Most organic pigment manufacturers uses their own test method and most of them are molding a chip in a homogeny thickness with a certain length and width (longer length) and then measures the shrinkage in length vs transverse direction to provide what could be called a warping index. The uniso-tropical shrinkage is a sign of warping and an iso-tropical index of 1 is of course the best (= same shrinkage in length as well as in transverse direction).
All pigments act as nucleators and will influence shrinkage and warping in semi crystalline resins. Some organic pigment chemistries are really powerful nucleators and will provide a high degree of warping (e.g. Phtalocyanine pigments). Besides general aspect as part- and mold design and a careful selection of pigments, nucleating agents may be added to overcome warping effects.
You will have to examine your application and choose accordingly, but I will try to explain briefly some of the advantages to a hot runner solution. As I currently work for a hot runner company I see many parts that benefit from this application. Here are a few basic reasons to look into hot runners:
- Less waste -which in turn reduces contamination
- Controlled melt temperature - reduces warping of part as well as plastic degrading
- Great gate vestige - Reduces the need for second operations
- Lower injection pressures - allows for more cavities per mold
- Shorter cycle times - improves production efficiency
The simulations results have a qualitative accuracy for this reason many companies and the academia are working in this area. The reason is that all models use strong assumptions that can be weak depending on the material, geometry and process conditions. I can give an example for fiber filled injection molded composites. For short fiber composites in a center-gated disk test sample, we have found that the actual models fail to predict the orientation near the gates and up to r/H =39. The actual model implemented in most software is reasonably good above 39, but still prediction of fiber orientation near the walls is challenging. Additionally, the quality of the predictions reduces noticeably as you increase the fiber content and the thickness of the parts. These two conditions reduce the validity of the assumption used in the simulation packages. The area of interest of many software developers is the prediction of fiber orientation of long fibers. This is an area of opportunity because we are still unable to predict a reliable fiber orientation due to the semi-flexibility of the fibers.
Specific decisions for the best approach (material, process, design detailing) can only be made with a detailed knowledge of all the constraints (structural, dimensional and financial) and all the must-have, nice-to-have and absolute must-not-have features and attributes. Is there not a web address where we can get more detailed information? Another process (actually, a hybrid process) came to mind and this might suit the combination of requirements that I think you have. This is offered by a couple of machine manufacturers. You'd need to contact the machinery manufacturers direct for contact details of processors offering this service. A thin skin (can be vacuum-formed) is normally inserted in the relatively low-cost mold, to give high-quality surface. The robotic head dispenses PUR foam with reinforcing fibers - that are chopped in the head also - into the open mold. Once the mold is closed, the "B" surface is formed, complete with any ribbing and fixing points, etc. However, the total thickness will tend to be significantly well above the 5mm maximum that you have defined. Rigidity is very impressive. Impact strength will depend on surface material and density/reinforcement variables in the backing material.
The easiest way to check the tonnage at mold parting line would be to use the pressure sensitive film. There are many factors that can cause the machine tonnage to change. Toggle clamps can vary the tonnage due to thermal expansion of the mold. Hydraulic clamps produce the same amount of clamp tonnage once they achieve full pressure position.
Pressure sensitive paper or die blue allows you to determine if the tonnage across the mold clamping surface is consistent. Factors that negatively affect squareness:
- Mold construction
- Platen surfaces
- Clamp squareness
- Tie bar stretch
- Tool offset
The thickness of skin layer is inversely proportional to mold temperature, melt temperature as well as injection speed. In principle, it is possible to obtain minimum thickness of skin layer with elevated mold temperature and melt temperature, at high injection speed. Typically, the skin layer is amorphous because of fast cooling of melt. In addition to that, the shear stress is low. Therefore, both the thermal induced and stress induced crystallization processes are inhibited.
If one likes to crystallize semicrystalline polymer with bare minimum or no skin thickness, shear induced orientation of injection molding could be adopted. The Brunel University and University of Minho use this technique to optimize the crystallization.
