Thread Design

Thread are used in plastic for the purpose of providing a secure anchorage or locking device for a mating plastic injected part. There are two types of threads are used in plastic injected part design: internal and external thread. External or internal screw threads can be molded. An internal thread is on the inside of the plastic injected parts which molded-in by a threaded core. An external thread is on the outside of a plastic injected part which molded by threaded pipe or threaded hold or by two half-molds.

Threads in plastic injected parts are obtained by four methods: molded-in; trapped; threaded metal inserts and thread inserts are pressed or cemented into plastic injected parts after injection molding.

Threads formed by thread plug or tube should be taken to eject the plastic injected parts from the mold. Usually, mechanical unscrewing or collapsible cores can be used in injection molding. There are different types of threads used in the plastic industry. It includes the V sharp, square, acme, buttress, round profile thread, unified screw thread. Fig 1 shows profiles of different types of threads which usually used in plastic injected parts.

Fig 1a shows a developed unified screw thread which is used frequently. The feature is that the tip or crest of the thread is flat, the root of the thread has a radius and does not leave a sharp edge. It can be quick and easy assembly of the plastic injected parts which are desired, and for all work where conditions do not use of fine-pitch threads. 

A square thread (Fig 1b) is used where the highest strength is desired such as in pipe fittings. The thread unscrewing is not as easy as unified screw thread during molding.

The acme thread (Fig 1c) is widely used in applications requiring strength because this type of thread is much easier to mold or unscrew than the square thread.

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Fig 1 Profiles of different types of threads

Designing with Weld Lines

Knit lines, known also as weld lines, are formed when two melt fronts converge and joined together, forming a thin, fine line in a plastic injected part. In a plastic injected part, weld lines can also be formed as a result of flow interruptions such thin sections, holes, slots, and other types of cored-out areas. Weld lines affect the strength and aesthetics of plastic injected plastic.

If there are holes, ribs, bosses, and opposite gates in plastic injected parts(Fig 1), the plastic flow in a mold is split by an obstruction such as a pin, insert, corner, or slot, weld lines will usually result when the flow fronts meet, as in Fig 1 a and Fig 1 b. Weld lines are particularly noticeable in transparent and translucent plastic materials.

Weld lines not only distort the aesthetics of plastic injected parts, but also affect the strength of plastic injected part for the point of potential failure. This problem occurs mostly to thermoset  and thermoplastic materials used in injection. Weld lines can be prevented by designing the mold so as to permit the material to move with maximum freedom.

An overflow located at the converging melt fronts is a technique. That permits the plastic material at the weld line to flow into a pocket, allowing the two melt fronts to bond together with higher injection pressure, as Fig 1 c. However, after molding, this overflow may need to be removed manually.

Setting a porous metal insert at the place of melt fronts converge and join is another technique. This can remove trapped gases at the converging melt fronts and providing a higher weld line strength, as Fig 1 d. This is recommended mostly for textured surfaces since on smooth surfaces a slight gloss difference can be found on the plastic injected parts.

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Fig 1 Forming and Solution of weld line

Plastic Moulds and Plastic Forming Technology

The mould used to make plastic products is called plastic mould for short. In order to get plastic products with certain sizes, shapes and properties, thermosetting or thermoplastic plastic is heated to a certain temperature, put into a mold cavity, and taken out after it cools. This is called plastic forming technology.

Plastic forming technology are classified mainly into compression forming, transfer forming, injection forming in accordance with method of moulding. Moreover there are hollow how molding forming, foaming forming, pouring forming, etc.

In accordance with plastic forming technology, plastic mould are classified into compression mould, transfer mould, injection mould and hollow blow molding mould, foaming mould, pouring mould, etc.

There are many kinds of plastic moulds, but they only can be decided into two main types, forming parts and structure parts. Now, it explain basic structure of mould with a  example of typical structure.

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Fig 1 Basic structure of Fixed injection mould

a) Mould closing state    b) Mould opening state

1-Sprue puller pin  2-Ejector pin  3-Headed guide pillar  4-Core  5-Cavity plate

6-Cooling channel  7-Locating ring  8-Sprue bush  9-Clamping plate of the fixed half

10-Fixed half of a mould  11-Moving half of a mould  12-Support plate

13-Spacer (mold leg)  14-Ejector retaining plate  15-Ejector plate

Inserts Design in plastic injected parts

Inserts generally serve an important functional purpose, but should be used sparingly because of the costs involved. They may be used in plastic injected parts to carry much higher mechanical stresses than plastic themselves, to transmit electric current, to take wear and tear, to decorate the plastic injected parts, and to aid in assembly work. Most of inserts may be made of copper, brass, aluminum and its alloy, or steel, ceramics and including plastics.

The great majority of inserts used in plastic injected parts are made by either automatic screw machines or metal stamping machines. Fig 1 shows variety of metal inserts used in molded plastic products, usually a medium or coarse diamond knurl. (a) Projecting rivet. The end of the insert to be imbedded in the plastic is rounded or chamfered. A chamfered or rounded end is desirable so that the plastics will flow easily around the insert and manufactory easily. The sharp corner on inserts which embedded may cause crack of plastic injected parts.

(b)Male insert as bolts and female inserts as nuts should be provided with shoulders to help prevent the plastic compound from flowing into the threads. Avoid using an insert that has not been provided with a shoulder which the flash may flow on the thread. A single sealing shoulder is better. A double sealing shoulder is the best, but it is more expensive.

(c)Blind female insert with internal threads. (d) Open ends female insert with internal threads. (e)Blind hole female insert with internal threads and counter bore.

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                                (a)                                                                    (b)

 
                       (c)                                                 (d)                                                  (e)

Fig 1 Various designed inserts

Voids in plastic injected part

Voids are formed inside the wall of a part and can adversely affect the structural performance of the plastic injected part. Except in transparent parts, voids can be invisible to the naked eye. Voids are formed as a result of shrinkage of the molten core after the wall has solidified on the cooler mold surface. The shrinkage of the molten core cause the layers of material to pull away, forming an opening in the part or void. Here are some causes and solutions below for voids of plastic injected parts.

Higher melt temperature slow down the cooling rate of the polymer in the plasticinjection mold, causing higher shrinkage in the hotter core, leading to voids. Solutions: Lower melt temperatures can help reduce the formation of voids.

Injecting the material at high speeds raising melt temperature due to share heating can also cause overheating. Solutions: Low packing pressures will allow voids to form since there is not enough resistance applied on the core to prevent shrinkage.

A gate that freezes-off too soon will prevent material from filling out the cavity completely, causing the molten core to shrink. Solutions: Measuring gate seal time can indicate whether the cavity is full before the gate closes.

Small runner diameters will cool faster and hinder cavity filling, creating the potential for voids to form in the part. Solutions: Larger-diameter runners, such as full round, trapezoidal, and modified trapezoidal are recommended.

A cold mold will only enhance the formation of the skin surface long before the core cools sufficiently. As a result, the core forms more slowly and shrinks more. Solutions: Increasing plastic injection mold temperature slows down the cooling rate, and the formation of the skin prevents voids. Improvements can be made in mold cooling, such as the addition of bubblers, cooling pins, thermal pins, or steel inserts to drive out more heat from the part.

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