Two level mold

Two-Level Mold n (double-decker mold) An injection mold having two layers of cavities, used in making low-mass articles with large projected areas, so as to make best use of both plasticating and clamping capacities. 

A two-mask process has been used to create a glass master containing two levels of positive photoresist relief structures. The master was used to cast a PDMS chip consisting of the channel/chamber (25-30 pm deep) and weir (7-12 pm clearance) [960]. Another two-mask process was used to create a Si molding master consisting of 3-pm-high Si relief stmctures and 25-pm-high photoresist relief structures [364]. 

A thin membrane having channel structures is molded on each face and sandwiched between two thicker, flat slabs that provide a structural support. Two masters are fabricated by rapid prototyping using two-level photolithography and replica molding and aligned face to face. The prepolymer is between these masters. Then the poly(dimethylsiloxane) is thermally cured. 

Table 5.4. Injection molding parameters used for a two-level factorial experimental design ...

A full factorial design has been carried out performing 2" =16 molding experiments (each parameter being varied between two levels) with 5 replications. In this way the influence of main effects as well as interactions was evaluated. Factors levels were established with the aim of keeping a realistic industry relevant perspective (see table 1). In particular ... 

Creep. The creep characteristic of plastic foams must be considered when they are used in stmctural appHcations. Creep is the change in dimensions of a material when it is maintained under a constant stress. Data on the deformation of polystyrene foam under various static loads have been compiled (158). There are two types of creep in this material short-term and long-term. Short-term creep exists in foams at all stress levels however, a threshold stress level exists below which there is no detectable long-term creep. The minimum load required to cause long-term creep in molded polystyrene foam varies with density ranging from 50 kPa (7.3 psi) for foam density 16 kg/m (1 lb /ft ) to 455 kPa (66 psi) at foam density 160 kg/m (10... 

The performance of the different polymers was based on the molding scrap levels obtained for two categories of scrap ... 

These SPI tables can also be used as the basis for establishing standards for molded products between the designer, molder, and customer. Users will find that two separate sets of values are represented. Commercial values represent common production tolerances that can be achieved at the most economical level. Fine values represent closer tolerances that can be held, but at a greater cost. The selection of one or the other will depend on the application under consideration and the economics involved. 

Residual stress There is a condition that develops, particularly in products with thin walls. This is a frozen-in stress, a condition that results from the filling process. The TP flowing along the walls of the mold is chilled by heat transferring to the cold mold walls and the material is essentially set (approaching solidification). The material between the two chilled skins formed continues to flow and, as a result, it will stretch the chilled skins of plastics and subject them to tensile stresses. When the flow ceases, the skins of the product are in tension and the core material is in compression that results in a frozen-in stress condition. This stress level is added to any externally applied load so that a product with the frozen-in stress condition is subject to failure at reduced load levels. 

This case study involved medical diagnostic parts manufactured from PC resin, which were beginning to break too easily. To determine the cause of the failure, a good sample was submitted for comparison to a cracked part. Two possible causes for the failures were postulated. These include brittleness due to an excess level of filler, or the presence of voids due to insufficient drying of the resin prior to molding. 

Let us illustrate the main theoretical results by discussing a mold tilling model when the mold contains a metal insert to be coated with a polyurethane compound.266 Two principal conditions must be met in this case. First, the total pressure drop must not exceed the limiting power of the molding machine. Second, the degree of conversion of the reactive mass at the moment of contact with the surface of the insert must not be higher than some critical level (3iim, which is determined by the requirement for sufficient adhesion between the polymer and the metal, because the adhesive... 

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