1) Moisture in the Granules
Many plastics absorb moisture from the atmosphere; how much they absorb depends on the type of resin. Moisture in the granules, even if it is only surface condensation, can cause problems in parts moulded with engineering polymers. Many kinds of undesirable effects can occur, including processing problems, poor surface on moulded parts, or loss of mechanical properties.
2) Feed System Too Small
Parts made of engineering polymers nowadays are designed with the help of complex methods such as computer-aided design, finite element analysis and mould-flow calculations. Though they are unquestionably useful, they sometimes fail to take enough account of the importance of the correct design of the feed system. This article considers the basic elements of correct feed system design for semi-crystalline polymers.
3) Wrong Gate Position
Since the position of the gate is usually specified by designers and mould makers, this article has been written especially with these persons in mind. Nevertheless, injection moulders should be also involved from the planning stage, to prevent predictable problems.
4) Hold Time Too Short
In practice many injection moulders, working from their experience of amorphous polymers, tend to use shorter hold pressure times and longer cooling times. Unfortunately, this approach also tends to be used for semi-crystalline polymers such as POM (acetal), PA (nylon), PBT and PET (polyesters). This article discusses the most important points to help machine setters chose the most suitable hold pressure time.
5) Wrong Melt Temperature
Choosing the right melt temperature is vital for part quality when moulding semi-crystalline engineering polymers. As a rule the margin of tolerance is less than when processing amorphous resins. The moulder at his machine directly influences the properties of the end-product. In the fifth chapter of this ten-part series, the authors consider the question of melt temperature when moulding POM (= acetal), PA (= nylon), PBT and PET (polyesters).
6) Wrong Tool Temperature
When moulding semi-crystalline engineering plastics such as POM (acetal), PA (nylon), PBT and PET (polyesters), it is important to make sure that the surface temperature of the tool is correct. The basic requirements for optimum processing are in the design of the tool. Only if the tool design is right can the moulder produce good quality parts with the help of temperature control equipment. This calls for close co-operation in the tool design and planning phase, in order to avoid production problems at a later stage.
7) Poor Surface Finish
Partially crystalline engineering thermoplastics such as POM (acetal), PA (nylon), PBT and PET (polyesters) are used primarily because of their outstanding mechanical, thermal and electrical properties. Further advantages over amorphous materials include their excellent chemical resistance and low tendency towards stress cracking. In many kinds of applications a high quality surface finish is an additional requirement. This article is intended to help eliminate possible surface defects.
8) Problems with Hot Runners
When injection moulding partially crystalline engineering thermoplastics, choice of the correct hot runner system determines the function of the mould and moulded part quality. Here, the temperature must be controlled much more strictly than in the case of amorphous materials. The type of hot runner system used, and its installation, decide the properties of the finished parts. This article deals with the most important points which have to be considered when choosing the most suitable hot runner system for POM (acetal), PA (nylon), PBT and PET (polyesters).
Partially crystalline substances such as POM (acetal), PA (nylon), PBT and PET (polyesters) tend to warp far more than amorphous ones. This point should be taken into consideration already when designing moulds and mouldings. If this is not done, it is almost impossible to rectify at a later stage. This article discusses the causes of warpage and steps that can be taken to prevent and reduce it.
10) Mould Deposit
These can occur with nearly all thermoplastics. As demands on end-products increase, so do the amounts of additives which have to be incorporated, e.g. modifiers, flame retardants, etc. These additives may quite often cause deposits to form on the mould cavity surface.
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