Getting started with injection molding

Hello,
My name is Sean and I have been designing a product that I eventually hope to get manufactured via injection molding and I have some questions on working with manufacturers. I have tried to design my parts so that they can all be made with a 2 part mold, but as a layman I can not be sure that I was successful. In addition to this my parts currently have varied wall thicknesses, no draft angles, and large sections of material that need to be hollowed out. In addition to this in order to reduce the number of molds needed (and hopefully the price as well) I wanted to lay out many parts on the same sprue like you would see with hobby model kits. The questions I have include:

When approaching a manufacturer do the parts need to be ready for injection molding, or will they work with me to get them to spec? Will this change the price of the service?

If I want to lay out multiple parts on the same sprue to I need to design the layout or does the mold maker take care of this?

Do manufacturers take a job if they are not confident that the parts will not come out correctly? If parts don't come out correctly do they ship them anyway or do they remake them? If they get remade is it at their expense or mine?

What is the typical size of a smaller run? I was hoping to start with 500-1000 units. Will this be a tiny order or fairly typical?

Any guidance on any of these questions would be hugely appreciated.

-Sean

Hi Sean,

What you are describing is a "family" mold. These are not good for tight tolerances because the small parts get filled and packed before the big parts.

"2 part mold" = 2 plate mold (?)

A manufacturer will usually only get the molds only after they are close to production ready , but many will work with you as long as you are paying for the sampling. So, you have 2 costs.... sampling costs and piece part price. You need to discuss both with the manufacturer as well as quality requirements. Generally speaking, the manufacturer will supply you with what is agreed to as "good" parts at the piece part price and will grind the rest at their expense.

500 - 1000 pcs is a fairly typical small run.

For low volume jobs, the biggest expense is the mold, tens of thousands of dollars.

Regards - Joel

Hey Joel,
Thanks for the info. In case it wasn't obvious I am figuring this stuff out as I go and I really appreciate your time.
When you say its no good for tight tolerances, can you provide an example of loose tolerances vs tight tolerances?
By a 2 part mold I mean that I designed my parts with no undercuts or anything else that couldn't be machined with an end mill (other then some sharp inside corners)

Are mold makers generally willing to help guide their clients through the process or is it more of an input/output type of operation? I make scale models for a living and am used to pouring over the details of a project with the client, but I'm not sure if that translates into other industries.

Thanks again,
Sean

Sounds to me like its a family mold, i.e. a mold that makes 2 or more parts that are different shapes. A mold that would make a left and right hand sides of some assembly would be an example. You sound like you are making something similar to a model toy kit that would be one shot of plastic containing all the pieces. The problem with this concept is you will have changing wall thicknesses that will make processing more difficult. If you are gated into a thicker area of one part, and then the part connected next to it is thinner and then the part after is thicker, the plastic in the middle part will freeze off before the last part does. If the middle part is frozen off and the last part is still molten, then you wont be able to pack it out and you will get warp, sinks, voids et.

My suggestion would be to contact and molding company that is experienced with these types of large family molds, like a model toy making company. That way you will get less resistance to the concept and more encouragement/help. Contact several different companies and ask what services they offer and what services they charge for. As Joel pointed out above, you may be charged for sampling the mold and you may be charged for their design expertice.

Hi Sean,

Good advice above. I am not the expert for you as I receive the tools after all the initial work has been done. Having said that, I will try to help.

You might look into an aluminum mold, this will save you money, but it wears out faster.

If you had a 5 inch part, and the length dimension was plus or minus .005, this would be tight, if it was plus or minus .050, this would be loose. You should google "injection molding tolerance guidelines" and this will give you all kinds of dimensions and tolerances.

You should call a mold builder(s) and discuss. They may direct you to a designer before they get involved.

The mold builder will generally sample and debug the tool at their expense, but engineering changes are generally your costs. Discuss this with any mold builder.

As a last bit of advice.. rapid prototyping will get you some parts in hand that may help you with fit/function and early design debugging.... and maybe (but not sure) that the design can be helpful for the mold design.

Hope that helps - Joel