Four major considerations, five major points to help improve plastic mold design

Time:2019-08-28 09:25:04 / Popularity: / Source:

According to research data from Research and Markets, global market value of plastic injection molding parts is currently US$325 billion. It is expected that market value will reach US$478.72 billion in 2025 and compound annual growth rate will be 5.7% in 2019-2025.
Driven by rapid development of Asian economy, advancement of process efficiency and new resin materials, plastic injection molding parts are widely used in more complex applications, which presents great opportunities and challenges for product designers and engineers.
A significant advantage of plastic injection molding is ability to produce very large quantities of product at a lower unit cost relative to development cost (tool). However, these same large-volume production requires a good plastic mold design.
Small, progressively improved plastic mold design can have negligible impact on production of small-volume plastic injection molding parts, but have a significant economic impact when producing millions of finished parts. This is why it is important to use reasonable design logic in design phase of any tool project.
As following are four key factors and five considerations to consider when designing two multi-cavity injection molds for products used in high temperature environments, and these considerations are generally applicable to any application.

Material design

Each thermoformed resin has unique chemical and mechanical properties that require mating with mold steel components that will be used to mold it. For example, when using polyetheretherketone PEEK, a high temperature engineering plastic for automotive, aerospace, and medical applications, mold needs to be able to withstand higher molding temperatures while maintaining dimensional stability.
plastic mold design 

1. Parts produced from high temperature PEEK plastic materials

For parts made from high temperature PEEK plastic materials, recommended tooling material choice is S136 stainless steel. Stainless steel is suitable for large-scale production at higher temperatures required by PEEK. In addition, stainless steel is resistant to abrasion of glass fibers and is highly polished to give plastic parts excellent surface quality.
Since PEEK must be injection molded at high temperatures and pressures, mold tool must be carefully heat treated. Heat treatment process is performed before final polishing and after CNC processing.
plastic mold design 

2. Rough grinding before mold polishing and heat treatment

Since S136 stainless steel is not an inexpensive tool steel, it is recommended to process only a portion of molding tool insert to reduce overall tooling costs. Once processed, insert can be placed into a modular mold of a larger standard molding tool material (such as P20 or NAK80 plastic mold steel) and then installed into machine.
Since PEEK requires injection molding temperatures that typically exceed standard heating loop temperatures of most injection molding machines, a separate electrical heating coil must be used to meet necessary high molding temperatures.
Electric heaters distribute heat very efficiently and evenly to achieve good molding results, but in turn put higher demands on correct design of cooling channels for fast heat dissipation and good cycle times.

Draft angle design

Due to unique shrinkage of resin, shrinkage occurs during cooling, which may cause plastic injection molding parts to stick into cavity and cause sticky mold conditions.
Manufacturer's spec sheet helps determine minimum draft angle, but it is also affected by surface texture of part. At some point, more textured (biting) injection molded parts require a larger draft angle.
plastic mold design 

3. Moving parting line needs to be drafted

It is common for product designers to determine parting line at right angle intersections of two vertical faces. However, if one of sides is appearance and any excess material appears in the mold, appearance surface may be damaged.
In order to avoid such damage, it is best to move parting line to adjacent non-design surface. Parting line should be slightly tilted along draft angle, not just 90 degrees. If excess material is present, it can be trimmed without damaging appearance of finished part.

Wall thickness design

Wall thickness management is critical to controlling stress marking while ensuring that design meets minimum wall thickness while maintaining maximum consistency with thickness of adjacent areas.
plastic mold design 

4. Apply high injection pressure near gate

Gate is area that represents high initial injection pressure, and narrow wall thickness also means that increasing injection pressure is limited. If unbalanced, these two forces can cause shearing of mold, and excess material may even damage effect. Therefore, it is a good design practice to increase wall thickness near gate, reduce injection pressure, or perform both actions simultaneously.
Second, likelihood of excess material will increase as pressure is accurately applied to dividing line. If there is excess material, it takes a lot of time and effort to remove it, and it will leave a corresponding mark on finished product.
Finally, uneven quality in adjacent areas will also result in dents. These damages not only make appearance of plastic parts unsightly, but also damage structural integrity of parts.

Demoulding design

Release force should be applied evenly to surface area of plastic injection molding parts. Considering thickness and mass to prevent part from warping or breaking. In addition, in the area around gate, it will be necessary to add a discharge plate or an additional thimble release device. These thimble-type demolding devices are necessary to clean gate in the case of short shots.
plastic mold design 

5. Is geometry difficult to balance?

Since area near gate will be under stress, it is best to design wall thickness to be as thick as possible, or to use a pad or other flat area to provide ejector pinning force.
As designers and engineers continue to move forward in direction of new product development, it is critical to grasp nuances of plastic injection molding and to properly adapt these differences.

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