screw boss
Solution Approach 1:
The screw boss is segmented from the main polymer body by using a separate insert component. The insert is positioned in a cavity within the mold tool, allowing the main polymer material to be injected around it. This segmentation enables independent optimization of the insert and the polymer body, preventing sink defects while maintaining the threaded fastener accommodation capability.
Solution Approach 2:
The insert acts as an intermediary element between the mold cavity and the final polymer part. By placing the insert in the cavity before injection molding, it serves as a mediator that defines the screw boss geometry while allowing the polymer material to flow around it through side passageways, ensuring proper bonding and preventing sink defects.
2Reliability
If the screw boss insert is positioned in the boss cavity portion, then the insert can be securely retained in the molded part, but the insert may rotate during the molding process
Solution Approach 1:
The insert includes non-circular surface features such as flat surfaces or protrusions that asymmetrically engage with corresponding features in the cavity. This asymmetry prevents the insert from rotating during the injection molding process, ensuring it maintains its correct orientation while being securely retained in the molded part.
Solution Approach 2:
The insert is preliminarily positioned in the cavity on a pin before the polymer material is injected. This preliminary positioning ensures the insert is correctly oriented and secured in place before the molding process begins, preventing rotation during injection and ensuring reliable retention in the final part.
3Strength
If molten polymer material flows into side passageways around the insert, then strong bonding is achieved between the polymer and insert, but the complexity of the mold tool increases
Solution Approach 1:
The mold tool is designed with side passageways that provide molten polymer material flow paths specifically at the regions where bonding to the insert is required. This localized approach ensures strong bonding at critical interfaces without requiring complex modifications throughout the entire mold tool structure.
Solution Approach 2:
The insert with its pin cavity is nested within the cavity of the mold tool, and the side passageways are integrated into the cavity structure. This nesting arrangement allows the complex bonding features to be incorporated within the existing mold tool geometry, minimizing overall complexity while achieving strong bonding.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method effectively prevents sink defects by securely integrating the screw boss insert into the molded polymer part, ensuring a strong and stable connection without rotation, thus enhancing the quality of injection molded components.
Implementation Method 1
positioning the screw boss insert at least partially within the boss cavity portion with the pin received in the pin cavity of the screw boss insert
Implementation Method 2
causing molten polymer material to flow into the cavity and into the side passageways along the outer surface of the screw boss insert while the screw boss insert is positioned on the pin
Implementation Method 3
The molten polymer material is allowed to solidify whereby the polymer in the side passageways bonds to the screw boss insert and retains the screw boss insert to solidify polymer material in the primary cavity
Implementation Method 4
a self-tapping threaded screw forms threads for secure interconnection
Data Source
AI summary
A method of forming a molded polymer part having a screw boss includes positioning a screw boss insert on a pin of a mold tool. The screw boss insert may contact guide surfaces of the mold tool. During molding, polymer material flows along the screw boss insert while the screw boss insert is positioned on the pin. The polymer material may bond to the screw boss insert. The screw boss insert may include one or more geometric features that prevent rotation and/or axial movement of the screw boss insert relative to the surrounding polymer material.