Injection Molding (Plastic)

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First point is size the crane to the MAX mould weight your largest press can support; you'll end up with that one mould that will be set in halves if you don't. Tie bar pullers for large machines. Standardize EVERYTHING! Water, power, valve gates, cores, air blow offs, weight blenders, mould mounting and ejection. I also recommend magnetic mould clamping. Think of material flow around the plant (raw material in and finished goods out the door) and machines (pallets, totes, boxes, racks). Consider keeping the smaller injection molding machines in the more land locked areas as a pallet can carry many different parts away but some 1000-2000t parts may only be 8 to a rack!! Data collection from the machines. Central drying and resin feeding.
Injection molding factory layout
Extend robot strokes to allow for auto-packing of parts. Climate control so people won't leave and the moulds in the factory sweat all summer plus summer/winter process parameters stay the same. Tie water temp controllers into press. I'd put chiller and tower to the injection molding machines for best cooling choice for that resin/part. Stay with one brand of press and controller.
Rapid Temperature Cycling, (also referred to as Rapid Heat Cycle Molding or Vario-thermal Molding) will enable you to heat the surface of the injection mold to 200C.

The technology uses Saturated Steam to heat the injection mold and Tower Water to cool the injection mold. At the end of the cooling cycle Compressed Air is blown through the injection mold cooling channels to remove the water. Steam at up to 235C is then flowed through the cooling channels to heat the surface of the injection mold PRIOR to plastic injection, the steam is subsequently removed by Compressed Air followed by cooling water to remove the heat from the plastic.
Labor: A whole line would require only two people (the whole line is compounded by compression molding machine, folding, slitting and lining (EVA) machines).
Maintenance: The compression molding and other lines machines have a different but, in my opinion, easier maintenance. The mold centenarian is also easier due to you deal with every cavity separately, no frame, no plates.
Quality: The compression molding machines has a per unit quality inspection on line, discarding any off specs closure from the production line - That is not possible using injection molding process - Also the closures has minimum stress compared with injection molding.
Raw materials: the common resin is PP, it's better to use a lower melt flow index but the range suitable for this process is wider that the required for injection molding.
Productivity: At the end of the line you will have a bigger and better production with compression molding compared with similar cavitation molds using injection molding. A usual rotary machine may have 64 cavities and its production may be as high as 60 MM caps/month.
Costs: Also at the end of the month your production costs will be lower using compression molding compared with Injection.
Market: Most Soft drinks producers and water bottlers around the world know and trust in compression molded closures.
Based on the grade you are using, check for the coefficient of linear thermal expansion (CLTE). It will depend on at which stage your weld lines are formed. If you have a high molecular weight material and the weld area is during the initial stages of fill, the weld strength may be good in both climate. If your weld area is towards the end of fill, a grade with higher elongation property might help. If you face the problem in an assembly, you have to check the CLTE of both the matching surfaces. Without having the complete details, it would be difficult to provide appropriate solution.

When you are molding a part, the mold temperature and melt temperature may be same irrespective of climate. So the environment at molding is uniform (assuming that the material is pre-dried if it has moisture). If moisture is present, the weld strength would be low and at the same time you will notice surface defects. The molecular chain would break and the material strength would be low; as good as low molecular weight material. The stresses developed due to climate change can cause problems. Thus, you have to analyze the problem whether it is in individual part or in an assembly.
There is no 1 rule of thumb. It will depend on several elements;
  • What type of Polymer and how they degrade (heat stability and moisture content)
  • whether filled and type of filler. Glass length will degrade making more brittle, lower modulus
  • whether it is a blend, PC:PBT interact with each other (transesterification)
  • can you add heat stabilizers, can increase the level of regrind
  • what kind of shear are you putting into the mix
  • you won't try to color match or all bets are off on color stability
Dry Ice Blasting is usually a process used to clean even aluminum molds without showing a change in the mold surface dimensions. The tire industry uses this in some factories. The upfront capital is high. The clean up is nothing since the CO2 pellets evaporate.

The sand blasting equipment has lower initial costs. If space is not an object then create a room where the floor is the sand and multiple parts can be staged. Again these limits clean up because the sand stays in the room on the floor and is pulled from the floor when shot at the part. You are using the economical approach maybe just not on a refined enough scale.
With respect to suck back/ decompression, other than decreasing to rule out air introduction into the nozzle, also check that the final screw position before each shot is repeatable. A variable gain control on the valve (hydraulic system) can cause a large variation that can be seen by the screw position repeatability after suck back. If at times the suck back is not enough then drooling can cause material solidification at the nozzle interface. You may see this as silver streak as the initial injection pressure may spike slowing screw speed initially until the pressure overcomes the cold slug and blasts the material thru the gate causing silver streaks and burning as you have occasionally noted.

