Tooling Know How Five Tips on Profile Die Design

A poorly designed profile die-one that does not let the part to come to be extruded with the same dimensions from set you back run-coupled with too little understanding of the extrusion method, is a recipe for scrap generation.

A poorly designed profile die-one that will not let the part to be extruded with the same sizes from set you back run-coupled with too little knowledge of the extrusion procedure, is a recipe for scrap technology. There are five things profile processors have to know about designing and maintaining a replicable program:

?Proper territory length: It is the area of steel that runs linear to the profile that provides "memory" to the required shape of the part. If the property length is too short, the flow changes with any noticeable change in the run conditions. Land length also helps offset any swell of the thermoplastic as it exits the die lip. Profiles with an equal wall thickness must have the very least land length 10 instances the intended wall thickness, for a land-length ratio of 10:1. If the designed final wall thickness is 0.100 in., the land length ought to be 1 in.

Of course this formula universally does 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 result in a land length of 0.400 in., that could result in extreme pressure buildup. From a wellbeing standpoint, the maximum continuous backpressure desired is 5000 psi.

?Terrain length ratio: On profiles with unbalanced wall structure thickness-i.e., with multiple wall structure thicknesses in different areas-the same guideline applies. Adjust the territory length by removing steel just. For instance, if one wall is 0.080 in. thick and another 0.060 found in., the land lengths will be 0.800 and 0.600 in., respectively.

If you¡¯re owning a low-viscosity resin or expect large output rates, land-length ratios can be increased up to 25:1 (Table 1).

? Drawdown: This is possibly the most misunderstood concept of profile die style. In profile extrusion, drawdown may be the percentage differential between the sizes of the die as built vs. the size of the final part. Without the drawdown to exert stress, the extrudate would drape and sag in the sizing medium. Hence, a drawdown is made by the die developer percentage into all of the dimensions to keep carefully the account taunt and aligned.

Since all thermoplastics swell, die designers may bring the wall height and thickness dimensions into tolerance with a percentage of drawdown, and if the proper proportion is added to the entire dimension, all dimensions would be to specification.

Another little well-known and rarely discussed concept close to drawdown is going to be that the extrudate continues to draw down until it reaches the glass-transition temperature (Tg) on crystalline polymers, or the heat-distortion temperature in amorphous resins. Hence if a share is added to accommodate polymer swell, however the cooling is normally inadequate to drop the melt temperatures to the Tg, the part will continue to draw down, resulting in a smaller than expected extrudate.

Each thermoplastic has a unique specific high temperature, defined as how much heat strength that must definitely be added or removed to change its temperature by way of a given amount. That affects the charge at which the resin could be cooled sufficiently to avoid further drawdown. This means each polymer has a varied drawdown ratio. Thermoplastics such as for example PP and LDPE happen to be crystalline with a high specific heat and require increased drawdown than an amorphous material such as for example rigid PVC. This explains why a die created for PP cannot end up being useful for other polymers. Table 2 reveals drawdown ratios for substances running on a 2.5-in. extruder at 100 to 125 lb /hr. A range is mentioned to reflect varied drawdowns for profiles versus tubes and to take into account different cooling media.

It¡¯s important to remember that amorphous resources such as rigid PVC cannot tolerate good sized drawdown percentages, simply because that will impart high residual stresses in to the extrudate, causing brittleness when impacted.

?Designing for sensitive materials: When designing with regards to polyolefin profiles, flat-plate dies operate fine because they tend to be very stable under temperature and don't develop high backpressure. Even flexible PVC, TPOs, HIPS, and Abdominal muscles can tolerate tools without any great degree of streamlining. Streamlining is the technique of creating generous transitions in the steel without the ¡°dead-spots¡± where material can melt away 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 in fact the polymer can stagnate.

?Decompression: Another important aspect of profile die style is to keep the melt under compression through the entire spreader, adaptors, transition plates, and die lip. Any section of decompression may cause problems of material stagnation and in the end degradation, leading plastic recycle machine to low quality. Angles ought to be kept to 60¡ã (30¡ã included angle)

Whereas the running period on a ¡°flat-back again¡± die can be from one to eight time with most polymers, a fully streamlined die can run for weeks without stopping to be washed. Profile dies for rigid PVC and PC are prone to degradation especially. For long runs, a completely streamlined die is your very best bet.

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