How to Make Injection Molds

Frontmatter

1. Materials for Injection Molds

Abstract

The injection molding technique has to meet the ever increasing demand for a high quality product (in terms of both consumption properties and geometry) that is still economically priced. This is feasible only if the molder can adequately control the molding process, if the configuration of the part is adapted to the characteristics of the molding material and the respective conversion technique, and a mold is available which satisfies the requirements for reproducible dimensional accuracy and surface quality. Therefore injection molds have to be made with the highest precision. They are expected to provide reliable and fully repeatable function in spite of being under extreme loads during the molding process, and a long service life to offset the high capital investment. Besides initial design and maintenance while in service, reliability and service life are primarily determined by the mold material used, its heat treatment and the machining operations during mold making [1.1, 1.2].

Georg Menges, Walter Michaeli, Paul Mohren

2. Mold Making Techniques

Abstract

Injection molds are made by a highly varied number of processes and combinations thereof.

Figure 2.1 demonstrates the relative costs for cavities made from various materials. Accordingly, steel cavities appear to be many times more expensive than those made of other materials. In spite of this, a cavity made of steel is normally the preferred choice. This apparent contradiction is explained with the consideration that the service life of a steel mold is the longest, and the additional costs for a cavity represent only a fraction of those for the whole mold.

Georg Menges, Walter Michaeli, Paul Mohren

3. Procedure for Estimating Mold Costs

Abstract

Injection molds are made with the highest precision because they have to meet a variety of requirements and are generally unique or made as very few pieces.

They are to some extent produced by very time- and cost-consuming procedures and are therefore a decisive factor in calculating the costs of a molded product. The mold costs for small series often affect the introduction of a new product as a deciding criterion [3.1]. In spite of this, in many shops, calculation not have the place to which it should be entitled.

Georg Menges, Walter Michaeli, Paul Mohren

4. The Injection Molding Process

Abstract

The injection molding process is one of the key production methods for processing plastics. It is used to produce molded parts of almost any complexity that are to be made in medium to large numbers in the same design. There are major restrictions on wall thickness, which generally should not exceed a few millimeters, and on shape – it must be possible to demold the part. This will be discussed later.

Georg Menges, Walter Michaeli, Paul Mohren

5. Design of Runner Systems

Abstract

The runner system accommodates the molten plastic material coming from the barrel and guides it into the mold cavity. Its configuration, dimensions and connection with the molded part affect the mold filling process and, therefore, largely the quality of the product. A design which is primarily based on economic viewpoints (rapid solidification and short cycles) is mostly incompatible with quality demands especially for technical parts.

Georg Menges, Walter Michaeli, Paul Mohren

6. Design of Gates

Abstract

The sprue gate is the simplest and oldest kind of gate. It has a circular cross-section, is slightly tapered, and merges with its largest cross-section into the part.

Georg Menges, Walter Michaeli, Paul Mohren

7. Venting of Molds

Abstract

During mold filling the melt has to displace the air which is contained in the cavity. If this cannot be done, the air can prevent a complete filling of the cavity. Besides this the air may become so hot from compression that it burns the surrounding material. The molding compounds may decompose, outgas or form a corrosive residue on cavity walls. This effect can occasionally be noticed in poorly vented molds at knit lines or in corners or flanges opposite the gate.

Georg Menges, Walter Michaeli, Paul Mohren

8. The Heat Exchange System [8.1, 8.2]

Abstract

The velocity of the heat exchange between the injected plastic and the mold is a decisive factor in the economical performance of an injection mold. Heat has to be taken away from the thermoplastic material until a stable state has been reached, which permits demolding. The time needed to accomplish this is called cooling time. The amount of heat to be carried off depends on the temperature of the melt, the demolding temperature, and the specific heat of the plastic material.

Georg Menges, Walter Michaeli, Paul Mohren

9. Shrinkage

Abstract

If plastics are processed by injection molding, deviations of the dimensions of the molding from the dimensions of the cavity cannot be avoided. These deviations from the nominal size are summarized under the term shrinkage.

Georg Menges, Walter Michaeli, Paul Mohren

10. Mechanical Design of Injection Molds [10.1]

Abstract

Injection molds are exposed to a very high mechanical loading but they are only allowed elastic deformation. Since these molds are expected to produce parts that meet the demands for high precision, it is evident, therefore, that any deformation of the mold affects the final dimensions of a part as well as the shrinkage of the plastic material during the cooling stage. Besides this, undue deformation of a mold can result in undesirable interference with the molding process or actuation of the mold.

