Materials, The basics’…  part 4

In the previous 3 articles or segments we discussed the various points between the types of materials. The final segment in the general view of the articles, is the cooling time. Let it be said that the cooling of the product does begin at the time of filling the part. What is meant is that the product starts to cool off or transfer heat ½ way though the fill stage, thus cooling is the largest segment of the cycle but least understood.

The question is at what point do we open the tool and eject the part? Everyone knows that it is at the point of no pin push and deformation of the parts. But what is the temperature?  The basic rule is that we need be below the HDT of the material to eject it from the tool. (HDT stands for heat distortion temperature and is given at two loads, 66 psi and 264 psi) Now barring any undercuts and or poor tool design issues we should be able to at the 66 psi temperature but if we have a long draw or texture we may want to lower that temperature or use the 264 psi temperature.

Now the differences become apparent in that the amorphous materials cool at a gradual rate and become stiffer and stiffer, just the opposite of when we heat them up. The crystalline materials cool too, but to solidify we need to pull out that additional heat which causes them to actually melt. Thus when looking at the chart below, note the times for the wall thickness and the materials describe.

The other aspect of the above statements would be do you know what the temperature of the part is when it makes a good part? There have been a few articles written on the subject of cooling and rate of cooling which you can review by visiting past issues, but below are some guidelines for estimation purposes. Basic stiffness of the materials and other aspects do come into play and the chart is meant only as a guideline, guess estimation.

Estimated cooling time from ½ of fill time.
 

Wall thickness mm	PS	ABS	PP	PVC	LDPE	Nylon	HDPE
1	2.9	2.9	4.5	3.3	3.5	3.8	4.5
2	9.3	9.3	12.5	10.1	10.6	11.2	12.5
3.2	20.5	20.5	25.5	21.7	22.5	23.4	25.5

 

Thanks for the time.
 

Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Distribution