Injection molding pressure

My firm has been working with a PC part that undergoes alot of stress. We had one batch of parts that seems to fail more often than other batches. The company doing the molding said that everything was the same between batches, except for the injection pressure. They assured us that the pressure makes no difference on part strength.

Does anyone have any experience with this? Does the injection pressure effect the strength of the part?

Just speaking in general hypothetical here. Not a moulder.

It makes sense to me that there would be a difference giving the fact that ALL the other factors are the same except for the pressure that the parts would be different.

Assuming that the parts that fail have lower injection pressure:
Seeing as how the melting temp would be the same and the same plastic used, therefore the plastic would flow the same.
Now, less pressure, generally would mean less speed of filling the mould.
Less speed in filling the mould would result in a more pronounced weld-line (where the coldfronts meet) Also less pressure would result (and this is more obvious in casting than in injection) in higher porosity. more gas bubbles, more prone to failure. so in conclusion, more bubbles and more ‘weld-lines’ = more chances the part will fail.

My 2 cents. once again I am not an engineer, not a molder.

Reading from my mass-production textbook (it’s good, I have to study this stuff for a midterm coming up):

“the use of LOWER INJECTION PRESSURES means:
– there is lower stress which increases chemical resistance
– less draft is required
– the cost of molds is lower
– larger parts, in excess of 45kg, can be molded”

I have no idea why lower stress increases chemical resistance, but it would seem to me that increased pressure increases the stress on the plastics. I know that when you buy acrylics (for example), the cast acrylic is a lot more expensive than the extruded stuff. The extrusion process (high-pressure of course) stresses the plastic, whereas the cast acrylic has no inherent stress points, so it’s stronger.

Incidentally, artur83 is right about the weld lines as well – if the plastic has a chance to cool more during the injection cycle, you’re going to get more pronounced welds. However, that’s a problem with the mold and gate design – the gates and pressure should be set up so that weld lines and other weak points are minimized.

Thanks for the responses. We still haven’t found the problem, but I believe it is larger than the injection pressure.

Wish me luck!

When you injection mold a part at one point in time and successfully produce parts at a given injection pressure and get acceptable strength there is no reason the molding pressure should have to be increased the next run. There is no real physical explanation that can be given that would give your molders statement any credibility, you can not change one condition in a system with as many variables as injection molding and end up with the same parts.
What causes a need for more molding pressure is typically some temperature, tooling or material change. when you use more force to push the material in it inherently will result in a part with more molded in stress and altered molecular structure due to the increased shear that usually accompanies higher molding pressure.
If your molder really thinks he can mold parts the same with different conditions like this I would bet he does not have a good enough handle on his process to maintain stability. You may want to reevaluate this vendor.
You can send me a PM if you have more questions and I hope this helps.LTR

So a higher pressure may increase the brittleness of the part?

The part has been injected on at least three different machines. We don’t deal straight with the vendor either, so this information is getting to us in the third person.

So a higher pressure may increase the brittleness of the part?

YES

It’s the same with just about any manufacturing…if you want the highest-quality product, you have to take it slow (as it were). High speed tools with a low feed rate give the best surface finish; lots of sanded coats of paint give the nicest gloss; and lower pressure molding takes longer, but gives less stress.

Ironically, low-pressure molding usually has much worse surfacing issues because of the melt swirling around the inside (like with RIM or structural foam).

I had some problems with a PC part and the problem was sharp corners in the runner/gate.

The PC sheared on the sharp edges as it was injected creating small cracks. The pressure may be inhancing this issue.

the molder rounded the corners of the gate and runner and it molded much better.

Mark

Hi, did u check the plastic melting temp? The material will become more brittle if melting temp is too high

Also, if the molded part is readlly sensitive to internal stress. Pls also check the drying time is not enough to keep the plastic is totally out of mositure.

Alex

Well, just about everything that has been posted seems to be involved in the part defects I’ve seen.

I received unpainted samples today. There are bubbles in the part. I had thought the molder didn’t dry the plastic properly, this seems to be the case. Also, the mold design and injection pressure made the problem worse.

I’ll let you know how it goes when we implement some solutions.

Arclight, what is the mass-production textbook that you are using? It sounds like some good info. Thanks.

agreed! :blush:

The part I’m working on has been injected on at least two machines under various conditions. The supplier is in China, so I don’t have too much control over what they do:/

Upon further examination of previous samples, it seems the bubbles are only in the new unpainted samples. Perhaps they changed another factor.

It’s my belief that the problems are a huge combination of part design problems, mold design problems and injection problems. It’s going to take awhile to sort it.