Plastic mould design for injection moulding


Applying draft and radii for a part is vital to a properly designed injection-moulded part. Draft helps a part release from mould with less drag on the part's surface since the material shrinks onto mould core. Limited draft requires an excessive amount of pressure on the ejection system that may damage parts and possibly the mould.

A good rule of thumb is to apply 1 degree of draft per 25mm of cavity depth, but it still may not be sufficient depending on the material selected and the mould's capabilities.

Protolabs uses CNC milling tomanufacture the majority of the features in the mould. The result of our manufacturing process drives a unique wall thickness and draft angle based on the end mill that we are using for each feature. This is where our design for

manufacturability (DFM) analysis becomes particularly helpful as our software looks at each part feature separately and compares it to our toolset. The design analysis highlights the part geometry where increased draft and thickness may be required.

On the other hand radiiisn't a necessity for injection moulding, but should be applied to the part for a few reasons—eliminating sharp corners on the part will improve material flow as well as part integrity.

Resin filling the mould cavity flows better around soft corners much like the flow of a river.Rivers don't have 90 degree corners as the water flow creates inside and outside corners so it moves easily towards its final destination. Similarly, plastic resin wants to take a path of least resistance to minimise the amount of stress on the material and mould. Radii, like draft, also aid in part ejection as rounded corners reduce the chance that the part will stick in the mould causing it to warp or even break.

Injection molded product design evolves through many phases of development before all the parts are finally documented and released for production. The last step in the development process is the most critical, since design changes or correctionscan no longer be made without significantly adding cost or project delays.Unfortunately, plastic part design mistakes will be uncovered only after first article parts are inspected and evaluated by the project team. Even with today’s sophisticated mold flow simulation, 3D CAD interference checks, rapid prototyping and numerous other development tools, it is impossible for anyone to predict every potential problem for an injection molded part. However, ther's a very simple, low-cost method for minimizing potential problems and virtually ensuring perfect parts. It’s called partnering with your molder, which is the focus on this article.

It doesn’t matter how well you think you know how to properly design parts for injection molding—you should always form a close partnership with your preferred molder as early in the design process as possible. Every molder has his or her own tooling preferences and techniques for molding parts, which will have a significant effect on part design. These subjective preferences can influence any of the following major design-related parameters affecting an injection molded part:

1.   Material options and consequences

2.   Critical tolerances

3.   Sink marks

4.   Steel safe areas

5.   Gate location

6.   Shut-off angles

7.   Draft angle orientation

8.   Texturing and draft

9.   Scheduling of critical start-up phases

10.  Secondary operations and fixtures

It’s difficult for designers/engineers to develop this relationship early in the design process, since the selection of a molder is often postponed until the design is completed and released for formal quoting by the purchasing department. In addition, many molders will not provide any input until they are assured the project will be awarded to them. This stalemate precludes designers from following these recommendations, often resulting in unacceptable delays or cost overruns because of tooling complexity or long cycle times. These policies are not cost effective in the long run, since they significantly reduce the efficiency of developing a product. However, there are some simple solutions for solving this paradox.

1st solution typically used by larger companies is to generate a short list of preferred vendors based on an extensive analysis of experts within their staff. This limited group of 3 to 4 preferred mold makers are typically accessible to engineers throughout the development because of their mutually beneficial business arrangements. Small companies can select one or two viable molders early in the process by establishing a good-faith business relationship. This informal handshake agreement requires both parties to be mutually honest about the estimated costs and terms of ultimately doing business with one another. Although there’re no guarantees, an alliance could be developed as molders and designers share their knowledge throughout the design process.

It should be noted that designing a quality injection moulded part requires designer to be knowledgeable about all the fundamental design parameters associated with injection molding and to be highly skilled. The molder/designer partnership is not intended to be an internship program—it’s supposed to optimize handoff of the final design to production with few or no changes. If completed successfully, final production parts typically are cost effectively molded precisely to specifications for the following reasons.

Hongmei mould will consider all the feasible ways of suitable mould design and discuss with customer before mould production to avoid further production problems.

Any mould design and manufacturing questions,feel free to contact us!