When using compression molding the cavity is slightly open when injecting and thus has more room for material. This way all the needed material can be injected rapidly into the cavity and instead of applying pressure in the material the mold clamps and thereby closes the cavity to the intended size (or volume). This reduces the internal stress in the part as you can image the molecules finding their own space so to say. Compression molding is very useful in thin wall parts to better be able to fill the part during injection and optical parts where stress will disturb the optical quality of the part, that's the pros.
However many beverage caps use continuous rotary compression molding which is altogether a different process. We used this process at a company I worked at for 35mm film cans and covers and it can be hypnotic to watch it run at full speed. If you look at a beverage cap and there is no indication of a gate mark on the part, it was most likely compression molded.
For manufacturing also other "sticking" phenomena will be important: most common problem can be vacuum in the cavity, while the core is well vented by all ejector pins and inserts. When using an ejector pin to determine adhesion, be aware of the dynamics of the ejection process, the possible pulling of vacuum on the pin and deformation of material surface at release (notch-effect). These effects can give quite a complex release force graph.
COF also can vary widely with mold temperature and the pre-heating of the sheet prior to injection molding. Also, no mold surface should be smooth, and the RMS or roughness chosen can make huge differences. Finally, I've seen the same mold used on different resins - e.g., ABS vs. PP-based TPO resins. Each had to be taylored with the proper release. This is done anyway for extrusion purposes, but subtle shifts in mold release additives for the sheet extrusion step can affect thermoform mold release as well. A simple ladder experiment would help, or you can design an injection mold with many different RMS finishes too and do a full designed experiment. This would include sheet temperature going into the thermoformer as well as heat dwell time. Dwell and temp affect the rate the mold release can bloom to the sheet surface, which affects release.
Even though N2 is green, for PP N2 assisted foaming is not the most cost efficient process. Nitrogen has too low solubility in PP. So much higher pressure is required to get consistent foam. Maintaining high melt pressure in PP is not easy. Higher pressure means also higher cost. At lab scale - all of these looks dandy. Proof is in producing at commercial scale.
Part you are considering is too large to use a typical structural foam injection molding process. Structural foam injection molding process can never achieve low densities of EPP. For large part (even much smaller than what is discussed here), Treacle process is not suitable.
There is a list of things to look at when you do troubleshooting on silver streaks. For example: 1) Material is not dry enough, masterbatch also need to be dry. 2) Gate size is too small to cause high shearing rate, then material break down. 3) Melt temperature not to be too high. 4) Back pressure too low make air come in. 5) Runner has shape edge. 6) Injection speed too fast. 7) Last factor maybe material is not good. Look on the web for many excellent troubleshooting guides.
First of all, this defect is commonly known as splay - it is often due to wet material or contamination. Before you go and start doing lots of troubleshooting in the processing set-up, I suggest that you look at whether the material is properly dried before processing, whether you may have contamination from other materials, or see if you purged adequately before the current job. When you look at the material to see if it is wet, also check that any color used is also dry if you are blending it in. Thorough cleaning of any dryers and feeders is also an absolute before material is placed into them for conditioning.
We spoke with a local injection molder who is very experienced and produces high end plastic parts for the medical industry. When they examined the parts, they noted the flow path, how it creates and positions the weld line, and was almost sure that the flow was trapping air in the part, at the weld line. There was a very slight discoloration along the weld line, but no voids or bubbles in the part. The air is compressed at the weld line and weakens the line. Several mold design features can be incorporated to resolve or prevent this (some of which we have incorporated into the mold modifications):
- move the gate to change the flow direction and path (done in the modification of our injection mold)
- add an ejector pin at the site of the trapped air, to vent the air (added pin at the remaining weld line after gate was moved, ensuring venting).
- vent the whole perimeter of the part mold.
- locate the gate at the thickest section of the part.
There are some reasons to cause the lack of material in final injection molded parts. First of all it starts with the mold design. Second we have the plastics. Third we have the injection molding machine. Forth we have the people involved orchestrating the whole concert. Fifth we have the machine and sixth we have the environment. Last but not least we have the process itself.
Starting with the injection mold. The mold has to have a design that allows you to produce good plastic parts. Clear for all of us every injection mold has a process window. That means that whatever your molding machine is doing at a certain viscosity the mold needs a certain pressure within a certain time limit in the cavity. This is unique for each and every injection mold like a finger print.