A little late to add anything about the difference between extrusion and injection molding, blow molding, but I want to add a practical market view including the factors of how much the molds cost and how many items are wanted.
Extrusion is a continuous process used to make sheet, pipe and profiles, film and coating, wire covering, filaments and fibers, feeding blow molders, and mixing/making pellets for other processes, including injection molding. Continuous extrusion dominates these markets -- there are ways to extrude into a closed mold, perhaps useful for low-volume applications. Almost all injection molding really does this, too, using a screw to melt the material, and then as a piston to force the melt into the mold. However, these processes are always called "injection," and the use of the phrase "extrusion molding" is confusing without further explanation.
We have manufactured a new Mold for an automotive part in our tool room. This mold consists of 2 cavity (LH+RH) with 7+7 lifters and 1+1 side core. The mold is quite a big one say nearly 2 ton. Now we are having the problem of Lifters getting jammed (scoring) and damaging the core insert as well. This scoring is really heavy. We could not produce a single good part till now. Because till the time there is half shot the mold works well. But when we fill the part completely then the scoring starts and the next shot cannot be done since the lifter has got this scoring and the machine cannot push the ejection forward any more due to lifter jam. The part material is PP. The lifters go in and out of the pocket smooth manually in mold dismantled condition.
We understand that with plastic parts, weight is the best method of controlling the process capability. Is there a guideline or standard for defining the tolerance?
The most common method is:
Shot to shot weight variation calculated as (three standard deviations) divided by (average weight) must be less than 1% and should be less than 0,5%. In general it is possible to achieve 0,1-0,4% weight variation using scientific molding.
What is the average operating temperature?
Material RTI value is critical for this
What is the peak operating temperature?
HDT is probably the most critical value
Is it wet or extremely dry?
Water Absorption rates can have an impact on mechanicals. Nylons have some of the highest rates.
Is it exposed to chemicals?
Chemical compatibility specifications must be considered
The undeniable, basic fact is that no-one really WANTS to have to purchase an injection mold. It is the plastic parts that it produces that are wanted and that generate revenue to pay for either the injection mold that made them, or the next mold. Strip away all the technicalities and consider the basic function of an injection mold - it is nothing more or nothing less than a shaped heat exchanger.
From the manufacturer's perspective: they need to be responsive, fast, and demonstrate their manufacturing expertise immediately. Turn-around time for small-to-medium orders should be really, really, fast. Simplicity -- no customs, no overseas shipping. Provide as much advice and expertise to their designs that will ease the manufacturing process. Production molds are usually built to last a long life with high quality steel and mold components view more plastic parts 50-800 Ton thermoplastic injection molding along with secondary operation services view. Mold price is unlikely to be a major sales point, but total cost (overseas shipping, extra lead time, excise tax, risk to product, etc.) brings the conversation back to favoring local mold suppliers.
Hot runner technology, with its advantages of low scrap rats, shorter the molding cycle times, has become an important direction of development of plastic injection molding process. In western countries, hot runner system are widely employed in injection molding process, in China mold making industry, hot runner system is also a trend since china injection molders face labor cost and raw materials increase.
There are many brands of hot runner system in the market, price, quality, delivery time, services are all need to be considered to choose the right one, Aco mold use different brands to meet customers' satisfaction, frequently to use hot runner brands includes Mold-master, husky, syventive, INCOE.D-M-E, etc.
It's a small plastic injected piece that will be submitted to short period of time (2 seconds) of high temperature (95-105ºC approx.) in every cycle; this high temperature is produced by a filament where electric current passes through (like a house light bulb but hotter) for no more than 2 seconds in the process cycle. A complete process cycle last for about 45 seconds where the piece has time to cool off again. This plastic piece has to work for about 45-50 complete process cycles.
I have used ULTEM (SABIC innovative plastics) PEI (polyetherimide) in high thermal service components such as hard drive parking ramps, insert molded side looking LED optics, motion sensor windows, and missile fuse optics in the past with good success. It also has high lubricity and it is easily used in molds designed for both commercial and engineering grade materials with shrinkage of 0.005 to 0.007in/in.. It comes in a variety of application grades with a 94VO rating.
Injection molding of phenolic has been around most of the 40 years I've been in the industry. Although it's not conventional (same as thermoplastic) equipment it is quite similar. It does take a special screw and a jacketed barrel, the basic concept is the same as thermoplastic injection molding and the same basic machine can be used with the proper modifications to the screw and barrel. In my opinion, this is the easy part and the part that is already being done by REHA, the complicated part is compounding the resin into a molding compound. Compounding is not really something that I would suggest a molder take on.
A lot of defects occur frequently can be minimized by good mold design:
- Sink mark
- Surface defects
- Unbalance filling
- Many other common defects
A good mold design can reduce the manufacturing cost as well as shorten the delivery time, it also increases mold quality, productivity and provides best performance stability.
Instead it is part thickness distribution with respect to injection point (gate) towards the extreme point on the melt flow path i.e, last fill point. A good injection molding part thickness should always decrease (not uniform) along the flow path and that thickness decrease magnitude is the prime factor responsible for sink marks.
Basically injection (processing) pressure and temperature are primarily polymer and then mold and machine dependent. i.e., injection melt state variables., So part form / features have very less to do with setting injection melt state.However a good control to prevent sinks can be achieved by increasing packing intensification rate, this technique can considerably compensate thickness distribution variance also, which is quite often.
if you have ANY draft, the force will only be a peak force at the instant of the core break away. Typically if we had a 4 cavity mold- we put one cylinder on center. This would be for the larger size nozzle. If we made 8 cavities we'd go with 2 cylinders out board near the guide pins. I can't ever remember building any of these with larger than 2.5"- 3" dia. cylinders. You might find some of these products at builders supply stores that sell building adhesives and epoxies for setting anchor bolts in concrete.