Technically, there's no doubt that roof structures — large, small and modular (small units to cover larger areas — CAN benefit from a twin-wall construction afforded by rotational molding, as the hugely increased flexural and torsional stiffness is a very welcome property for this application. However, as with any application, the designers, developers and funders must satisfy themselves that the financial figures also stack up when considering process, material(s), numbers, operating environment, building regulations (not least fire performance), affordability by the purchaser, cost of ownership over its lifetime and design options.
Rotational molding process is unlimited in end product applications, whatever the roof size or shape make sure you study the environment in detail in which you plan to place it and design it accordingly based on all the controlling elements and the materials available to you in the rotational molding process, limited materials are available vs. the other plastic processes. Always know all of your options, cost, and market demands before choosing you end product process. Keep in mind if the rotational molding process does not fit for the long haul sometimes it will fit for the short run, i.e. (R/D phase, minimum capital on the front end, and allows for initial market studies).
The major culprit that I've found over the years, for electrostatic discharge during injection molding is caused by the friction of the plastic pellets through the delivery hoses for the hopper. The injection molding machines should definitely be grounded with copper rods in the floor and I believe they should reach nearly to the water table for optimum grounding. I have seen the "lightning bolt" jump about 6 inches between a hose and material handlers head when he was working near the hose while it was loading the hopper with PP. Obviously we weren't using the grounded hoses that have the internal grounding wire, which must be exposed to grounded metal at the connection to the hopper. Another source of shock for the operators is when molded parts (esp. PP and PE) are conveyed to a plastic tote or bin. You can see the small parts try to "climb" the wall of the container as the charge builds. I've attached bolts to a grounding wire to the conveyor and put them in the totes to help dissipate the charge so operators wouldn't get zapped when they reached into the tote.
It is no surprise to me that its uptake over the last three decades or so that it's been generically available has been limited compared to injection molding process simulation. The latter process involves very high-cost tooling, with (typically) many weeks of lead-time and is highly complex, impossible-to-see and susceptible to hugely expensive mistakes. In contrast, most thermoforming projects require low-cost, short lead-time, relatively simple tooling and you can see the forming process in real time. Mistakes of poor tooling and/or part design can be relatively quickly resolved. Ratios of cost of simulation studies to total capital expenditure are much more favorable for injection molding than thermoforming. Ditto for time of simulation studies to total lead-time before appearance of first-off samples.
This does not mean that I do not support the use of thermoforming process simulation: it can certainly allow more efficient use of material and energy. You can “tweak” your design to make it more process-friendly as well.
In the commodity products area, there are lots of production buckets, pails and containers co-injected using the method described (Two shot), because it can be adapted to most any injection molding machine. Savings realized not only on the resin but color and additives like UV inhibitor. In many cases, the less expensive core constitutes over 60% of the total part volume.
More exotic (or lower volume) applications include soft over rigid, like a TPE over anything with some modulus. Also, structural foam parts with cosmetic skins; likewise, fiber or glass filled parts with cosmetic surfaces.
Co-injection or overmolding for a "soft-touch" or for a cosmetically clean surface over a fiber reinforced core is understandable. There is a value added feature there.
As for the quantities. 1000-10.000 units/ year might not be enough to justify investing in an injection molding tool. Depending on the complexity of the parts (e.g. undercuts with slides...) your mold can get rather expensive. While the finished products might not necessarily have better properties than your current one.
As a guideline I would not invest in hard injection molding tools if the part quantities are below 10,000 a year. Between 5,000 and 10,000 we sometimes stick with an aluminum mold that we design ourselves, and build ourselves. Where we just outsource the milling. If you study mold design a bit, this is doable + gives you a tremendous amount of knowledge for future injection molded parts that need to be designed.
The process consists of the following steps, but the detail may vary depending on how formal you want to be about the project:
- Establish the cost incentive using quotes or cost estimation tools.
- Understand everything about the function of the "target" component to be replaced.
