Tips needed on injection molding my product

Hi everyone,

I’m developing a product, I can’t exactly tell what it is because it’s not yet patented, but it’s quite a simple storage item for convenience, similar to a toothbrush case (see attached). It’s going to be injection molded, probably in China.

It would be great to have some tips on the following points:

  • The design specifies a high gloss white finish, similar to say, Apple or Nintendo Wii products. And scratchproof if possible. For material I’m thinking ABS, or could PP also give a similar appearance, or maybe another material?
  • A Chinese molder i’ve been in contact with says they need 1 degree draft for glossy parts (usually 2 even), is that true for most Chinese molders? Because Protomold says they can do 0.5 degree. But I’ve even measured a draft as low as 0.35 degrees over 75mm on a simple ballpoint pen, and it doesn’t show split lines of a multicavity mold or sliders or other types of tooling. It’s probably PP (although I should do one of those burning tests) and the pen looks fine.
    I attached a picture of what I’m trying to achieve . It’s a 0.4 degree draft max. on the cavity without side actions to prevent witness lines.
  • As the part’s draft angles are different for the inside and outside surface, the wall thickness decreases linearly from 1.65 to 1.1 mm. Is that an ok thickness for an ABS part?

Any help on these points would be great, since I don’t have too much experience with injection molding.


Ralph Zoontjens
toothbrush cases.jpg

On highly polished surfaces although lower draft is possible, they tend to ask for more due to yield. If they start scratching the parts when ejecting them, they don’t want the customer to start rejecting them due to the tight design constraints. 1-2 degrees is fairly typical, especially over that long of a haul.

Your wall thickness should be OK as well.

from your sketch, it’s unlikely the part would be tooled as drawn, you’re putting the entire part onto the core tool. The part would be molded with matching positive / negative draft angles in the core and cavity tools therefore producing an equal wall thickness from top to bottom. Regardless, the comments on degree of draft angle are accurate.

I have never regretted being conservative with draft angles. Even with Protomold I try to hover around 2deg, it’s just not worth the potential failures to get that extra degree.

Not if you want a 0.4 draft and your Chinese vendors want 1-2.

The 50% increase in in wall thickness is somewhat problematic, but I’ll assume the gating is on the cavity side and won’t be to much of an issue.

I think the problem is the 0.4 on the cavity side. The core with 1 will release first and you will have a stuck part. My guess is that our tooling guy will want to create an undercut on the core side to ensure hold.

Are you starting with a prototype tool or are you jumping right into production tooling?

ABS can sometimes be molded with no draft angle, and your thin wall at the top can certainly be done.
However, whether or not your Chinese supplier can do it depends on the type of molding press they are using, whether they have optimized the mold construction, and whether they can achieve optimal process conditions.

There’s a reason why Protomold and the pens you observed have less draft and thinner walls.
It’s because they’re not molding in China.

Those Chinese kids are fast and they work hard, but they’re not using the best tools and equipment.
My experience is you’ll need to allow them the widest processing window possible, and that means maximize draft and make your walls at least .060" thick.

Thanks for all the replies, much appreciated.

  • the draft angle is quite critical for aesthetical purposes, it needs to look straight and to me anything above 0.4 degrees is no longer acceptable. I guess I am going for the Steve Jobs style of trying to push manufacturing limitations in order to get that extra sharp look. I also don’t want a split line down the center as produced with split-cavity molds (seen for example on the front of some electric toothbrushes), so it probably means that I will need to look for a supplier here in Europe.
  • The gating is on the cavity side, I will keep the wall thickness like this.
  • The core side does have an undercut -not visible in the image- since there will be a groove for a snap fit on either side. That will prevent the part from sticking to the cavity on opening of the mold.
  • We will probably need to make a prototype tool first (or a multicavity mold with the first cavity being the prototype tool). Currently a 3D printed model is being created to judge the overall design, but dimensions will need to be verified in the actual material as well.

Thanks again for the advice.

One word of advice is be careful when pushing vendors into areas they don’t feel comfortable with. I’ve had many interactions with overseas cm’s and the issue I run into more often than not is a lack of feedback when they have questions or concerns.

I would advise you to get samples as frequent as possible when trying to push back on methods your counterparts may not be comfortable with. They may agree to your standards but in reality it may never be accomplished or the consistency you want can’t be met with their equipment and skill-sets of their employees.

It varies from vendor to vendor but make sure to ask any questions or concerns you have upfront before you agree on something that can’t be revised.

Also is there any way you could revise the actual design of the product to achieve the same aesthetic? You say you want straight no draft walls on certain areas, is it possible you could divide this into separate parts and accomplish this with an extrusion?

Thanks, extrusion is a good option we can investigate. It has to be a good glossy surface finish but I am sure this is possible.
In the meantime we have found vendors that can produce this item. 0 degrees is possible in some cases with air assisted ejections.

I am now working on another item that requires a special side cam. I am wondering if you can design side cams to rotate into the mold cavity rather than a linear sliding motion, to create hook-like cavities in a part. Let’s discuss some of these special features people have used in injection molds.

I’m not sure if it’s possible to do what you’ve suggested, but I have seen a few images around that look like they use a rotating core.

I’ve grabbed this one off of google, and it looks like it’s has a rotating core on the end part. I’ve no idea how this would work, but it looks a bit rudimentary.

Interesting, my idea is just that but for rotating cam actions inside a mold half.
It will require some new tooling solutions but if successful this allows for the creation of new features, such as hinge axes internal to a part.

The tooling guys might not like you very much :laughing:

I imagine sliders of this kind would require a lot of maintenance, have you asked a shop if they would have any ideas for the insert?