Introduction
Stretching molding is a common manufacturing process for forming a variety of shapes and sizes of plastic products. The process involves forming a plastic sheet into a mold, applying heat and pressure and allowing the sheet to cool and harden, thereby creating a solid plastic product. While stretching molding is a reliable and cost-effective way to produce plastic components, there are several common defects associated with the process.
Types of Defects
The most common stretching molding defects are blisters, splay or splash, weld lines, and air entrapment. Blisters are protrusions on the surface of a part, and are caused by trapped air bubbles and pockets of trapped pressure. Splay or splash is when overly applied pressure causes plastic to be extruded out of the mold, and is typically seen in parts with fine details. Weld lines, which occur when areas of molten plastic don’t completely fuse together, can also appear in areas with fine details. Air entrapment occurs when, due to incorrect gate locations or high injection pressures, air is trapped inside the part and causes air pockets and voids.
Insufficient Part Filling
Insufficient part filling can occur in cases where the plastic material is not filled with enough heat, pressure or time. This can lead to parts that are too thin or too weak, or that may lack details or have an uneven wall thickness. While this is not technically a defect per se, it can lead to parts that are not practical or useable because they don’t meet the specifications required.
Overmolding
Overmolding occurs when parts are formed with too much heat and pressure, causing them to take on a distorted or excessively curved shape. Overmolded parts can be too large or too thick, or may have sharp edges that can cause injury if handled. This can be avoided by using lower temperatures and pressures when forming parts, as well as using appropriate mold materials that can better support plastic shrinkage.
Poorly Designed Molds
The design of the mold is an important factor in ensuring that plastic parts are formed correctly. Poorly designed molds can result in parts that have inconsistent surfaces, inconsistent thickness, and are difficult to remove from the mold. This can also lead to parts that are off-spec, and are not usable for the intended application.
Conclusion
Stretching molding is an effective and reliable way to produce plastic components but, like any process, it is not without its defects. Common defects that can occur include blisters, splay or splash, weld lines, air entrapment, insufficient part filling, overmolding, and poorly designed molds. Taking the appropriate steps to ensure that a mold is properly designed and the correct materials and processes are used can help reduce the potential for defects and ensure a more successful result.
