Canadian Plastics

Thinking Thermoforming

A sk a Canadian engineer or manufacturing manager in our business what they understand as "processing" and you'll generally get "injection molding" or "extrusion" as an answer.

February 1, 2008   By Jim Anderton, Technical Editor



Ask a Canadian engineer or manufacturing manager in our business what they understand as “processing” and you’ll generally get “injection molding” or “extrusion” as an answer.

Those processes are the heavyweights in both image and pounds-on- the-ground, but they’re far from the only way to make money with resin.

Take thermoforming, for example. At it’s simplest, thermoforming involves heating a sheet of resin and using vacuum to draw the softened sheet into a negative mold. Post-mold trimming then finishes the part. Advantages are primarily the low cost of the tooling and the ability to make very large parts such as hot tub liners, small boat hulls, bathtubs, etc. Disadvantages include poor surface detail, waste and slow cycle times for large parts.

Matched metal molds using pressure either alone or with vacuum can address the detail issue, and using a “pre-stretch” technique can help mold deep parts like vessels or tanks. A popular version involves pre-blowing the soft sheet into a bubble, then enclosing the matched metal mold at a measured rate to produce a deep part with consistent wall thickness and good surface detail.

Multi-layer products are possible when coextruded sheet is used as the raw material, allowing a thin layer of expensive engineering resin at the surface and a cheap commodity grade behind for support. Pre-stretch can also orient the molecules in the bulk resin helping optimize physical properties to suit the application.

For simple, shallow parts like the ubiquitous PS coffee cup lid, the stunning speed of the process overcomes the need to use an expensive semi-finished input material, extruded sheet.

For big stuff with moderate volumes, like canoe hulls, thermoforming is not just affordable compared to injection molding, it’s the only way to market. This only scratches the surface of this process, but if your shop wants to bid on bigger parts or smaller volumes, thermoforming is worth a look. CPL

———Jim’s Buzzword of the Month: Aspect Ratio

The difference between the length and width of an object, usually expressed as a ratio or percentage. In plastics, usually refers to the shape of filler/reinforcement particles. Low aspect ratio (roughly spherical) fillers give physical properties that are the same in any direction, while high aspect ratio fillers (needle-like) can be oriented to give more strength in one direction or plane of the molded part.

———Jim’s Tip of the Month: Cleaning contacts on phenolic cards

Most “mother” and driver boards on modern computer-controlled equipment use phenolic cards with printed copper edge contacts that press fit into female connectors. Even a slight contaminant film can create intermittent contact or poor conductivity through the connectors.

A great way to clean the delicate contacts without chemicals or abrasives is to use a soft gum art eraser. Blow or wipe off any rubber residue and you have bright clean conductive tracks without risking the circuitry or leaving behind flammables or corrosives.

You need the tan coloured soft gum art eraser, not the pink kind found on wooden pencils. Staples/Business Depot, Grand & Toy and any art supply shop are good resources.

I used to source mine from the drawing board in the engineering manager’s office, but CAD means we’ll have to spend the two bucks on our own.


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