Many oversea buyers intend to send their inquiries to a bunch of Chinese plastic injection mold makers before they placed order to one of them, choices sometimes just simply made base on price. If you are lucky enough, you could probably get a qualified molded product at the cheapest price. But what about the next time? Your business success should not be hanging on odds.
Whenever you get a quotation from a China mould supplier, you better have your own sense to estimate if it's competitive or reasonable or it's over-charged. It's very much necessary to understand the actual mold price in China.
Cost of a plastic molded part includes raw material, injection molding, labor cost, packing, transportation, management, and profit. Let ‘s get into each items one by one.
Plastic parts produced by the injection molding process are usually long lived consumer products. It's suitability depends on the properties of the finished part and surface quality, it required a lot of experiences of raw materials, processing condition, injection molding parameter to get optimum results. The reasons to cause problems could not be a certain answer as the processing condition is always different, on the basis of an individual case a decision has to be made as to whether simple and rapid fault correction is possible or whether intervention is the design of the parts or the layout of the mold and gating is necessary.
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.
I've never relied on a mold company salesman or rep to try to explain what is the best choices for my company or my customer. I find that 95% of the time the mold salesman is just that, a salesman with little or no practical knowledge of the molding process or industry and certainly not the best source of information that works best for my plant or my customers' plant. There are a lot of companies that rely on the mold maker to decide what to quote and how to quote it. Including what materials, components to use and design criteria. If one goes this route and allows each supplier to quote the job as they see it, you'll end with a wide range of prices, and probably quire different molds from each. Now you need to compare apples and oranges and sense out of it.
Buyer and supplier always hunting each other, there is always a good mold supplier with reasonable price somewhere, that's why we need to do more sourcing and survey, nothing is VERY cheap and VERY good, only reasonable price, a number accepted by the mold buyer and supplier both party. Take it, leave it or go somewhere else.
You need to watch what you get and follow up with what you ask for. That is what we do at Aco Mold, we have such a large group of experienced staff here stateside to manage your tooling and molding projects so that you are getting what you expect. It can be difficult at times but we travel there and build relationships to better the outcome for your companies.
If the resin is in powder form then used high power magnets to remove them, and if it is in the form of granules then melt it in big volume in any bucket type (in this way metal parts will moved down in bottom) then in its molten state put it in extruder for re-granules or processing (molding).
Heat transfer from inside a thick molding will be far more limiting compared to heat removal from the tool. That is why conformal cooling or chilled air or more common systems that use nitrogen (very common in Germany) will not produce any substantial reduction in cycle time. Tool side cooling technologies work only when heat is delivered to tool fast enough and that is only in small thickness parts. Having rapid chilling can produce stress.
One thing that may really benefit with cooling time reduction will be to use as low as possible melt temperature. If melt is reduced by 20-30 deg. C you will get amazing cycle time reduction (combined with as high a practical ejection temperature). Look at frozen skin criterion for ejection, not text book % and adjust core cavity temperatures (differential) so that highest temperature is in geometric mid plane of part.
Gas Assisted Injection Molding chairs have been produced successfully for the last 20 years. My company (Aco Mold) has recently supplied gas assisted molding equipment for a customer in Brazil who is making Stadium Seating for the 2014 World Cup. We also supplied all of the gas assisted molding equipment for the London 2012 Olympic Stadium Seating and the Kiev Stadium seating for the last Euro championships. We have one customer in UK who is making all polymer (PP) chairs for schools and offices using our equipment. The MOST IMPORTANT thing is to get the chair properly designed for the gas assisted injection molding process.
However, it is very unlikely that it will be cost effective to modify an existing chair mold to a Gas Assist part. To get the best out of the process you would need to re-design the chair using a new mold tool. The use of "box beam sections" with gas assisted molding does enable you to reduce the General Wall Section of the part in many cases (stadium seating is a good example) which would offer a potential overall weight reduction.
Your Part size and Weight will also determine Press Size which will narrow the selection of manufactures, you will see that among your list of plastic injection molding machines, there are some that are very well suited for smaller parts whereas other excel in very large tonnage presses. The Resin used in your applications will also determine your choice of Barrel specified on your selection of the injection molding machines, this will relate to your part weight as it must be calculated to resident time in the barrel.
Because inserts are manually placed by an operator before the injection cycle you blow the tool surface with cold air, increase the residence time (furthermore not the same amount from one shot to another!) and bring potential pollution through the inserts and manual operation (even though operators usually wear gloves in this case). This makes a good number of possible root causes. I'd see the "over-shearing" as an interesting one to investigate (after moisture content of course) for 2 reasons: the PA12 you use has a high viscosity / low fluidity (it is hard to inject) + it first flows around an insert, that is made of metal. Trying to pre-heat these inserts (at least on 30 consecutive shots), this should ease the material flow near the sprue.
You are not alone with your difficulty finding talented injection molding technicians. With the industry down turn cycles and mass closing of many injection molding companies
most of the junior technicians got laid off and found work in other industries. Now that our industry has taken a positive upturn there is no one in the talent pool to fill these roles. I blame the bean counters for this problem. They sacrificed the technical floor personal to secure their profit margin, knowing that at some time in the future the industry would pick up. Now they are scratching their heads wondering what they are going to do.
There's a big difference between making a good mold and saving money on manufacturing. You can make a good mold anywhere in the world. However having a good mold made is only a small part of the equation. It takes 6-12 weeks to make a tool. Material, product and production quality is a much bigger part and it can span years.
It's easy to save money on mold and lose tenfold on parts. What we do is make the mold in the most cost effective place worldwide but we always run production here in China. We tried to run production overseas only to come back with our tail between our legs after having to criss-cross the globe flying engineers to scratch their heads over supplier based failures spending weeks in hotels and eroding any potential savings.