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The main advantage of injection molding is the ability to produce on a collective scale. Once the initial cost has been paid for at the injection molding production unit is extremely low.Consider some of the following things as you strive to make part of the production through injection molding.
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.
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.
Generally a short shot will be due to man, machine or process. If you can define some of the operating parameters, material, personnel, cavities, manifold type, material drops, pressure, valve gating, cycle time, etc. and monitor the variables in the process. To determine process capability you will have a good start as to where your issues are and why. The data will point you in the right direct to the root cause, and when you can repeat the defect on demand by changing a variable you have successfully identified root cause.

Try using Fish bone diagram and covering the 6M. Also try to understand what is this short shot. Do you get it with flash? If yes, then this is because of pressure spikes due to unbalanced filling or switch over pressure. If the section thickness is less, then it is due to fast cooling and you need to increase injection rate and if it is due to air trap, you have to modify injection location/ part thickness. All these are solutions; but if you really understand the preliminary cause, then using the six sigma techniques would really help.
Sorting out this type of issue can be quite complicated and although the advices given above are all very helpful and sound, one would require a full picture of the issue (part, machine, material, processing conditions etc.) to resolve it. My first suggestion would be to hire someone who is experienced in resolving this type of issues. Here are four tips:
  1. Change in pellet shape (was stranded, now underwater) implies a different compounding line, so maybe mixing/uniformity is less, degradation more, etc.
  2. Grind both the parts made from old (good) compound and the new (suspect) compound, and test them the same way (can you tell the bad ones from some quick test like tensile or impact)? Record screw amps or torque to compare viscosities.
  3. Infrared on such a mix may be too complicated to read clearly. DSC is better. Test solid density, melt index (dry samples), other rheometry.
  4. Once you prove that the materials are substantially different, it's the material supplier's problem to figure out why. Look for another supplier, at least until the supplier pays for more testing, replaces the bad lot, or any other solution that you agree to. If it's a "bargain" supplier who is charging less than the average elsewhere, you are getting what you have paid for, and still should look elsewhere.
Before making an estimate of the required flow rate of the installation, you have to decide the method of cooling the water and thus the operating temperature. Water chillers can provide 10 - 13 oC input, while cooling towers in a hot summer not less than 30 oC. The coldest the water the less you need. Gather information concerning the quantity of water needed by each injection molding machine for the oil coolers and feed throats. This information is provided by the manufacturer of the injection molding machine. A good option is to bypass water cooled oil coolers with air cooled. In this way you provide heating during winter in the plant and at summer you reject this heat out.
Next is to estimate the quantity of water needed to cool the injection mold. This depends on the nature of the process and the quantity of plastic processed per hour.
With regards to ZERO sink-marks, however, this is a Holy Grail that is practically impossible to achieve with basic injection molding, as soon as you locally thicken the general wall section by adding ribs or any other protrusions on the opposite surface. However, following the well-proven guidelines on ratio of rib-to-base wall thickness will always help minimize the physical dimensions of sink-marks. Conversely, the higher the gloss level, the more you will see sink-marks, for any given depth. On a high gloss surface, even a dip of a few micro-meters will be seen under certain light conditions. The nearer to feed points, the more you will be able to make the packing pressure do its job of "inflating" the slightly thicker section as volumetric shrinkage pulls it inwards. You can see often this effect very clearly along the length of a rib.

Another means, other than packing pressure, of resisting the pull caused by volumetric shrinkage is to use fluid-assisted molding (gas or water). Great results can be achieved compared to conventional injection molding. Also, plastic parts produced using Mucell technology benefit from microcellular pockets of inert gas to maintain the internal pressure needed to resist the localized sinking. Use of the Rapid Temperature Cycling technology can even "iron out" the otherwise "grainy" surface caused by bubble collapse, resulting in superb gloss (if the material is intrinsically glossy, that is).
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