Purpose of making a viscosity curve is to select a suitable injection speed, so that when there are slight fluctuations in various parameters, melt viscosity will not change greatly. Fluctuation between each mold should be as small as possible to ensure repeatability of product quality.
Of course, there are special cases where this optimized speed cannot be used, such as reducing gate halo. In this case, appearance is of course priority, but this optimized speed should be used as a reference for injection curve, such as starting to pass through gate at a low speed to reduce gate halo, then quickly increase to this optimized speed.

 

 

An important parameter to measure fluidity of plastics is viscosity. Non-Newtonian fluid characteristics of plastics are that as shear rate increases, viscosity decreases.
When polymers flow, there is always a certain velocity gradient between liquid layers. If thin and long macromolecules pass through several liquid layers with different flow rates at the same time, different parts of same macromolecule will move at different speeds. This situation is obviously unsustainable. Therefore, when flowing, each long-chain molecule always tries to make itself enter flow layer with same flow rate. Parallel distribution of liquid layers with different flow rates leads to the orientation of macromolecules in flow direction. This phenomenon is like a rope (thin and long) flowing with water in a river. They are always naturally arranged longitudinally along direction of water flow. During flow of polymers, as shear rate or shear stress increases, viscosity decreases due to orientation of molecules.
Multi-stage injection
As long as product quality allows, use as few as possible;
Injection speed segmentation
Cancel ineffective segmentation;
Purpose is to reduce change of plastic viscosity caused by change of injection speed and ensure a more stable process
Parameter setting and optimization during filling phase
Set injection pressure should be 10% higher than actual peak pressure;
Try to complete filling at a consistent and fast speed;
Purpose: Try to use a fast filling method and ensure that injection speed is as stable as possible.
Speed segmentation as little as possible:
Control screw position to ensure that product is only 95-99% full at the end of filling.

 

This test is only required for multi-cavity cases, such as 2 cavities or more. Purpose is to check maximum percentage deviation between cavities at different filling stages.

 

1. Short shot
2 Flash
3. Burning
4. Sink mark
5. Different gloss
6. Different dimension
7. Stress mark
1. If product can be fully held and molding window is large, check whether product dimensions are within tolerance. If they are, filling imbalance is acceptable.
2. If molding window is small and the first cavity filled has flash, while other cavities have short shots or sink marks, find out cause of filling imbalance.
Factors leading to unbalanced injection
1. Different runner sizes
2. Different gate sizes
3. Different exhaust sizes
4. Inconsistent product structure thickness
5. Different cooling, but this reason often has little effect when machine is just started
6. Another situation is imbalance caused by shear, especially for 8-hole cold runner molds.

 

 

Purpose of doing a pressure drop test is to evaluate pressure loss at different stages of filling. This usually includes machine nozzle, vertical runner, main runner, secondary runner, gate and filling end.

 

Molding process should not use maximum pressure of machine. For example, if maximum pressure of machine is 1800Bar, maximum pressure required for filling should not reach 1800Bar. If this is the case, it means that screw needs more pressure to achieve set injection speed, but it cannot be achieved due to pressure limitations. This situation is called "pressure limitation".
Usually, injection process should not exceed 90% of maximum pressure of machine. In pressure drop curve, if it is in "pressure limitation" or exceeds 90% of machine pressure, find a steeper section in pressure curve and try to reduce pressure loss here.

 

Molding window is a very important test. Usually, appearance molding window is composed of holding pressure and material temperature (amorphous material), holding pressure and mold temperature (crystallized material). Appearance molding window tells how much room there is to adjust process while obtaining a product with acceptable appearance. Ideal situation is to have a relatively large molding window. If molding window is relatively small, it will be easier to produce quality defects. For example, in right picture, if molding window is relatively small, it is easier to have short shots or burrs due to fluctuations in process itself. A robust process is to have a relatively large molding window to compensate for fluctuations in process itself.

 

 

 

For cold runner molds or semi-cold runners (hot to cold), to ensure repeatability between molds, holding pressure must be maintained until gate is completely cooled.
After curve is generated, select a time after product weight stabilizes. In above figure, product weight does not increase after 3s, so for safety reasons and to compensate for fluctuations of process itself, holding time should be set to 4s or 3.5s.
It should be noted that extra 1s in holding stage does not increase cycle time, because during this time, gate should have frozen and only briefly hold cold runner, while product has already begun to cool. Therefore, extra 1s should be subtracted from cooling time to ensure same cycle time.

Heat transfer path from plastic to mold:
☆ Pass to injection molding machine mold plate
☆Through air convection and radiation
☆Cooling water is taken away
◆Cooling system of mold takes away most of heat in mold.

 

 

 

Once plastics part has been formed in mold, heat that was put in to get it to flow has to be removed. Heat goes from plastic into mold. Heat is then removed from mold largely through drilled holes in mold called cooling channels.
Critical dimensions for cooling channels are:
>Diameter (of cooling channel in cooling area, not water line going to mold)
>Depth (average distance from center of a cooling channel to the surface of part)
>Pitch (average distance between cooling channels)

 

o One of important tools of statistics that has been widely used in evaluating quality of a product, identifying sources that affect quality, setting up values of parameters that will optimize response variable,is Design of Experiments.
o Designing an experiment is like designing a product. Purpose should be clearly defined to begin with.
o Experiment should be set up to answer a specific question or a set of questions.