Guidelines and Best Design Practices for Tooling of Vacuum Formed Automotive Interior
Have you ever heard customers talk about the car interior as “Too much plastic”? Chances are they are thinking of a vehicle from a few decades ago when Polyvinyl chloride (PVC), Polypropylene (PP), Nylon, ABS were commonly used. In the last couple of decades, these materials tend to have Thermo Plastic Olefins (TPO) placed over the top of the plastic substrate to create a soft feeling in some of the automotive interior components. TPO skins are commonly used in the interior for applications such as door panels, instrument panels (IP), dashboards, armrests, and IP side covers. TPOs are now a go-to material when car designers want to give economy cars a premium feel. It is not luxurious like leather, metal, or wood, but it adds a soft luxury touch to the interior within a compressed budget. The biggest advantage of TPO is that it can be recycled completely.
In spite of so many advantages of TPO material, there is a need to carefully evaluate the design and manufacturing process for making automotive interior parts using TPO. This article lays down several considerations while designing the part using advanced graphical software such as CATIA.
Vacuum Forming Manufacturing Process
The process of manufacturing an automotive component that uses TPO material is primarily thermoforming, which uses a heated sheet of TPO that is fitted on a custom-designed tool using an external force that fits the plastic against the tool to form the desired shape. Depending on the type of external force used, this process is called Vacuum Forming, Pressure Forming, or Drape Forming.
Vacuum Forming is a type of Thermoforming, where a sheet of plastic is heated to a forming temperature, stretched onto a mold, shaped against the mold by sucking the air inside it (creating a vacuum). This process can be used to form heated plastic into permanent objects. You can learn about the basics of vacuum forming in our previous post here: Vacuum Forming Process Basics. One additional component of a vacuum forming process is understanding In-mold graining lamination (IMG-L) and In-mold graining skin (IMG-S). The TPO Sheet for IMG-L is plain, and the texture is formed in the tool, while the TPO Sheet for IMG-S comes with a textured sheet. It is important to note that during the (IMG-L) process, A-surface should be completely clear for a 3-degree draft, On the other hand, for the (IMG-S) process, B-surface should be completely clear for a 3-degree draft.
Vacuum Forming Tool Design
There are two types of Vacuum Forming tools:
- Male Tool (primarily used for Automotive components) provides design details, texture, and features on the inside of the part.
- Female Tool (primarily used for disposable cups, plates, custom trays, and others, and sometimes used in automotive components) offers design details, texture, and features on the outside of the part.
The typical process for the design of a Vacuum Forming Tool involves the following steps:
- Feasibility Analysis — This is an essential step for any engineering problem to decide the viability of the early fundamental design. It involves conducting draft analysis of the surface received from the customer (OEM) followed by radii and surface quality check to ensure that the part geometry meets the fundamental design guidelines for vacuum forming.
- Evaluate critical areas — A localized simulation or soft tool for certain critical areas is built to ensure that the secondary processes such as trimming, and edge wrapping are done easily. Critical areas include corners, deep draws, bumps, and other areas. In some cases, the soft tool is built to understand the wrapping of the part.
- Generation of A-surface — A-surface is extracted from the designed part’s surface (typically a styling surface). Any holes or pockets in the designed part surface are filled and then the extracted surface is extended as per vacuum forming design guidelines.
- Generation of B-surface — This surface is normally created by offsetting the A surface, for the most part, extensions are built as per design guidelines and merged at the base plane. This is also referred to as the tooling surface.
- Tool layout creation — After creating the A and the B surfaces, the overall tool layout is prepared using a combination of LH (left hand) and RH (right hand) parts. The purpose of the layout creation is to ensure that cost of material and tooling are optimized.
Nomenclatures and Design Considerations
While designing the tool, there are some important design considerations and nomenclatures to be aware of. Common practice is to tackle the nomenclatures first to help the product designers, and the tooling engineers/designers to get on the same page.
As shown in the graphic below, the A-surface is generally extracted from the part, with borders extending through the R-tangent of the part. The B-surface is offset of A-surface with the required thickness. Extension surface (extension of A or B surface) is built depending on the design considerations for skin tooling or laminate tooling. The base surface is the outer perimeter with a given offset dimension & given width throughout. Clearance surfaces are created after the extensions to reduce pressure on unwanted areas. The clearance surface is merged to the base surface which is held in the frame of the machine. Guide cones are placed at the corner of the base surfaces and used for minor adjustment of the gap between the A & B surfaces. The trim line is generally the total perimeter of the wrapping material to create a wrap of 10 to 12mm around the part’s inner surface.
It is important to note that each part being designed is different in shape, size, structure, and geometry. A common fixed design guideline is NOT possible but almost 50 to 60% of the basic intent of design remains the same.
Our Goken Expert:
Abhijit Kulkarni is a Senior Design Engineer at Goken’s India office. He has more than 20 years of experience in the automotive industry. He specializes in the design & development of interior & exterior trim parts. He has extensively worked with automotive OEMs in North America, Europe, and India. In his current role at Goken, he works closely with clients on tool and product design for several interior components.
Keywords
Automotive process, Manufacturing Process, Vacuum Forming, Thermoforming, Interiors, Premium feel, soft-touch
Sources:
https://journals.sagepub.com/doi/abs/10.1177/152808379302300109?journalCode=jitb
