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Weld lines of injection molded plastic parts

Weld lines is something you tend to get in an injection molded product with multiple flows. Question is 1) Is the weld line strong 2) Is the appearance acceptable. If there is only one/ two weld lines which can not be accepted, in addition to what is already covered, I would add sequential valve gating. Open the valve gate just as it is about to form a weld. I have done many projects simulating on Moldflow by changing thickness (flow leaders/ deflectors) and converting to a meld line which is less severe. The results can not be guaranteed as the flow pattern can change with the viscosity of material and if product design has thickness variations. We have observed that in Moldflow, the weld lines are seen but in product, they are not noticed. This happens if the inner layer material shear heating is high which heats the surface material.

There is a RHCM process for the weld lines elimination. There are at least six different technologies of RTC/RHCM molding, that require different equipment, different design of injection molding tool with different cycle time of the plastic part molding. All RTC/RHCM processes extend cycle time by about 30-400% (depends on process) compare to the standard molding of the same part. Investment costs to get those technologies in-house can be different by up to 20 times. All those RHCM/RTC technologies can eliminate weld lines.

But, the first question is what is a purpose of the weld line elimination on your plastic part. Is that surface decorative and exposed as molded on assembly, or weld line is eliminated only for the plastic part painting, since deep weld lines are still visible on some paints. If you need weld line reduction for painting then RHCM is not needed. You can try different material grades from different manufacturers since some materials allow for easier elimination of weld lines than similar other materials, and some colorants affect slightly those weld lines depth/visibility as well.

The higher the melt skin temperature and injection mold surface temperature at time of contact with melt and higher the pressure, the better the appearance and strength retention. Weld-lines are an inevitable consequence of any tool feature that splits flow before it rejoins, just as night follows day. Their visibility and physical properties versus the rest of the molding will vary depending on the many variables mentioned. Avoid mold release spray like the plague, unless you want dreadful loss of strength. High aspect ratio metallic and pearlescent pigments are bound to accentuate appearance. Post-machining a real option, but adds cost, creates material swarf or cut blanks that can (with good control) be re-fed into the material feedstock (at a slight energy penalty, of course). No magic solution, just common-sense application of one or other of the approaches mentioned.

Our company (Aco Mold) specializes in automotive decorative products. We have in-house and perfected the most effective RHCM technology for our needs. We use that process for the high gloss surfaces and for paint replacement surfaces. We molded high gloss parts with up to 7 openings, many features at the back and 5 gates with no weld line existing on "A" surface at all. Additionally we developed our own technology, different than RHCM, that is effective for the weld lines elimination as well. If you are interesting in producing your plastic parts with no weld-lines using our technologies then please contact Aco Mold.

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