Design Considerations
Good parts begin with a good design. This design guide identifies broad areas that you need to keep in mind when developing part design and determining the most appropriate manufacturing method for production. The design of a part will often determine which thermoforming technique should be used, or if thermoforming should be used at all. Depth of draw, level of surface detail required, ribbing, fillets, stress concentration, shrinkage, expansion, and undercuts are all factors that must be carefully considered when creating your part design.
Depth of Draw
The relationship of a parts depth to its width is called the "depth of draw ratio." While most thermoformed parts are much shallower than they are wide, draw ratios of one-to-one are fairly common, and ratios in excess of one to one (where a part is deeper than its width) can be achieved with careful part design, tool construction, and manufacturing controls.
The depth a thermoplastic material is drawn is an important factor in determining the best thermoforming technique, and depth of draw ratio is a prime factor affecting the final average part wall thickness. For moderately deep draws, male drape forming will give more uniform wall thickness than straight vacuum female forming. For very deep draws or depth-to-width rations exceeding one-to-one, plug assisted female forming will often produce the most uniform material distribution. Using a "pre-draw" bubble to stretch material prior to forming can also extend the draw ratio for many forming approaches.
Reproducing Detail
For reproducing detail in the part, equal results can be achieved with both the straight female vacuum and male drape methods. Since the surface of the sheet which is in intimate contact with the mold receives the most detailed impression, it is the design of the part which determines the technique to use - provided other considerations are equal. As a rule of thumb, the male drape method should be used for inside detail, and straight female vacuum method for outside detail. It is important to remember, however, that the degree of gloss produced on a smooth surface is dependent on the properties of the plastic material used; it is not usually imparted by the mold surface. A poor mold surface can mar or detract from the finished surface of the plastic part being formed.
Ribbing
Another important design consideration is ribbing in the formed parts. Ribs can be placed to add rigidity to the part as well as to enhance the looks of the design itself. By proper ribbing, thin gauge sheet can be used successfully for a broad range of applications requiring rigidity, thus leading to a reduction in material cost as well as heating cycle time.
Fillets
In order to produce formings of maximum strength and serviceability, adequate fillet radii should be used. The radius should be at least equal to the wall thickness of the sheet, and never less than 1/32 of an inch.
Lack of adequate fillets will result in excessive concentration of mechanical stress. Engineering experience has shown that the service life and structural strength of a part may only be 30% of design when stress concentration factor is high.
In a structural part having any sort of notch or groove or any abrupt change in cross section, the maximum stress in that region will occur immediately at the notch, groove or change in section. It will be higher than the stress calculated on simple assumptions of stress distribution. The ratio of this maximum stress to the nominal stress based on simple stress distribution is the stress concentration factor "K" for the particular shape. It is a constant, independent of the material, except for non-isotropic materials such as wood:
K = Stress (maximum) / Stress (nominal)
Thus it can be shown that the maximum (or actual) stress in a given part under load is greater than the nominal (or calculated) stress by a factor "K". For many simple parts of a flat section without fillets, the value of "K" may be as high as 3.0 under bending loads.
Shrinkage
Shrinkage is a vital factor in large precision formings and allowances should be made for it in the part design for vacuum formed items. Shrinkage takes place in three basic forms: mold shrinkage, after-mold shrinkage, and in-service shrinkage.
Mold Shrinkage
When a thermoplastic material is heated and formed to a mold, shrinkage of the material occurs during the cooling cycle. The dimensions of the formed part, after its surface reaches a temperature at which it can be unloaded, are slightly less than the dimensions when first formed. This difference is referred to as mold shrinkage; it is usually expressed in terms of "inches per inch per F". It varies with the processing and design factors as well as with different materials.
Experience indicates that the shrinkage as related to final part dimensions is less critical with male drape forming. This is due to the fact that, while cooling, the material shrinks onto the rigid mold, thus retarding the shrinkage action. Although this phenomenon improves final part dimensions, it requires molds with proper draft angles in order to extract the part from the mold. Conversely, in straight female vacuum forming, the material shrinks away from the mold against the negligible resistance of the outer air.
After-Mold Shrinkage (Secondary Shrinkage)
After unloading, the part will shrink due to heat loss from the part going from temperature at unloading to room temperature. The hot part continues to shrink as the hot center or core of the plastic cools. This shrinkage ceases when temperature equilibrium is reached in the cooled material.
In-Service Shrinkage & Expansion
This is the normal expansion or contraction in the dimensions of an object which will result from changes in temperature and humidity. It is considered a significant factor only where tolerances are extremely critical, or where the formed plastic is rigidly fastened to a material with markedly different coefficient of expansion. Each type of thermoplastic material has a different coefficient of thermal expansion or contraction.
Undercuts
Undercut sections can be formed by use of hinged mold sections, cammed sections, and by loose pieces in the mold. Often used in female molded, pressure formed parts, undercuts can produce inset handle sections, mating lines, and a variety of other features that can be both cosmetic and functional.
Now you should have a good grasp of the various major factors that can effect part design as it relates to thermoforming. Next, please move on to our materials section for a good overview of how thermoformable sheet is made, the types and characteristics of the most commonly used materials, and the tests used to measure material performance.