Nozzle misalignment or interface/ seating wear can also have the same effect as well as a poorly designed, screw, compression section or faulty check ring. For the check ring, I would check to see the screw final position at hold stage. You should have a sufficient cushion to allow repeatable shot size and hold pressure change, but not so much to have a high residence time of material in the screw, depending on your injection molding process conditions.
As far as drying goes you could dry the material for 12 hours but you need to make sure that the hose that goes from your dryer to your hopper is not too long. I have seen cases where the mold will run fine for a while and once you get into a cycle the material has a chance to sit in the hose and gather moisture. If the problem is a burning issue you may want to make sure that your nozzle orifice on the injection molding machine matches the orifice on the hot runner. If your injection molding machine orifice is larger than the one on the hot runner you are causing shear heat when the material passes over the edge of the hot runner inlet. Other factors that the hot runner may be causing is degrading of the material in the system.

You need to make sure that you follow the proper start up and shut down procedures recommended by your supplier. You should also make sure that the hot runner supplier sized the flow channels correctly and you are not holding too much material in the system allowing it to either degrade the material or the colorant. This could also be happening if the system is allowed to sit and the material is cooking in the system. If your melt channels are too small you may be creating too much shear heat. You should be able to determine this by the injection pressures that are required to fill the parts.
I trouble shot splay and blistering for 12 years (until three years ago). Silver streaks were (in our blended products) always a first sign of marginal mixing, and of course contamination (this covers humidity, larger filler than specs allowed, other contaminating materials). I was the optical and electron microscopic morphologist. These silver streaks (some of the thinnest and hardest surface defects to microscopically visualize), splays (ultra-thin), then blisters (largest) all contained separations that could easily be seen from the surface with the correct microscopic conditions, and in cross section as an interface that at one end usually displayed a thin cavity usually at or very near the skin layers.

It was very difficult in the first 3-4 years to convince traditional staff at first that it was not only humidity, understandably, but repetitive micro-analysis seemed to produce very predictable results, even before surface silver, or small or this, splay could be seen on production lines. So predictability and rapid solutions became an important issue.
Couple of things I usually recommend to get good bonds of TPE (Thermoplastic Elastomer) and PC:

a. Melt temperature is critical for a good bond. The melt temperature for overmolding is usually 390- 440F for overmolding over polycarbonate. The TPE has to be over the minimum melt temperature of 390F from start to finish. If it goes below this melt temperature at any point, the TPE will not bond to the PC beyond this point.

b. The flow ratio is critical as well - this is the ratio of the flow length and thickness of the TPE. Typically it is suggested to have an 80 - 120 flow ratio for overmolding. If the flow ratio is higher than 120, then multiple gates should be used. If flow ratio is too high, then it will be difficult to maintain the right melt temperature across the length of the substrate.
The root cause of sink marks is almost always either poor part design or poor gate design. Make your gate diameter (subgate) or thickness (edge gate) should be 80% of the nominal wall thickness to avoid the gate freezing off too soon during packing. Wall thickness should be uniform (no thick sections). The ribs and bosses should be short, not tall, and their base thickness less than 2/3 of the nominal wall thickness for low-shrinkage plastics like ABS and less than 1/2 of the nominal wall thickness for high-shrinkage plastics like nylon or PP.

Begin by recognizing that paint usually accentuates flaws in the injection molded part, any sinks, flow lines, scratches, dust, oil, etc. will show up so take special care-concentrate on your part design. Minimize ribs, bosses, focus on basic part design rules and good process will take care of the rest. Minimize the use of mold release, keep it clean and you should have good success.
I will not go down the road around the tooling or processing assumptions in this matter but concentrate on the fact that it's a black colored part, and the black color has been introduced by using a masterbatch. There are a number of theoretical possibilities linked to the coloration as such and around the formulation of the masterbatch. The black colorant is likely a carbon black but could potentially also be a back iron oxide.

Suggestions:
Establish qualitative knowledge of the details of the masterbatch used linked to above mentioned factors. By a good co-operation with your masterbatch supplier, this should not be a problem to discuss.
Investigate by DSC if the colorant introduced influence the Tc of the resin vs the neat resin. May be complemented by actual measurements on shrinkage between colored and non-colored parts as especially LDPE is difficult to measure statistical Tc differences on.
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