Holes

Louvers, holes, grills and other openings through the wail of thermoformed parts cannot be provided during the forming operation. These openings have to be cut, stamped or routed as a secondary operation. In this regard, thermoforming is the same as blow molded, lay up or spray up reinforced plastic, rotationally molded and sheet metal parts. This limitation has always been considered as one of thermoforming's disadvantages in comparison to injection molding. However, the comparatively recent advent of computer controlled, three- and five-axis routing equipment such as COMPUTRIMSM has minimized this limitation.

These fully automated machines can often accomplish the full routing process as a part of the procedure whereby the formed part is trimmed out of the original sheet. In many cases the reproducibility of the location and dimension of the routed surfaces are better than that attainable with large parts from closed molds.

Thermo Pressure Forming tends to concentrate on parts which are produced in small-to-medium volumes. Routing side holes and other intricate shapes on these limited quantities is often preferred to the increases in tool cost that would be required to provide side cores, stepped parting lines and kiss-off coring on injection molds.

A side benefit of Thermo Pressure Forming parts with their machined openings is that they do not contain the inherent weakness and surface appearance problems that are present on injection molded parts where the plastic separates to flow around an opening and then reunites on the opposite side to create a weld line.

Routed openings through the side walls of Thermo Pressure Formed parts give the designer an opportunity to use his imagination. The sizes, shapes and locations of routed openings are almost endless. The size, position or configuration of these openings can be changed with no more effort than modifying the program. This allows the production of very small quantities of customized parts that would not be practical if mold modification were required for each special part.

A few different types of openings that are routinely provided on Thermo Pressure Formed parts are shown in Figure 16. The round hole shown Detail A is the simplest type of opening. This hole can be round or square or any other shape imaginable. It can be a straight-through or angled hole. Its side walls can be straight or beveled.

The holes shown in Details B and E2 are routed through recessed areas. This approach has several advantages. The top outside appearance surface can have a nice smooth as formed surface. A radius or recess or trim bead can be formed around the hole. The cut edges are recessed below the appearance surface. The walls that form the vertical portion of the recess provide reinforcement to the hole and make it stronger.

The inward projecting horizontal flanges on hole E2 can add even more strength to that hole. The vertical walls which can be routed to precision dimensions, can also be used to provide non-drafted bearing surfaces. Shallow inside threads can also be cut into these vertical walls as in Detail C. Shallow, course pitch threads can be formed during the thermo pressure forming process. However, they require an unscrewing mechanism in the die.

The two outward projecting bosses shown in Details C and D have the same attributes. However, they can provide other additional features on the outside of the part.

Small diameter, recessed holes of the type shown in Details B, C and D can be used as shallow, hollow bosses that can accept screws, ultrasonic inserts or other types of fittings. Long, smaller diameter hollow bosses and solid pegs are often added to Thermo Pressure Formed parts by adhesive bonding or heat sealing. These machined or molded bosses and pegs can be given additional strength by providing flanges, gusset plates or interconnecting ribs.

The openings described so far have all been provided in the top of the formed part. However, routing can give you holes through the side wall of a part as shown in Details E1, F, G, H and I without incurring any increase in the cost of the die.

If the final part is to incorporate a multiplicity of similar holes such as a series of louver openings, it is helpful to design these openings as shown in Details B or C. Openings of this type can be made with the router cutter simply passing across the top at the openings in one continuous movement.

The openings shown in Details A, D and E2 require the router to move to that location, stop, cut the hole and then move on to the next position, stop, cut that hole and then move to the next position, etc.

Blind holes or recesses of the type shown in Detail F can also be provided. The cut portions of the remaining wall could be shaped to selectively weaken certain areas. This approach could be used to provide a knock-out type feature.

Thermo Pressure Formed parts are normally produced using female dies. This allows the process to produce its exceptionally good-looking surfaces on the outside appearance walls of the finished product. There are no cores on the inside of a female formed part and as a result, these inside surfaces are free-formed. Inside dimensions are therefore dependent upon the size of the die, minus the final thickness of the sheet after it has been thinned out due to being stretched into its final shape. Precise inside dimensions are therefore difficult to achieve and maintain. It is common practice to provide critical inside dimensions and locating surfaces by the use of the same computer controlled routing procedures that are used to produce openings in formed parts. More often than not, the important dimensions can be established during the same procedure that provides the opening. In the case of large parts, these machined dimensions will often be more reliable than as-molded dimensions.