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Short Shot is a customer concern or symptom, and root cause could be one of many or combination of other process parameters. I have been out of injection molding for few years, but recall a DOE (design of experiment) completed in late 1990's to resolve the same issue. It could be mold temperature, injection pressure, barrel temperature, inconsistent cycle time (manual part removal).

Based on cavity pressure you will detect every short shot produced and will be able to sort out the respective parts as well. Additionally your people after a very short period of time will understand what they are doing and how to establish a stable process within a very short time. Have in mind your machine is not reliable as well as your operators are not. Cavity pressure monitoring will show you what an opened window or a changed injection speed, or a defective heater band for your quality produced means. So give them a try and you will learn a lot.
As an engineering and manufacturing company in the China specializing in plastic injection tooling and molding we have been battling what we call 'purchaser apathy' ever since low labor cost countries entered the market in the 80/90's.

Injection MoldIt has been too easy for the manager with the sign-off to pick the 'cheapest' quote without understanding what quality of engineering, tooling manufacture and partnership he is getting. The specifics of what we do does not lie now in actually 'cutting metal' this is largely computer controlled as "metal is metal" it is the service experienced before and after that adds the value.

What we find our customer value is us telling them no they can't do that because if they do it will impact total product quality or yes they can have an extra rib for strength in there but it will create a weld line right through their bose where the insert is going. Also find a company that has actual manufacturing on site as we do because it automatically aligns the customer and supplier goals (to make the tool as quickly as possible without compromising quality) why? Well because that's what we have here and if we quote a customer for tooling we must make it work and work to the best of it's ability for it to be a profitable and sustainable project.
Impact additives add strength but also add cost, and may reduce stiffness (rigidity) which will be important during stacking. The injection mold is built so you aren't going to change the dimensions. The key then is cycle time -- relates to available machine hours, your accounting system, financing and things like that. There is little virtue in running faster if you're not selling out the machine time.

There are plenty of lower-MFI grades that are stronger, but they will mold slower, even at higher melt temps and injection mold temps. What is the real cost of running more slowly? If the products are cracking in service, the current cycle time is not a goal but a wish. HDPEs for pipe, milk bottles, bags and the well-known (in USA) post office tote boxes are made from fractional-melt grades, but you may not have to go that low. If you do use these, beware creating flow-line weaknesses which will encourage cracks in some places and not others, based on mold design.
Regarding Continuous Compression Molding of plastic caps and containers, while talking about "compression" in general I'd be talking out of school. Continuous compression molding is a continuous extrusion process where the extruded plastic is cut in small gobs ("dose" o "pellet"), delivered into a cavity of a rotary machine (not index machine, it is a continuous rotary machine) and molded to the final shape.

This technology is extremely efficient time wise and energy wise and the parts produced are very consistent.

The plastic (HDPE or PP) is extruded at lower temperature than injection molding since there is no need to pump it through the hot runners and through the gate valves into the cavities. That saves power when melting the plastic and when cooling the part. Moreover colder parts means better dimensional consistency because you leave less to out-of-the-mold uncontrolled shrinkage.
In most cases injection mold shop will end up making "acceptable" parts by "shrink fixing" it. There goes all profit and residual stress is introduced.

I am asking Chinese mold flow experts- have you produced mold flow simulations with tools where either part thickness varies and or cooling channels are not uniform, or are not conformal with uniform pitch- how do you model uneven tool temperatures and uneven part thickness average temperature? Would you deliver a mold flow simulation with differential mold shrinkage so tool can be cut with compensation? Otherwise we are back to surrogate tool or prototype too?
On prototype mold worth remembering the tool material and cooling geometry can have vast difference in shrinkage and be disaster when proceeding to production tool from Al tool with minimal cooling.
I work for an OEM company, and we generally prefer United States injection molds for the following reasons. We have China injection molds in production now, but some of our experiences tend to make us prefer to keep tooling stateside.
  • Mold Design - We tend to get better designed molds in the USA. If the injection mold can be designed in the US and build in China, this has produced superior molds for us.
  • Lead Times - Maybe we just had a bad experience, but one injection mold seemed to take the slow boat across the ocean. The lead time wasn't competitive with a stateside mold.
  • Mold Trial - Generally not possible to be on hand for the initial injection mold trial in China.
  • Mold Modifications - We have gotten better/faster injection mold modifications when the mold was able to be sent back to the original mold maker in the USA. Sending the mold to China to be modified is cost prohibitive, so a 3rd party mold maker has to make the injection mold modifications.
I have traveled worldwide to many molding facilities in US, UK, Europe and China and found that it is quite common for cooling channels in injection molds to be clogged with various mineral deposits that accumulate over short periods of time. In some cases over a few weeks. This of course is a function of local water quality and whatever secondary filtering and/or water treatment that is at the injection molding factory. As you can imagine, the cleaning out of these mineral deposits can be very troublesome and typically requires the use of acids that are indeed risky to use on high quality precision tooling. It is a rather simple process to test for the increased % of flow restriction and may be a more valuable parameter to be used to verify cooling efficiency if indeed you are seeing variations in your parts that appear to be cooling related.
Injection Mold Mold price = materials cost + design cost + processing cost and profit + VAT + tryout cost + packing and shipping costs

The typical ratio is:
Materials cost: materials and standard parts proportion are 15% - 30% of total cost;
Processing cost and profits: 30% - 50%;
Designing cost: 10% - 15% of total cost;
Tryout: large and medium size molds can be controlled within 3%, small precision molds within 5%;
Packing and shipping costs: according to the actual calculation or by 3% of total cost;
VAT: 17%
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