According to research data from Research and Markets, the current global market value of plastic injection molded parts reaches 325 billion U.S. dollars. It is estimated that the market value will reach 478.72 billion U.S. dollars as early as 2025, with a compound annual growth rate of 5.7% from 2019 to 2025.
Driven by the rapid economic development in Asia, as well as benefiting from process efficiency and the advancement of new resin materials, plastic parts are widely used in more complex applications, which brings huge opportunities and challenges to product designers and engineers.
A significant advantage of plastic injection molding is the ability to produce very large quantities of products at a relatively low unit price (tool) relative to the development cost. However, these same large-volume productions require the use of good mold design.
The small, incrementally improved mold design may have a negligible impact on the production of small batches of injection molded parts, but it will have a significant economic impact when producing millions of finished parts. This is why in the design phase of any tool project, it is important to adopt reasonable design logic.
The following are the four key factors and five precautions that need to be considered when designing two multi-cavity injection molds for products used in high-temperature environments. These considerations are generally applicable to any application project.
Material design
Each thermoforming resin has unique chemical and mechanical properties and needs to be matched with the mold steel parts that will be used to mold it. For example, when using polyetheretherketone PEEK (a high-temperature engineering plastic used in automotive, aerospace, and medical applications), the mold needs to be able to withstand higher molding temperatures while maintaining dimensional stability.
1. Parts produced with high-temperature PEEK plastic materials For parts produced
with high-temperature PEEK plastic materials, the recommended mold tool material selection is S136 stainless steel. Stainless steel is suitable for mass production at the higher temperature required by PEEK. In addition, stainless steel is also resistant to the abrasion of glass fiber, and is highly polished, which can give plastic parts an excellent surface quality.
Since PEEK must be injection molded under high temperature and high pressure, the mold tools must be heat treated carefully. The heat treatment process is performed before the final polishing and after the CNC machining.
2. Rough grinding before mold polishing and heat treatment
As S136 stainless steel is not a cheap tool steel, it is recommended to process only part of the molded tool inserts to reduce the total tool cost. Once processed, the insert can be placed in a modular mold of a larger standard molding tool material (such as P20 or NAK80 plastic mold steel), and then installed in the machine.
Since the injection molding temperature required by PEEK usually exceeds the standard heating circuit temperature of most injection molding machines, in order to meet the necessary high molding temperature, a separate electric heating coil must be used.
Electric heaters can distribute heat very effectively and evenly to obtain a good molding effect, but in turn put forward higher requirements for the correct design of the cooling channel in order to quickly dissipate heat to obtain a good cycle time.
Draft angle design
Because the resin has a unique shrinkage rate, it shrinks when it cools, which may cause the molded part to stick to the mold cavity and cause a mucosal condition.
Although the manufacturer's specification sheet helps determine the minimum draft angle, it is also affected by the surface texture of the part. At a certain point, more textured (bited) injection molded parts require a larger draft angle.
3. The draft angle is required to move the parting line
It is common for product designers to determine the parting line at the intersection of two vertical planes at right angles. However, if one of the surfaces is the exterior surface, and any excess material appears in the mold, the exterior surface may be damaged.
In order to avoid this kind of damage, it is best to move the parting line to the adjacent non-appearance surface. The parting line should move slightly obliquely along the draft angle, not exactly 90 degrees. If excess material appears, it can be cut out without damaging the appearance of the finished part.
Wall thickness design
Wall thickness management is essential to control stress marks, while ensuring that the design meets the minimum wall thickness while maintaining maximum consistency with the thickness of adjacent parts.
4. Apply high injection pressure near the gate
The gate is the area that represents the high initial injection pressure, and the narrow wall thickness also means that increasing the injection pressure will be limited. If unbalanced, these two forces will cause shearing of the mold, excess material and even damage the effect. Therefore, it is good design practice to increase the wall thickness near the gate, reduce the injection pressure, or perform both actions at the same time.
Secondly, the possibility of excess material will increase because the pressure will be accurately applied to the dividing line. If there is excess material, it will take a lot of time and effort to remove it, and a corresponding mark will be left on the finished product. Finally, the uneven quality of adjacent areas will also cause dents. These damages not only make the appearance of plastic parts unsightly, but also damage the structural integrity of the parts.
Demoulding design
The demolding force should be evenly applied to the surface area of the injection molded part, while considering the thickness and quality to prevent the part from warping or breaking. In addition, in the area around the gate, it will be necessary to add a stripper plate or an additional ejector pin release device. These thimble-type demolding devices are necessary to clean the gate in the case of short shots.
5. Is it difficult to balance geometric shapes?
Since the area near the gate will be under stress, it is best to design the wall thickness as thicker as possible, or use a gasket or other flat area to provide the pushing force of the ejector pin.
As designers and engineers continue to advance in the direction of new product development, it is important to grasp the nuances of plastic injection molds and adjust them correctly.