Georg Menges, Walter Michaeli, Paul Mohren

11. Shifting of Cores

Abstract

A problem in the mechanical design of molds is the determination of core shifting in parts which are like cups or sieves or more complex parts containing such configurations.

An eccentric mounting of a core due to inaccuracy during production or an asymmetric gating causes a lateral loading of the core. The resulting deformation of the center axis leads to a shift at the tip of the core. Dimensional accuracy of the molding and the demolding process are adversely affected.

Georg Menges, Walter Michaeli, Paul Mohren

12. Ejection

Abstract

After the molding has solidified and cooled down, it has to be removed from the mold. It would be ideal if gravity could separate the part from cavity or core after mold opening. The molding is kept in place, however, by undercuts, adhesion and, internal stresses and, therefore, has to be separated and removed from the mold by special means.

Georg Menges, Walter Michaeli, Paul Mohren

13. Alignment and Changing of Molds

Abstract

Injection molds are mounted onto the platens of the clamping unit of the injection-molding machine. The clamping unit opens and closes them during the course of the molding cycle. The molds have to be guided in such a way that all inserts are accurately aligned and the mold halves are tightly closed. Without proper insert alignment, molded parts would exhibit deviations in wall thickness; they would not have the required dimensions.

Georg Menges, Walter Michaeli, Paul Mohren

14. Computer-Aided Mold Design and the Use of CAD in Mold Construction

Abstract

Development work on the simulation of the injection molding process started in the mid-1970s when the first simple programs for programmable pocket calculators became available for calculating the pressure loss in specified flow channels. The geometry options then available were cylinders for the gate system and plates and circular segments for the molded part, depending on whether the melt flowed through a constant or a divergent channel (Figure 14.1).

Georg Menges, Walter Michaeli, Paul Mohren

15. Maintenance of Injection Molds

Abstract

Injection molds represent a major investment for plastics processors. They constitute a large position of the company ‘s assets and are the basis for production, economic success, and technical development. For these reasons, injection molds must be in good working order and ready for use.

Georg Menges, Walter Michaeli, Paul Mohren

16. Measuring in Injection Molds

Abstract

Crucial to the quality of injection moldings is the state of the melt after plastication and how the mold-filling process went, which is characterized by the change in pressure and temperature.

It is therefore advisable to incorporate appropriate sensors into the molds for monitoring and perhaps control purposes.

Georg Menges, Walter Michaeli, Paul Mohren

17. Mold Standards

Abstract

Injection molds are always made in accordance with the same rules. Therefore, it should not come as a surprise that their design approach is always similar. This holds particularly true for the basic components. A great number of companies are specialized in manufacturing such basic elements. They produce these elements on a large scale and in a variety that a detailed discussion of these products would be beyond the scope of this chapter. We emphasize, therefore, to contact a supplier of mold standards. Suppliers offer extensive and very informative catalogs. Sometimes the information is also available as a software database. Figures 17.1–17.3 and Table 17.1 show the most common mold standards and their application areas.

Georg Menges, Walter Michaeli, Paul Mohren

18. Temperature Controllers for Injection and Compression Molds

Abstract

Temperature controllers have the function to bring up molds connected to them to processing temperature by circulating a liquid medium and keep the temperature automatically constant by heating and cooling.

Georg Menges, Walter Michaeli, Paul Mohren

19. Steps for the Correction of Molding Defects During Injection Molding

Abstract

In the following a number of molding defects, which are mostly caused by faulty mold design, are listed along with steps to remedy them. A first summary is presented with Figure 19.1. It shows a point system, which has been developed to remove visible defects from acrylic moldings [19.1]. However, it can also be applied to other thermoplastic materials.

Georg Menges, Walter Michaeli, Paul Mohren

20. Special Processes – Special Molds

Abstract

The injection molding process may be used to produce on the one hand extremely small parts with molded-part weights of less than 1000th of a gram and, on the other hand, parts with structured areas each measuring just a few square micrometers. Both applications enable mass production of molded parts intended specifically for microsystems technology.

Georg Menges, Walter Michaeli, Paul Mohren

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