- Develop thermoplastic concepts which reflect the required functions, including environmental considerations, temperature, structure, etc.
- Validate the preliminary concept using simulation tools.
- Validate the cost incentive - make sure the project is a net cost save given the geometry, material, and annual volume (i.e. tooling amortization).
- Finalize design.
- Validate with testing.
Just as well I would recommend you that any software you must verify first an structural analysis prior to validate your injection simulation to ensure you will not find bubbles, air trap and weld lines in areas of high or low strength. And also is very important to document the evidence you found, and take care later in a meeting with the tolling that will develop the tooling and take this into account in the development of possible new flow leaders, vents or put some specific flow to improve special parts of your model, don't forget to invite, or call to your resin representative, because they have some tunning recommendations regarding final gate designs as well processing guide lines.
We are investigating an alternative method of making 200 small flat curved parts that are symmetrical relative to the midplane. Both sides are identical so the two mold cavities are identical. We are looking for an easy to cast material that has strength/toughness properties as HDPE or even as UHMW PE. The parts should never break but are allowed to bend (somewhat or even a lot) under exceptionally heavy loads. It can be a net casting or machined after casting but tolerances are not important as the parts are not attached to anything.
One of your choices can be Casting Polyurethane. Based on the situation, you must use Polyether - TDI base casting material if the environment is wet or these parts will work in water. This type of Polyurethane has moderate to high tensile properties and excellent resistant against water. But if you need a high property such as tensile strength or abrasion resistant, it's better to use Polyester- MDI or Polyester - TDI. You can use every colorant for this purpose.
I work for a company that sells Moldflow and provides Moldflow services nationwide, but we have been seeing that Moldflow is making a move towards the front end of the design cycle and more design engineers are getting involved using simulation up front. It can help them in a number of ways, and it is much more cost effective to do your simulations early on. With tool makers doing less of it, and simply moving to creating molds per the specs of the designers, many make additional money off reworks - which everyone is looking to do in this economy. Thus, companies are taking back control of their designs...it is an interesting shift from the way Moldflow was used in the past vs. how its usage is evolving.
Some suggestions for effectively using in-house scrap, whether for injection molding, extrusion, vacuum forming (which generates a lot of offcuts).
1. For companies producing multiple products, a good strategy would be to use up in-house scrap in components/molding with lesser service severity requirements molded from the same polymer. e.g. scrap from moldings of high aesthetic requirements could be used up in molding where aesthetics are not so important/darker in color. Load bearing molding regrind could be used up in non-load bearing items.
Sometimes it is safer to use up all the regrind in a dark, non-critical item if such an item is in the manufacturing mix in sufficient numbers to be a scrap sink.
There is a bit you can do to help improve adhesion between PC and TPE; two materials that don't typically love each other. I am working on a fix right now on one of our products having just this problem.
- Make sure the material has been dried properly to obtain the best adhesion possible (which may not be great).
- I believe you should expect better results in a two shot process as the initial shot is still warm when it receives the TPE attaining a better bond. In an insert mold operation, the part is rarely warm and sometimes has had the opportunity to take on moisture if it has been sitting around.
Slow cooling relieves internal stresses of metals. Plastics, especially ones with more crystaline (less amorphous) structure shrinks more as they cool slowly. Slower cooling promotes more internal crystaline structure, which is more brittle and shrinks more. If you want a more dimensional stable part, when the injection mold opens, drop the part into cold water (it will have less internal stress. Think of the melted plastic having no internal stresses. Then as it cools in the mold with varying rates, stresses are formed. Plastic parts shrink more when then they cool slowly.
Slower cooling does usually relieve some of the molded in stress. Many products are annealed after injection molding just for this purpose. The other factor that affects the linier stress is the velocity. At slower velocities a greater amount of stress in the direction of flow occurs. Of course faster velocities used to achieve a consistent viscosity will tend to entrap more air. Venting is very important. There must be vents along the entire flow path, not just at the end of fill. Weak weld lines may also become more evident after annealing, shrinkage will increase.