Some I/M myths
By Jim Anderton,technical editor
Here at Canadian Plastics I get the occasional phone call from entrepreneurs who want to get into injection molding, usually to produce a new product or invention. They typically want to spend ten to...
Here at Canadian Plastics I get the occasional phone call from entrepreneurs who want to get into injection molding, usually to produce a new product or invention. They typically want to spend ten to fifteen thousand dollars, set up presses in their garages, and become the next Rubbermaid or Samsonite. When I give them the rough guess about what it’ll really cost, the answer is usually stunned silence. Molds are a whole series of columns by themselves, but on the machine side of the equation, there’s a persistent belief that injection molding presses are simply giant heated syringes squirting resin into a mold. Part of that mythology is caused by the way we teach I/M basics.
Here are a few basic misconceptions that I encounter frequently:
A used “universal” press will get me going cheaply. If you need real production efficiency, there really isn’t a truly universal machine. Screw designs, ‘L/D” (more about screws later), machine throughput, shot size, platen dimensions, tiebar design, clamping, controls and other factors can only be optimized if you spec the machine at the same time that the mold is jobbed, or preferably at the product design stage. The only ‘cheap’ way to do it is to buy a complete job, contract, mold, press and accessories as a package, but if it’s so profitable, why is the owner selling?
Better to buy a bigger machine than you need to accommodate growth. This one sounds logical enough, but what about cycle time? Swinging a massive platen to mold swizzle sticks “six up” will kill profitability. Besides, a small shot size can be difficult to control, and the extra residence time that the resin spends in the barrel can bring its own problems. Just getting the processor set up to control a job that’s at the margins of a machine’s capability can cost a fortune in lost time. Think about just enough machine for the job, then trade up later.
All-electrics are better than hybrids or hydraulics. The injection molding machine doesn’t know what power source spins the screw, shoots the resin or closes the mold. If you’re molding heart valves under clean room conditions, doing away with hydraulic oils has a definite appeal. If you’re Canada’s leading golf tee manufacturer, you have essentially a commodity product, so it’s all about low costs and “pounds on the ground”, so you might balk at the higher price of all-electrics. On the other hand, with energy prices climbing, electrics or hybrids may make sense for some simple molded products too. No matter what the motive power, however, the right screw, controlled by modern software, shooting into a well-designed and properly cooled mold is still the road to what it’s all about: profitability.