Molded Parts and Mold Design July 29, 2009
Introduction Importance of Proper Mold Design Design Considerations Overview of Design Process SolidWorks & Mold Design
Overview of Mold Design Most common method of creating plastic parts Tooling is expensive Mistakes can be catastrophic Complex geometries are possible
Design Considerations Simplify design (KISS principle) Avoid sharp corners Avoid stepped parting lines Material shrinkage Possible cooling system required Fluid flow
Process Plasticizing melting the plastic Injection filling the mold with the melted plastic Chilling removing heat from the melt to bring it back to its solid state Ejection removing the part from the mold
Plasticizing The raw material is called resin and is usually in pellet form first and than melted. Different materials have different thermo characteristics. Melting Point Chilling Stage
Injection The molten plastic is forced in to the mold filling the cavities. It is desirable to have a fast injection phase. The last 15% of the injection process is pressurized to ensure the molten plastic fills the entire mold.
Chilling Temp Depending on the T1 material being molded t1 the rate of cooling will change. time Stay out of degrading area. T2 Degraded material have poor properties. Degrading All materials show degradation by changing color to yellow or brown or black. t2
Ejection In order to be separated the mold needs to be in at least two halves. Care must be taken in the design to ensure none of the part is locked in.
SolidWorks Draft Analysis Undercut Detection Draft Scale Parting Lines Shut off Surfaces Parting Surfaces Tooling Split Core
Draft Analysis Although you can use the Parting Line tool to check for draft this will become problematic with more complex parts so to avoid issues use draft analysis. Since thermoplastics shrink as they cool they grip to cores or male forms in the mold making normal ejection difficult if draft is not included in the design. If careful consideration is given to the amount of draft and shutoff in the mold it is often possible to eliminate side actions and save on tool and maintenance costs.
Undercut Detection Identifies areas that require a side core. As seen below a simple mold with just two halves cannot work so a small core slides out perpendicular to the main cavity leaving the phone clip hole. Hole for cord clip Undercut faces highlighted in red
Draft Scale First, verify that all faces include sufficient draft with the Draft Analysis tool. (Draft is a slight taper on selected model faces that facilitates removal of the part from the mold tooling.) Direction of Pull defines the direction that the tooling body (cavity or core) is pulled to separate the mold. It controls the direction in which the draft angle is applied
Draft Scale Compensates for plastic shrinkage. Scales only the geometry! (Not dimensions, sketches, reference geometry). Disadvantage in SolidWorks compared to other 3D modeling software is that you have to type in the shrink factor and there are no suggested values for different materials.
Parting Lines Parting lines separate the core from the cavity. All faces must draft away from the parting line which divides the core from the cavity. Cavity side surfaces must display a positive draft and core side surfaces must display a negative draft. All surfaces must display a draft angle greater than the minimum specified by the design specifications.
Shut off Surfaces To cut the tooling block into two pieces, you need two complete surfaces (a core surface and a cavity surface) without any through h holes. Shut off surfaces close up the through holes. Complex through Shut off surface patch Shut off surface patch tangent hole. tangent to the walls ofthe to the surface that the hole hole. penetrates.
Parting Surfaces Parting surfaces extrude from the parting line and are used to separate the mold cavity from the core.
Tooling Split The surface where the mold will split into its two halves. Interlock Surfaces The interlock surface surrounds the perimeter of the parting surfaces in a nearly perpendicular direction at (in most instances) a 5 taper. Seal the mold properly to prevent liquids from leaking. Guide the tooling into place during the molding process. Maintain alignment between the tooling entities.
Core Molds from two solid halves restrict the geometrical possibilities for the part. More complicated parts require geometry extraction from a tooling solid by using a core feature. Side Core a hole in the side of a core, the faces of the hole are undercut faces (trapped areas that would prevent the part from ejecting from the mold).
Conclusion Creating molds is expensive Using SolidWorks or a similar 3 D modeling program can prevent the unnecessary production of bad molds Draft Analysis Undercut Detection Draft Scale Parting Lines Shut off Surfaces Parting Surfaces Tooling Split Core
Questions?