A poorly designed account die-one that will not let the part to end up being extruded with the same dimensions from run to run-coupled with a lack of knowledge of the extrusion method, is a recipe for scrap generation. There are five things account processors have to know about creating and retaining a replicable device:
?Proper territory length: This is actually section of steel that runs linear to the profile that provides "memory" to the desired form of the part. If the land length is too brief, the flow changes with any modification in the run conditions. Land length also facilitates offset any swell of the thermoplastic since it exits the die lip. Profiles with the same wall thickness must have the very least land length 10 occasions the intended wall structure thickness, for a land-length ratio of 10:1. If the designed final wall thickness is certainly 0.100 in., the land length should be 1 in.
Needless to say this formula universally will not apply. For example, when owning a thin-wall account (say 0.40 found in.) of rigid PVC or another high-viscosity polymer, a 10:1 ratio would create a land amount of 0.400 in., that could result in increased pressure buildup. From a safeness standpoint, the maximum continuous backpressure desired is 5000 psi.
?Area length ratio: On profiles with unbalanced wall structure thickness-i.e., with multiple wall structure thicknesses in different areas-the same rule applies. Just adjust the land length by removing steel. For instance, if one wall is 0.080 in. solid and another 0.060 found in., the land lengths would be 0.800 and 0.600 found in., respectively.
If youre running a low-viscosity resin or expect substantial output prices, land-length ratios can be increased up to 25:1 (Table 1).
? Drawdown: This is perhaps the most misunderstood concept of profile die style. In account extrusion, drawdown is the percentage differential between the dimensions of the die as built vs. how big is the final portion. Without the drawdown to exert tension, the extrudate would drape and sag in the sizing moderate. Hence, a drawdown is made by the die custom made percentage into all the dimensions to keep the profile taunt and aligned.
Since all thermoplastics swell, die designers may bring the wall thickness and height dimensions into tolerance with a percentage of drawdown, and if the proper proportion is added to the overall dimension, all dimensions is always to specification.
Another little well-known and rarely discussed concept nearly drawdown is definitely that the extrudate continues to draw straight down until it reaches the glass-transition temperature (Tg) in crystalline polymers, or the heat-distortion temperature in amorphous resins. Hence if a share is added to accommodate polymer swell, however the cooling is inadequate to drop the melt temperatures to the Tg, the proper part will continue to draw down, resulting in a smaller sized than expected extrudate.
Each thermoplastic includes a unique specific heating, defined as how much heat energy that must definitely be added or removed to change its temperature by a given amount. That affects the fee of which the resin could be cooled sufficiently to avoid further drawdown. This implies each polymer has a different drawdown ratio. Thermoplastics such as for example PP and LDPE will be crystalline with a high specific heating and require greater drawdown than an amorphous material such as for example rigid PVC. This explains why a die created for PP cannot get used for other polymers. Desk 2 shows drawdown ratios for components running on a 2.5-in. extruder at 100 to 125 lb /hr. A range is stated to reflect different drawdowns for profiles versus tubes and to account for different cooling media.
Its important to note that amorphous materials such as for example rigid PVC cannot tolerate good sized drawdown percentages, as that will impart high residual stresses into the extrudate, leading to brittleness when impacted.
?Designing for sensitive substances: When designing with regards to polyolefin profiles, flat-plate dies do the job fine because they are generally very stable under high temperature and don't develop high backpressure. Even flexible PVC, TPOs, HIPS, and Abdominal muscles can tolerate tools without any great amount of streamlining. Streamlining is the technique of fabricating generous transitions in the steel without the dead-spots where materials can burn off and degrade. If very good aesthetics are required, without any dark-colored specks or flaws, it is important not to let any transition create areas where the polymer can stagnate.
?Decompression: Another essential requirement of profile die style is to keep the melt under compression through the entire spreader, adaptors, transition plates, and die lip. Any certain section of decompression may cause problems of material stagnation and ultimately degradation, leading to low quality. Angles ought to be kept to 60 (30 included angle)
Whereas the running period on a flat-back again die can be in one to eight hours with plenty of polymers, a completely streamlined die can work for weeks without stopping to be cleaned out. Profile dies for rigid PVC and Personal computer are especially prone to degradation. For long runs, a streamlined die is your best bet fully.