Pressure Forming 101
This checklist is a guideline for the design and development of pressure formed parts in order to facilitate manufacturability, quality assurance and cost containment. There are several aspects in designing pressure formed parts which should be considered in product development. Attention to these aspects in the design and development of a pressure formed part will help provide a part which can be manufactured to the highest quality in a cost-effective manner.
- Part Size and Draw Ratios
- Surface Radii
- Draft Angles and Part Shape0+
- Undercuts and Ribbing
- Textures and Finishes
- Part Dimensioning
In looking at materials, consideration should be given to the environment the part is exposed to including U/V, temperature and chemical exposures as well as structural requirements. In addition, tolerancing requirements should be taken into consideration since some materials are not as stable and tight tolerance will be difficult to achieve. Cosmetic requirements should also be reviewed when selecting raw materials in order to accommodate necessary esthetics such as color, textures as well as paint, shielding or part marking. Different raw materials will have varying positive or negative effects on the final product with respect to cosmetics and finish.
Common materials include: ABS, Flame Retardant ABS, ABS/PVC, Acrylic/PVC, Polycarbonate, PETG, Polypropylene, Polyethylene, and HMWPE.
Part Size and Draw Ratio
While both large and small parts can be produced with the process, Pressure forming tends to be most competitive in medium to large parts, due to the competitive nature of small injection molded parts produced in very large quantities. One factor in determining cost is the draw ratio of the part being designed. This is the relationship between the sizes of the opening at the top of the tool and the surfaces the material must cover inside the tool cavity. For example: a female tooled part 12” L x 12” W x 6” D has a 12”x12” (144 sq.in.) opening at the top of the tool and the sheet material must be stretched down all 4 sidewalls 6” (4x (6”x12”) = 288sq.in.) and across the bottom 12” (1x (12”x12”)=144sq.in.). This area of 288 + 144 = 432 sq.in. must be covered by the 12” x 12” (144 sq.in.) of material at top opening of tool. This represents a draw ratio of 3.0 (432sq.in. / 144sq.in.). If a nominal wall thickness of .100 was desired on the finished part the starting gauge of material need to form the part would be approximately 3.0 times the nominal wall thickness or .300 thick.
Although a 3 /1 ratio is an industry maximum for manufacturability, parts with greater draw ratios can be achieved through alternative process techniques and engineering adjustments. This information is also used to determine the estimated material cost requirements which can be economized by taking these factors into account in the design process.
As draw ratios increase so does the need to manage formed-in corner radii in order to: lessen the thinning effects of deep drawn parts, reduce stress and increase strength of sharp corners, as well as reduce costs associated with material starting gauges required to form into deep cavities with sharp corners. As a general rule, the greater the draw ratio the greater the radii that are required on the part. Parts that are 12’ deep or more should have radii of .50 or more depending on details. Conversely, sharp radii in shallow parts (less than 3”) can achieve as tight as .015. Designing sufficient radius in the part will not only increase the strength and esthetic quality but reduce overall cost associated with material thickness reductions in the require material starting gauge.
Draft Angles and Part Shape
Draft angles and part shape are an important factor in releasing parts from the tool as well as promoting consistent forming characteristics. Pressure formed parts are generally formed into female cavities which require less draft than forming over male’s tools. This is because in a female cavity the part shrinks away from the tool surface and parts formed over male tools shrink to the tool. The general rule it is suggested that 1-3 degrees of draft be use on female cavities, depending on texture (if applicable) and 3-5 degrees on a male tool. When designing a pressure form part consideration should be taken for the relationship between part shape and formability. Draft angles, draw ratios and radii all work together to increase the formability and manage material costs. Angles less than 90 degrees can create costs due to added material thickness requirements, additional backfilling and fabrication requirements and quality issues due to material thinning and esthetics. Managing draft angle also allows for the nesting or stacking of parts to accommodate shipping constraints or functional use in conveying or warehousing applications.
Undercuts and Ribbing
The integration of undercuts, flanges and ribbing in pressure formed parts not only allows for detailed and controlled locating and mating of design lines and critical surfaces but also adds strength and rigidity to surfaces and corners. Typical undercuts should not exceed .375” especially where adjacent radii are less than .125”, undercuts less than .125” can be formed with sharp radii. Ribbing is typically used on large parts to create rigidity across flat surfaces or corners and designing in crowned surfaces with as little as .020/in rise will also create rigidity across the crowned surface. While ribbing and crowning are very simple to incorporate into thermoform tooling, undercuts will require additional tooling development in order to be automated for large production or designed to be manually loaded for small or short run production requirements.
Textures and Finishes
Pressure forming allows for the forming of specific textures directly onto the part surface. While many textures are formable, fine textures .002” deep or less or not recommended as they can “wash out” in areas and become inconsistent. Course textures of .003” deep or more will form with better detail and consistency. While formed in texture may eliminate the need for painting, there will be fallout due to inability to repair any damage in processing which might affect the textured surface. In addition, minimum quantities of custom colored raw material could be cost prohibitive, depending and quantities and colors required. The other option is to paint the required surfaces; this option creates greater yields and also allows for refurbishing of parts in the field. Painting also gives the flexibility to produce endless colors in small quantities, and creates a finished look that can be matched to adjacent parts made from other process such as powder coated sheet metal.
Other coating such as EMI / RFI shielding are typically use in electronic applications to control radio or electronic emissions. If part marking is required, very course textures well have an adverse effect on detailed printing or screening.
In designing pressure formed parts tolerancing can play a key role in manufacturability and costing. That said over tolerancing a formed and trimmed part can make a simple part costly or even non-manufacturable. Always create dimensional datum points from tooled surfaces or machined surfaces only. There are 2 basic tolerancing positions to consider: formed in tolerances which will be larger and machined in tolerances which will be smaller.
Standard Formed Tolerances:
- +/- .030in up to 12.00in plus an additional +/- .001in for each inch thereafter
Standard Machined Tolerances:
- Tool Surface to Machine Surface +/- .015in up to 12.00in plus an additional +/- .0008in for each inch thereafter.
- Machine Surface to Machine Surface +/- .010in up to 12.00in plus an additional +/- .0008in for each inch thereafter.
Closer tolerance can be achieved in specific areas of a part with use of dimensionally stable raw materials as well as upgraded tooling and process control. Designing flexibility in attachment of pressure formed parts to the mating frame or base component will reduce the require tolerances for functionality and control costs and quality in the manufacturing process.
In creating part drawing or cad file it is necessary to determine what the critical surface is, inside or outside. This decision will determine whether the part will be formed with male or female tooling. Once that is determined all dimensions should datum off of the tooled surface. Dimensions taken from non-tooled surfaces will require extra tolerance or should be dimensioned as reference only.
These are guidelines only for purpose of helping in the design and development of pressure formed parts. Specifics part details and features should be discussed with your manufacture in order to address all the requirements of the part to optimize quality, productivity and cost.
Plastics Design & Manufacturing