Injector pin marks suggest you are not letting the part cure enough prior to its release, flow marks are a more serious issue and are possibly a result of the gate size or position(s) being in the wrong spot. Worse the mold for the part may have been misdesigned. Flow marks are a sign of the polymer flowing under stress which can lead to part failure. Depending on the direction of the flow marks you can better understand the cause.

Injection Mold Design Engineering by David O. Kazmer sold on Amazon has some very nice information of flow rates and issues that could really help you. It covers several chapters, but really explains the problem and offers potential solutions.

Since each mold is different, each part will have its own issues. Where the gate is, how big the gate opening is and the size of the runner are points that often solve these problems. Remember with mutiple gates the smaller the opening the higher the shear rate and friction. The greater the friction the greater the temperature which keeps the polymer flowing. To large an opening and the polymer slows before filling the mold. Polymer flows from various directions and temperatures collide, one way of creating flow marks. Pushing material through a broad area in a mold can allow the polymer to cool slightly not allowing enough heated material to properly fill thinner walls leaving flow marks.

To tell you what is specifically causing your flow marks I would need to see the mold design including gates and runners. Another possibility is that you could be running your polymer outside the viscoelastic range which can contribute to strange behaviors in the mold from one section to the next. Make sure you are considering the shear effect in the sprue, runners, gates and the mold cavity itself. And finally using too much pressure to force the material into the mold can cause flow marks.

Questions like these are why we started publishing a free on-line magazine from our commercial WEB site. Better understanding of polymers lead to better products, better vendors and better customers a win/win solution for all of us.

Polymer manufacturing is a science grounded in chemistry and mathematics, but there is a point where science stops and human understanding must prevail. Even commercial software to aid in designing molds can have problems with fill and flow marks, it is indeed an interesting challenge that varies with each part made.


PMESTL

We use evoprene, an elastomere (evoprene) on a medical grade very small overmold. Do the same rules apply to this?
Also what causes bubbling in this plastic?

Evoprene is indeed a thermoplastic elastomere. It is manufactured by AlphaGary. AlphaGary offers a wide range of compounds which will vary in performance based on their makeup.

Evoprene when considered in general as a thermoplastic has similar properties as other thermoplastics and will behave as listed in the prior comments. Some of the Evoprenes have high friction rates which will suggest high shear rates and higher temperatures at shear points.

Bubbling (voids) with some products can be overcome by increasing the compression (after pressure).

You may also consider decreasing the melt temperature and increasing the mold temperature within the limited range of the overmold. Or increasing the after pressure time. With a higher friction material you may be creating to much heat from shear points, this could impact the initial material in the overmold process (assuming the material in discussion is for the overmold). Consider dropping the melt temperture and mold temperature until you get true voids or underfill, then increase mold temperature until you acquire a complete fill without flash. Let the heat from properly designed shear points do the work. Finally are you venting air pockets properly.

Go to the Alphgary commercial WEB site and download their injection molding specifications for additional information found on the bottom right of the screen.

Since we know nothing of the exact material you are using or the cavity size/mold design, the above suggestions are at best general suggestions.

You can also learn more about plastics polymers and how they are molded at our commercial WEB site where you can link to our FREE on-line magazine. There are so many polymers out there and part variances it is hard to discuss each one; which is where the challenges start and general concepts leave off.

Best of Luck,

PMESTL