Learning Center
The Society of Plastics Engineers
Materials Ease of Machining Chart
Rule of Thumb Wall Thickness Chart
As a general rule wall thickness should be as thin and uniform as possible. Part design, function and size generally will help dictate wall thickness and the material required. If the parts are going to be subject to any significant loads they should be studied for stress and deflection. Increased wall thickness, alternate materials and contours, gussets and ribs can be used to address many design concerns. Changes can bring with them other issues to address: sink, voids, and material flow disruptions. A gradual wall thickness transition (typically 3 to 1), coring, and proper radii are helpful tools here. Sometimes wall thickness must consider insulation (heat and/or electrical), impact, and Agency requirements. In the end the bottom line is to minimize the material used along with the molding cycle time.
Mold Design/Mold Making
The standard guide for molds is: “Customs and Practices of the Mold making Industry.” If you do not have it or fully understand it you can get it from SPI (Society of the Plastics Industry Trade Association) Plastics E-Store under the Moldmakers Division category.
Whether a buyer, a designer or a mold maker a key roadmap should always be a Mold Data Sheet. A proper Mold Data Sheet makes everyone's job easier by eliminating misunderstanding, improving focus and accuracy.
Typically the buyer of the mold should supply the specifications for the press, the material, the number of cavities, and shrinkage. You might be surprised to know that molds get built where they do not fit between the intended tie bars, or where the shrinkage is wrong—it happens. Whether the buyer or the mold maker is furnishing the mold design, the design review is extremely important.
Another area where close attention is necessary is in the class of the mold to be built. Depending on the anticipated production volumes there are molds that absolutely should be Class 101 (or Class 401) tools and others that should not. It is easy to find mold racks with tools that will never use but a small percentage of their total cycle life. Getting the mold class wrong can be costly.
Schedule is usually tight with mold builds. A common problem encountered today is issues with the part design files: draft, wall thickness, undercuts, and even a failure to factor in parting line and appearance/gating considerations. Files with issues are common. Prior reviewing the part files between the designer and mold maker can help minimize schedule and other changes down the line.
Other obvious steps that are helpful to the mold maker and ultimately the molder are that your product is Designed For Manufacturing and it has also been prototyped, tested and refined. Changes during or after the mold is built can be costly.