Plastic products are used widely in our day-to-day life. In recent decades the use of plastics is increased by the end user due to demand of lightweight alternatives in automotive, aerospace, consumer goods, electrical and electronic sectors.
The plastic parts which we using in our daily life are injection molded parts which are manufactured in higher volumes. The regular using gadgets like wi-fi dongles, cameras, ear pods, wearables are made out of plastic parts by considering the user comfort.
The design of plastic parts can be done by anyone, but when coming for the mold design there are some constraints which has to be covered for better part quality. The following points are
1.Uniform Wall Thickness:
This is the main criteria to be considered while designing the plastics. Maintaining non-uniform wall thickness will results in uneven filling of plastic inside the cavity. The thinner thickness areas will fill first and cooled rapidly, while the thicker areas take some time to cool which results in warping and also some formation of air bubbles, formation of excess material inside the part, which will affect the aesthetic and quality of the end product. Sudden change in the thickness will results in short fill also the formation of sink marks, shrinkage and warpage of parts. It is highly recommended to maintain uniform wall thickness to avoid the issues and the curing time.
2.Wall Thickness Transitions:
The sudden wall thickness transitions of the part will experience different rate of cooling and shrinkage. This will cause warping In molding. Avoiding solid sections will results better quality of parts.
This is the most important design requirement for the plastic part design; draft angles are needed in part design to eject the part from the mold. Minimum 1° draft angle is needed for easy ejection of parts. If the design has no draft angle, the parts will be stuck inside the mold. Radius at the corners recommended for smooth ejection. Also, for a texture of more than 30 on VDI 3400 chart, we would recommend 3° draft angle. The below image will explain why draft angles are required.
Ribs are added to strengthen the bosses, walls of the also it is recommended to avoid warpage in the part. Rib thickness should never exceed or same as per the wall thickness. If rib thickness exceeds wall thickness which results in sink marks and cause filling problems. Maintaining 60 % of rib thickness to the wall thickness will avoid sink mark issues. Add minimum radius in the rib area for easy material flow. The recommended condition for ribs is shown below,
Adding radius will helps the material flow than the sharp corners. The radius should be minimum and within the ratio. Providing radius more than the wall thickness ratio will leads to a chance of sink marks (condition 2). If the internal radius is 0.5(t), the outer radius should be 0.5(t) + t.
6.Sink mark control:
Sink marks are depression in plastic parts due to excess thickness and uneven cooling. There are several ways to avoid sink mark issues in plastic design. Creating a recessed area near the bosses and ribs will reduces thickness in that area and avoid sink mark issues in the part.
7.Removing the undercuts:
Undercut is a feature in mold parts, which will affect the ejection of parts from the mold. They are protrusion, holes or recessed surface in parts which can be taken using some additional mechanism. Reduction in undercuts will make the mold cost efficient.
8.Material Design Consideration:
While designing the plastic, the material consideration should be important, every thermoforming resin has separate mechanical and chemical properties. Shrinkage rate will vary from material to material. This consideration will avoid issues in molding stage.