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It's crucially important to note the relationship between process-induced stresses and warpage/distortion. A geometrically acceptable part, with no obvious distortion from design intent, can be highly stressed through its wall thickness, if those stresses are evenly balanced. Think of pre-stressed concrete beams as an example: the re-bars are under huge tension, while the actual concrete is in compression and the loads are in equilibrium. Think also of those toys that collapse when the base is pressed and the internal tension member (string!) is loosened. Warpage occurs when stresses in some areas are not balanced and the stronger ones "win," so to speak, as the part cools. The part shape reaches a state of equilibrium only when the stresses balance, in the distorted shape, when cool. Counter-intuitively, you may actually get lower stress levels through the section in an obviously distorted part. Minimize both flow-induced and cooling-induced stresses AND warpage by:
  • Following good component design practice on wall thickness relationships and radii;
  • Simulation, simulation, simulation (to paraphrase a recent UK Prime Minister); i.e., FEA if necessary to keep load-induced stresses reasonable; flow simulation and simulation of the proposed cooling circuits (with design reiterations to even out shear stress and part surface temperature-upon-ejection variations); and
  • Using the same material and moulding conditions as used during the simulation (or as near as is practical).
I have considerable experience using dye, post moulding for plastics. Most of this was done for actual knife handles. The dye does not penetrate far into the part, so removing some material by a process like buffing yields a mix of the dye color and the substrate color. This is frequently used to create a simulated horn or bone appearance. Care and control are required to produce a consistent result. Nylon is also quite dyeable post moulding and I have used that to give different colors to parts moulded together in a family mould.
Each Factory has its own rule. Generally speaking I am "by my finger rule" using between 10-15%, more often recycling only the runner in cold runner moulds and 5% in hot runner system moulds.

Process you are using, the machine plasticizing capacity, the cycle time, the part thickness, weld line formation at critical areas. The reason being, if you are grinding the wastage and using again, the pellet structure may not be uniform and plasticizing time may be different. Any contamination in the product can come out near weld line and the strength may go down.
Adding recycled material simply reduces the effective molecular weight of the material you are processing. So, whatever product characteristics are affected by lower effective molecular weight will suffer. Even of the effects on the final product are minimal or acceptable, it will affect processing since it affects the viscosity of the melt -- the same as if you changed resins or resin formulation. In many extrusion blow moulding processes or any other process where it is an inherent part of the process, the key is to keep it consistent. Statistically designed experiments are efficient and effective at helping you optimize the settings for a given blend of virgin and recycled material. Statistical process control charting is your best friend for providing guidance to the effects of the recycled material. Control chart the injection or extrusion pressures, fill times (injection moulding), melt temperatures, product weight and other key quality characteristics like shrinkage. Use Xi/R charts and EWMA charts to best determine the need for adjusting the process to accommodate variation in the mix.
There are a number of things that could be the root cause of the problem. The first thing that comes to mind is the surface being plated was not chemically clean before the plating was done. There may be a problem with the chemistry of the chromic acid bath at the time of plating. I don't think the environment the part is being used in is a problem. I have used hard chrome plated surfaces on cutting tools. Cutting tools are extreme conditions compared to what you are describing. There are some basic design rules.
  1. Discuss your issues/concerns with the platter.
  2. The exact alloy being plated makes a difference. Some alloys plate better than others.
  3. Avoid sharp corners, both internal and external. Corners need radii.
  4. Avoid deep pockets. It is difficult to get uniform plating in deep pockets.
  5. Avoid long and non-uniform distances between the Anode and the surfaces being plated.
I assume separation is required for the difference in melt index. I think any picker could be trained in about 15 minutes to differentiate extruded parts from moulded parts. The processes are so dissimilar it should be easy. ie a garbage can cannot be extruded and a garden hose cannot be injection moulded. Scale that concept down to tubes, profiles and film vs chairs and most toys. Blow moulded bottles are probably easy for them to separate. Large hollow parts could be rotomoulded and that is another variable for the pickers.
Annealing helps in releasing stresses frozen in during injection moulding of highly crystalline materials like PA6. This generally increases values for mechanical properties. If annealing is done in water, PA6 absorbs some of it and gets plasticized. This increases flexibility of the component, and thereby impact strength also. However, this is a reversible process, i.e., this water may get lost on exposure to high ambient temperatures, over a period of time. This may reduce component flexibility but stress relaxation would anyway have happened. For PA, annealing is done at around 80 deg C.
Annealing

As a cost reduction strategy, injection moulders sometime avoid annealing and optimize cycle time to minimum for higher production rate, which leads to the possibility of high residual internal stresses in the part. Solution is to ask your supplier for a specific PA grade which has minimal tendency for stress or may be ask you moulder to re-optimize injection mould pressure and temperature conditions. This would minimize stress but not eliminate it.
There is no generic formula for this and wall thickness is only one of many variables. All of the following and more are other variables that can greatly change the cycle time:
  • number of cavities in the mould
  • type of material
  • any slides, core pulls, or unscrewing mechanisms will all add time
  • robot for part or runner removal will add time
  • geometry of the part
  • design of the mould
  • type of injection moulding machine that is running it
  • runner type
  • gate type
  • the shop that runs it
When you are moulding a part, the mould temperature and melt temperature may be same irrespective of climate. So the environment at injection moulding is uniform (assuming that the material is predried 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.

I used to assist periodically in a moulding shop in Nigeria, with >90% RH at >40°C (104°F) and had not been made aware of this problem. That is not to say, of course, that it wasn't there.
Injection Mould ClampFor big PP parts with undercuts, how do you evaluate the additional clamping force needed to keep the wedges (slides) closed and avoid flash? I want to develop a method to predict Injection Mould Clamping force for big PP parts with a lot of undercuts (8 big wedges in the mould). At the moment, I am using moldflow to predict tonnage but I am always missing 25% in comparison to the real life! I know that this is due to the side pressure on the wedges which makes them moving backward and the part is flashing. Do you have any idea of how to predict the right clamping force? Or what mould design change (bolster plates thickness, wear plate angle,...) could reduce the clamping force?
In comparison to the shear stresses in the flowing polymer melt, the gravity force is very small. (You can do a simple order of magnitude comparison to confirm this). However, I have seen cases in Gas Injection Moulding where the melt sagged around the gas core during the gas pressure holding phase. This is reasonable. If all melt in the cavity is pressurized to approximately the same pressure, then the shear stress are zero (or near zero) and so the gravity force is the only significant directional force acting on the melt. (To put it another way, the buoyancy of the gas core causes it to rise up through the polymer melt. Of course it won't displace the frozen skin layer).

I once had a Chinese injection moulder argue with me about this on a project where the weld lines where located in a bad location for the part's mechanical performance. He added gates (From one to four) and rotated the tool in the press. When the weld lines moved on the part he naturally said "See! Gravity does make a difference!"
In the injection moulding process, the mould cavity is filled by pressure-driven flow, in fact the material is injected very fast into a mould through small gates. The melt is at high temperature in the filling phase and some viscous heating also occurs due to friction. This increases the melt temperature even more and thus change (lowers) the viscosity. The gate dimension so effect a lot the material behavior.

As viscosity is a function of pressure, the pressure dependence of viscosity may become important in this kind of process. In addition to shear also extensional deformation has a significant effect, due to viscoelasticity.

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