Assembly
The assembly process involves taking a thermoformed part and assembling it into an application. Plastic assembly is different from assembling wood, metal, fiberglass and other materials. There are two major distinctions: 1) plastic to plastic, and 2) plastic to non-plastic.
For more detailed information about designing parts for assembly, see the Bayer Engineering Polymers Joining Techniques Design Guide and this Design for Assembly chapter of a Bayer guide (registration required).
For plastic-to-plastic assembly, the concern is attack on the plastics from the attaching mechanism. When bonding plastic together, the adhesive, glue or tape must bond and not attack the plastic. This section lists various adhesives that have been tested on Bayblend ET1000 material. If the bonding is mechanical, such as nails, staples, bolts or screws, then the concern is chemical attack. Chemicals such as cleaners, oils, grease and other materials that may be on the fastener could attack the plastic once bonded. The amount of attack is related to temperature, time and amount of stress as a result of the bond.
If the assembly involves bonding the plastic to non-plastic, there is an additional concern. When bonding plastic to a non-plastic material, one needs to account for the expansion and contraction of the plastic with changes in temperature. Plastics typically expand and contract with temperature changes more than wood, metal or most other materials. Thermal coefficient of expansion data (CLTE) of Bayblend ET1000 is 3.1 x 10E-5 in/in/°F (machine direction) and 3.9 x 10E-5 in/in/°F cross machine direction .
Plastics will expand and contract depending on the end-use temperatures. Expansion must be planned for, and the stress developed by the constraint must be evaluated under, the end-use environment. There are ways to allow for expansion. For example, use predrilled, oversize holes when bolting, screwing and attaching the plastic part. This is very important if attaching to other materials that may expand and contract differently. Also, allow the part to bow as it expands with temperature. This is a factor of attachment location and design.
Another fabrication consideration is notches. A notch can be a site where a crack starts. A notch can be from a mounting device such as a screw or staple used without a predrilled hole, or riveting/screwing so tightly that the plastic is cut in the process. A notch can come from routing, cutting or sawing a part out of the thermoformed sheet. These can sometimes be eliminated by sanding the cut surface or sharpening the tool. A notch probably will not result in a crack if the notch area is not placed under stress.
